Dear friends: This is an article, that an Auiogoner send to me yesterday, from Audio Perfectionist Journal about the subwoofers subject and doing a Vandersteen review about: this is not the issue, only the statements about the whys of the subs:
ntroduction
Suppose I told you that you could add two
components to your system that would reduce
intermodulation distortion in the midrange by a
factor of two or more, dramatically improve the
resolution of midrange and high frequency detail,
double or triple the dynamic range capability of
your system without changing your existing
amplifier or speakers and improve imaging more
than you can imagine. You would probably be
interested, right? But wait, theres more.
These same components would allow the
amplifier to maintain tighter control over the
speakers in the mid-bass and lower midrange.
They could extend bass response to infrasonic frequencies
while lowering bass distortion and
improving the systems ability to accurately convey
the rhythm and pace of music. And these
same components could virtually eliminate the
uneven response at lower frequencies caused by
room standing waves.
Does all that sound too good to be true?
Are you concerned about the possible cost of all
this improvement? If all this is so easily achievable,
are you wondering why youve never heard
about it before?
Let me assure you that all these sonic
improvements can be yours and Ive been conservative
in my estimates of the level of audible
improvement youll get. You can have all this for
$2,500 and you can upgrade in two steps of
$1,250 each. If you are starting from scratch, you
may actually reduce the cost of a complete system
by purchasing a less expensive amplifier and a
lower cost speaker model, along with these components,
and end up with better overall performance.
Few people have figured this out and fewer
have spread the news, but its all true.
Of course the components Im talking
about are a pair of powered subwoofersbut not
just any subwoofers. These subwoofers need to
Page 12 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
Page 13 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
have some special characteristics which well get
to in a minute.
Subwoofers?
Im sure you are shaking your head in disbelief
right now, and thinking that Ive lost it. You
may have auditioned some popular subwoofer
models and been less than impressed with their
performance and I wont disagree. Most subwoofers
available today are simply unacceptable
for use in a system designed for critical music listening.
Yes, we have all heard those thunderous
thudpuckers, commonly called subwoofers, that
add to the excitement of movie sound and simply
ruin the sound of music. How can I claim that
these things can actually be beneficial in a highend
audio system?
Here are two reasons that your experience
may conflict with my statements: most subwoofers
werent designed for good musical performance,
and most dealers set subwoofers up
poorly, on purpose.
When properly integrated with the system,
subwoofers blend seamlessly with the main
speakers and dont make their presence known.
But thats a very hard sell to the average consumer
and selling is the name of the audio game.
Subwoofers are supposed to add bass, right?
After their initial forays into the market,
few manufacturers continue to try to make subwoofers
that accurately represent music. Why try
to educate consumers when its easier to just give
them what they think they want? Boom!
Subwoofer makers soon learned what
dealers had already figured out: if they cant hear
it woof they wont buy it. Manufacturers started
to build subwoofers with high-Q alignments and
vents in order to provide more slam. Dealers
started to set up their demonstrations for maximum
thump, and maximized sales figures.
Awareness of the basic concepts of specialized
bass reproducers faded or was suppressed.
Home theater exacerbated this situation.
People today expect a subwoofer to rattle their
fillings and the exaggerated bass that most subwoofers
deliver is incompatible with accurate
music reproduction.
But there is more to bass than boombass
is the foundation of all music. And there is more
to subwoofers than bass. They reproduce bass frequencies
to be sure, but bass extension is possibly
the least of the sonic benefits offered by good
powered subwoofers.
Why Good Subwoofers Improve Sound
In order to provide the benefits mentioned
at the beginning of this article, subwoofers must
utilize a dedicated bass amplifier, and the main
amplifier and speakers must be high-pass filtered
using a passive, first-order device.
A high-pass filter does just what you
would expect: it allows frequencies above the cutoff
point to pass, and blocks frequencies below
that cut-off point. The attenuation of a first-order
filter is 6dB per octave. That means that the signal
will be reduced in amplitude by 6dB, one octave
below the crossover or cut-off point. If the
crossover point is 80Hz (-3dB), the signal level at
40Hz will be -6dB relative to the signal level at
80Hz, and -9dB relative to the signal level in the
midrange. The signal amplitude will continue to
fall at a rate of 6dB per octave as the frequency
decreases.
A passive, single-pole filter at the input to
the amplifier is the only sonically transparent way
to high-pass the main speakers.
The subwoofer amplifier will require a
low-pass filter to prevent frequencies above the
selected crossover point from being sent to the
subwoofer. A low-pass filter passes low frequencies
and blocks higher frequencies.
A subwoofer with an internal amplifier,
commonly called a powered subwoofer, will
require electronic compensation to allow both
infrasonic response and acceptable dimensions for
home use.
Given these stipulations, a pair of powered
subwoofers can provide the following benefits:
Page 14 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
1. Better performance from your speakers.
Full range loudspeakers utilize the same
driver to reproduce both the bass range of frequencies
and at least part of the midrange. For
optimum reproduction of midrange frequencies
little cone movement is required, and a relatively
small driver is necessary to provide quick
response and good dispersion.
Low frequencies require lots of air movement,
demanding greater cone area and more cone
movement. In engineering terms, the back-andforth
movement of the cone is called excursion.
Cone excursion quadruples with each halving of
frequency.
Good midrange reproduction requires the
use of moderately-sized drivers and good bass
reproduction requires lots of cone area, so most
full range speakers compromise the quality of
both bass and midrange by utilizing woofers that
are too small to provide good bass yet too large to
deliver the best midrange quality.
The cone of the 8-inch or 10-inch woofer
typically found in a full range loudspeaker will be
required to make peak-to-peak excursions of perhaps
an inch to deliver audible levels of output at
40Hz and it will have to do this while producing
300Hz (or higher) midrange signals at the same
time. A 6.5-inch woofer will make a better
midrange driver but it will have to work even
harder to deliver low frequencies and IM distortion
in the midrange will rise.
Intermodulation distortion occurs when
one frequency modulates (alters by its frequency)
another. Peak-to-peak cone excursions of an inch
or more, which may be required to reproduce a
40Hz signal, will have a substantial effect on a
signal at 300Hz. The 300Hz signal will increase
slightly in frequency when the cone is moving
towards the listener to reproduce the 40Hz portion
of the signal, and decrease in frequency when the
cone is moving away from the listener. This is
only one mechanism of IM distortion, which is
sometimes called Doppler distortion. There are
other forms of IM distortion.
All dynamic drivers exhibit some nonlinearity
in outward versus inward cone movement.
High cone excursion exacerbates nonlinear driver
response and causes harmonic distortion.
Harmonic distortion occurs when a harmonic
(multiple) or side-band of the desired signal is
produced due to nonlinear behavior of the electrical,
magnetic or mechanical mechanism of the
driver. If you want to reproduce 40Hz and you get
some output at 160Hz as well, thats harmonic
distortion.
The results of high excursion of the
woofer cone are intermodulation distortion of the
midrange signal and increased harmonic distortion
of the bass signal. And theres more.
The small woofers required to maintain
reasonable midrange performance in a full range
speaker dont do a very good job of reproducing
the lowest bass frequencies but they do put a lot
of energy into the speaker cabinet structure and
this is very detrimental to sound quality.
As the woofer cone makes these large
mechanical movements to pressurize and rarefy
air, an equal and opposite force is applied to the
woofer basket, or frame, which is attached to the
speaker structure. This force excites resonances in
the cabinet structure and tries to move the whole
speaker back and forth. Cabinet resonances color
the sound in the midrange. Cabinet movement
distorts high frequencies.
A backward and forward motion of just a
few thousandths of an inch may represent a major
percentage of the total excursion of the tweeter
diaphragm as it attempts to reproduce subtle high
frequency details. The result of structural movement
is IM distortion of the midrange and high
frequencies.
If you are skeptical about the sonic consequences
of woofer energy moving the speaker
cabinet, think about speaker spikes. A reduction in
cabinet motion is the main reason that spikes
beneath the speaker improve sound. Remove the
spikes and see (no, hear) what happens.
As you can see, a full range loudspeaker is
a bundle of compromise. It is asked to perform
many conflicting tasks. There is an old Chinese
Page 15 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
proverb that goes something like this: man who
chase two rabbits have no meat for dinner. By
the same token, a speaker that tries to provide
both bass and the rest of the spectrum compromises
the quality of both.
A single-pole, passive high-pass filter at
the input to the amplifier can cure or minimize all
these speaker problems and improve performance
dramatically. This sonically transparent filter will
reduce woofer cone excursion which will reduce
distortion in the bass, midrange and treble as
described above. The result will be better definition,
better imaging, tighter control, greater
dynamic range and a better presentation of the
rhythm and pace of music. The only thing missing
besides distortionwill be low bass and that
will be reproduced by specialized devices
designed just for that purposepowered subwoofers.
2. Better performance from your amplifier.
The major energy demands in music occur
at low frequencies. The major current demands
from an amplifier are at low frequencies. When an
amplifier distorts because of demands for power
that it cannot meet, the output waveform is flattened
at the top and bottom. This distortion is
called clipping because the positive and negative
signal peaks have been clipped off.
Amplifier clipping becomes evident at
high frequencies but clipping is almost always
caused by energy demands at low frequencies that
exceed the capability of the amplifier.
Clipping is the primary cause of speaker
damage because a clipped waveform fools the
crossover network in the speaker which then passes
high power to the high frequency drivers.
An amplifier in normal use will be clipping
at least occasionally. The percentage of time
that the amplifier is driven to the point of clipping
or beyond will have a profound effect on sound
quality. As the amplifier approaches clipping the
sound will become slightly hard, then harsh, and
then, as the amplifier clips, a shattering distortion
will be heard. This distortion eventually destroys
tweeters and crossover networks.
A single-pole, passive high-pass filter at
the input of the amplifier can eliminate all these
distorted sounds and make the amplifier sound
smoother and more relaxed. The amplifier may
seem to be three times more powerful. Removing
the huge low frequency current demands from the
amplifier, by reducing the level of the input signal
at low frequencies, allows the amp to coast along
with lots of power in reserve. The system will
play at much higher levels with much lower distortion,
providing a greatly improved listening
experience.
The high current necessary for accurate
bass reproduction will be provided by specialized
amplifiers designed just for this purposethe
amplifiers in the powered subwoofers.
3. Better bass.
Designing a product to perform a very
specific task requires less compromise.
Subwoofers are designed to reproduce a small
range of frequencies at the lowest audible range.
Thats about as specific as it gets in audio.
When compared to full range speakers,
powered subwoofers can provide the following
advantages: more cone area, greater linear excursion
capability, more amplifier power at low frequencies,
and electronic compensation for falling
output at the lowest frequencies. Subwoofers can
also have smaller, stiffer, less resonant enclosures
and can be placed in the optimum position to
introduce bass energy into the room.
Eliminating the compromised bass output
from the main speakers by high-pass filtering the
input signal to the amplifier will dramatically
improve the quality of reproduction in the midbass
range. Improving the mid-bass provides a
better sense of rhythm and pace and makes it easier
to follow the tune of the bass.
4. Better room interface.
The pressure-zone microphone (PZM) was
developed after it was determined that smooth frequency
response at lower frequencies could not be
Page 16 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
obtained from a stand-mounted microphone due
to interactions with the room boundaries. Placing
a conventional microphone on the floor smoothed
the response curve but caused a gradually rising
bass output. Compensating for this bass rise gave
us the PZM microphone. A similar effect occurs
with speakers.
For good imaging and midrange detail full
range speakers must be placed well out into the
room. Bass response from these speakers will be
uneven due to room interaction. This phenomenon
is frequently attributed to standing waves.
Removing bass from these speakers and
redirecting it to a subwoofer placed in the corner
of the room will ameliorate most of these room
anomalies. The subwoofer will load the room
from a pressure zone, smoothing response across
the bass range. Adding a second subwoofer,
placed in a second corner, allows low frequencies
to be introduced from two different positions
within the pressure zones of the room virtually
eliminating bass irregularities. (You must remove
other sources that store and release energy at low
frequencies as described in the room treatment
article.)
5. Reduced system cost.
In a given manufacturers amplifier line,
the more expensive models usually offer more
power and little else. In fact, smaller amplifiers
frequently sound better than their big brothers and
they always cost less.
The Levinson 33H mono amps that I use
cost about $15,000 less than the Reference 33
amplifiers from the same company. Both models
are essentially the same design, with the larger
version offering only higher output power.
Most loudspeaker manufacturers offer a
range of models that differ only in their ability to
produce bass. Bigger, more expensive models provide
extended bass response with bigger woofers
and larger cabinets. Except for bass extension, its
not unusual to find that the smaller models in a
given line of speakers actually sound better
because they have smaller woofers that offer better
midrange performance and the smaller cabinets
add less box sound. Compare the smaller Dunlavy
models to their larger brothers for example.
The Vandersteen 3A Signature speakers
that I use in conjunction with a pair of 2WQ subwoofers
deliver 90% of the performance of the
Vandersteen Model 5s for 60% of the price (3A
Sigs and two 2WQ subwoofers cost about $6,000
and Model 5s sell for about $10,000). My speaker
system delivers a time- and phase-accurate
response over a usable range of 18Hz to 30kHz.
What other speakers can offer that for $6k?
Some reviewers claim that the 3A
Signatures lack the detail and definition of the
Model 5s. You may find this puzzling because
both models share identical midrange and tweeter
drivers and use essentially the same crossover network
in this range. Why the perceived performance
difference? Model 5s have a slightly more
inert cabinet structure and they have built-in, powered
subwoofers.
The use of powered subwoofers can allow
a smaller amplifier and a pair of lower-priced
speakers to equal or outperform their more expensive
counterparts. The result is better sound for
less money. Who doesnt want that?
Why Most Subwoofers Dont Work Well for
Music Reproduction
Not so many years ago, few people were
aware of the concept of specialized bass speakers.
Explaining what a subwoofer was and the sonic
benefits it could provide were difficult tasks
before the home theater craze hit the public.
Today, people are rushing to add subwoofers to
their audio systems to provide the visceral excitement
that only thunderous bass can supply.
Thunderous bass output makes an onscreen
explosion or gun shot more physically
involving but it can also alter the tonal balance, as
well as the rhythm and pace, of music. Most subwoofers
seem to march to the beat of a different
drummer instead of the one who is playing with
the orchestra.
Today the average consumer believes that
Page 17 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
These illustrations compare frequency response (black trace), phase response (purple trace), impedance (yellow trace), and
group delay (blue trace) for the same JBL 2235H 15-inch driver mounted in a second-order sealed enclosure (above) and in a
fourth-order vented enclosure (below). You can learn a lot about the performance trade-offs involved in the choice of vented
versus sealed enclosure design from studying these graphs. In my opinion, the trade-offs make vents acceptable for use in full
range speakers and totally unacceptable for subwoofers that are meant to reproduce music. A conventional vented design offers
inferior performance in every area except one: output level capability.
the only purpose of a subwoofer is to add bass
and many music purists derisively refer to them as
fart boxes.
There are many reasons why boom-box
subwoofers may do a good job of reproducing
explosions and perform poorly when reproducing
music.
1. Phase shift and group delay cause subwoofers
to start late.
Picture a marching band with the bass
drum following about a block behind the rest of
the players and youve got a pretty good image of
the major problem with most subwoofersthe
sound they produce is just out of step with the rest
of the music. There are many reasons for this but
most revolve around phase shift and group delay.
Acoustic phase in this instance has to do
with the time relationships of the launch of air
pressure waves towards the listener. Phase shift
that varies with frequency alters the time relationships
between different frequencies. A resonance
in the pass band causes phase shift. Filters cause
phase shift. A speaker in a box is a filter.
When two elements like a subwoofer and
a main speaker have an overlapping frequency
range, or are reproducing different parts of a single
note, such as the fundamental and the harmonics
of that fundamental, you want both to be in
step. If the subwoofer cone pushes out when the
main speakers woofer cone is moving inward,
things get out of sync.
Group delay is a complex concept. It is the
negative of the derivative of the phase curve with
respect to radian frequency. Group delay describes
how well the time relationships between a small
group of frequencies are preserved within a narrow
range of frequencies. Time delay and group
delay are not necessarily equivalent but a delay to
one group of frequencies changes its time relationship
to the rest of the spectrum.
Look at the illustrations on page 17 comparing
phase response of vented and sealed enclosure
designs. Trying to synchronize the phase of
the main speakers and the subwoofer will be diffi-
cult with a sealed enclosure subwoofer design and
virtually impossible with a vented design because
of phase shift as the system passes through resonance
in the pass band. Note the 17ms group
delay at the 32Hz tuning frequency of the vented
design.
Subwoofers with a fundamental resonance
in the pass band and a steep slope low-pass filter
at the input will produce output that is delayed in
time relative to the main speakers, and this delay
will vary with frequency.
Subwoofers that start late sound slow and
plodding. They distort the overall waveform even
if their own distortion products are low.
2. High Q makes subwoofers stop late.
An electrical filter will oscillate or ring, to
some extent, after the signal stops. The steeper the
slope of this filter, the more it will ring. The higher
the Q of this filter, the more it will ring.
Mechanical filters work the same way. In
fact, all the mechanical properties of a loudspeaker
can be expressed with electrical equivalents and
modeled by electrical circuits.
A woofer in an enclosure is a high-pass filter.
It passes frequencies above the cut-off or low
frequency limit of the design and the signal rollsoff
below this point at a rate determined by the
design. A sealed box acts as a nominal secondorder
high-pass filter and a vented enclosure will
typically display fourth-order high-pass characteristics.
The vented design will ring (oscillate)
about twice as much as the sealed design after the
signal stops.
The Q of the mechanical system affects
oscillation, too. System Q defines the shape of the
response curve and the amount of damping to overshoot
or ringing (oscillation after the signal stops) that
the system will provide.
Asealed enclosure with a Q of .5 is considered
a critically damped alignment with a step response
that has no overshoot. For a given driver, a Q of .5
requires the largest box. This low-Q alignment has a
downward-sloping response curve but offers the best
possible transient performance and the lowest frequency
extension at -10dB.
Page 18 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
Asystem Q of .577 is a Bessel alignment
which has the most linear phase response and offers
slightly less damping.
When Q =.707 we have a Butterworth alignment
with the flattest amplitude response. This is the
most common alignment for high-end subwoofers
because it offers a full sound which is still well controlled.
System Q near 1.0 delivers a peaked response
but allows the smallest box size still considered by
some to be high-fidelity. Asubwoofer with a system Q
over 1.0 is a boom box with a peaked response curve
and lots of overhang. Guess where most home theater
subwoofers fall.
Subwoofers that play on after the signal has
stopped (due to oscillation), sound slow and muddy.
The Q and slope of the high-pass filter formed
by the subwoofer acoustical system has a major effect
on the sound of the bass the subwoofer produces, but
theres more. The subwoofer is a mechanical high-pass
filter but it must be used with an electrical low-pass filter
and those cause problems as well.
3. Steep filter slopes and direct-radiating drivers.
Most subwoofer designs include one or
more drivers that radiate directly into the room.
Its commonly assumed that subwoofers shouldnt
be allowed to encroach on the midrange where
they perform poorly so the conventional wisdom
mandates a steep-slope low-pass filter to prevent
output above the bass region. There are some
drawbacks to this approach.
Page 19 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
This illustration shows the effect of system Q on frequency response. Notice that the low-Q alignments start sloping down sooner but
ultimately extend lower in frequency at the usable low frequency limit of -10dB. Remember that the low-Q alignments provide far better
damping to oscillation after the signal ceases.
A low-pass filter with a fourth-order slope
will cause another complete phase rollover,
increasing signal delay. Even with this steep attenuation
curve, side band distortion will still be
audible if the driver radiates directly into the
room.
All drive elements will break-up (display
non-pistonic cone behavior) at some frequency.
Filtering below this frequency prevents the signal
from stimulating this behavior. Many other anomalies
such as cone resonances, surround reflections
and flapping, magnetic nonlinearities and basket
ringing will remain audible as side-band distortion
even without frequency-specific stimulation. And
some midrange frequencies will still pass through
the filter at attenuated levels.
Midrange signals (even at low levels) and
side band distortion detract from the quality of
sound from the main speakers and draw attention
to the position of the subwoofer which should be
spaced away from the main speakers.
4. System resonance in the pass band.
Removing resonances from audio components
is generally considered to be a good idea but
subwoofers are designed to create resonance.
A vented subwoofer has two resonances
right in the middle of its pass band. The vent resonance
is tuned to play at frequencies where the
output of an unassisted driver would be falling.
Much of what you hear from a vented design is a
production of the subwoofer rather than a reproduction
of the signal.
Sealed enclosures are better with only a
single resonance in the pass band. One is better
than two, as you can see from the graphs on page
17, but none is better yet as well see later.
When the subwoofer passes through a resonance
a big shift in phase occurs. Look at the
graphs of phase response for sealed and vented
enclosure designs and see the effects of resonance
on phase. (System resonance occurs where the
impedance peaks. Note the single peak in the
graph of the sealed enclosure and the dual peaks
in the graph for the vented enclosure. The dip
between the impedance peaks indicates the tuning
frequency of the vented enclosure.)
Subwoofers with a resonance in the pass
band will tend to emphasize the frequencies
around this resonance. The higher the Q the
greater the emphasis. Ever hear the term onenote
bass?
5. Dissimilar amplifiers for high and low frequencies.
A powered subwoofer may have an internal
amplifier that is designed for optimal performance
when driving the specific load of the subwoofer
drivers. This amplifier will almost always
have electrical compensation for the falling
response of the subwoofer driver(s) which will
typically be housed in an enclosure that is smaller
than idealbecause nobody wants a subwoofer
the size of a refrigerator in their living room.
A perfect subwoofer amplifier and the
amplifier that is best suited for driving the main
speakers may be very different electrically and
sonically. For instance, a bipolar solid-state ampli-
fier will offer the best performance for bass but a
MOSFET or tube amplifier may sound better driving
the main speakers.
Transfer function is a measurement that
compares the frequency and phase response of the
output from a device under test to the input signal.
If the transfer function of the main amplifier is
very different from the TF of the subwoofer
amplifier, this sonic dichotomy may have a negative
impact on overall sound quality.
Making a Subwoofer to Play Music
A subwoofer should march in step with the
rest of the band and stop playing when the song
has ended. Most do neither.
Subwoofers should be positioned in the
room corners to properly load the room from pressure
zones, creating the smoothest bass response.
If the subwoofer has a Q higher than .5 (most
do) it will exhibit a rising response when placed
in a corner.
So how can a subwoofer be designed that
Page 20 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
doesnt suffer from the performance deficits
described above?
We can eliminate the problem of subwoofers
that start late by blending the subwoofer
and the main speakers using a first-order
crossover for transient-perfect phase response.
This filter system can be implemented in an
unusual way: the high-pass section can be placed
at the input to the amplifier driving the main
speakers and the subwoofer amplifier can sample
the output from this amplifier, including its sonic
signature and phase characteristics. The subwoofer
amplifier can have its frequency response
tailored to compensate for the falling response of
the input signal and the falling response of the
acoustic system that is operating primarily below
system resonance. (More about that in a minute.)
Blending the amplifiers in this way will
ameliorate the discontinuous sound created by
dissimilar amplifier designs driving different parts
of the spectrum.
We can eliminate the overhang of subwoofers
that stop late by designing for a target Q
of .5 to achieve critical damping, along with the
greatest usable bass extension, and to allow corner
placement without a rising low-end response.
The driver(s) can be slot loaded to
mechanically filter out side band distortion and
midrange frequencies. And the system can be
designed to operate primarily below the fundamental
resonance so that no resonance can cause
sonic emphasis or phase shift in the pass band.
This all makes perfect sense and Id like to
take credit for thinking it up all by myself. But I
didntRichard Vandersteen did. What I have just
described is the Vandersteen 2WQ powered subwoofer
which has been subtly but continuously
refined since it was first brought to market sixteen
years ago. It is the most sophisticated product of
its kind available today.
The Vandersteen 2WQ Subwoofer
The Vandersteen 2WQ subwoofer is completely
unique in a number of ways. It takes
advantage of the fact that loudspeakers in sealed
enclosures offer very predictable amplitude and
phase response characteristics at frequencies
below the fundamental resonance of the system.
The 2WQ operates primarily below fundamental
system resonance to provide frequency and phase
linearity that cannot be achieved by conventional
designs with resonances in the pass band.
It uses a phase-perfect first-order crossover
with special characteristics. The 2WQ samples the
output from the amplifier that is driving the main
speakers for better system integration.
It uses feed-forward error correction to
prevent output errors before they occur and a
unique protection circuit that does not compress
signal dynamic range. The 2WQ will not produce
audible distortion regardless of the frequency or
level of the input signal.
It utilizes three small drivers instead of
one larger unit for greater power-to-weight ratio
and better diaphragm control. The 8-inch drivers
in the 2WQ are slot-loaded to linearize pressure
on the front and rear of the cones and to mechanically
filter side-band distortion.
The cabinet is constructed using constrained-
layer lamination techniques and crossbracing,
resulting in the most inert, resonance-free
subwoofer enclosure that Ive ever seen.
It has adjustable Q. You can adjust the output
response curve of the 2WQ to suit room
acoustics and placement.
No other commercial subwoofer offers all
these features and virtually all high-quality competitors
cost more.
The 2WQ Operates Primarily Below System
Resonance
Conventional subwoofers operate above
system resonance. They exhibit uneven response
and lots of phase shift as they pass through the
fundamental system resonance which typically
Page 21 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
occurs at the lower end of the pass band.
Designers try to get the resonance frequency low
because output falls off steeply below this frequency.
A vented design with a B4 alignment falls
at 24dB per octave and a sealed design with a Q
of .7 falls at 12dB per octave below resonance.
Take a look at the illustrations in the previous article
to see what happens to phase.
A driver mounted in a sealed enclosure
with a Qtc (total system Q) of .5 will have an
amplitude response curve that falls in a linear
fashion at about 8dB per octave below system resonance
with limited and linear phase shift. Output
will theoretically extend down to DC without the
sudden drop-off and phase shift that occurs when
conventional systems pass through resonance.
This predictable and linear frequency and phase
response is easily compensated for with electronic
correction in the amplifier to produce ruler-flat
output to subsonic frequencies.
Since output is more linear below system
resonance and flat response can be achieved with
amplifier compensation, why not design a subwoofer
that operates below resonance rather than
above it? Thats just what Vandersteen has done.
The result is a subwoofer with virtually no
resonance in the pass band, minimum group delay,
linear phase response, flat amplitude response to
subsonic frequencies, critical damping and a low
system Q making it suitable for corner placement.
A Better Blend with the Main Speakers
The Vandersteen 2WQ subwoofer is integrated
with the main speakers using a unique system
that is not a crossover in the usual sense.
Transitions between the subwoofer and the main
speakers are made with gentle 6dB per octave
slopes using phase- and transient-perfect firstorder
filters that are completely transparent.
Page 22 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
This illustration shows the amplitude and phase characteristics of a sealed enclosure subwoofer with a Qtc of 0.5,
at frequencies below system resonance which, in this example, is 40Hz.
A passive, first-order filter is inserted at
the input to the main amplifier. This filter causes
the signal to the main amplifier and speakers to
roll-off at 6dB per octave below 80Hz.
A 300 watt subwoofer amplifier, designed
specifically to deliver high current into the low
impedance load of the three drive elements, samples
the signal at the output of the main amplifier
and compensates for the roll-off to produce flat
output from the subwoofer. The output from the
subwoofer amplifier is tailored to produce firstorder
low-pass response above 80Hz and a rising
response below 80Hz to compensate for the
falling response curve of the filtered input signal
as well as the falling output response of the subwoofer
which is operating primarily below system
resonance.
Output that could exceed the power limits
of the amplifier, or the linear excursion limits of
the drivers, is prevented by dynamically raising
the low frequency cut-off point rather than compressing
the signal. A unique circuit analyzes the
input signal and dynamically alters the feed-forward
error correction characteristics to accomplish
this feat. The 2WQ cannot be driven to produce
audible distortion under any conditions, yet
it never compresses the dynamic range of the signal,
maintaining the natural rhythm and pace of
music regardless of level.
A passive, first-order high-pass filter at the
main amplifier input is completely transparent so
the sound from the main speakers is not negatively
impacted in any way and all the positive bene-
fits listed in the Subwoofers From a High-End
Perspective article can be realized.
Sampling the output from the main ampli-
fier passes along the sonic and electrical characteristics
(particularly propagation delay) of that
amplifier to the subwoofer system for a better
blend between the subwoofer and the main speakers,
even if the main amplifier is a tube or MOSFET
design. This results in superior integration
between the subwoofer(s) and the main speakers.
Less Audible Distortion
In addition to the feed-forward error correction
system and the specially-designed internal
amplifier, the 2WQ uses several other distortion
reducing techniques.
Three 8-inch drivers have the combined
cone area of a single 14-inch unit but three motors
can provide far better control over the lighter,
stiffer cones. Smaller cones produce less sideband
distortion than larger, more flexible cones,
and any distortion that remains will be at higher
frequencies which can be mechanically filtered by
the indirect radiation path.
These three drivers are slot-loaded providing
an indirect radiation path into the room. Slotloading
the front of the drive elements equalizes
the pressure on the front and back of each
diaphragm making resistance to fore and aft
movement more linear.
A driver in a sealed enclosure sees a
diminishing volume of air and increasing pressure
within the box as the cone moves inward, and an
increasing volume of air and reduced pressure as
the cone moves outward. Covering the front of the
driver(s) with a plate so that radiation from the
front of the drive elements enters the room
through a slot or slots between this plate and the
enclosure is an attempt to compensate for this
phenomenon.
Slot-loading provides a reduction in distortion
by linearizing cone motion and also acts as a
mechanical low-pass filter to absorb residual distortion
products at higher frequencies. This
mechanical low-pass filter is far more effective
than a steep-slope electrical filter for the reasons
described earlier.
The cabinet is elaborately constructed
using constrained-layer laminates and cross bracing
to completely eliminate panel resonances and
spurious noise. The 2WQ enclosure feels like a
solid block of material. Rapping on any panel is
like banging your knuckles against a rock. Panel
flexing is simply out of the question.
Page 23 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
Caveats
The Vandersteen 2WQ subwoofer provides
tightly-controlled bass that is critically damped
and limited in output level compared to a typical
home theater subwoofer. Two units will be
required in all but the smallest rooms to provide
the THX-recommended output level of 105dB at
35Hz. I recommend using two subwoofers anyway
and 105dB is much too loud for music listening
so I dont see these as problem areas.
Tightly-controlled bass that is perfect for
music may not satisfy explosion fans who use
their audio systems for both music and home theater.
Vandersteen makes another subwoofer, the
V2W, for these folks. It looks the same but trades
some control and integration for the ability to play
much louder.
Other subwoofers that offer excellent performance
for those with a strong home theater
bias include many of the M&K models and the
Bag End InfraSub. These subwoofers will still
perform well on music and deliver more visceral
output. Dont use their high-pass filters. Choose a
passive single-pole filter instead.
Best Value
In my opinion, the Vandersteen 2WQ is
the best subwoofer available for reproducing
music regardless of price, and the price is a mere
$1,250 each! If thats not a bargain, Im a bad
shopper. I have four 2WQ subwoofers and theyre
not on long term loan; I bought them. I want the
best possible performance and Im willing to pay
for it, but if the product that offers the best sound
quality also costs less, I wont complain.
Page 24 Audio Perfectionist Journal Issue #2 Copyright © 2000 R. L. Hardesty
Regards and enjoy the music.
Raul.
