Full-Range Speaker Drivers Explained in Detail
Why discuss speaker drivers? Simply because the sound you hear originates from them. No matter how impressive the speaker cabinet (or lack thereof) or how perfect the crossover, without a good driver, everything is in vain. Therefore, the driver is crucial; this should be beyond doubt.
So, what's so remarkable about a midrange driver? It can't play very high or very low. But many say the midrange is the most important frequency band for sound reproduction, and I wholeheartedly agree. If you've ever been bored enough to try listening to Cai Qin sing through a single tweeter, or Paganini's violin through a single woofer, you'll deeply appreciate the charm of a midrange driver. I think you'd also agree that if forced to listen to music with only one driver, you'd choose something that looks like a midrange unit. The reason is simple: you know (or guess) it will reproduce the midrange frequencies where human hearing primarily lies and where the core of music resides.
Midrange Driver Design
The "One Finger Jiang" concept for tweeters can extend into the midrange domain, as any driver can be deconstructed into a vibrating diaphragm, its suspension, and a motor system. However, due to different operating frequency ranges, these components have evolved over many years towards specific size ranges. Yet, their shapes and materials show considerable variation, especially diaphragm materials, which have become highly diverse recently. Let's examine them one by one:
Paper Cone Diaphragms
This is likely the oldest material. Essentially, a paper pulp suspension is poured into a cone-shaped mesh mold. The pulp deposits onto it, and once the desired thickness is reached, it's lifted out for drying and further processing, forming a paper cone. The composition of the pulp (fiber types, lengths, fillers) and the papermaking process and post-treatment (e.g., air drying vs. hot pressing) all affect the final product's characteristics and thus its sound. Naturally, these are closely guarded trade secrets (Note 1)...
(Note 1: Years ago, I read an article by Mr. Hong Huaigong detailing paper cone manufacturing. Besides marveling at the profound knowledge involved, I deeply admired Mr. Hong's research spirit. My few sentences here cannot capture the essence accumulated by pioneers over many years.)
Generally, paper cones offer a smooth, natural sound, clear and lively without being nervous. Because countless intertwined fibers quickly absorb energy within the cone, damping is excellent. This minimizes pronounced cone breakup resonances at the high end of its operating range, resulting in a smooth roll-off. This is a good trait, allowing simple crossovers without extra tailoring, leading to healthy system integration. Additionally, paper has good rigidity, benefiting transient response and perceived detail. Don't be fooled by common soft paper; with appropriate shape and thickness, paper can achieve excellent rigidity. Furthermore, well-designed and made paper cones can be very light, even lighter than the lightest plastic cones by 15% or more. While slightly heavier than the latest high-tech synthetic fibers, the difference is small, leading to high efficiency. Audax's 6.5-inch paper cone midrange PR170 series boasts 100dB/W efficiency.
A potential weakness is that characteristics can change with ambient humidity. Absorbed moisture increases density (making it heavier) and reduces rigidity (making it softer), altering its sound. Whether this change is good or bad is debatable; members of the British Lowther club claim their speakers sound better on rainy days.
A greater concern might be material fatigue over repeated dry-wet cycles, changing its original properties. However, many vintage paper cone drivers still perform well after decades, suggesting this change is gradual and steady, perhaps maturing into a new stable state, which shouldn't be a major concern for users.
Many modern paper cones incorporate various improvements for greater stability, such as surface coatings or specialized pulp formulations. Some manufacturers claim their paper cones are waterproof, as seen in some outdoor PA speakers. Of course, as mentioned, we laypeople mostly see the surface; understanding the intricacies is harder. Also, never equate paper's long history with "outdated." From an industry perspective, paper cone drivers dominate the market. Check your TV, portable radio, bedside stereo, computer... aren't most using paper cone drivers in small speakers? You say, "Hey! These can't compare to my high-tech Hi-End speakers!" But consider: if these "inferior" products used non-paper cones, they'd sound worse and cost more. Paper is mature, offering excellent cost-effectiveness. Moreover, many time-tested legendary and cutting-edge flagship speakers feature paper cones: WE/Altec 755A full-range, Goodmans Axiom 80 full-range, Altec A5/A7, AR 3a, Lowther full-range, TAD... the list is long. Devotees declare: "Give me paper, nothing else!" Many consider paper cone making more art than science, highlighting its captivating appeal.
Plastic Diaphragms
With the petrochemical industry's advancement, plastic products are ubiquitous. Low-cost raw materials and simple processing naturally attract various industries, including audio.
Here, plastic diaphragms refer to integrally molded cones, usually via injection molding. The most common material is Polypropylene (PP). We encounter PP daily in microwave-safe containers and food storage boxes, typically injection molded. PP is also used in strapping tape. This reveals a key trait: extreme toughness. Most polymers are very tough. Their massive, irregular molecular structures quickly absorb and dissipate mechanical energy, providing excellent damping. Like paper, this results in a smooth high-end roll-off, subjectively smooth, and allows simple, low-order crossovers. We can sense these good traits in many European two-way bookshelf speakers. ProAc's SCAN series uses a 6.5-inch transparent PP cone mid-bass, arguably the best example.
However, PP has relatively poor rigidity and is heavier than other materials. While a food container hurts when dropped on your head, it doesn't mean it has good rigidity under the microscopic, high-speed, small-excursion conditions crucial for drivers. PP's weaker rigidity causes inconsistency in transmitting voice coil energy across the entire cone during high-frequency operation (cone breakup). Good damping suppresses breakup resonances, but imperfect piston motion increases distortion. Subjectively, this can mean smoothness at the cost of resolution and dynamics. Some two-way speakers using 8-inch PP cones can sound sluggish and dull in the mid-to-upper bass. Smaller cones can mitigate this issue, as the required thickness for sufficient rigidity increases weight disproportionately with larger areas. Thus, you won't find high-efficiency speakers using PP cones.
Unlike paper, PP isn't hygroscopic, but its characteristics can change with temperature. This shouldn't worry us much, as changes are gradual, like paper and humidity.
Despite PP's drawbacks (poorer rigidity, higher mass), its use depends on compromises. As mentioned with the Scan driver, criticized PP can still make successful products. Or, PP can be improved by adding modifiers to enhance rigidity. This brings varying degrees of improvement in dynamics, distortion, detail, and efficiency. Dynaudio and Infinity/Genesis use such treated cones. Different additives and processes yield noticeable results.
Convenient petrochemicals and molding invite new materials beyond PP, like Bextrene, TPX, or Neoflex. Their chemistry is often proprietary, but they look similar to PP. Better rigidity and lower mass offer improved dynamics and resolution. You'll see these materials in ads and catalogs; verify their claims if possible.
Metal Diaphragms
Since weak rigidity harms dynamics and resolution, using high-rigidity metals should yield good results. Excluding compression drivers for horns, aluminum and its alloys are common for direct-radiating midrange/woofers. The main advantage is high rigidity. Within its operating range, it doesn't deform, resulting in low distortion and excellent detail. However, high rigidity means low internal damping. Like the "One Finger Jiang" tweeter, energy isn't absorbed by the material, so cone breakup produces pronounced resonant peaks at the high end of the frequency response. If not handled properly, "metallic sound" easily occurs.
Proper handling involves crossover design suppressing this peak, placing it within the filter's stopband or beyond. This "hides" the peak. Achieving this usually requires at least a second-order crossover slope. A first-order slope is too gentle. Lowering the crossover point sacrifices usable bandwidth, which is unhealthy. Therefore, high-order slopes and careful crossover point selection are crucial for metal diaphragms.
Alternatively, instead of avoiding the issue, actively improve damping: sandwich structures or damping coatings help. Many successful products exist, like Elac or the expensive Swiss Ensemble.
Besides high-frequency challenges, diaphragm weight is another disadvantage. Cost prevents titanium midranges. So, metal cone mid/woofers offer excellent dynamics under strong drive but generally have lower efficiency, requiring more power.
Synthetic Fiber Materials
Historically, advanced materials often debut in weapons, a human tragedy. Using them in audio for music appreciation would foster harmony. Boron/carbon fiber and honeycomb sandwich structures, proven effective in fighter jets years ago, are now used in audio.
Being aerospace-grade, these materials combine lightness and high strength. They can be lighter than paper and stronger than metal (even exceeding steel strength). They seem ideal for speaker diaphragms. Manufacturers of Kevlar or carbon fiber drivers heavily promote high rigidity, low mass, and high damping. The first two are true, but self-damping depends on conditions and isn't necessarily better.
(Note 2: This refers to optimal results achievable with other forming methods, not meaning a thin driver cone is harder than your kitchen knife – not yet, at least.)
Without proper treatment, these high-rigidity fibers face issues similar to metal cones: high-frequency breakup resonance. Less severe than metal, it's still audible and easily annoying. Years ago, a review criticized Kevlar midrange harshness.
With enhanced damping (sandwich or coatings) and proper crossover, such drivers exhibit excellent detail, fast transient response, superb macro/micro dynamics, requiring minimal power. Focal's Audiom 7K, using a Kevlar/polymer foam sandwich with latex coating, achieves 98dB/W efficiency, respectable compared to Audax paper's 100dB/W (Note 3).
(Note 3: Comparing specs, Focal Audiom 7K has a larger magnet (1132g vs. 880g) and lower moving mass (7.3g vs. 9.1g), yet efficiency is lower than the "less powerful" Audax. This shows other factors like suspension compliance, magnetic circuit design, voice coil, cone shape involve much compromise.)
Beyond common Carbon and Kevlar Fiber drivers, a special synthetic fiber diaphragm emerged years ago – HAD (High Definition Aerogel) by Audax. Made from acrylic polymer gel and various synthetic fibers (including Carbon and Kevlar), its properties are excellent. Measurements show superb transient response, very low distortion, and smooth high-frequency roll-off without resonant peaks. Current products have lower efficiency than paper or Kevlar, likely due to magnetic circuit design choices. Other aspects are formidable. Swans' Allure, designed by Stereophile's Martin Colloms, uses this driver. My brief listening: relaxed and natural like fine paper, with more modern resolution and dynamics, no apparent flaws – a very successful design (system integration deserves credit).
(Note 4: This gel/fiber process is unique. From start to finish, the gel volume shrinks to one-tenth. Crucially, polymer chains grow along pre-added fibers, making molecular alignment controllable. This yields excellent rigidity and self-damping.)
Other Materials
Many other lightweight, strong materials make speaker diaphragms: fiberglass, celluloid fiber, graphite fiber, Bakelite, silk fiber, foamed polystyrene, various foamed plastics, vacuum-sintered precision ceramics... Many show promise, some for tweeters, some for mids, some for woofers, some full-range. Someone in Japan even developed a process using a special plant (mold) to "grow" a cone on a mold! Reportedly, its natural sound surpasses any material. However, mass production seems unlikely due to high cost (time cost).
(Caution: Many drivers disguise their diaphragm material or make it resemble another. This borders on counterfeiting. As helpless consumers, we must be cautious.)
Magnetic Circuit System
Having explored diverse diaphragms, let's examine the magnetic circuit. Previous articles covered magnet materials. We'll focus on the overall magnetic circuit design, including the voice coil, as they function together.
Simply put, the cone moves via the voice coil. The voice coil moves because its current-induced magnetic field interacts with the fixed field from magnets and poles. The voice coil design and magnetic gap width/length deserve discussion.
Voice Coil Design
As the name implies, it's a coil for sound. Enameled wire is tightly wound onto a former using special adhesive. Wire materials include copper, aluminum, silver, or alloys. Cross-sections are often rectangular or hexagonal to maximize winding density. More turns mean stronger magnetic force, better driving power, higher cone acceleration, efficiency, and dynamics. For flat wire, a 1:5 aspect ratio rectangle wound with the short side against the former can provide 30% higher acceleration, efficiency, and dynamics than round wire.
(Note 5: Voice coil length is the axial length of the wound coil, not the unwound length.)
The winding tension is immense. Try winding string tightly around your finger ten times; you'll quickly want to remove it. Some voice coils exert tons of pressure on the former! So formers must be very strong. Also, to withstand voice coil heat, formers must be heat-resistant. Aluminum (alloy), Kapton, or other lightweight, strong, heat-resistant materials are common. Reputable manufacturers subject wound coils to multiple heat treatments for stability.
Klipsch's Jim Hunter mentioned in a "Speaker Builder" interview receiving a repaired speaker where a horn driver fell from its melted plastic throat, indicating extreme heat. Yet the voice coil assembly inside was still intact!
Voice coil size involves trade-offs. For high driving force (efficiency, dynamics), a large-diameter, long coil is suitable. But this increases weight and inductance, harming transients and highs. A long coil means only part is within the magnetic gap, reducing control and increasing distortion from voice coil field modulation. A very small coil is light but lacks driving force, compromising efficiency, control, and power handling. So size must be optimized with cone area, shape, and magnet strength.
Magnets and Magnetic Circuit
Traditionally, speaker magnets are axially magnetized (poles parallel to the hollow cylinder's axis). Ferromagnetic pole pieces guide flux into the gap, completing the circuit. The voice coil needs radial flux in the gap (parallel to the radius). Gap flux density stems from magnet strength, related to magnet type/size. Ferrite ceramic magnets dominate (Iron Oxide). They resist temperature change, demagnetization, offer good mechanical strength/corrosion resistance, and crucially, low cost. But achieving unit flux strength requires large size/weight. High efficiency demands large magnetic structures.
Tweeters/horn drivers always have magnets larger than the diaphragm. Some 6-7" midranges have magnets nearly matching diaphragm size. Some professional 10-12" mid-woofers also have matching magnets!
High magnetism is desirable (efficiency, dynamics, control). But large magnets look impressive yet offer little else, potentially harming sound propagation. A large magnet blocks the cone rear. Rear sound waves must squeeze out sideways; some reflect back onto the cone. If mounted on thick baffles, the problem worsens. The gap between cone and magnet might match baffle thickness. Without treatment, rear waves "jet" out a narrow ring. The cone back then faces strong near-field reflections and pressure changes, severely harming frequency response and distortion.
Using large-magnet drivers necessitates proper baffle treatment – channels carved inside to guide rear waves out (e.g., Theil). Or use strong, thin metal baffles.
Driver frames pose similar challenges. Older stamped steel frames have wide supports. If close to the cone, they increase rear wave reflection and coloration. New cast aluminum frames achieve better shapes, balancing strength, aesthetics, and low coloration.
Alternatively, use high-strength, compact magnets (e.g., Neodymium) for better rear wave dispersion. Vendersteen used Vifa-made mids with small Neodymium magnets. Wilson Benesch's flagship Bishop, using face-to-face Isobaric woofers with magnets facing out, employs newer, stronger Neodymium Iron Boron magnets. Poles are rounded, and frames minimize frontal area. Lowther, a legendary full-range driver, despite its age, carefully addresses this. Though magnets are large, their shape is streamlined, freeing space behind the cone. Frame struts are narrow-edged to minimize rear wave obstruction.
Another factor affecting performance is voice coil/magnetic circuit interaction. Strictly, the voice coil and magnetic circuit push/pull each other. The magnetic circuit is fixed, making it seem like magnets drive the voice coil.
(Note 6: Vendersteen's philosophy is sound, investing in unseen areas. Packaging is simple, sound is balanced, musical. Sadly, its shape isn't favored locally.)
Acknowledging this reveals issues: 1. Voice coil field can demagnetize magnets. Magnets must withstand this to maintain dynamics, power, efficiency. Magnet demagnetization resistance affects sound; Alnico magnets' mid-high charm relates to this. 2. Voice coil field modulates gap flux, causing distortion. Copper-plated poles or copper shorting rings eliminate flux modulation, drastically reducing distortion. This especially improves mid/woofer intermodulation distortion, as long woofer excursions and fast midrange movements complexly modulate the field.
Magnetic Circuit Dilemma vs. Innovative Polarization & Pole Structure
Earlier, I mentioned traditional axially magnetized structures. But the voice coil ultimately needs radial flux. Why not magnetize radially initially? Manufacturing difficulty and high cost delayed radial polarization until ~5 years ago.
Disadvantages of axial magnetization: 1. Larger size; 2. Difficulty achieving high flux density with deep gaps. Size was discussed. Now, gap intricacies.
In traditional systems, gap length roughly equals the top plate thickness at the gap. For higher flux density, narrow the gap. But this complicates voice coil assembly, increasing cost. Also, pole saturation must be avoided, requiring consideration of material/thickness.
Long-gap/short-coil configurations face lower flux density. Combined with a short coil, efficiency drops significantly. While offering good power linearity, achieving high efficiency requires overcoming many compromises.
Altec 515 series and TAD 160X series use short-coil/long-gap designs, achieving excellent power linearity and ultra-high efficiency – a triumph of engineering.
Radially polarized magnets easily achieve high flux density and long gaps (cost remains high). Gap length with equal flux density can be several times longer than traditional designs, meaning several times more linear excursion! Distortion at high SPL is very low. This suits bass reproduction. Existing products are professional 18" woofers (Note 7), claiming maximum linear SPL beyond human tolerance with still-low distortion.
(Note 7: Aura Sound 1808. Not B&W's Aura brand.)
Sadly, no radially polarized midranges exist yet. Though mids don't need long excursions, this compact, high-strength design benefits mids too. Likely in some lab; expect products soon.
Stepping into Full-Range
Huh? Isn't this article about full-range drivers? Why spend so long on mids?
Don't blame me! Full-range operation faces many challenges. Starting from mids provides clearer context.
An ideal driver (tweeter/mid/woofer) should:
1. Low Distortion;
2. Good Power Linearity;
3. High Efficiency;
4. Wide Effective Bandwidth.
Maximizing point 4 creates a full-range driver. Next time, I'll discuss extending a midrange driver to full-range operation, the compromises involved, and ingenious solutions – equally fascinating. Stay tuned.
At first glance, it seems simple: make a midrange driver play higher and lower frequencies, and it's full-range? Look at car audio, computer speakers, portable radios, bedside systems – aren't "full-range" drivers everywhere? Not a big deal!
It's not that simple. Do you know the bandwidth of those generic drivers? You don't need measurements to hear that clear vocals are an achievement; drums/cymbals are often barely recognizable; bass/high percussion is faint. Pipe organ? String harmonics? Piano decay? Forget it!
What constitutes full-range? See the sidebar. Now, let's explore the design challenges of one driver covering the entire audio spectrum.
Low-End Extension
Visually, similarly sized (e.g., 6" or 7") cone mids and woofers look similar. Woofers need larger excursions, so they have wider, softer surrounds. Otherwise, they "look" similar. But this is a generality.
Can you modify a 6-7" midrange to play bass? If only output matters (not SPL/distortion), yes. Generally, a driver's low-frequency limit is indicated by its free-air resonance frequency (fs).
How to lower fs? Acoustic impedance (Note 2), moving mass, magnetic strength, and suspension compliance are key. Acoustic impedance relates to radiating area and frequency. For similar size/direct radiation/similar band, it's comparable. So, focus on other factors.
Low-frequency operation involves slow reciprocating motion. Basic physics: Acceleration is inversely proportional to mass under constant force. So, heavier cones generally have lower fs. Comparing driver specs confirms this. 15"+ woofers with fs below 25Hz often exceed 100g moving mass.
Lowering fs by adding mass is easy but bad: heavy cones reduce efficiency and worsen high-frequency extension. Dead end. Next, reduce external damping – mechanical and electrical. Damping applies braking force.
Car suspension analogy: Traditional American cars have soft suspension (low spring rate, compliant shocks) for comfort. This yields low system tuning frequency, absorbing bumps (high-frequency pulses) effectively. But long-wave undulations (low-frequency pulses) cause 2-3 cycles of oscillation – system resonance excited.
Similarly, increasing suspension compliance lowers resonance frequency. Straightforward.
But this approach has issues:
Mechanical Damping: Refers to braking force from the surround and spider. Besides damping overall motion, the suspension (especially the surround) suppresses cone breakup. Different surrounds drastically change tonality. Reducing damping to lower fs increases coloration, especially in the mids. Adjust carefully.
Electrical Damping: Refers to magnetic control over the voice coil. Stronger magnets mean more driving force and braking. We want strong drive for efficiency/low distortion, but high damping prevents lowering fs. Here's the dilemma. Adding high-end extension issues complicates compromise.
High-End Extension
Factors affecting high-frequency performance include electrical and mechanical elements, similar yet different. Electrical factors mean voice coil inductance. Voice coils are inductors. Alone, it's an air-core inductor – low inductance, linear. Unfortunately, the voice coil operates within the magnetic circuit, becoming an iron-core inductor. Inductance increases significantly, and the inherent low-pass characteristic attenuates highs. Worse, the relative position of voice coil and pole piece constantly changes, causing complex inductance/field interaction and modulation. Distortion rises sharply at high volume/wide bandwidth. Subjectively: blurred, rough, flattened texture, collapsed imaging, compressed soundstage. Solution: Copper plating the pole piece or inserting copper rings "shorts" the field, drastically reducing modulation and inductance. This extends highs and lowers distortion.
Mechanical factors involve basic physics: Force = Mass x Acceleration (F=ma). Acceleration is the rate of velocity change. Imagine a cone decelerating during forward stroke, stopping at peak excursion, then accelerating backward. At 20kHz, this entire process happens in 1/40,000th of a second! Calculate the peak acceleration for a half-cycle harmonic motion; it's immense!
Reproducing such highs requires immense cone acceleration. From F=ma, only two paths: reduce cone mass or increase driving force. But this brings dilemmas and contradictions.
Cone Mass
Adding mass lowers fs easily, but harms high-frequency response and efficiency. Instead, let the driver "see" a heavy cone at low frequencies and a light one at highs. Sounds weird?
This is a clever trick in full-range design: "mechanical" crossover. In practice: at low frequencies, the entire cone moves. Toward highs, cone breakup causes the heavier, acoustically resistive outer part to "lag." Only the inner part moves with the voice coil, significantly lighter than the whole area. Thus, the "effective" moving mass varies with frequency, enabling both high and low response.
"Cone breakup" sounds simple, but controlling which parts lag and preventing uncontrolled movement (causing coloration/distortion) is extremely difficult. Music contains wide, constantly changing frequencies. Uncontrolled breakup causes horrific distortion.
Driving Force
Strong driving force is needed for high acceleration. Sources: voice coil and magnetic circuit. More voice coil turns increase magnetic force for interaction, but increase inductance and mass, harming highs. Not feasible; compromise is needed. "Small is beautiful" beats "large and clumsy."
We must increase magnetic strength. Though strong magnets cause high damping, raising fs, high acceleration demands stronger magnets than typical drivers to push the "not-light" cone (Note 4). Over-damping is compensated by relaxing mechanical damping.
(Note 4: Lowther cones are relatively heavy.)
System Integration Issues
Only one driver – what "system" integration? Two aspects: 1. Fine-tuning tonal balance; 2. Enclosure tuning. They are interrelated.
Theoretically, an ideal full-range driver should connect directly to an amp on a suitable baffle and produce heavenly sound. But given the compromises designers make, perfection is impossible. Like instrument making, achieving perfect tone and even loudness across the range demands immense effort; speaker drivers are mere imitators.
A full-range driver covers the range but may not be flat. Common problems: A broad midrange bump (upper or lower mids) causes coloration; Some have a gentle high-end roll-off, sounding dark; Over-damping causes low-end roll-off, sounding thin/tight with weak bass.
A slight bump causing unacceptable coloration can be flattened with a notch filter. If mild, results are often satisfactory. Don't disdain this; it's simple frequency correction. Multi-way speakers have complex overlaps and phase distortion. Notch filters exist in many speaker crossovers.
High-end roll-off usually stems from insufficient magnetic strength or a large cone overwhelming "mechanical crossover." Vintage 12" or 15" full-rangers often suffer this. Adding a tweeter is the only solution. You say, "What full-range is that?" Don't judge. Properly implemented, crossing the tweeter in gently (e.g., -6dB/octave from 16-18kHz or higher) yields good results. The crossover point avoids sensitive hearing ranges; first-order maintains phase coherence, retaining most full-range benefits. (If you have Altec 412Cs and dislike their lack of highs, contact me. After I fix them, you can't buy them back.)
Low-end roll-off indicates strong damping. Bass sounds tight/short. Benefit: clear detail. Proper enclosure tuning or horn loading can increase low-end acoustic resistance, boosting efficiency. If done well, this combination offers the best full-range performance.
Enclosure tuning: Over 90% of commercial speakers use sealed or ported (bass reflex) enclosures. Full-range drivers benefit from minimized cone excursion at low frequencies. Large excursions increase distortion and affect mids/highs. Imagine small, fast mid/high movements "riding" large, slow bass movements – high intermodulation and Doppler distortion. All drivers face this, but full-range drivers cover wider bands, making it more pronounced. Avoid or minimize it.
Of the two main types, ported enclosures suit full-range drivers better. They significantly reduce cone excursion near system resonance (typically 30-50Hz). This reduces distortion, increases power handling, and improves efficiency. Most full-range drivers work acceptably this way.
Purists argue good drivers are "tainted" by cabinet resonance, preferring open baffles. Drivers with sufficient bass work well here, achieving purest sound (e.g., WE/Altec 755C). Reportedly, midrange transients rival electrostatic speed with better dynamics. Drawbacks: Large footprint (baffle size determines bass extension; at least 1m²); Lower efficiency/power handling/ bass response; Dipole radiation complicates room interaction; Two large panels aren't widely acceptable.
Finally, the most complex: horn loading. We'll detail horns later. Briefly, a horn is a flared pipe. The wide end is the "horn mouth," the narrow end the "throat." The shape creates higher acoustic resistance at the throat than mouth, coupling diaphragm and air molecules well, yielding high efficiency.
Back-loaded folded horns, properly implemented, effectively boost mid-bass to bass efficiency, perfectly matching overdamped drivers. Driver Highlights
Many manufacturers have produced full-range drivers. I can't list them all. Below are some notable examples, some still in production, others vintage.
Jordan Watts
Unique design: Aluminum cone. Abandons traditional corrugated spiders for special linear suspensions with high compliance. My introduction was through "Vases" – bought on clearance, attracted by quaint looks. Surprisingly good sound: 6" aluminum cone delivers surprisingly "full-range" sound in my 10-ping room, bass is decent. Medium volume with small ensembles: pure, charming. Cons: Mid-bass coloration (fatty sound), but I stop noticing after 30 minutes. Low efficiency affects clarity at higher volumes. Their 2" aluminum driver is a classic – limited bass/efficiency, but excellent otherwise. Superb pulse response tests; subjectively fresh and delightful.
Diatone P-610 Series
Historically popular. 6.5" paper cone, Alnico magnet, 90dB/W efficiency, bass to 50Hz – good for full-range. Surface ridges control cone breakup for "mechanical crossover." Original P-610 (up to Mk.4) discontinued in 1993. Limited commemorative editions released. I haven't heard them, but reliable sources call them the most "comprehensive" full-range – cleverly compromised. Smooth, sweet sound, superb imaging, fine microdynamics, easy to use (ported enclosure works). Reportedly, a perfect match for single-ended triodes, especially 2A3.
"WE/Altec 755A/C" Legendary 8" paper cone full-range, high efficiency. 755A specs: 70Hz-13KHz, 8W power handling; 755C: 40Hz-15KHz/15W. A raised ring on the cone front acts as mechanical crossover control.
Vintage WE units are rare. Altec units I've only seen incomplete. Looks unremarkable, frame/magnet potentially cause rear reflections. Yet, DIY enthusiasts abroad highly praise its purity, comparing it to Quad electrostatics with better dynamics. Violinist/DIYer Joseph Esmilla (Sound Practices) used them on simple open baffles with 2A3/300B amps for impeccable musicality.
Goodmans Axiom 80
Another legend! I frequent friend Li Jiande's "vintage museum." Over a year ago, I spotted half a dark green driver (magnet/frame). Familiar? "Goodmans," he said casually. GOODMANS!!! I grabbed it, examined reverently. "Looks like it sounds wonderful" (Note 5), hearing imagined soundscapes. Suddenly, he snatched it back, handed me a mop: "Wipe the drool." I planned to buy them, but he returned them to the owner. Devastated! Still sighing today...
Axiom 80: UK-made 50s-60s classic. Unique frame/suspension, small Alnico(?) magnet. Specs: 20Hz-20KHz(!!), 6W power handling. Haven't heard it, but reliable sources confirm specs! Like Lowther, needs excellent back-loaded horn for best performance; amp preferably 2A3 (300B too powerful!).
Lowther Series
Iconic Lowther, over 50 years history. Traces to P.G.A.H. Voigt's 1920s moving-coil driver and 1930s double-cone patent.
Notable: Distinctive white paper cone; elegant, structurally sound frame; high-end models have mushroom phase plugs. Double-cone structure enables mechanical crossover: Entire cone moves at low frequencies. Toward highs, outer cone breaks up, inner cone continues. Phase plug prevents inner cone high-frequency cancellation, improving dispersion. Mushroom plug forms a slot load with inner cone, boosting high efficiency for front horn loading. A new UFO-shaped phase plug reportedly improves high-frequency response significantly, fitting all Lowthers.
Cone material/construction: Still uses hand-made laminated paper cones. Lowther believes flat paper offers best thickness uniformity vs. molded cones. Uneven thickness causes local resonances/coloration; thick spots add weight. Precise craftsmanship.
Magnetic circuits: Ferrite ceramic, Alnico, Neodymium. All Lowthers are strong/efficient, but differ. Ceramic magnets are cheapest, specs least impressive; sound is easiest to listen to, less fussy. Alnico is most expensive/strongest (PM-4A: 2.4 Tesla flux density, highs to 22kHz). Neodymium aims for Alnico strength in smaller/lighter/cheaper packages. Top models price similarly. Neodymium offers modern specs/sonic character; sound vs. Alnico is subjective.
Requires back-loaded horns for best performance (cone excursion only 1mm). Recently, Lowther American Club published ported designs claiming flat bass to 40Hz with better mid-bass detail. Debate awaits.
Sound: Almost all Lowthers share traits: Superb presence, stunning detail, lightning transients. Measurements often show slight upper-mid bump; poor off-axis response; small sweet spot. Polarizing: Devotees swear by them; detractors find them un-HiFi. Europe has Lowther Clubs. The trend spread to the US. Meticulous Japanese embraced them early. Now you know. Join the club?
Conclusion
Full-range drivers, used appropriately, offer unmatched musical satisfaction. Full-band phase coherence, no crossover eating signal, exquisite microdynamics/musical expression, superior soundstage/imaging – unattainable by multi-way speakers. But remember, nothing's perfect. If you listen to heavy metal at 120dB, use speakers for AV gunfire, or sing karaoke loudly, avoid full-range drivers – both you and the drivers will suffer.
Cherish these treasures. Play simple music at modest volumes for maximum soul-stirring impact. The music itself becomes profoundly moving; volume becomes irrelevant.
So, what's so remarkable about a midrange driver? It can't play very high or very low. But many say the midrange is the most important frequency band for sound reproduction, and I wholeheartedly agree. If you've ever been bored enough to try listening to Cai Qin sing through a single tweeter, or Paganini's violin through a single woofer, you'll deeply appreciate the charm of a midrange driver. I think you'd also agree that if forced to listen to music with only one driver, you'd choose something that looks like a midrange unit. The reason is simple: you know (or guess) it will reproduce the midrange frequencies where human hearing primarily lies and where the core of music resides.
Midrange Driver Design
The "One Finger Jiang" concept for tweeters can extend into the midrange domain, as any driver can be deconstructed into a vibrating diaphragm, its suspension, and a motor system. However, due to different operating frequency ranges, these components have evolved over many years towards specific size ranges. Yet, their shapes and materials show considerable variation, especially diaphragm materials, which have become highly diverse recently. Let's examine them one by one:
Paper Cone Diaphragms
This is likely the oldest material. Essentially, a paper pulp suspension is poured into a cone-shaped mesh mold. The pulp deposits onto it, and once the desired thickness is reached, it's lifted out for drying and further processing, forming a paper cone. The composition of the pulp (fiber types, lengths, fillers) and the papermaking process and post-treatment (e.g., air drying vs. hot pressing) all affect the final product's characteristics and thus its sound. Naturally, these are closely guarded trade secrets (Note 1)...
(Note 1: Years ago, I read an article by Mr. Hong Huaigong detailing paper cone manufacturing. Besides marveling at the profound knowledge involved, I deeply admired Mr. Hong's research spirit. My few sentences here cannot capture the essence accumulated by pioneers over many years.)
Generally, paper cones offer a smooth, natural sound, clear and lively without being nervous. Because countless intertwined fibers quickly absorb energy within the cone, damping is excellent. This minimizes pronounced cone breakup resonances at the high end of its operating range, resulting in a smooth roll-off. This is a good trait, allowing simple crossovers without extra tailoring, leading to healthy system integration. Additionally, paper has good rigidity, benefiting transient response and perceived detail. Don't be fooled by common soft paper; with appropriate shape and thickness, paper can achieve excellent rigidity. Furthermore, well-designed and made paper cones can be very light, even lighter than the lightest plastic cones by 15% or more. While slightly heavier than the latest high-tech synthetic fibers, the difference is small, leading to high efficiency. Audax's 6.5-inch paper cone midrange PR170 series boasts 100dB/W efficiency.
A potential weakness is that characteristics can change with ambient humidity. Absorbed moisture increases density (making it heavier) and reduces rigidity (making it softer), altering its sound. Whether this change is good or bad is debatable; members of the British Lowther club claim their speakers sound better on rainy days.
A greater concern might be material fatigue over repeated dry-wet cycles, changing its original properties. However, many vintage paper cone drivers still perform well after decades, suggesting this change is gradual and steady, perhaps maturing into a new stable state, which shouldn't be a major concern for users.
Many modern paper cones incorporate various improvements for greater stability, such as surface coatings or specialized pulp formulations. Some manufacturers claim their paper cones are waterproof, as seen in some outdoor PA speakers. Of course, as mentioned, we laypeople mostly see the surface; understanding the intricacies is harder. Also, never equate paper's long history with "outdated." From an industry perspective, paper cone drivers dominate the market. Check your TV, portable radio, bedside stereo, computer... aren't most using paper cone drivers in small speakers? You say, "Hey! These can't compare to my high-tech Hi-End speakers!" But consider: if these "inferior" products used non-paper cones, they'd sound worse and cost more. Paper is mature, offering excellent cost-effectiveness. Moreover, many time-tested legendary and cutting-edge flagship speakers feature paper cones: WE/Altec 755A full-range, Goodmans Axiom 80 full-range, Altec A5/A7, AR 3a, Lowther full-range, TAD... the list is long. Devotees declare: "Give me paper, nothing else!" Many consider paper cone making more art than science, highlighting its captivating appeal.
Plastic Diaphragms
With the petrochemical industry's advancement, plastic products are ubiquitous. Low-cost raw materials and simple processing naturally attract various industries, including audio.
Here, plastic diaphragms refer to integrally molded cones, usually via injection molding. The most common material is Polypropylene (PP). We encounter PP daily in microwave-safe containers and food storage boxes, typically injection molded. PP is also used in strapping tape. This reveals a key trait: extreme toughness. Most polymers are very tough. Their massive, irregular molecular structures quickly absorb and dissipate mechanical energy, providing excellent damping. Like paper, this results in a smooth high-end roll-off, subjectively smooth, and allows simple, low-order crossovers. We can sense these good traits in many European two-way bookshelf speakers. ProAc's SCAN series uses a 6.5-inch transparent PP cone mid-bass, arguably the best example.
However, PP has relatively poor rigidity and is heavier than other materials. While a food container hurts when dropped on your head, it doesn't mean it has good rigidity under the microscopic, high-speed, small-excursion conditions crucial for drivers. PP's weaker rigidity causes inconsistency in transmitting voice coil energy across the entire cone during high-frequency operation (cone breakup). Good damping suppresses breakup resonances, but imperfect piston motion increases distortion. Subjectively, this can mean smoothness at the cost of resolution and dynamics. Some two-way speakers using 8-inch PP cones can sound sluggish and dull in the mid-to-upper bass. Smaller cones can mitigate this issue, as the required thickness for sufficient rigidity increases weight disproportionately with larger areas. Thus, you won't find high-efficiency speakers using PP cones.
Unlike paper, PP isn't hygroscopic, but its characteristics can change with temperature. This shouldn't worry us much, as changes are gradual, like paper and humidity.
Despite PP's drawbacks (poorer rigidity, higher mass), its use depends on compromises. As mentioned with the Scan driver, criticized PP can still make successful products. Or, PP can be improved by adding modifiers to enhance rigidity. This brings varying degrees of improvement in dynamics, distortion, detail, and efficiency. Dynaudio and Infinity/Genesis use such treated cones. Different additives and processes yield noticeable results.
Convenient petrochemicals and molding invite new materials beyond PP, like Bextrene, TPX, or Neoflex. Their chemistry is often proprietary, but they look similar to PP. Better rigidity and lower mass offer improved dynamics and resolution. You'll see these materials in ads and catalogs; verify their claims if possible.
Metal Diaphragms
Since weak rigidity harms dynamics and resolution, using high-rigidity metals should yield good results. Excluding compression drivers for horns, aluminum and its alloys are common for direct-radiating midrange/woofers. The main advantage is high rigidity. Within its operating range, it doesn't deform, resulting in low distortion and excellent detail. However, high rigidity means low internal damping. Like the "One Finger Jiang" tweeter, energy isn't absorbed by the material, so cone breakup produces pronounced resonant peaks at the high end of the frequency response. If not handled properly, "metallic sound" easily occurs.
Proper handling involves crossover design suppressing this peak, placing it within the filter's stopband or beyond. This "hides" the peak. Achieving this usually requires at least a second-order crossover slope. A first-order slope is too gentle. Lowering the crossover point sacrifices usable bandwidth, which is unhealthy. Therefore, high-order slopes and careful crossover point selection are crucial for metal diaphragms.
Alternatively, instead of avoiding the issue, actively improve damping: sandwich structures or damping coatings help. Many successful products exist, like Elac or the expensive Swiss Ensemble.
Besides high-frequency challenges, diaphragm weight is another disadvantage. Cost prevents titanium midranges. So, metal cone mid/woofers offer excellent dynamics under strong drive but generally have lower efficiency, requiring more power.
Synthetic Fiber Materials
Historically, advanced materials often debut in weapons, a human tragedy. Using them in audio for music appreciation would foster harmony. Boron/carbon fiber and honeycomb sandwich structures, proven effective in fighter jets years ago, are now used in audio.
Being aerospace-grade, these materials combine lightness and high strength. They can be lighter than paper and stronger than metal (even exceeding steel strength). They seem ideal for speaker diaphragms. Manufacturers of Kevlar or carbon fiber drivers heavily promote high rigidity, low mass, and high damping. The first two are true, but self-damping depends on conditions and isn't necessarily better.
(Note 2: This refers to optimal results achievable with other forming methods, not meaning a thin driver cone is harder than your kitchen knife – not yet, at least.)
Without proper treatment, these high-rigidity fibers face issues similar to metal cones: high-frequency breakup resonance. Less severe than metal, it's still audible and easily annoying. Years ago, a review criticized Kevlar midrange harshness.
With enhanced damping (sandwich or coatings) and proper crossover, such drivers exhibit excellent detail, fast transient response, superb macro/micro dynamics, requiring minimal power. Focal's Audiom 7K, using a Kevlar/polymer foam sandwich with latex coating, achieves 98dB/W efficiency, respectable compared to Audax paper's 100dB/W (Note 3).
(Note 3: Comparing specs, Focal Audiom 7K has a larger magnet (1132g vs. 880g) and lower moving mass (7.3g vs. 9.1g), yet efficiency is lower than the "less powerful" Audax. This shows other factors like suspension compliance, magnetic circuit design, voice coil, cone shape involve much compromise.)
Beyond common Carbon and Kevlar Fiber drivers, a special synthetic fiber diaphragm emerged years ago – HAD (High Definition Aerogel) by Audax. Made from acrylic polymer gel and various synthetic fibers (including Carbon and Kevlar), its properties are excellent. Measurements show superb transient response, very low distortion, and smooth high-frequency roll-off without resonant peaks. Current products have lower efficiency than paper or Kevlar, likely due to magnetic circuit design choices. Other aspects are formidable. Swans' Allure, designed by Stereophile's Martin Colloms, uses this driver. My brief listening: relaxed and natural like fine paper, with more modern resolution and dynamics, no apparent flaws – a very successful design (system integration deserves credit).
(Note 4: This gel/fiber process is unique. From start to finish, the gel volume shrinks to one-tenth. Crucially, polymer chains grow along pre-added fibers, making molecular alignment controllable. This yields excellent rigidity and self-damping.)
Other Materials
Many other lightweight, strong materials make speaker diaphragms: fiberglass, celluloid fiber, graphite fiber, Bakelite, silk fiber, foamed polystyrene, various foamed plastics, vacuum-sintered precision ceramics... Many show promise, some for tweeters, some for mids, some for woofers, some full-range. Someone in Japan even developed a process using a special plant (mold) to "grow" a cone on a mold! Reportedly, its natural sound surpasses any material. However, mass production seems unlikely due to high cost (time cost).
(Caution: Many drivers disguise their diaphragm material or make it resemble another. This borders on counterfeiting. As helpless consumers, we must be cautious.)
Magnetic Circuit System
Having explored diverse diaphragms, let's examine the magnetic circuit. Previous articles covered magnet materials. We'll focus on the overall magnetic circuit design, including the voice coil, as they function together.
Simply put, the cone moves via the voice coil. The voice coil moves because its current-induced magnetic field interacts with the fixed field from magnets and poles. The voice coil design and magnetic gap width/length deserve discussion.
Voice Coil Design
As the name implies, it's a coil for sound. Enameled wire is tightly wound onto a former using special adhesive. Wire materials include copper, aluminum, silver, or alloys. Cross-sections are often rectangular or hexagonal to maximize winding density. More turns mean stronger magnetic force, better driving power, higher cone acceleration, efficiency, and dynamics. For flat wire, a 1:5 aspect ratio rectangle wound with the short side against the former can provide 30% higher acceleration, efficiency, and dynamics than round wire.
(Note 5: Voice coil length is the axial length of the wound coil, not the unwound length.)
The winding tension is immense. Try winding string tightly around your finger ten times; you'll quickly want to remove it. Some voice coils exert tons of pressure on the former! So formers must be very strong. Also, to withstand voice coil heat, formers must be heat-resistant. Aluminum (alloy), Kapton, or other lightweight, strong, heat-resistant materials are common. Reputable manufacturers subject wound coils to multiple heat treatments for stability.
Klipsch's Jim Hunter mentioned in a "Speaker Builder" interview receiving a repaired speaker where a horn driver fell from its melted plastic throat, indicating extreme heat. Yet the voice coil assembly inside was still intact!
Voice coil size involves trade-offs. For high driving force (efficiency, dynamics), a large-diameter, long coil is suitable. But this increases weight and inductance, harming transients and highs. A long coil means only part is within the magnetic gap, reducing control and increasing distortion from voice coil field modulation. A very small coil is light but lacks driving force, compromising efficiency, control, and power handling. So size must be optimized with cone area, shape, and magnet strength.
Magnets and Magnetic Circuit
Traditionally, speaker magnets are axially magnetized (poles parallel to the hollow cylinder's axis). Ferromagnetic pole pieces guide flux into the gap, completing the circuit. The voice coil needs radial flux in the gap (parallel to the radius). Gap flux density stems from magnet strength, related to magnet type/size. Ferrite ceramic magnets dominate (Iron Oxide). They resist temperature change, demagnetization, offer good mechanical strength/corrosion resistance, and crucially, low cost. But achieving unit flux strength requires large size/weight. High efficiency demands large magnetic structures.
Tweeters/horn drivers always have magnets larger than the diaphragm. Some 6-7" midranges have magnets nearly matching diaphragm size. Some professional 10-12" mid-woofers also have matching magnets!
High magnetism is desirable (efficiency, dynamics, control). But large magnets look impressive yet offer little else, potentially harming sound propagation. A large magnet blocks the cone rear. Rear sound waves must squeeze out sideways; some reflect back onto the cone. If mounted on thick baffles, the problem worsens. The gap between cone and magnet might match baffle thickness. Without treatment, rear waves "jet" out a narrow ring. The cone back then faces strong near-field reflections and pressure changes, severely harming frequency response and distortion.
Using large-magnet drivers necessitates proper baffle treatment – channels carved inside to guide rear waves out (e.g., Theil). Or use strong, thin metal baffles.
Driver frames pose similar challenges. Older stamped steel frames have wide supports. If close to the cone, they increase rear wave reflection and coloration. New cast aluminum frames achieve better shapes, balancing strength, aesthetics, and low coloration.
Alternatively, use high-strength, compact magnets (e.g., Neodymium) for better rear wave dispersion. Vendersteen used Vifa-made mids with small Neodymium magnets. Wilson Benesch's flagship Bishop, using face-to-face Isobaric woofers with magnets facing out, employs newer, stronger Neodymium Iron Boron magnets. Poles are rounded, and frames minimize frontal area. Lowther, a legendary full-range driver, despite its age, carefully addresses this. Though magnets are large, their shape is streamlined, freeing space behind the cone. Frame struts are narrow-edged to minimize rear wave obstruction.
Another factor affecting performance is voice coil/magnetic circuit interaction. Strictly, the voice coil and magnetic circuit push/pull each other. The magnetic circuit is fixed, making it seem like magnets drive the voice coil.
(Note 6: Vendersteen's philosophy is sound, investing in unseen areas. Packaging is simple, sound is balanced, musical. Sadly, its shape isn't favored locally.)
Acknowledging this reveals issues: 1. Voice coil field can demagnetize magnets. Magnets must withstand this to maintain dynamics, power, efficiency. Magnet demagnetization resistance affects sound; Alnico magnets' mid-high charm relates to this. 2. Voice coil field modulates gap flux, causing distortion. Copper-plated poles or copper shorting rings eliminate flux modulation, drastically reducing distortion. This especially improves mid/woofer intermodulation distortion, as long woofer excursions and fast midrange movements complexly modulate the field.
Magnetic Circuit Dilemma vs. Innovative Polarization & Pole Structure
Earlier, I mentioned traditional axially magnetized structures. But the voice coil ultimately needs radial flux. Why not magnetize radially initially? Manufacturing difficulty and high cost delayed radial polarization until ~5 years ago.
Disadvantages of axial magnetization: 1. Larger size; 2. Difficulty achieving high flux density with deep gaps. Size was discussed. Now, gap intricacies.
In traditional systems, gap length roughly equals the top plate thickness at the gap. For higher flux density, narrow the gap. But this complicates voice coil assembly, increasing cost. Also, pole saturation must be avoided, requiring consideration of material/thickness.
Long-gap/short-coil configurations face lower flux density. Combined with a short coil, efficiency drops significantly. While offering good power linearity, achieving high efficiency requires overcoming many compromises.
Altec 515 series and TAD 160X series use short-coil/long-gap designs, achieving excellent power linearity and ultra-high efficiency – a triumph of engineering.
Radially polarized magnets easily achieve high flux density and long gaps (cost remains high). Gap length with equal flux density can be several times longer than traditional designs, meaning several times more linear excursion! Distortion at high SPL is very low. This suits bass reproduction. Existing products are professional 18" woofers (Note 7), claiming maximum linear SPL beyond human tolerance with still-low distortion.
(Note 7: Aura Sound 1808. Not B&W's Aura brand.)
Sadly, no radially polarized midranges exist yet. Though mids don't need long excursions, this compact, high-strength design benefits mids too. Likely in some lab; expect products soon.
Stepping into Full-Range
Huh? Isn't this article about full-range drivers? Why spend so long on mids?
Don't blame me! Full-range operation faces many challenges. Starting from mids provides clearer context.
An ideal driver (tweeter/mid/woofer) should:
1. Low Distortion;
2. Good Power Linearity;
3. High Efficiency;
4. Wide Effective Bandwidth.
Maximizing point 4 creates a full-range driver. Next time, I'll discuss extending a midrange driver to full-range operation, the compromises involved, and ingenious solutions – equally fascinating. Stay tuned.
At first glance, it seems simple: make a midrange driver play higher and lower frequencies, and it's full-range? Look at car audio, computer speakers, portable radios, bedside systems – aren't "full-range" drivers everywhere? Not a big deal!
It's not that simple. Do you know the bandwidth of those generic drivers? You don't need measurements to hear that clear vocals are an achievement; drums/cymbals are often barely recognizable; bass/high percussion is faint. Pipe organ? String harmonics? Piano decay? Forget it!
What constitutes full-range? See the sidebar. Now, let's explore the design challenges of one driver covering the entire audio spectrum.
Low-End Extension
Visually, similarly sized (e.g., 6" or 7") cone mids and woofers look similar. Woofers need larger excursions, so they have wider, softer surrounds. Otherwise, they "look" similar. But this is a generality.
Can you modify a 6-7" midrange to play bass? If only output matters (not SPL/distortion), yes. Generally, a driver's low-frequency limit is indicated by its free-air resonance frequency (fs).
How to lower fs? Acoustic impedance (Note 2), moving mass, magnetic strength, and suspension compliance are key. Acoustic impedance relates to radiating area and frequency. For similar size/direct radiation/similar band, it's comparable. So, focus on other factors.
Low-frequency operation involves slow reciprocating motion. Basic physics: Acceleration is inversely proportional to mass under constant force. So, heavier cones generally have lower fs. Comparing driver specs confirms this. 15"+ woofers with fs below 25Hz often exceed 100g moving mass.
Lowering fs by adding mass is easy but bad: heavy cones reduce efficiency and worsen high-frequency extension. Dead end. Next, reduce external damping – mechanical and electrical. Damping applies braking force.
Car suspension analogy: Traditional American cars have soft suspension (low spring rate, compliant shocks) for comfort. This yields low system tuning frequency, absorbing bumps (high-frequency pulses) effectively. But long-wave undulations (low-frequency pulses) cause 2-3 cycles of oscillation – system resonance excited.
Similarly, increasing suspension compliance lowers resonance frequency. Straightforward.
But this approach has issues:
Mechanical Damping: Refers to braking force from the surround and spider. Besides damping overall motion, the suspension (especially the surround) suppresses cone breakup. Different surrounds drastically change tonality. Reducing damping to lower fs increases coloration, especially in the mids. Adjust carefully.
Electrical Damping: Refers to magnetic control over the voice coil. Stronger magnets mean more driving force and braking. We want strong drive for efficiency/low distortion, but high damping prevents lowering fs. Here's the dilemma. Adding high-end extension issues complicates compromise.
High-End Extension
Factors affecting high-frequency performance include electrical and mechanical elements, similar yet different. Electrical factors mean voice coil inductance. Voice coils are inductors. Alone, it's an air-core inductor – low inductance, linear. Unfortunately, the voice coil operates within the magnetic circuit, becoming an iron-core inductor. Inductance increases significantly, and the inherent low-pass characteristic attenuates highs. Worse, the relative position of voice coil and pole piece constantly changes, causing complex inductance/field interaction and modulation. Distortion rises sharply at high volume/wide bandwidth. Subjectively: blurred, rough, flattened texture, collapsed imaging, compressed soundstage. Solution: Copper plating the pole piece or inserting copper rings "shorts" the field, drastically reducing modulation and inductance. This extends highs and lowers distortion.
Mechanical factors involve basic physics: Force = Mass x Acceleration (F=ma). Acceleration is the rate of velocity change. Imagine a cone decelerating during forward stroke, stopping at peak excursion, then accelerating backward. At 20kHz, this entire process happens in 1/40,000th of a second! Calculate the peak acceleration for a half-cycle harmonic motion; it's immense!
Reproducing such highs requires immense cone acceleration. From F=ma, only two paths: reduce cone mass or increase driving force. But this brings dilemmas and contradictions.
Cone Mass
Adding mass lowers fs easily, but harms high-frequency response and efficiency. Instead, let the driver "see" a heavy cone at low frequencies and a light one at highs. Sounds weird?
This is a clever trick in full-range design: "mechanical" crossover. In practice: at low frequencies, the entire cone moves. Toward highs, cone breakup causes the heavier, acoustically resistive outer part to "lag." Only the inner part moves with the voice coil, significantly lighter than the whole area. Thus, the "effective" moving mass varies with frequency, enabling both high and low response.
"Cone breakup" sounds simple, but controlling which parts lag and preventing uncontrolled movement (causing coloration/distortion) is extremely difficult. Music contains wide, constantly changing frequencies. Uncontrolled breakup causes horrific distortion.
Driving Force
Strong driving force is needed for high acceleration. Sources: voice coil and magnetic circuit. More voice coil turns increase magnetic force for interaction, but increase inductance and mass, harming highs. Not feasible; compromise is needed. "Small is beautiful" beats "large and clumsy."
We must increase magnetic strength. Though strong magnets cause high damping, raising fs, high acceleration demands stronger magnets than typical drivers to push the "not-light" cone (Note 4). Over-damping is compensated by relaxing mechanical damping.
(Note 4: Lowther cones are relatively heavy.)
System Integration Issues
Only one driver – what "system" integration? Two aspects: 1. Fine-tuning tonal balance; 2. Enclosure tuning. They are interrelated.
Theoretically, an ideal full-range driver should connect directly to an amp on a suitable baffle and produce heavenly sound. But given the compromises designers make, perfection is impossible. Like instrument making, achieving perfect tone and even loudness across the range demands immense effort; speaker drivers are mere imitators.
A full-range driver covers the range but may not be flat. Common problems: A broad midrange bump (upper or lower mids) causes coloration; Some have a gentle high-end roll-off, sounding dark; Over-damping causes low-end roll-off, sounding thin/tight with weak bass.
A slight bump causing unacceptable coloration can be flattened with a notch filter. If mild, results are often satisfactory. Don't disdain this; it's simple frequency correction. Multi-way speakers have complex overlaps and phase distortion. Notch filters exist in many speaker crossovers.
High-end roll-off usually stems from insufficient magnetic strength or a large cone overwhelming "mechanical crossover." Vintage 12" or 15" full-rangers often suffer this. Adding a tweeter is the only solution. You say, "What full-range is that?" Don't judge. Properly implemented, crossing the tweeter in gently (e.g., -6dB/octave from 16-18kHz or higher) yields good results. The crossover point avoids sensitive hearing ranges; first-order maintains phase coherence, retaining most full-range benefits. (If you have Altec 412Cs and dislike their lack of highs, contact me. After I fix them, you can't buy them back.)
Low-end roll-off indicates strong damping. Bass sounds tight/short. Benefit: clear detail. Proper enclosure tuning or horn loading can increase low-end acoustic resistance, boosting efficiency. If done well, this combination offers the best full-range performance.
Enclosure tuning: Over 90% of commercial speakers use sealed or ported (bass reflex) enclosures. Full-range drivers benefit from minimized cone excursion at low frequencies. Large excursions increase distortion and affect mids/highs. Imagine small, fast mid/high movements "riding" large, slow bass movements – high intermodulation and Doppler distortion. All drivers face this, but full-range drivers cover wider bands, making it more pronounced. Avoid or minimize it.
Of the two main types, ported enclosures suit full-range drivers better. They significantly reduce cone excursion near system resonance (typically 30-50Hz). This reduces distortion, increases power handling, and improves efficiency. Most full-range drivers work acceptably this way.
Purists argue good drivers are "tainted" by cabinet resonance, preferring open baffles. Drivers with sufficient bass work well here, achieving purest sound (e.g., WE/Altec 755C). Reportedly, midrange transients rival electrostatic speed with better dynamics. Drawbacks: Large footprint (baffle size determines bass extension; at least 1m²); Lower efficiency/power handling/ bass response; Dipole radiation complicates room interaction; Two large panels aren't widely acceptable.
Finally, the most complex: horn loading. We'll detail horns later. Briefly, a horn is a flared pipe. The wide end is the "horn mouth," the narrow end the "throat." The shape creates higher acoustic resistance at the throat than mouth, coupling diaphragm and air molecules well, yielding high efficiency.
Back-loaded folded horns, properly implemented, effectively boost mid-bass to bass efficiency, perfectly matching overdamped drivers. Driver Highlights
Many manufacturers have produced full-range drivers. I can't list them all. Below are some notable examples, some still in production, others vintage.
Jordan Watts
Unique design: Aluminum cone. Abandons traditional corrugated spiders for special linear suspensions with high compliance. My introduction was through "Vases" – bought on clearance, attracted by quaint looks. Surprisingly good sound: 6" aluminum cone delivers surprisingly "full-range" sound in my 10-ping room, bass is decent. Medium volume with small ensembles: pure, charming. Cons: Mid-bass coloration (fatty sound), but I stop noticing after 30 minutes. Low efficiency affects clarity at higher volumes. Their 2" aluminum driver is a classic – limited bass/efficiency, but excellent otherwise. Superb pulse response tests; subjectively fresh and delightful.
Diatone P-610 Series
Historically popular. 6.5" paper cone, Alnico magnet, 90dB/W efficiency, bass to 50Hz – good for full-range. Surface ridges control cone breakup for "mechanical crossover." Original P-610 (up to Mk.4) discontinued in 1993. Limited commemorative editions released. I haven't heard them, but reliable sources call them the most "comprehensive" full-range – cleverly compromised. Smooth, sweet sound, superb imaging, fine microdynamics, easy to use (ported enclosure works). Reportedly, a perfect match for single-ended triodes, especially 2A3.
"WE/Altec 755A/C" Legendary 8" paper cone full-range, high efficiency. 755A specs: 70Hz-13KHz, 8W power handling; 755C: 40Hz-15KHz/15W. A raised ring on the cone front acts as mechanical crossover control.
Vintage WE units are rare. Altec units I've only seen incomplete. Looks unremarkable, frame/magnet potentially cause rear reflections. Yet, DIY enthusiasts abroad highly praise its purity, comparing it to Quad electrostatics with better dynamics. Violinist/DIYer Joseph Esmilla (Sound Practices) used them on simple open baffles with 2A3/300B amps for impeccable musicality.
Goodmans Axiom 80
Another legend! I frequent friend Li Jiande's "vintage museum." Over a year ago, I spotted half a dark green driver (magnet/frame). Familiar? "Goodmans," he said casually. GOODMANS!!! I grabbed it, examined reverently. "Looks like it sounds wonderful" (Note 5), hearing imagined soundscapes. Suddenly, he snatched it back, handed me a mop: "Wipe the drool." I planned to buy them, but he returned them to the owner. Devastated! Still sighing today...
Axiom 80: UK-made 50s-60s classic. Unique frame/suspension, small Alnico(?) magnet. Specs: 20Hz-20KHz(!!), 6W power handling. Haven't heard it, but reliable sources confirm specs! Like Lowther, needs excellent back-loaded horn for best performance; amp preferably 2A3 (300B too powerful!).
Lowther Series
Iconic Lowther, over 50 years history. Traces to P.G.A.H. Voigt's 1920s moving-coil driver and 1930s double-cone patent.
Notable: Distinctive white paper cone; elegant, structurally sound frame; high-end models have mushroom phase plugs. Double-cone structure enables mechanical crossover: Entire cone moves at low frequencies. Toward highs, outer cone breaks up, inner cone continues. Phase plug prevents inner cone high-frequency cancellation, improving dispersion. Mushroom plug forms a slot load with inner cone, boosting high efficiency for front horn loading. A new UFO-shaped phase plug reportedly improves high-frequency response significantly, fitting all Lowthers.
Cone material/construction: Still uses hand-made laminated paper cones. Lowther believes flat paper offers best thickness uniformity vs. molded cones. Uneven thickness causes local resonances/coloration; thick spots add weight. Precise craftsmanship.
Magnetic circuits: Ferrite ceramic, Alnico, Neodymium. All Lowthers are strong/efficient, but differ. Ceramic magnets are cheapest, specs least impressive; sound is easiest to listen to, less fussy. Alnico is most expensive/strongest (PM-4A: 2.4 Tesla flux density, highs to 22kHz). Neodymium aims for Alnico strength in smaller/lighter/cheaper packages. Top models price similarly. Neodymium offers modern specs/sonic character; sound vs. Alnico is subjective.
Requires back-loaded horns for best performance (cone excursion only 1mm). Recently, Lowther American Club published ported designs claiming flat bass to 40Hz with better mid-bass detail. Debate awaits.
Sound: Almost all Lowthers share traits: Superb presence, stunning detail, lightning transients. Measurements often show slight upper-mid bump; poor off-axis response; small sweet spot. Polarizing: Devotees swear by them; detractors find them un-HiFi. Europe has Lowther Clubs. The trend spread to the US. Meticulous Japanese embraced them early. Now you know. Join the club?
Conclusion
Full-range drivers, used appropriately, offer unmatched musical satisfaction. Full-band phase coherence, no crossover eating signal, exquisite microdynamics/musical expression, superior soundstage/imaging – unattainable by multi-way speakers. But remember, nothing's perfect. If you listen to heavy metal at 120dB, use speakers for AV gunfire, or sing karaoke loudly, avoid full-range drivers – both you and the drivers will suffer.
Cherish these treasures. Play simple music at modest volumes for maximum soul-stirring impact. The music itself becomes profoundly moving; volume becomes irrelevant.