|general theory||The purpose
of a loudspeaker is to produce sound (move air). The most
common way to attain this goal is an electrodynamic
transformation. All electrodynamic drivers are based on
the same concept; a cone is set in motion by an
electromechanical motor system.
is composed of 4 systems:
|Full-range (or wide-range) drivers
come in two general types: the center cap and the whizzer
cone. The center cap type looks like your normal cone
speaker with a dome center cap. The whizzer cone type has
two cones, a large cone (as in normal cone speakers) and
a smaller whizzer cone inside the large cone. Driver
units have quite different characteristics depending on
the design and manufacturer. Some (like Lowther) have
very high efficiency, low excursion, and a frequency
response graph that shows lowered sound pressure levels
at medium to low bass (below 400 Hz). For this reason, to
be an effective full range speaker, these drivers usually
need special cabinet construction designed to enhance the
bass output, such as a rear-loaded horn. However, the
high efficiency allows the use of very low power
amplifiers (such as a two Watt 2A3 tube amp). Other
drivers have lower efficiency but flatter frequency
response. These drivers can often be used in an open
baffle or bass reflex cabinet, but require higher
amplifier power to drive them.
There are VERY FEW true full range drivers that radiate from one source. The Diatones almost qualify, but are a bit weak in the bass, and a little weak in the highs. Some old drivers such as the Goodmans Axiom 80s, and maybe Telefunkens and Norelcos and others go down to low bass (30s) and pretty high (12 - 16 kHz) making them more full range. These all radiate "from the front cone", making a true point source and having no or little time delay errors. Finding or making these drivers is very difficult. Most "full-range" drivers end up needing two radiation sources, one for low and one for high frequencies.
Lowther drivers, for example, radiate the high and mid frequencies from the cone, but the bass come out of a horn and are delayed in time and 180 degrees out of phase, as well as coming from the horn mouth rather than from the cone. So in a way, you have two frequency sources, with a crossover between them. The crossover is acoustic and needs to be tuned properly, and at the crossover frequency there is still potential for interaction of the two wave fronts.
Another option with a Lowther-style radiator (which goes from high to mid frequencies) is to add a separate bass system. Bert Doppenberg's Oris 150 front horn for Lowthers needs a separate bass system, Bert uses bass horns with dedicated drivers. Subwoofers might work but are often not "fast" enough to integrate properly. This philosophy puts the (one) crossover at the low frequencies (150 - 200 Hz for Bert's, approx 300 Hz for most rear-loaded Lowther drivers).
Another philosophy puts the crossover frequency high up, at 12 kHz, or 15 kHz, the higher the better. This method uses a driver that runs from low bass (30 or 40 Hz say) all the way up to the crossover frequency (10.5 kHz say), then lets a supertweeter take over from there. The big question is, where is it better to put a crossover? Low frequencies or high?
A second question is, what kind of bass do you like? Horn-loaded bass (whether from a Lowther horn or a dedicated bass horn) is regarded as being "fast", having good transient response characteristics, a very natural sound. Direct radiators have a harder time making that kind of bass. Bass drivers with long throws and polypropylene cones are particularly bad. A bass driver cone made from paper (or sometimes kevlar or other modern material) usually sounds better. But a full-range driver such as a Diatone cannot have too large a diaphragm and also cannot have too large a cone movement (xMax) because otherwise high frequencies will be attenuated and intermodulation distortion will result from the long throw. However, to make more bass, you can increase the surface area (cone size) without increasing the travel (xMax). Now you can have more bass and higher treble without too much intermodulation distortion, as long as you can control the larger cone (need good design for the cone). With the shorter travel, bass will become more controlled, crisper and more responsive compared to a long travel bass driver. (You won't get as much bass out, but it will sound nice).
Cone material can be made from paper (the classic material), plastic (often polypropylene), synthetic fiber/resin such as kevlar, and metal (often aluminium). Paper tends to have less stiffness than some other materials (kevlar and metal) but doesn't exhibit the ringing resonance often found in kevlar and metal. Some paper cones can be stiffened up with varnish-like substances such as Damar.
Bob Stout at LDSG - good discussion on driver theory is by
Lynn Olson on speakers - almost everything you wanted to know,but you did not dare to ask!
|There are two main surround types
used today, half-roll and accordian.
Half-roll (positive, negative) is a commony used in home speakers, but high excursion can pull the cone to one side.
Accordion surrounds are generally made of cloth, and are used in high-powered audio setups. Accordion surrounds can move more than half-roll, but at the surround resonance dips in frequency response can occour.
Butyl rubber and foam surrounds are used to aleviate some of the problems of paper surrounds. Foam surrounds do disentagrate after about 30 years, but they are light and allow high excursion. Rubber surrounds are heavy which reduces effencency, but they do not disentagrate. On a sidenote, the Qms is generally a measure of friction losses from the suspension, with values from 1-5 indicating a surround that will control a cone well.
The surround, which acts as an air seal between the cone and the basket, adds to the restoring force of the spider. Another function of the surround, is to absorb cone flexure waves as they are transferred up the cone.
|The spider is needed to center the
voice coil in the magnetic gap. Spiders are generally
made of cotton and are steam-pressed. The spider can
introduce noise into a loudspeaker system from air moving
through its holes.
There are some unusual aplications, where spider is action is achieved with springs.
The spider is attached to the voice coil former and the basket. The spider acts as a centering device and a restoring force for the voice coil.
|The basket is usually made of stamped steel or cast
aluminum. Although it does not directly affect the sound
of the speaker, it does play a critical role in aligning
the voice coil and the magnetic circuit. Speakers with
large magnet structures sometimes require the use of cast
aluminum baskets. Care should be taken when mounting a
speaker so that the basket is not bent, or a rubbing of
the voice coil may result, causing failure.
Driver Baskets: Driver baskets may be constructed from various types of metals depending on budget, and design goals. Steel baskets tend to be cheaper in design and have some disadvantages if not properly implemented. They must be designed with special care so that the driver magnetic strength will not be compromised by the baskets magnetic properties resulting in a less powerful motor structure which will weaken response time and increase overhang distortion. The most critical disadvantage of steel baskets is their low resonate properties (80Hz or so) which if not properly damped may couple to the cabinet causing unwanted coloration in sound. Cast Baskets are always better in design as they do not suffer from the previously mentioned disadvantages of Steel Baskets. In most cases, this should not be an issue providing that the drivers are properly designed and implemented. However it is important to note the potential of this problem.
|The works of Neville Thiele and
Richard Small are considered to have the most impact on
the loudspeaker design field as far as predictability of
frequency responce at small excursions is concerned. A
method was found, so that one could predict the frequency
response performance of a loudspeaker system, based on
its physical characteristics.
Understanding the response parameters allows calculation of predicted frequency response for a given speaker system in a uniform way. The formulas that accomplish this are rather lengthy and complex, and are best left to a computer. There are a number of computer programs and java applets that automate the design process of building an enclosure.
The three parameters that primarily determine the frequency response of a loudspeaker are compliance, free-air resonance, and Q.
The compliance, Vas, is a measure of the overall stiffness of the cone, surround (the part the attaches to front of the cone), and spider (the part that attaches to the rear of the cone). It is specified as the volume of air having the same compliance as the driver. A small number corresponds to a small volume of air, which is stiffer than a larger volume of air. Thus, compliance and stiffness are inversely proportional. Optimum enclosure volume is proportional to Vas.
Free-air resonance, Fs, is the resonant frequency of the driver's voice coil impedance with the driver suspended in free air (no enclosure). The -3 dB frequency (F3) of an enclosure is proportional to Fs.
The Q, Qts, is a measure of the sharpness of the driver's free-air resonance. It is defined as (Fh-Fl)/Fs, where Fh and Fl are the upper and lower -3 dB points of the driver's voice coil impedance in free air. Optimum enclosure volume is related to Qts but is not directly proportional. It is accurate to say that the volume gets larger as Qts gets larger. Likewise, F3 gets smaller as Qts gets larger, and for the sealed box enclosure, F3 is inversely proportional to Qts.
The Physical Characteristics of a speaker are:
Re: The D.C. resistance of the voice coil measured in Ohms.
Sd: The surface area of the speaker’s cone.
BL: The magnetic strength of the motor structure.
Mms: The total moving mass of the speaker including the small amount of air in front of and behind the cone.
Cms: The stiffness of the driver’s suspension.
Rms: The losses due to the suspension.
By understanding the relationship of these physical parameters and how to change them, we may alter the response parameters to fit the desired goal.
The Thiele/Small Response parameters are:
Re: The D.C. resistance of the voice coil measured in Ohms.
Sd: The surface area of the speaker.
Fs: The resonant frequency of the speaker.
Qes: The electrical “Q” of the speaker.
Qms: The mechanical “Q” of the speaker.
Qts: The total “Q” of the speaker.
Vas: The volume of air having the same acoustic compliance as the speaker’s suspension.
|Low end - how much air is displaced
in order to achive good reproduction of bass without excessive IM distorsion in mids and treble and retain capability of huge air displacements, ratio between excursion and effective cone area must be changed. Long throw chasis is not welcomed here, excursion must be as possibly short and cone area big (V=Sd*Xmax).
Note that most wideband drivers tend to have a rising response above about 1kHz (much depends on driver specifics) and that for example the FE208 will reach about 104db in the 2 - 4kHz range. This is a seperate issue to be dealt with. Some notes on that compiled by James from seperate post of myself and others are on the "single driver" webpages.
It is higly instructive to compare the kind of parameters found in Drivers traditionally used in sealed or vented Cabinets (Altec 755, Diatone 610A and similar) with those traditionally used in Horns (Fostex Sigma, Lowther, Pro Audio Drivers explicitly designed for Horns).
Ergo, Drivers with a Qt above 0.5 go into sealed or reflex Boxes, Drivers with a Qt below about 0.3 go into horns. Anything inbetween is not really happy in either...
Many modern Drivers use a very short phaseplug. Lowthers and similar use long Phaseplugs and longer Phaseplugs seem to improve smoothness at the cost of HF Extension. If in doubt get more cigars and use one set as long as you want and reduce the length in 5mm (or so) sections, noting the changes in sound. There will come a point where the sound does no longer improve but get's worse.
|Excursion How loud a speaker can
play depends on how much air it can move without
overheating. How much air can be moved is determined by
the surface area of the cone and the excursion capability
of the motor system.
Xmax is defined as the width of the voice coil that extends beyond the front plate plus 15% (See Diagram 2). This relates to how far the speaker can move in either direction without appreciable distortion.
Diagram 2 - Xmax The amount of power required to move a speaker to its maximum excursion is referred to as the displacement limited power handling. Please note that this number varies with enclosure size and frequency.
Power Handling Loudspeaker power handling ratings are one of the most commonly quoted, but most poorly understood of specifications given by loudspeaker manufacturers. It seems that every company has its own way of measuring and specifying power handling. That’s because Marketing departments are always looking for ways to be able to list higher numbers for power handling in order to impress their customers with the apparent ruggedness of their products. It is sometimes difficult for product users to understand how these specifications relate to real world amplifiers or how they relate to the way they listen to their favorite kinds of music on loudspeaker systems. Loudspeakers fail in one of two ways - mechanically or thermally. Mechanical failures occur when one of the moving parts of the speaker such as the surround, spider, or cone become fatigued, tear, or break from the effects of continued long excursions. Thermal failures occur when the electrical power dissipated in the voice coil as heat causes the adhesives holding the turns of voice coil wire together to break down, or the insulation on the wire to fail, resulting in shorted turns. Also, the wire itself can melt, which means an open voice coil, or the coil support can melt or burn, again meaning failure of the loudspeaker.
On the other hand, many audio companies ascribe to the Electronic Industries Association (EIA) standard for loudspeaker power testing called RS-426A. This test calls for a white noise signal to be applied to a band limited filter from 40 Hz to 318 Hz with a six decibel crest factor. However, the resulting signal, when displayed on an RTA or any constant percentage bandwidth analyzer, has a bandwidth from approximately 500 Hz to 8 kHz with a peak near 2 kHz. This means that it has relatively little low frequency content. The other test conditions include operation in free air without an enclosure, and a test period of eight hours. While useful for testing general purpose loudspeakers, it has limited application with large woofers. In addition, because its test signal is centered around 2 kHz, where the impedance of most woofers has risen to two or three times its nominal value, the actual power delivered to the speaker is much less than is indicated by the calculation of power using the nominal impedance. The result is that most failures using this test are thermal and the applied voltages required to cause the failures are excessive and difficult to generate with normal power amplifiers. More importantly, the power handling ratings that come from this type of testing are unrealistically high and do not represent actual usage conditions.
The results of testing loudspeakers in this way has been to stress drivers both mechanically and thermally to nearly the same extent, with a test signal that is a close representation of real music. The power handling ratings that would result give the customer a very good idea of the size of power amplifiers that could be safely used with a given loudspeaker.
hysics of Full Range Drivers
By John Wyckoff
Any attempt to explain full range cone-and-coil loudspeakers must begin with the fact that there is no such thing. Rather, such drivers are systems of two or more drivers on the same motor assembly. This may seem to be splitting hairs, but it is a very important concept in understanding drivers designed to operate full range.
Many smaller broadband drivers (200mm and under) are rear loaded into horns, in order to extend bass. There are several problems related to this type of loading: Size, delay, coloration, and the fact that this approach is the application of feedback. Striving to produce amplifiers without feedback, and then introducing it into the speaker design, seems odd engineering.
Another Item is the so called Baffle-Loss, which results is a 3db loss at about 250Hz for a 12" Wide Baffle, theoretically reaching -6db and then leveling out. All these issues are relevant for ANY application of Drivers in non-hornloaded (and even hornloaded) boxes.
The result is that I can take (for example) a Driver with a Qt of about 0.5 to 0.7 (say an 8" Audax Fullrange Driver) and mount it into a nice Box of about 50L Volume with a suitable bass reflex or sealed. This Driver will actually under anaechonic conditions peak up at about 100Hz (and by doing so fill in at least partially the baffle loss) and roll off quite quickely below that.
In room and with the speaker free-standing we will have a fairly evenly balanced Frequency Response (up to a few kHz at least where cone breakup and resonances between whizzer and main cone make things go scanbark) and about 93 to 94 db of genuine sensitivity down to about 45Hz.
Now let's take a Fostex FE208 with a Qt of about 0.27 and place the same Driver in a similar Box. Not only is there no peaking, the Driver already shows a rolloff by itself in the Box from the 96db in the midrange going down to about 91db at 100Hz where the SPL becomes "shelved" untill it hits about 50Hz and looses it completely. This is the low Q reducing the possible reinforcement from the box and at the same time lifting the midrange. BTW, the Driver in our example could also be a Lowther.
In the end we get a speaker which in room will play at 96 to 98 db/W/m in the midrange, will dip below 90db in the lower midrange and will recover to about 92db and carry down to about 50Hz....
It would be the Polepiece (Voicecoil) Diameter. I'd try about twice that for a starter, if good midrange smoothness and good HF extension are your aim. Note that these recommendations are puerly empirical and where found on Lowthers but worked well on the Axiom 80.
As explained to me recently, a phase plug works by
preventing out of phase HF energy from causing peaks and
The advantage they have though is that they don't have a big cone driving them, so no major IM or Doppler distortion. For this reason, WB drivers need to be power limited to ~2mm of cone travel. So either you play it at modest volume, make it very efficient, or limit it's LF BW.
You can do that. But you will still need some front Suspension. Be Yamamura uses Strings and Carbon Fibre Ringes (depends on the Specific Driver)....
I use the tractrix, hyper or expo calculations to obtain the best audible result in the phase plug design. The lenght is automatiquely dependant of the base diameter of the phase plug. This diameter is less than the pole piece of the drivers (- 2mm).
Try it !
Recipe to lathe your own phase plug ;
1) Find a cork ring or a fishing rod cork handle (you can buy it in fishing store)
2) With a long bolt, insert it in the cork ring. Glue it and bolt the cork ring with one nut.
3) Insert the bolt in a drill or press drill and push to start.
4) With a sandpaper make your shape.
1) Buy a little bag of White plaster
2) Make a mix of water and Plaster to obtain a good texture in a plastic cup
3) Insert your cork phase plug in the mixture and wait to dried.
4) When the plaster is dry pull out gently your phase plug.
5) Cast our phase plug with epoxie resin or polyester resin
forces involved in loudspeaker's action:
inertial force opposes acceleration
mms - weight of a complete moving system - vc, vc former, cone, part of a susspension and last but not least air
friction force opposes speed
inner friction in a cone and air drag
elastic force opposes movement
stiffness of a system
Large amounts of harmonic, IM, and crossmodulation distortions combine with mechanical driver resonances to concentrate spectral energy at certain frequencies. Driver damping techniques usually improve spectral characteristics (the frequency response curve looks better as a result) but do not provide much improvement for the underlying breakup modes, so the distortions may actually be spread over a much broader frequency range. The narrowband nature of resonant distortions in loudspeakers is why a single-frequency THD or IM measurement is useless; it takes an expensive tracking-generator type of measuring system in order to create a usable frequency vs. harmonic distortion graph. In order to eliminate these resonant distortions, the driver diaphragm needs to have a density equal to air and absolutely uniform acceleration over the entire surface at all frequencies. As you can imagine, we are nowhere close to meeting this criterion. As a result, all speakers have tonal colorations ranging from subtle to gross, with some types of colorations present at all times, and other types of colorations appearing only at high or low levels. A reviewer's preferences in music can easily mask the presence of these problems, so make sure to get a second (and third) opinion. * Standing-wave resonant energy is stored in drivers (all types, except for "massless" exotics), cabinets, and in the listening room itself. The unwanted mechanical energy must be quickly removed in two ways: rigid, low-loss mechanical links to the earth itself (a rigid path from the magnet to stand to floor to ground), and also dissipated as heat energy in high-loss, amorphous materials such as lead, sand, sorbothane, etc. The energy that is not removed is slowly released from every single mechanical part of the speaker, each of which has its own resonant signature. In any real speaker system, regardless of operating principle, there are hundreds of standing-wave resonances at any one time, which are gradually released over times ranging from milliseconds to several seconds. These resonances continually overlay the actual structure of the music and alter the tonal color, distort and hide the reverberent qualities of the original recording, and deform and blur the stereo image. In speakers that measure "textbook-perfect", this type of "hidden" resonance is the dominant source of coloration. This is also the reason that 1/3 octave pink-noise measurement techniques have fallen out of favor, being replaced by much more sensitive techniques such as TDS, FFT, MLSSA, and others. * Radiation patterns shift dramatically with frequency, and change sharply at crossover points; in addition, the radiation pattern is further deformed by diffractive re-radiation at every sharp cabinet edge (regardless of cabinet size or type - this includes planar types of loudspeakers). Diffraction, which occurs at every sharp cabinet boundary, creates delayed, reverse-phase phantom sources that combine with the direct sound from the actual driver. These secondary phantom images create significant ripples in the midrange response (up to 6 dB) and create delayed sounds which interfere with the timing cues necessary to perceive stereo images. These dispersion problems are audible as room-dependent colorations, harsh or dim midrange and treble, fatiguing stereo, and a "detenting" effect that pulls images in towards the loudspeaker cabinets. * This list only covers some of the problems of contemporary loudspeakers. There are other problems, not as severe, but still quite audible to a skilled listener. These problems occur in all loudspeaker types - dynamic direct radiator, horns, ribbons, electromagnetic planar, electrostatic planar, you name it. They all have lots of THD and IM distortion concentrated at certain frequencies, they all store and release significant amounts of resonant energy, and they all have frequency-dependent dispersion further degraded by diffractive re-radiation.
The speaker driver determines the ultimate potential of the entire loudspeaker, and plays a dominant role in the sound of the entire high-fidelity system. As mentioned in the first part of the series, there is no perfect driver at the present state of the art, and that goal is many decades away, since it requires a driver with a density equal to air, completely uniform motion at all frequencies, and no distortion of any kind. We have a long road ahead of us ... but take heart! Major advances in materials sciences are happening right now, with many improvements due to occur in this decade. I confidently expect we'll be seeing breakthroughs every 2 or 3 years at the present rate of progress. This is thanks to major advances in computer modelling of mechanical behaviour and the research conducted in the aerospace, automotive, and sports/recreation industries for lightweight, high-performance materials to replace costly and heavy traditional materials. We now have Kevlar, carbon fiber composites, and forged aluminum drivers; we can expect synthetic diamond, ultralow density aerogel-silica glasses, new types of metallic and carbon monocrystals, and new classes of composite drivers before the turn of the Millenium. *
Why Drivers Sound Like That The major challenge facing the driver designer is combining uniformity of motion (rigidity) with freedom from resonance at mid and high frequencies (self-damping). This is the major tradeoff in speaker systems of all types (except the "massless" group discussed in the first part of this series). There are additional 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 objectivist engineers. "How can can a mid or woofer possibly be fast, since the crossover limits the pulse risetime to a fifth, or tenth, of what any tweeter can do?" This quickly leads to what diplomats call a "full and frank exchange of views", in other words, a mutual exchange of misunderstandings. As usual, both sides are right, and both sides are wrong. They're just speaking about different things. What the audiophile is actually hearing is uniform cone motion; this phenomenon can be measured by the absence of IM distortion, a flat frequency response in the working range, and good pulse response with a clean 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 bronze, perhaps? That's nice and strong, and it can be formed into nearly any shape. You can see the direction this is taking. Bells are made out of bronze. Another problem raises its head .... resonance! After all, why does a bell, or any other rigid metal, ring so long, for many thousands of cycles? The answer has two parts, one obvious, one not so obvious. First, the metal is rigid, and formed in a shape that increases the rigidity even further. Second, the only path for the bell to release mechanical energy is to the air itself, which takes a long, long time, since the density of air and bronze are quite different, resulting in very weak coupling, and very little damping by the air load. This leads us to another desirable property for the speaker driver, which is ...
* Self-Damping We also want the voice coil to stop the cone or dome, not have the cone or dome play a tune all by themselves. Unfortunately, the most rigid materials (traditionally metals) have very little self-damping, resulting in vibrations of very long duration (high Q). One way to control the problem is to extend a heavy rubber surround partway down the cone, and pay a lot of attention to the damping behaviour of the spider and surround materials. At the present, though, even the best Kevlar, carbon-fiber, or aluminum designs show at least one high-Q peak at the top of the working range, requiring a sharp crossover, a notch filter, or preferably 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. Self-damping results in an absence of coloration, as well as contributing to a relaxed, natural, and unfatiguing quality. Interestingly, many audiophiles (and reviewers, too!) are unaware of the particular sound of driver-material resonance, calling it "amplifier sensitivity", "room sensitivity", or similar term that seems to point away from the loudspeaker itself. There are highly-reviewed (by the large-circulation "underground" magazines) 2-way speakers that use 7" Kevlar drivers crossed over to metal-dome tweeters. Technically, these loudspeakers operate with uniform motion over the range of both drivers; in practice, though, the crossovers are hard pressed to remove all of the energy from the Kevlar breakup region between 3 and 5 kHz. The reviews of these particular 2-way speakers go on at considerable, and amusing, length about the trials in finding an amplifier that "revealed" the full quality of the loudspeaker. In reality, the reviewer was forced to use an amplifier that was particularly free of coloration in the region where the Kevlar driver was breaking up. Since most audiophiles and reviewers are unfamilar with the direct sound (and measurements) of commonly-used raw drivers, they can't evaluate how much "Kevlar sound", or "aluminum sound", remains as a residue in the finished design. This is a problem, by the way, that plagues all current 2-way Kevlar, metal, or carbon-fiber loudspeakers ... at the current state of the art, the 6.5" or 7" drivers are forced to operate right up to the edge of their working ranges in order to meet the tweeter in a moderate-distortion frequency range. If you lower the crossover frequency, tweeter IM distortion skyrockets, resulting in raspy, distorted high frequencies at mid-to-high listening levels; if you raise the crossover frequency, the Kevlar 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). This presents the designer with a tough choice: rough sound in the entire treble region, or the characteristic Kevlar forwardness, which can at times actually give a snarly sound to the speaker system. At the present, the best choice is a fourth-order (24dB/Oct.) crossover with a sharp notch tuned to the Kevlar resonance. I should add, by the way, that I like Kevlar and carbon-fiber drivers very much ... but they are difficult drivers to work with, with strong resonant signatures that must be controlled acoustically and electrically. As mentioned above, rigid cones have advantages, but are difficult to damp completely. A different 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 most 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 cone has fairly high IM distortion since it is quite soft. In addition, it is quite difficult to make a material that has perfectly linear mechanical attenuation; in practice, distortion creeps in when you actually want a progressive attenuation of energy over the surface of the cone. I think what is really happening is similar to many soft-dome tweeters; the cone is actually breaking up throughout the entire frequency range, but the heavy damping hides this from the instrumentation (but not the ear). To overcome this subjective effect, the best drivers of this type (Dynaudio, Scan-Speak, and Vifa) are actually composites, adding silica, talc, or metal dust to the plastic, 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 resonant cavity. One example of this was the (in)famous 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 number of problems, such as low efficiency, limited power-handling, a broad one-octave peak centered at 1.5 kHz (corrected by the crossover), and group of 3 very high-Q peaks centered around 4.5 kHz (only slightly attentuated 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, also had similar resonances between 9 and 16 kHz, which were partially damped by a felt pad nearly filling the space between the dome and the polepiece. Since the soft-domes were much more lossy than the stiff B110 dustcap, the resonances were much broader and only 1 to 3 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 more mundane speakers of the day. (Ah yes ... who can remember such paragons of excellence as the BIC Venturis? The Cerwin-Vegas? The Rectilinears? The JBL L100's? In a prior life, I actually had to sell these awful things! "Wait'll you hear 'Dark Side of the Moon' on these babies!") Returning to the present, the best midbass and tweeter drivers now sidestep this problem in two ways: a vented polepiece assembly, used by the Scandinavian manufacturers Dynaudio, Scan-Speak, Vifa, and Seas; and a bullet-like extension of the polepiece, which replaces the midbass dustcap entirely, used by the French manufacturers Audax and Focal. The Dynaudio Esotec D-260, Esotar T-330D, and Scan-Speak D2905/9000 tweeters are the most notable examples of using a vented polepiece loading into a tiny transmission-line to damp the backwave from the tweeter dome; their use in the Sonus Faber Extrema and ProAc Response Threes is a comment on how successful this technique can be. By contrast, the Focal T120 and T120K, which use a rigid fiberglass or Kevlar inverted dome directly above an undamped polepiece cavity, show a series of high-Q resonant peaks at the top of their operating range, which are caused by the resonant cavity coupling to the first breakup region of the rigid dome. I must admit I was rather puzzled by the public acclaim for these drivers when they first came out; I didn't like the way they sounded, and I wasn't too impressed by their measurements. >From all accounts, though, the new Focal titanium-dome T120Ti and titanium-dioxide T122Ti-O2 are excellent, and I liked what I heard when I auditioned a speaker that used the Focal T120Ti at a recent Triode Society meeting. 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 (or "choke"). 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 determing the upper rolloff frequency of the driver, and its resulting acoustic delay (relative to the tweeter). Vary this inductance, and the rolloff frequency and acoustic delay move along with it. When does this happen? Whenever the driver moves a significant proportion of the linear region of voice-coil travel, which is less 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 distortion 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 loss of resolution that is depends on the bass content of the program material, which may be masked by problems in the power amplifier (such as output transformer saturation or inadequate power supplies). Are there solutions? Yes. The best drivers from Scan-Speak (SD System) and Dynaudio (DTL-System) 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 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 carry bass and midrange at the same time. The inductance figure also has a another hidden meaning; 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 desirable; it means that the interaction between the two rolloff mechanisms is going to be small. Other drivers (and this is true of most drivers) are going to 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. Not surprisingly, this type of cartridge usually sounded much less transparent than its high-end moving-coil brethren, which had less than one-tenth the inductance and a much flatter, more accurate mechanical system. In the section that follows, I'll show you how you can make your own decisions about which drivers you like (and second-guess the manufacturer, reviewer, and your audio friends). *
Selecting A Driver I use a method that's so crude it might sound kind of dumb; I put the driver on large, IEC-sized baffle (135cm by 85 cm) and listen to it. No crossover, no enclosure, and if it's a tweeter, not very 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 how much potential resolution the driver posseses). 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. In addition, I very 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 it 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) The Group Delay vs. Frequency Response. (How ragged is the frequency range above the first breakup? Can it be fixed in the crossover?) 3) The Waterfall Cumulative Decay Spectrum. (Can I accept the resonances that can't be fixed in the crossover? If crossover correction is required, how complex is it going to be?) 4) The flatness of Frequency Response in the working band. (Can I accept the broad, low-level colorations that may appear here?) Listening and measurements are equally essential. Both give only a partial picture of the actual driver. Even the finest modern audiophile system will have very serious sonic deficits 5 years from now; measurements provide a reality-check on colorations that present-day equipment may not reveal. In turn, the MLSSA system can point out troublesome colorations to listen for; some are much more audible than others. The thoughtful designer is obliged to be as careful as the craftsman (or craftswoman) who lavishes a full measure of care and attention on even the hidden parts of their creation.