The Family of Direct Radiators
Ahem. I must reluctantly draw the curtain on this depraved scene of electro-motive-force before it proceeds any further, and gently but firmly steer our attention back to the topic at hand.
As we descend from the ethereal realm of charged plasmas, we must once again contend with solid materials ... the same materials that Rice and Kellogg used to build their first direct-radiator cone loudspeaker in 1928. Back then, they made the cone from paper, and paper is still used today. We also have new materials that spring from high technology, such as Kevlar, carbon-fiber, ceramic, and impact-forged aluminum, magnesium, and titanium. In the years to come, we can expect new composites, synthetic diamond, ultralow density aerogel-silica glasses, and new types of monocrystalline materials.
The direct-radiator cone has only one task to perform: transform the accelerations of the voice coil into acoustic power over the desired frequency range. To accomplish this deceptively simple task, the driver designer must balance uniformity of motion (rigidity) with freedom from resonance at mid and high frequencies (self-damping). This is the number one sonic tradeoff in all drivers (except the plasmas). There are other problems introduced by cavity resonances and magnetic non-linearities, which are discussed later.
Uniform Motion
Rigidity means accelerations from the voice coil are accurately translated into cone or dome acceleration over the entire driver surface; this translates to ruler-flat frequency response, fast pulse risetime, low IM distortion and a transparent, "see-through" quality to the sound.
Audiophiles usually describe this type of sound as "fast," much to the dismay of measurement-oriented engineers. "How can a woofer possibly be fast, since the crossover limits the pulse risetime to a tenth of what any tweeter can do?" This leads to what diplomats discreetly call a "full and frank exchange of views," in other words, a shouting match.
As usual, both sides are right, and both sides are wrong. They’re just speaking about different things. The audiophile is unwittingly describing uniform cone motion, and it can be indirectly measured by the absence of IM distortion, a flat frequency response in the working range, and good pulse response with a smooth and quick decay signature.
Well, that’s great, you might think, just make the cone, or dome, or whatever as rigid as possible. How about a metal, like aluminum, perhaps? That’s nice and strong, and it can be formed into nearly any shape.
You can see the direction this is taking. Remember, bells are made of metal! Another problem raises its head ... resonance! After all, why does a bell, or any other rigid material, ring so long, for many thousands of cycles?
We need to take a close look at how the mechanical energy gets released (if it didn't get released the bell would ring forever). Well, obviously there are some resistive losses in the bell itself; even in a vacuum the bell will quit ringing after a while. The major loss path is through the air; in effect, the air discharges the stored mechanical energy of the ringing bell. But since there is a very large mismatch between the density of the air and the metal, the coupling is very inefficient, and the bell rings for a long time before all of the energy gets discharged.
Well, guess what? All of these things happen in a speaker cone, too! The cone is much denser than air, resulting in the typically low efficiency of most direct-radiators. (89dB at one metre with 1 watt input corresponds to an absolute efficiency of a mere 0.5%) In addition, the air is so weakly coupled that it doesn't help much with damping the cone (unlike a large-area electrostat or magnetic-planar). We can only look for help from two sources; amplifier damping, which controls the voice-coil, and the intrinsic self-damping properties of the cone and the surround.
Self-Damping
We’d like the amplifier, acting through the voice coil, to stop the cone or dome, not have the cone keep playing a tune all by itself. Unfortunately, the voice coil represents only a small portion of the cone area, and the rest of the cone may have almost no self-damping, particularly if it is made of metal, carbon-fiber, or Kevlar. One way to control the problem is to extend a rubber surround partway down the cone, and pay a lot of attention to the damping behavior of the spider and surround materials. (I have heard from several sources that Kurt Mueller of Germany makes rubber surrounds with superior damping qualities.)
At present, though, even the best Kevlar, carbon-fiber, or aluminum cones show at least one high-Q peak at the top of the working range, requiring a sharp crossover, a notch filter, or both to control the peak. Unfortunately, this peak usually falls in a region between 3 and 5 kHz, right where the ear is most sensitive to resonant coloration.
Most audiophiles and magazine reviewers are unaware of the sonic signature of Kevlar or carbon-fiber resonance, misidentifying it as "amplifier sensitivity," "room sensitivity," or other problems that point away from the real culprit. Since few reviewers have auditioned the raw, unmodified sound of commonly-used drivers, they can’t evaluate how much "Kevlar sound," or "aluminum sound," remains as a residue in the finished design. It is the task of the designer to skillfully manage the crossover and cabinet profile to minimize the driver coloration. Despite advertising claims or the opinions of nationally famous reviewers, the characteristic signature of a driver can never be removed completely.
When working with rigid-cone drivers, there are some hard choices to make: if you lower the crossover frequency to minimize driver coloration, tweeter IM distortion skyrockets, resulting in raspy, distorted high frequencies at mid-to-high listening levels; if you raise the crossover frequency to improve the sound of the tweeter, the rigid-driver breakup creeps in, resulting in a forward, aggressive sound at moderate listening levels, and complete breakup at high levels. (Unlike paper cones, Kevlar, metal, and carbon fibers do not go into gradual breakup.) With the drivers we have today, the best all-around compromise is a 2nd, 3rd, or 4th-order (12-24dB/Oct.) crossover with an additional notch filter tuned to remove the most significant HF resonance of the midbass driver.
I should add, by the way, that I like Kevlar and aluminum drivers very much ... but no question about it, they are very difficult drivers to work with, with strong resonant signatures that must be controlled acoustically and electrically.
As mentioned above, rigid cones have advantages, but they are difficult to damp completely. An alternative approach is to use a cone material that is made from a highly lossy material (traditionally, this was plastic-doped paper, but this has been supplanted by polypropylene in most modern loudspeakers). The cone then damps itself, progressively losing energy as the impulse from the voice coil spreads outwards across the cone surface. The choice of spider and surround are then much less critical.
This type of material typically measures quite flat and also allows a simple 6dB/Octave crossover; personally, though, I don't care for the sound of many polypropylene drivers, finding them rather vague and blurry-sounding at low-to-medium listening levels. Without access to a B&K swept IM distortion analyzer, I have to resort to guesswork, but I strongly suspect that this type of driver has fairly high IM distortion since it is a soft cone material.
It is quite difficult to make a material that has perfectly linear mechanical attenuation. In the electrical world, we expect resistors to have almost zero distortion. In the mechanical world, though, lossy (soft) materials tend to have weird hysteresis modes, and linear behavior cannot be taken for granted. This is the source of the IM distortion in the midband of a driver's frequency range, where the displacement is low, and it is operating in a constant-acceleration regime. In short, it has moderate cone (or dome) flex, but it isn't the all-or-nothing gross breakup that people see in the acoustic holography pictures.
I suspect (without proof) this is the problem for many soft-dome tweeters and midrange domes; the driver is actually flexing throughout the entire frequency range, but the lossy damping material hides this from the instrumentation (but not the ear). To overcome this, the best cone drivers (Scan-Speak, Vifa, and Seas) are actually composites, adding silica, talc, or metal dust to the plastic cone, which significantly improve rigidity without losing the characteristic polypropylene smoothness.
Cavity Resonances
Even though the dust cap in a mid/woofer (or the dome in a tweeter) looks pretty harmless, the space between dustcap and the polepiece of the magnet creates a small high-Q resonant cavity. One example of this was the KEF B110 Bextrene midbass driver dating from the early Seventies (as used in the BBC LS 3/5a).
Although this driver was probably the one of the first high-quality midranges available, it also had a host of problems, such as low efficiency, limited power-handling, a broad one-octave peak centered at 1.5 kHz (corrected by the BBC crossover), and group of 3 very high-Q peaks centered around 4.5 kHz (only slightly attenuated by the BBC third-order crossover). These upper peaks, which reviewers mistakenly ascribed to the tweeter, were also very directional, which is typical of dustcap resonances.
The popular tweeters of the 1970’s, including the Audax and Peerless 1" soft-domes, had similar resonances between 9 and 16 kHz, which were partially damped by a small felt pad almost filling the space between the dome and the magnet polepiece. Since the soft-domes were much more lossy than the stiff B110 dustcap, the resonances were much broader and only 1 to 2 dB in magnitude ... but they were still there, and they were responsible for some of the fatiguing quality noticed by attentive listeners.
Not surprisingly, the problems were much worse in the phenolic, fiberglass, and hard paper domes used in the nastier speakers of the day. (Ah yes ... who can remember such paragons of excellence as the BIC Venturis? Cerwin-Vegas? JBL L100's? Once upon a time I actually sold these things!)
Returning to the present, the best midbass and tweeter drivers now sidestep the dustcap/dome problem in two ways: a vented polepiece assembly, used by the Scandinavian manufacturers Scan-Speak, Vifa, and Seas; or a bullet-like extension of the polepiece, which replaces the midbass dustcap entirely, used by the French manufacturers Audax and Focal.
The Scan-Speak D2905 series of tweeters are the most notable examples of tweeters with vented polepieces that load into a tiny transmission-line behind the magnet assembly. (The line progressively absorbs the backwave from the tweeter dome, improving the impulse response and power-handling of the tweeter.)
Magnetic Non-linearities
Most audiophiles are aware that loudspeaker drivers are inductive; after all, the voice coil is wound around a ferrous polepiece, and that’s how you make an iron-core inductor. Not as many audiophiles know about the myriad of problems this creates.
If the inductance were constant, like an air-core inductor, there would be no problem; just adjust the crossover design to allow for it (using a simple R-C network) and off you go. Unfortunately, this is an iron-core inductor, and much worse, the inductance varies with the position of the voice coil.
The varying inductance has profound consequences, since the inductance is actually a important factor in determining the upper rolloff frequency of the driver, as well as its acoustic delay (relative to the tweeter). Vary this inductance, and the rolloff frequency and acoustic delay vary too. When does this happen? Whenever the driver moves a significant proportion of the linear region of voice-coil travel, which is a shorter distance than you'd think. In the excellent 8" Vifa P21W0-12-08, this linear region is only 8 mm (plus/minus 4 mm either way). A more typical figure for linear travel would be 6 mm for most 8" drivers, and 1 to 3 mm for most midranges.
Play some deep bass, and the effects of inductance modulation begin to show, creating IM and FM sidebands over the entire frequency spectrum. This is a genuine problem for 2-way systems and 3-way systems using a low midrange crossover; it means that any time you can actually see the drivers move, there are quite significant amounts of IM and FM distortion. What does this sound like? You can expect a "grayish" coloration and a blurriness that will change depending on the type of music you play.
Are there solutions? Yes. The drivers from Scan-Speak (SD System), Dynaudio (DTL-System), and the new Seas Excel series plate the polepiece with copper to short out eddy currents induced within the magnet structure by the voice coil. The specification that gives this away is the lower-than-usual voice coil inductance.
The 8" Scan-Speak 21W/8554, probably one of the best 8" drivers in the world, has an inductance of 0.1mH, which is far lower than the 8" Vifa P21W0-20-08, which has in inductance of 0.9mH. Both are excellent drivers; the Scan-Speak, though, is almost certainly going to have more transparent sound when asked to play bass and midrange at the same time.
The inductance figure has another hidden consequence; remember, the upper rolloff frequency of the driver is the combined function of the mechanical rolloff and self-inductance of the voice coil. If you calculate the electrical rolloff frequency by using the VC inductance and the DC resistance, a few drivers have an electrical rolloff well above the measured acoustical rolloff. This is good; it means that the interaction between the two rolloff mechanisms is going to be small.
Most drivers, though, have an electrical rolloff well below the measured acoustical rolloff. How is this possible? The mechanical system actually has a broad peak which is masked by the self-inductance of the voice coil. This is not good; any change in either the mechanical system or the electrical system is going to strongly modulate the frequency and transient response.
This, by the way, is the same kind of problem found in the old moving-magnet phono cartridges. Most moving-magnets (typically Shure and Stanton) were mechanically peaked, then rolled-off electrically by the combination of cable capacitance and cartridge inductance. By contrast, moving-coils have less than one-tenth the inductance, a much flatter and wideband mechanical system, and are much less susceptible to cable coloration.
The same applies to loudspeakers; it is always preferable to have a flat mechanical system and avoid compensating with electrical equalization; the trick is to know when this has been done within the driver itself by generous amounts of self-inductance.
Selecting A Driver
I use a method that’s so crude it might sound dumb; I put the driver on large, IEC-sized baffle (135 cm by 85 cm) and listen to it! No crossover, no enclosure, and if it’s a tweeter, not loud at all. I listen to pink noise (to assess the severity of the peaks that may appear in the sine-wave and FFT waterfall measurements) and music (to get a sense of the driver's musicality and resolution).
This does take an educated ear, though, since you have to listen around the peaks that the crossover might notch out, and not hold the restricted bandwidth against it. However, this listening process tells you a lot about how complex the crossover has to be, particularly if you remember that the crossover can never totally remove a resonance ... it can just make it a lot more tolerable.
I also carefully assess the results of the MLSSA PC-based measurement system (using the same IEC baffle), looking at the:
1) Impulse Response. How fast does the driver settle to zero? Is there chaotic hash in the decay region or is it a single, smooth resonance? Are there two or more resonances?
2) Group Delay vs. Frequency Response. How ragged is the frequency range above the first breakup? Can it be corrected in the crossover?
3) Waterfall Cumulative Decay Spectrum. Can you accept the resonances that can't be fixed in the crossover? If crossover correction is required, how complex is it going to be?
4) Flatness of Frequency Response. Take a good look at the Fletcher-Munson curves; these show where the ear is most sensitive to small deviations in response. The most critical region is between 1 and 5kHz; any peaking in this region, even as small as 1/2dB, is audible as an unpleasant "canned" quality. By contrast, small valleys are much less audible, so long as they are not caused by reflections or resonances.
Paying attention to these small details is the difference between the cheerful DIY builder with a table saw and the serious, dedicated craftsman(or craftswoman) who loves the art. As in traditional crafts, a deep knowledge of technical means is combined with a sense of beauty and purpose.
Direct-Radiator Drivers
It helps when you start listening and comparing to have a good grasp on the basic characteristics of the driver, so you can determine if it is a good example of its type. By listening carefully to the driver in an open baffle with no crossover, and examining all of the important specifications, you can find out just how well the designers solved the problems of making that particular type of driver.
Soft-Dome Tweeters
These tweeters, using silk domes with damping compounds, came into common use in the early Seventies with the introduction of the Peerless 1" soft-dome (remember the tweeters of the original Polk speakers?), followed by the superior Audax 1" tweeter, which found its way into many British and American designs during the 1970’s and early 1980’s.
These designs fell into disfavor with the introduction of modern titanium and aluminum domes, which swept the Audax-class soft-dome drivers off the audiophile market.
Over the last ten years, the soft-domes have made a surprising comeback with the gradual improvement to the Scan-Speak D2905 series of 1" tweeters, which compete on even terms with any metal-dome around. These tweeters combine vented pole-pieces with sophisticated transmission-line back-loading, new dome profiles, and new coating materials. As a result, they have the sonic resolution and detail of the best metal domes without the characteristic 22 to 27 kHz metal resonance.
Strengths are: Intrinsic self-damping and potential for extremely flat response and first-class impulse response. Potential for natural, open sound without intrusive and fatiguing resonances, a valuable quality when listening to many digital recordings.
Weaknesses are: The first-generation of soft-dome tweeters had a dull sound with a hard-to-pin-down fatiguing quality, as well as quite limited power-handling which required a high-slope 18 dB/Octave crossover. Modern soft-domes have solved these problems, with the best examples comparing to the best of all other technologies, including electrostatic and ribbon tweeters.
Best Examples are: The Scan-Speak D2905 family of 1" dome tweeters. I’ve used the Scan-Speak D2905 in the Ariel loudspeaker and am very pleased with the sound.
Soft-Dome Midranges
These are enlarged (2 to 3 inch) versions of soft-dome tweeters, using similar construction techniques with a half-roll surround acting as the combined surround and spider. Unfortunately, what works for a tweeter doesn’t work so well when scaled up for midrange use. In a tweeter, excursion requirements are modest (0.5 mm is plenty), but the requirements for the 3rd derivative of excursion (jerk) are severe, since the tweeter handles the very top of the spectrum, and is occasionally exposed to ultrasonic clicks from amplifier clipping, phono cartridge mistracking, or high-frequency noise and distortion from digital converters.
By contrast, the midrange (or midbass) driver experiences much greater demands for excursion and acceleration for two reasons: if you halve the frequency, you need four times as much excursion, and the musical spectrum carries most of its power in the lower midband. Both factors combine to make the midrange driver a device that must handle much more power than a tweeter. This imposes harsh demands on the rigidity of the diaphragm, and it exposes the simple suspension to rocking modes.
The reason conventional cones have a separate surround and an inner spider is to constrain the cone travel to a back-and-forth piston motion. Only very expensive mid domes intended for professional studio monitors (like the ATC) use a separate spider; as a result, most consumer-grade domes have serious problems with side-to-side rocking and other spurious motions. In addition, the doped-silk diaphragm is easily deformed by the high acceleration loads in the power band of the midrange. You don’t see bass drivers made out of doped silk, after all.
As a result of these problems, soft-dome midranges measure well, but sound a lot worse than conventional steady-state measurements would indicate. Even if you stick to measurements and discount all of the foregoing, they are limited bandwidth drivers, requiring a 12dB/octave crossover no lower than 500Hz (800Hz is better) thanks to a linear excursion of no more than 2mm. You’d expect a big tweeter to do well at high frequencies, but all of the soft-domes I’ve seen start to roll off at 4 to 5kHz, which is no better than good modern midbass drivers.
Of course, there are exceptions to what I’ve mentioned above. For example, there are cone-dome hybrids, such as the 5" Scan-Speak 13M/8636 and 13M/8640, and the 5" Dynaudio 15W-75. These new drivers are actually designed as high-quality miniature cone drivers, not as midrange domes. The only thing they have in common with the traditional soft dome is a large dustcap, which does indeed act as a dome radiator at the higher frequencies.
These new cone-domes have much more excursion, much lower distortion, and a much wider frequency response than the older soft-dome midranges. The cone-dome drivers are capable of realistic and transparent sound. They are described in more detail in the other sections, since they use Kevlar, paper, and polypropylene cone materials.
Another "special case" is the English professional-grade ATC 3" dome with an integral short horn. This driver uses a dual spider to eliminate the rocking problem that plagues most soft-domes, reducing the IM distortion very significantly. Ron Nelson (of Nelson-Reed) recommended this driver as one of the very best midranges around, and I take his recommendation seriously. This is a very expensive driver (around US$300/each). They also need to be hand-selected so the resonant frequencies of the left and right channels match.
Strengths are: None. Metal-dome midranges have some potential, but they require sharp crossovers on both ends with an additional sharp notch filter at high frequencies to remove the first (and worst) HF breakup mode. Note: This does not apply to the cone-dome hybrids or the prosound ATC driver.
Weaknesses are: High distortion, fatiguing sound, high crossover frequency, limited bandwidth, limited power-handling, and misleading frequency response measurements. It takes a detailed swept IM distortion measurement and laser holography to get the full story on these drivers. Note: As before, this does not apply to the cone-dome hybrids or the prosound ATC driver.
Best Examples are: ATC 3" professional-series - a totally different animal than the usual soft-domes. About 4 times as expensive, though (so what did you expect?). The Scan-Speak 8640 and 8636 are also excellent wideband midrange drivers.
Metal-Dome Tweeters
Advances in German metallurgy (at Elac and MB) resulted in thin profile titanium and aluminum domes in the mid-Eighties, with drivers from several vendors in Germany, Norway, and France now available. This type of driver can offer very transparent sound, rivaling the best electrostatics if correctly designed.
The downside is the lack of self-damping, with aluminum coming a little ahead of titanium in being better behaved in the ultrasonic region. At the present, all metal-dome drivers have significant ultrasonic peaks, ranging in magnitude from 3 dB (excellent) to 12 dB (not so good).
There’s controversy about the significance of this ultrasonic peak, since the engineers of Philips and Sony have gone to great lengths to ensure that none of our wonderful new "Perfect Sound Forever" recordings ever have any musical information above 20kHz. Not withstanding limitations of the signal source, power amplifiers (and CD players) can generate spurious ultrasonic signals, especially at or beyond clipping. These ultrasonic signals can excite the metal-dome resonance, causing IM distortion to fold down into the audible region.
Strengths are: Uniform piston action right up to the HF resonance, providing sound of very high resolution, transparency, and immediacy if correctly designed. Dispersion is typically excellent, since the metal domes have flatter profiles than soft-domes.
Weaknesses are: Potential for (dare I say it) "metallic" coloration caused by the HF peak intermodulating with the inband sound. Some early designs have restricted power-handling. If overloaded, breakup distortion can be extremely harsh.
Best Examples are: Vifa D25AG-35-06 1" aluminum dome, which is even better with the plastic phase disk removed. This dome has a vented pole piece, so power handling is quite good, and the ultrasonic peak is only about 3 dB even with the phase disk clipped off (recommended). The Focal tweeters are reputed to be even better.
Ribbon Tweeters
The best-known true ribbon tweeter is the rare Kelly Ribbon of the Fifties, but other types appear every now and then. These are the only dynamic tweeters with the low mass, uniform drive, and low distortion of electrostatics. True ribbon tweeters are in a category of their own, since the of the design compromises of conventional drivers don’t apply. Of course, that means they have a whole new set of problems! No free lunches in audio!
The biggest drawback of true ribbons is the one-turn "voice coil," freely suspended in the side-by-side magnetic gap. This means the impedance and efficiency approach zero, unless a transformer is used. Even with a matching transformer, the efficiency of the ribbon was still pretty low, which is why Kelly added a short horn to their tweeter. Unfortunately, the short horn compromised some of the best traits of the ribbon, which are accurate pulse response and freedom from resonance.
By combining rare-earth magnets with an integral transformer, Raven has raised the efficiency of their ribbons to an astonishing 95dB/metre. This is ten times higher than traditional ribbons, and without horn loading! The MLSSA waterfalls look impressive too, but that’s to be expected with ribbons, along with low distortion. (Raven claims less than 1% distortion at 105dB continuous output, a very good figure.)
The only drawback I can see for the Ravens is the requirement for a high-slope crossover. This is a potentially serious issue, since a 4th-order (acoustic) slope is already on the threshold of audibility, with a 360-degree phase rotation at the crossover frequency. The most direct way around this is raising the crossover frequency, and selecting a very wideband midbass driver.
Paper Cone Midbass & Full-Range
This class of drivers go right back to the original Rice & Kellogg moving-coil patent of 1925. Paper-cone drivers range in quality from terrible to wonderful; from a ten-cent speaker glued to a computer motherboard to the superb Scan-Speak 5" cone/dome midrange, the classic horn-loaded Lowthers, and the 12 and 15-inch Tannoy Dual Concentrics.
This oldest of cone materials is actually a composite structure, and changes properties significantly when treated with an appropriate material (the makeup of the additive is invariably a trade secret of the manufacturer). The cone treatment is quite important, since paper undergoes significant alterations with changes in humidity if left untreated; the additive stabilizes the material, improves the self-damping, and extends the HF response of the cone.
Strengths are: Good-to-excellent self-damping, potentially excellent resolution and detail, very flat response potential, and a gradual onset of cone breakup. It can be used with low slope linear-phase crossovers without much trouble. Paper is a material that sounds better than it measures ... this is an genuine asset, not a disadvantage.
Weaknesses are: Not as rigid as the Kevlars, carbon fibers, and metals, so it lacks the last measure of electrostatic-like inner detail. Doesn't go as loud as the materials above, but the onset of breakup is much more gentle and progressive. Paper-cone drivers may require modest shelving equalization in the crossover for the best results.
Paper is not as consistent as synthetics, so pair-matching isn’t quite as exact, which may affect imaging, depending on the precision and quality of manufacture. Properties may slowly change over time, depending on the composition of the cone.
Best Examples are:
Scan-Speak 8640 5" cone/dome midrange, with linear response from 300Hz to 13kHz, very low distortion, excellent pulse response, and excellent inner detail.
Audax PR170M0 6.5" high-efficiency midrange. (100 dB at 1 meter!)
Diatone PM610A 6.5" (Anniversary Edition) from Mitsubishi in Japan. These are very wideband drivers, covering 70 Hz to 20kHz in a conventional enclosure.
Various Lowther models. These require horn-loading for correct operation, and to prevent over-excursion damage. If correctly horn-loaded, they cover a wide range from 50Hz to 18kHz.
Bextrene Midbass
This is an acetate plastic derived from wood pulp, and is typically damped by a layer of doping material on the front of the cone to control the strong first resonance it displays around 1.5 kHz. It was originally developed by the BBC in 1967 to replace paper with a more consistent and predictable material for monitoring purposes. It came into widespread use in the early Seventies, with the typical audiophile speaker using a 8" KEF or Audax Bextrene midbass driver with an Audax 1" soft-dome tweeter.
The BBC-derived designs always employed notch-filter equalization to flatten the Bextrene driver in the midband; the most famous (or infamous, depending on whether you were the listener or the designer) driver was the KEF B110 used in the BBC LS 3/5a minimonitor. Not everyone knows that this speaker, which is legendary for its sweet midrange, employs a deep notch filter with 6dB of attenuation at 1.5kHz to correct the B110.
Over time, Bextrene has been replaced by BBC-developed polypropylene, which gives much flatter response, does not require a layer of doping material, and provides a 3-4 dB increase in efficiency due to the decrease in cone mass. Bextrene is now considered an obsolete material by nearly all speaker designers.
Strengths are: Consistent batch-to-batch, excellent potential imaging (by mid-Seventies standards). Inner resolution higher than many paper cones.
Weaknesses are: Very low efficiency (82-84 dB at 1 meter), requires a strong notch filter in the midband, a "quacky" coloration by modern standards, sudden, unpleasant onset of breakup at not-so-high levels, and numerous resonances at the top of the working band.
Best Examples are: None. Modern designers are not willing to tolerate the low efficiency and the complex notching and shelving equalization required to make these drivers acceptable. Although some traditionalists revere the KEF B110 used in the Rogers LS 3/5a, the uneven response of this driver requires the LS 3/5a crossover to be very complex. Having worked with the B110 for many years, I feel the modern Vifa P13WH-00-08 is superior in every way.
Polypropylene Midbass
This material was developed and patented by the BBC in 1978 (my dates may be off) as a replacement for Bextrene. Since it is intrinsically self-damping, a correctly designed polypropylene driver is capable of flat response over its working range without little or no equalization. In addition, they typically attain efficiencies of 87 to 90 dB at 1 meter, which is a major improvement over Bextrene.
This material has become nearly universal, since it requires a minimum of hand treatment to assemble a loudspeaker - the only difficult problem was finding the cyanoacrylate adhesives that would stick to a slick material like polypropylene. That problem was solved in the beginning of the Eighties.
As with paper, this cone material is used in speakers ranging in quality from mass-fi rack-stereo systems to the first-rank ProAc Response series. The cone profile, termination at the edge of the cone, and additional materials added to the polypropylene mix strongly determine the ultimate quality of this type of driver.
Strengths are: Very flat response if correctly designed, very low coloration, good impulse response, gradual onset of cone breakup, good efficiency, and a crossover that can be as simple as one capacitor for the tweeter. The best examples can be as transparent as the best paper-cone drivers, which is a very high standard.
Weaknesses are: Not quite up to the standard of transparency set by the rigid-cone class of drivers and the planar electrostatics. Many poly midbass units do not mate well with the popular metal-dome tweeters, with differences in resolution that can be obvious to the skilled listener. Not a good choice for woofers 8 inches or larger unless the polypropylene is reinforced with another, more rigid, material. Woofers 10" or larger are better served by stiff paper or carbon fiber.
Best Examples are: The Scan-Speak 18W/8543 7" midbass, as used in the ProAc Response Threes, is probably the finest polypropylene driver in the world.
Another closely-ranked contender is the Dynaudio 17W-75 Ext. 7" midbass, as used in the Hales System Two Signature.
The Vifa P13WH-00-08 5.5" midbass/midrange unit is a superb performer, well suited for midrange or minimonitor use. It is unique in having a textbook-flat midrange combined with a completely smooth Bessel 2nd-order rolloff. I use these drivers in the Ariels, and I’m very pleased with the results. The Vifa P13WH does not have the typical "poly" sound, sounding much more like a top-rank paper-cone than other polypropylene drivers.
Rigid Midbass
Aluminum and Magnesium Drivers
The first rigid drivers to find limited use in high-fidelity applications were the small Jordan Watts 2" aluminum-cone units. The hand manufacture, high price, and low efficiency limited the market for these drivers, and very few appeared in the United States (I have heard of them by reputation but have never auditioned them personally).
There is a new generation of British and German 2-way minimonitors that use proprietary 5" to 6.5" aluminum-cone midbass drivers. These drivers typically have very low efficiency (82-84 dB/metre) and almost certainly have a high-Q peak at the top of the working range. Since these drivers are not being sold on the open market, I have not seen any detailed information on them.
The new Seas Excel series uses magnesium cones with an intriguing solid-copper "bullet" replacing the usual dust-cap. They certainly look beautiful, and have a quite usable efficiency around 87dB/metre, unlike the older aluminum-cone units above. The preliminary data I've seen, though, shows a whacking great 16dB peak at 4.9kHz, so you better be a pretty good crossover designer!
Expanded-Foam Drivers
The next generation were the expanded-foam bass units, with the KEF B139 being the most famous example. This class of driver offered piston-band operation through the midbass, but suffered from very low efficiency, limited power-handling, and severe high-Q resonances in the midband.
(It was not generally known that the B139 had a 12dB peak at 1100Hz with a very high Q. Many reviewers blamed the midrange for problems that were actually caused by the lack of a notch filter for the B139.)
They were quite popular in 3 and 4-way transmission-line systems in Britain (IMF) and the United States (Audionics) in the Seventies, which is where I enter the story ...
I remember working with the KEF B139, B110, Richard Allen 7" and front and rear KEF T27's in my very first commercial design, the Audionics TLM-200 4-way system. My baptism into the mysterious art of speaker design went as follows:
Charlie, my boss: "Hey Lynn! You remember what Laurie Fincham was talking about when we visited KEF in England? All that slide-rule stuff about driver impedance correction and frequency response target functions?"
Me: (warily) "Well, I didn't write it down, but I remember a bit of it."
Charlie: "Great! You can do the crossover for THIS!" pointing at a gigantic six-foot-tall loudspeaker with the 4 aforementioned drivers.
The original designer had high-tailed it to Seattle and disappeared without a trace, leaving behind the two monstrous prototypes, which at least were finished in an attractive walnut-veneer cabinets. No crossover had even been started, and even I, a rank novice, knew that crossover design was the single hardest part of making a new loudspeaker.
Six months later, with a 57-component crossover sitting on the floor, the TLM-200 was done. How did it sound? For 1973, not too bad. I think we sold maybe 10 of them. Fortunately, the other models I designed for Audionics sold in the mid-hundreds ... and I wasn’t stuck with the marketing department choosing the drivers.
Carbon-Fiber Drivers
The next generation or rigid-cone drivers were the Japanese carbon-fiber units, which made their first appearance in the pro studio monitor (prosound) 12" TAD units with very high efficiencies and very high prices (around $300 each in 1980). Carbon fiber prices have now dropped, and Vifa, Audax, and Scan-Speak make good examples of this type of driver. The Japanese make lots more of them, having pioneered the technology, but they are very difficult to obtain if you are a non-Japanese small-run specialist manufacturer.
These drivers have true piston action, outstanding bass and midbass response (the best I have ever heard), but also have a characteristic double-peak region at the top of the working range. Unfortunately, these peaks are grossly audible in most carbon-fiber drivers, and worse, cannot be removed by a notch filter tuned between the two peaks; it requires two notch filters to control the peaks, or a low crossover with a very sharp rolloff (24dB/octave) to remove them from audibility.
Although I very much dislike drivers that require filters as complex as this (after doing the TLM-200, I vowed never again to design a 57-component crossover), I must admit that carbon fiber woofers are the only direct radiators where I've actually felt tactile bass.
The Scan-Speak 18W/8545 looks pretty interesting; although it has the classic double-peak signature of a carbon-fiber driver, they look quite well-damped, and the breakup region above these two peaks looks pretty smooth. It may even be possible to use the Scan-Speak 18W/8545 with a simple 2nd-order filter.
Kevlar Drivers
Kevlar drivers made their appearance in the mid-Eighties with the French Focal and German Eton lines, with the Eton having superior damping due to the higher-loss Nomex honeycomb structure separating the front and rear Kevlar layers.
At the time of this writing, the 7 and 8-inch Scan-Speaks have the best frequency response and the lowest IM distortion of any Kevlar driver. Another desirable property of this family of drivers is a well-behaved rolloff region above the characteristic Kevlar peak. All of the other Kevlar drivers (that I have measured and listened to) have chaotic breakup regions; by contrast, the Scan-Speaks are the only ones that appear well-controlled in this region. This is certain to provide a significant improvement in smoothness and transparency.
Composite Drivers
Audax has an unusual composite technology, called HD-A. This is an acrylic gel containing a controlled mix of grain-aligned carbon-fiber and Kevlar fibers. The response over the main frequency range is impressively flat, with only a moderate peak at the top of the range.
Another intriguing series of composite drivers are the Focal Polyglass paper-fiberglass cone drivers, with the 6V415 midbass looking most interesting, with very flat response and quite good excursion capability. For the East Coast triode fans who are into the no-crossover full-range driver thing, a stack of four 4V211’s might do the trick, with a response flat from 60Hz to 12-14kHz.
Strengths & Weaknesses of Rigid Drivers
Strengths are: Best available transparency, imaging, and depth presentation of any type, equaling or exceeding electrostats if carefully designed. High efficiency, high peak levels, and very low IM distortion in the best examples. This class of drivers is considered at the state of the art by many designers, and this field is expected to advance quite rapidly as material technology advances.
Weaknesses are: Older designs have severe peaking at the top of the working band, and most have a uncontrolled chaotic breakup region above this high-frequency peak. This would cause fatiguing sound over the long run and a compression of depth perspective and "air."
Loudspeaker systems that does not use correctly designed notch filters with a metal, Kevlar or carbon-fiber drivers can be considered faulty, since the narrow HF peak does not lend itself to correction with a standard low-pass filter. The sound of this peak will be obvious to any listener familiar with the sound of an unequalized Kevlar or carbon-fiber driver. (Tap the cone to hear this.) The new 7-inch Scan-Speak 8545 and 8546 may be the first of a new generation of moderate-peak drivers that won't need the usual notch filter.
Although these types play quite loudly, the onset of breakup can be harsh and unpleasant, akin to clipping in a amplifier. Some Kevlar and carbon-fiber drivers require an extremely long break-in period (>100 hours) to soften the fibers in the cone and the spider.
Best Examples are: The Scan-Speak family of 5", 7", and 8.5" Kevlar and the new 7" carbon-fiber/paper drivers. These are the only rigid drivers that have well-damped peaks and a reasonably well-behaved rolloff region above the main high-frequency peak. In addition, the Scan-Speak drivers also have vented pole-pieces that are copper-coated, reducing inductive types of IM distortion by tenfold or more.
The new Seas Excel series with magnesium cones and solid-copper phase plug look interesting if the designer is willing to meet the challenge of designing a very deep and accurate notch filter to correct the first resonance of the magnesium cone.
Some Closing Thoughts On Speaker Design
Don’t be led astray by marketing literature ... all, repeat all, drivers have a sonic signature, which can only be controlled, not eliminated, by equalization in the crossover network. Even though crossover equalization can straighten out the driver in the frequency and time domains, the IM distortion still undergoes a shift in character when the diaphragm or suspension goes into a resonant mode. All physical materials have resonant modes, so if the driver is constructed of physical materials, it’ll have resonant modes!
Since we all have to work with imperfect materials, here’s a set of design guidelines that can help us get from the world of abstract ideas and concepts to happy listening.
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Aim for a reasonably smooth frequency and impulse response, and take steps to eliminate any reflections from the front of the cabinet or from the interior surfaces. Reflections are much more audible, and much more unpleasant, than you’d imagine from looking at the little wiggles they make in the frequency response curves. On the inside of the cabinet, intelligent use of felt (85% wool) and Deflex pads can usefully damp reflections. On the outside, never put the driver in any kind of cavity, since even the best felt absorber has only a modest absorption capability. It’s far better to mount the faceplate precisely flush with the front panel and radius the cabinet edges if at all possible.
Reduce driver and cabinet resonances to levels where they aren't too obtrusive. Cabinets in particular must be rigid first, with damping a secondary priority. Interior crossbraces of 3/4" high-grade plywood that run the full interior width of the cabinet can make all the difference here.
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Avoid crossovers in the critical 300Hz to 3kHz region. The telephone company was correct in selecting this region as the most important part of the spectrum; this is the region that must be really spotless, with flat response and very low distortion. Even if the crossover is brilliantly executed, using the most modern computer tools and months of subjective balancing, it is still slightly audible. That is why it is better to keep the crossover out of this sensitive frequency range.
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A well-controlled and peak-free response in the rolloff region of the crossover is very important; if this can be done, you get a smooth phase and amplitude hand-off between the drivers, relaxed and sweet midrange, and greatly improved image quality.
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Last but not least, reduce the IM distortion across the spectrum, with greatest emphasis on the 500 to 5 kHz region. This means selecting drivers with well-designed magnetic systems and using midranges and tweeters that are generously sized for the intended frequency range (5" to 7" midrange and 1" tweeters).
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Resources Test and Measurement
DRA Labs MLSSA
The standard of the industry and the system I use myself. Not cheap, and requires a separate calibrated lab-grade condenser microphone and preamplifier from Aco Pacific.CLIO from Audiomatica
The CLIO hardware/software system includes a calibrated microphone and is a professional-grade instrumentation system. This is an important feature, since calibration accuracy and data-file compatibility with separate crossover-optimizer programs is essential for serious design work.Pen-Strobe from MLS Instruments
Very useful for analyzing what a speaker cone is really doing. Click the "Voicecoil Article" for an interesting description of using the strobe to detect rocking modes, standing waves in lead-out wires, tweeter dome resonances, direct-observation phase testing, etc. An essential tool if you plan to modify a driver.
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© Lynn Olson 2001. All Rights Reserved.