Introduction
Q. What is the best material
for loudspeaker cones?
Q. What are the characteristics
of various cone materials?
Q. What is the best material
for tweeter domes?
Q. Why aren't the L/1 and
L/2 floor-standing towers?
Q. What is a "shielded"
speaker and when do I need one?
Q: Is an open-air/free-air/top-mount
high frequency driver configuration the best for all situations?
Q. What is the relationship
between Biro Technology and Audio by Van Alsine, Inc.?
Introduction
One of the aims of Biro Technology as a company is quite simply
to make the whole of the audio world a better place. In our opinion,
a major obstacle keeping audio from moving forward and better assuming
its proper role in the universe -- as a servant to truer and deeper
musical experiences -- is the fact that it is treated by a preponderance
of the industry as essentially a marketing-oriented activity rather
than as an engineering or music-oriented one. What this means is that
the less you, the audio consumer, know about how things really work,
the easier it will be for the marketing people to go to work on you
and make you buy (literally) what they want you to. The flip-side
of this is that the more you know, the more easily you'll be
able to spot the exaggeration, hyperbole, or outright lies in a marketing
effort, and the better decisions you'll be able to make. Ultimately,
this means that the deserving products and companies will get rewarded,
and the ones who survive by exploiting your ignorance, neuroses, and/or
insecurities feel the pressure to get real.
It was with this in mind that we decided to publish this FAQ. From
the outset we must apologize for being unable to break things down
into bite-sized factoids that you can easily tuck away in your brain.
The truth is, loudspeaker drivers and systems are deceptively complicated
things. And while we have tried to simplify things as much as possible,
our primary concern has been to not give so little information as
to create a bigger problem than the one we are trying to solve. Especially
true in audio, a little knowledge can be a dangerous thing.
[back]
Q. What is the best material
for loudspeaker cones?
A. (Short) There is no best material for loudspeaker cones.
A. (Long) As anyone shopping for speakers is undoubtedly aware,
loudspeaker cones are made from a variety of materials, each one being
claimed to have some property or another that makes it better than
all the rest. Unfortunately, in spite of what the ad copy writers
would like you to believe, there is no single "best" cone
material for loudspeaker applications. Different cone materials have
different mechanical and acoustical properties that result in various
performance tradeoffs, making them better or worse suited for various
situations. However, almost always the choice of material involves
some kind of compromise. The basic material parameters that affect
the acoustic performance of a cone material are its density, stiffness,
and internal lossiness (i.e., the internal damping). Very loosely
speaking, the stiffer and lighter a cone material is, the wider the
bandwidth of the cone will be. The more lossy it is, the smoother
the response. Unfortunately, the above parameters are typically interactive,
and it is very difficult to optimize all three parameters simultaneously.
To find out why, we need to understand a little better what happens
in a speaker cone when it is making music.
At low and very low frequencies, a loudspeaker cone moves essentially
as a homogeneous unit, and there is only one cone parameter that has
significant impact on the performance of the driver: the total cone
mass (which is itself a function of the cone material's density and
the total amount of material used).[1] All other parameters being
equal, the greater the cone mass, the lower the frequency of the fundamental
resonance of the driver, the less damped the resonance, and the less
sensitive the driver will be. However, other driver component parameters
-- such as the suspension compliance and lossiness -- also affect
the resonance, damping, and sensitivity of the driver. All these variables
must be considered when performing a design analysis to get you to
the desired result. Fortunately, the mathematics describing the low-frequency
behavior of loudspeakers is not terribly complicated, and so the modeling
of loudspeaker drivers at low frequencies is a fairly straightforward
task.
At higher frequencies (where the wavelength of the sound wave becomes
comparable to the radius of the cone) the cone ceases to move as a
homogeneous unit, and our low frequency model breaks down. At these
frequencies, you'd do best to think of the sound wave as starting
at the base of the cone (at the voice-coil former/cone joint) and
propagating outward towards the edge of the cone. When the wave hits
the edge of the cone, it is reflected back toward the base of the
cone (towards the voice-coil former). When the wave hits the voice-coil
former, it is again reflected back towards the edge of the cone, and
the whole process starts again. This process is similar to sound waves
traveling in a room, hitting a wall, reflecting back, etc. In both
situations, significant standing waves result, and these standing
waves can produce really, really huge peaks and dips in the response
of a driver unless steps are taken to counteract it. (For example,
the first standing wave resonance for a good 6-1/2" driver typically
falls in the upper midrange, well within the range where it would
be contributing significantly to the output of a two-way system.)
Fortunately, most cone materials have a degree of lossiness in them
-- meaning that they are imperfect sound conductors. A portion of
the wave energy travelling through the material is converted to heat,
and the wave is gradually attenuated as it travels down the cone.
Such lossiness reduces the intensity of the standing waves by reducing
the intensity of the reflected wave energy, thereby smoothing the
response of the driver. Different cone materials vary greatly in the
amount of internal damping they have, ranging from almost none (metal)
to a lot (some plastic materials).
Another means of controlling the intensity of standing waves in a
cone is the cone surround. Typically, a speaker cone is supported
around its edge by some kind of material -- usually a rubber-like
elastomer or foam, but sometimes cloth or even accordioned paper.
One function of this surround is to allow the cone to move back and
forth with relative ease at low frequencies while providing an air-tight
seal. At higher frequencies, it can be used to absorb some of the
cone's standing wave energy. As the wave travelling out from the base
of the cone hits the surround/cone interface, a portion of the wave
energy is actually transmitted into the surround material, with the
remaining energy immediately reflecting back into the cone. Depending
on how lossy the surround material is, the portion of the wave energy
transmitted into the surround may be converted into heat (effectively
damping resonances), or it may be bounced around inside the surround
and then back into the cone (creating a more complicated series of
resonances). Synthetic rubber-like surround materials are typically
formulated to have very high internal losses, although there are a
few that are surprisingly low. Foam surrounds are typically less lossy
than "rubber" ones, although I'm betting that someone, somewhere
makes a foam that is very lossy. In either case, the amount of loss
in a surround (or a cone for that matter) may or may not be constant
with frequency.
The correct amount of damping in the cone and surround depends on
the demands of the situation. Generally speaking, you want enough
loss in the combined cone/surround system to produce a smooth and
well-controlled high frequency response, unless all the standing wave
resonances occur well outside the bandwidth under which the driver
will be used.[2]
The usable bandwidth of a cone is determined largely by the frequency
of the first standing wave, and the faster the wave travels through
the cone material, the higher in frequency this will occur. The primary
mechanical properties which determine the rate of sound propagation
through the material are its stiffness, density, and thickness: stiff,
light, thick cones producing faster rates of sound propagation than
limp, dense, thin ones. Unfortunately, the general tendency is that
the lighter and stiffer a material is (yielding wider bandwidths)
the less internal loss it has -- meaning that the less damped the
standing waves and rougher the frequency response will be. In addition,
the more dissimilar the cone material is from the surround, a stiff
metal cone with a loose and lossy elastomer surround, for example,
the less wave energy will be transmitted into the surround, and the
less effective it can be at damping the standing waves. These characteristics
make it very difficult to get wide bandwidth and smooth response simultaneously
from a cone. To further complicate matters is the very annoying phenomenon
that the lossier a surround material is, the less linear it tends
to become at high excursions -- as it might experience when the driver
is reproducing large low-frequency signals. So if on top of a wide,
smooth bandwidth you also want good low frequency performance, you
are stuck with a very complicated juggling act where no one can be
completely happy, but with luck nobody will be overly let down.
While the above is certainly not an exhaustive description of every
aspect of loudspeaker cone behavior, it does hit on some of the major
ones. Indeed, one could write several books on the subject. (And we
wish someone would!)
______
[1] Having said that,
we would be amiss if we did not mention that there might be additional
mechanical properties which influence the low frequency behavior of
cones in subtle, difficult to measure ways. Unfortunately, opinions
vary widely on this subject, and there is very little research backing
up any claims one way or the other.
[2] Another option is to use active or passive
equalization to compensate for the resonances. This turns out to be
a pretty bad idea since the actual frequencies of the resonances move
around a little depending on manufacturing variances as well as ambient
conditions. A compensation circuit that works for one driver on one
day may or may not work on a different driver or on a different day.
[back]
Q. What are the characteristics
of various cone materials?
A. If you haven't done so, you should probably read "What
is the best material for loudspeaker cones?" before consuming
the material below.
Paper
Paper is the traditional material for speaker cones and is widely
(though mistakenly) considered to be an outdated technology inappropriate
to high performance audio applications. Among its virtues are that
it can easily be formed into a wide variety of shapes without overly
complex or expensive tooling and its mechanical properties can be
varied over a usefully wide range. Unfortunately, untreated paper
is very sensitive to environmental conditions -- humidity in particular.
As the ambient humidity changes, the moisture content in the paper
also changes, and this leads to changes in cone mass and other parameters.
Also, while it is possible to manufacture paper to be stiff enough
to get extended frequency response, the paper itself is usually insufficiently
lossy to achieve a smooth rolloff. Finally, paper is not the easiest
material to manufacture consistently, with the possible result that
there will be wide production variances.
The first two of the above shortcomings can be greatly alleviate by
the application of various types of surface treatments including latex
and PVA-based coatings and impregnations. These coatings help to isolate
the cone from ambient environmental conditions while increasing transmission
loss, thereby smoothing out the upper range of the driver. (Note:
many coated paper cones are coated largely for aesthetic purposes
using God-only-knows what kind of materials that do God-only-knows
what to the acoustical performance of the driver.) The potential for
wide production variances can be ameliorated by tightly controlled
production processes and quality material sourcing.
Despite the seeming low-techness involved, a well engineered paper
cone can deliver a combination of bandwidth and smoothness that is
at least as good as any "higher-tech" material. Additional
research into new paper formulations, manufacturing methods, and treatments
continues. Don't be surprised if some of the newest "breakthroughs"
in cone technology are based on lowly, old-fashioned paper.
Polypropylene
Polypropylene is probably the most common plastic material used in
speaker cones. Most cones advertised as being made of polypropylene
are in fact a combination of polypropylene and a mineral or other
filler (e.g., carbon fiber and Kevlar®). These fillers can be
used both to control costs and to alter the mechanical properties
of the material. Polypropylene cones tend to be inherently well damped
with the result that they can deliver smooth, if not terribly extended,
frequency responses. They are also largely immune to changes in ambient
humidity. The material itself and the methods used in manufacturing
cones with it are such that tight tolerances are easily achieved.
In fact, polypropylene is the material of choice for may researchers
involved in finite element analysis (FEA) of drive units because it
is easy to reliably characterize.
Polypropylene acquired something of a bad reputation in its early
days due to fact that it's a difficult material to get things to bond
to. Luckily, modern adhesive technology has completely solved this
problem. However, this is not to say that polypropylene is free of
problems. While a quantitative study has never been published (not
to my knowledge at any rate), there are some who feel that drivers
made with polypropylene cones tend to exhibit an audible degree of
hysteresis or hysteresis-like behavior. (Hysteresis is a kind of nonlinearity
where the parameters of a system which should be constant vary depending
on the system's recent history.) The most common thinking is that
it is the viscoelastic creep present in all plastic materials that
is responsible. (Viscoelastic creep refers to the tendency of plastic
materials to slowly stretch when under stress. This process may or
may not be linear and typically is related to the lossiness in the
material.) One colleague whom I respect greatly feels that the joint
between the voice—coil former and the cone may be to blame. He
suggests that the heat generated by the voice-coil and dissipated
by the former may soften either the plastic cone material or the glue
at the joint -- the amount of softening depending on how much power
the coil is dissipating.
Despite these actual or imagined problems, polypropylene cones remain
a popular choice for high performance systems largely because of their
well-behaved high-frequency response and consistent performance.
Other plastics
Apart from polypropylene, there are numerous other plastic and plastic-based
materials that have appeared over the years including TPX, HD-A, and
HD-I (all manufactured by Audax), Neoflex (manufactured by Focal),
and Bextrene (which polypropylene largely replaced). All these represent
attempts at finding combinations of stiffness, lossiness, density,
and sound velocity that are somehow optimal for a given application.
They generally have the same virtues and potential pitfalls as polypropylene.
Resin-bonded high-strength woven fibers
In this class of materials belong most carbon-fiber, fiberglass and
Kevlar® cones. These cones are made from a fabric of fibers bonded
together with an epoxy or similar resin. The fibers themselves have
a high degree of tensile strength; when embedded in an appropriate
resin, a material of considerable stiffness results. Not surprisingly,
these woven cones tend to have extended bandwidths. However, it comes
with the cost of quite a bit of roughness as the internal losses of
the basic resin-bonded material are quite low. It has been suggested
the random orientation of the fibers helps to break up standing wave
patterns on the cone, thereby smoothing the response of the driver.
In our experience, this phenomenon has at best a minor influence on
the high frequency response of the driver as every woven-cone driver
we've examined has exhibited a rather rough high-frequency response.
Attempts have been made to improve on the basic construction of simple
woven fabric cones. One manufacturer of raw driver units employs two
thin layers of Kevlar® fabric bonded together with a resin and
silica microball combination. The laminated structure is purported
to be very stiff and the core material has the potential of introducing
a controllable amount of damping. Another driver manufacturer employs
a similar sandwich structure but with a honeycomb Nomex core. While
these technologies are very exciting, they tend to be extremely costly
and suffer, to greater or lesser extent, from the same high frequency
roughness as their simpler cousins.
It is highly unlikely that a woven fabric cone will have any hysteretic
properties. (Although the surround and spider -- even the motor system
-- may still suffer from hysteresis, but that's another issue.) So,
while they may not generally be the best choice for wide-range applications,
woven fabric cones are well suited to low-frequency applications owing
to their inherent stiffness and immunity to hysteresis. In addition,
woven fabric cones typically are insensitive to environmental conditions
and are not likely to be bothered very much if exposed to a direct
heat or light source (e.g., the sun). Thus they may be well suited
to a variety of mobile or even outdoor applications as well.
Metal
Metal is seeing something of a surge in popularity as a cone material.
Of all the materials we have discussed so far, it is the least well
damped and so suffers from extreme peakiness in the high frequency
region -- peaks of 12 dB at 5 kHz for a 6-1/2" driver being not
uncommon. However, below their first breakup mode, metal cones tend
to be very well behaved, and this is a major source of the attraction
to metal cones.
The most common materials used in metal cones are aluminum (and its
alloys) and magnesium. Given the broad range of forming and surface
treatment options possible with these materials, it is not inconceivable
that we may one day see the advent of a well-controlled metal cone
driver. However, even with the best crossover design, the high-frequency
peaks present in currently available cones make them a poor choice
for wide-range applications.
Everything else
Driver manufactures are constantly experimenting with new permutations
of basic materials and constructions in an attempt to find (at best)
a better compromise for a given application, or (at worst) a product
that merely has greater market appeal. Laminates of all sorts, Kevlar®
and paper composites, and Kevlar® and plastic composites are but
a few of the materials that have recently been made available. As
with any new technology, all claims made for or against such new materials
must be considered very, very carefully.
The Bottom Line
I hope that by now it is clear that the "best" cone material
to use for high performance audio depends on what you need to do with
it and that at best it will only be some kind of compromise. It is
also important to bear in mind that a loudspeaker driver is much,
much more than the material from which its cone is made. The profile
of the cone and distribution of material, the properties of the surround
and spider at various frequencies, the voice coil geometry and materials,
the magnetic structure, etc. all play a large role in the final performance
of the driver. What all this means, dear reader, is that you simply
cannot judge a driver by its cone material.
[back]
Q. What is the best material
for tweeter domes?
A. (Short) There is no best material for tweeter domes.
A. (Long) Coming sooner or later. But many of the kinds of
issues that are true for woofer cones (above) apply here as well.
[back]
Q. Why aren't the L/1 and
L/2 floor-standing towers?
A. One of the recent trends in loudspeaker marketing has been
the move toward floor-standing systems that have footprints similar
to more conventional stand-mounted designs. This is chiefly due to
a perception in the market that larger boxes necessarily deliver "better
bass". By simply packaging a system in a floor-standing format
the seller can easily increase the "perceived value" of
a product -- no matter what the technical consequences.
Unfortunately, the premise that bigger boxes necessarily
deliver better bass is by no means universally or generally true.
In reality, for a given alignment (e.g., second-order Butterworth,
fourth order Bessel, etc.), the size of the box merely fixes an efficiency-bandwidth
figure. The exact relationship is:
n0 = k * f3 * Vb
where n0 is the system efficiency,
k is a constant determined by the system's alignment, f3 is the system's
low-frequency 3dB cutoff, and Vb is the size of the box. This equation
tells us the following:
* If we increase the box volume and keep the low-frequency
cutoff the same, the system's efficiency will increase.
* If we increase the box volume, we can decrease the low-frequency
cutoff and maintain the same efficiency.
* If we decrease the box volume, we can keep the same low-frequency
cutoff by reducing the system efficiency. (In all cases we've kept
k the same so that the bass quality is unchanged.)
This last result -- which probably comes as a surprise
to most -- shows that a small box is capable of the exact same extension
and quality as any large box as long as you're willing to pay for
it with efficiency [1]. Furthermore, this cost is not astronomical:
halving a box's volume while keeping the extension and bass quality
constant will reduce the system efficiency by about 3dB.
But perhaps more important than the size vs. extension
vs. efficiency issue is the annoying fact that there are two significant
factors conspiring against a system effectively taking advantage of
the additional volume a floor-standing configuration has to offer.
The first (and possibly most obvious) factor is cost. A large, well-braced
floor-standing cabinet is appreciably more expensive to build, handle,
and transport than a comparable stand-mounted one. This extra cost
must be passed on to the customer as either an increase in the selling
price of the finished system or a reduction in the quality of the
drivers, crossover components, or even the cabinet itself. Second,
a floor-standing cabinet of the format suggested (e.g., 90cm (36")
high with a footprint matching our L/1's 26cm (10.25") by 29cm
(11.5")) when left to its own devices will possess a very strong
1/4-wave column resonance around 100 Hz. This resonance will manifest
itself as a spectacularly unpleasant coloration and will thoroughly
perturb the system's low-frequency alignment.
There is very little you can do about the cost issue.
But if you're still really committed to the floor-standing concept,
there are a few things you can do to mitigate against the column resonance.
Unfortunately, all of these solutions drive up the cost even further,
and most of them eliminate or drastically reduce any advantage the
additional box volume may have contributed in the first place.
For example:
You can resistively damp the airflow inside the
cabinet. While this approach can eliminate the column resonance it
will also largely nullify the benefits that the larger volume may
have given you vis-a-vis low frequency extension or system
efficiency.
Or...
You can incorporate an angled surface to break up
the column. To be effective, the angled surface must be comparable
in size to the sides of the cabinet. (A simple angled top is not sufficient!)
Such a construction, whether it happens as an additional panel inside
the cabinet or as an integral part of the cabinet exterior, is a very
expensive proposition, and it robs you of a good chunk of the extra
volume.
Or...
You can block off the lower part of the cabinet
with an additional panel, but now you've lost all the extra volume
you're trying to gain.
Or...
You can divide the cabinet into top and bottom halves
and use two drivers, one in each section. Not only does this approach
increase the crossover complexity, it also means your low-frequency
driver budget now needs to buy two inexpensive units rather than one
really good one. It can, however, have the advantage of increasing
the radiating area of the system at low frequencies.
Or...
You can increase the width and height of the cabinet
to distribute the resonant modes and then deal with those distributed
resonances with a moderate amount of resistive damping. The cabinets
of our L/1 and L/2 are currently narrow enough that we can very precisely
compensate for the acoustical influence of the cabinet baffle on the
frequency response of the system. With a wider baffle, this would
no longer be possible -- at least not without dramatically increasing
the crossover network's complexity and cost.
Of course, one of the hidden costs in a stand-mounted
design is the price of the stand itself. Suffice it to say that you
can spend anywhere from $50 on a stand that gets the job done effectively
and not too unattractively to as much as you'd ever care to spend
for a pair of custom sculpted marble pedestals that would make even
Brancusi envious. The important thing to keep in mind is that you,
the end customer, are in control of how much to spend on utility and
how much on aesthetics.
To conclude, the primary benefit of a stand-mounted
cabinet over a floor-standing cabinet of equal footprint is that it
permits the use of construction methods, drivers, and crossover components
of an appreciably higher quality for the same end-cost. Nature being
what it is, there is a downside associated with using the smaller
cabinet and that is that if you want the same bass extension and quality
as the (marginally) larger floor-standing box, you need to sacrifice
a little system efficiency. However, in this day of relatively inexpensive
amplifier watts, we don't consider this a significant compromise.
In fact, we barely consider this a compromise at all. In our 3200
cubic foot auditioning room we are consistently able to drive our
L/1 and L/2 to very satisfying listening levels using a high quality,
yet modestly powered 85 wpc solid-state amp and an equally high quality
35 wpc tube amp.
______
[1]There is a practical limit on the minimum
size of the box that is determined by the maximum required sound level
and the nonlinear compressibility of air. The air inside the box cannot
be compressed more than a few percent before its nonlinearity becomes
noticeable. A system reproducing low frequencies will typically reach
this point when the cabinet approaches 3 liters (0.1 cubic feet).
As a frame of reference, our L/1 has an internal volume of just under
19 liters (0.67 cubic feet).
[back]
Q. What is a "shielded"
speaker and when do I need one?
A. Read this.
[back]
Q: Is an open-air/free-air/top-mount
high frequency driver configuration the best for all situations?
A. (Short) No.
A. (Long) An open-air mounting technique as used in our own
L/1 loudspeaker and by others when implemented competently
can simultaneously solve three loudspeaker design problems. The first
problem involves cabinet edge diffraction and the effect it has on
high-frequency system behavior. In short, a conventional system's
cabinet edges introduce radiation discontinuities, and these discontinuities
introduce time and frequency response aberrations. One way to reduce,
but not eliminate, these aberrations is to place the high-frequency
driver on top of the box. This isssue increasingly becoming less of
an issue at Biro Technology as our Diffraction Optimized Positioning
driver placement technology matures. Diffraction Optimized Positioning
allows us to create conventional systems whose diffraction behavior
is essentially as good as an open-air mounting configuration.
The second issue involves the placement in time of the low and high
frequency drivers. High-frequency drivers are physically shallower
than low-frequency drivers. When the both drivers are mounted on a
common plane (as in a conventional system), the low-frequency driver's
physical and acoustical center will be behind the high-frequency driver.
This introduces a small time delay between the drivers. With an open-air
configuration, the high-frequency driver can be physically set back,
thereby bringing the acoustical centers of the two drivers back into
perfect synchronization.
The third issue follows directly from the above. When the acoustical
centers of two drivers are identical, it becomes possible to use "textbook"
crossover configurations that are optimized for smoothest off-axis
response and high out-of-band rejection. Our crossover target function
optimization technology lets us develop system-specific "non-textbook"
crossover configurations for conventional systems whose performance
is very close to that of the ideal crossover configurations attainable
with open-air configurations. However, the open-air configurations
still has a slight advantage in this respect.
The above three benefits do not come without cost. In particular,
open-air configurations have a frequency-dependent radiation pattern
that may present problems in some situations. In both open-air and
conventional systems, the low frequency driver radiates in a forward-facing
hemispherical pattern in the midrange, with the radiation narrowing
as the frequency increases. The high-frequency driver's radiation
in conventional systems is essentially forward-facing hemispherical
up to very high frequencies, making it a reasonably close match to
the low-frequency driver's radiation pattern throughout the crossover
region. In contrast, in an open-air system the high-frequency driver's
radiation pattern is mostly hemispherical but upward-facing
from below the crossover frequency up to very high frequencies.
If your listening room is reasonably absorbent, the differences in
radiation patters of the two drivers in an open-air system will be
of no consequence. However, if your listening environment is particularly
reflective, the differences in radiation patterns may lead to subjectively
greater brightness, potential hardness, and possibly compromised imaging.
Therefore, the overall conclusion is that open-air mounted configurations
when implemented competently will be of greatest advantage
in listening environments that are relatively absorbent. However,
in acoustically reflective environments their frequency dependent
radiation pattern may introduce perceived response and imaging artifacts
that overshadow the advantages that this configuration facilitates.
[back]
Q. What is the relationship
between Biro
Technology and Audio
by Van Alsine, Inc.?
A. Biro Technology and Audio by Van Alstine, Inc. are two inpedendent
companies that share a close working relationship. Audio by Van Alstine
assembles and sells Biro Technology products under an exclusive licence.
AVA has an exceptionally flexible production capacity that suits Biro
Technology's manufacturing needs very well. At the same time, Biro
Technology founder Mithat Konar provides technical design services
to AVA, being responsible for most of the innovation implementations
at AVA since 2001.
[back]
