I found this pretty relevant and interesting

 

 

audiopulse`s site... enjoy

SUBWOOFER MYTHS

# 1 Subwoofers have an RMS rating

Speakers
actually have a very complex thermal compression relationship and
certainly can not be quantified by just one or two numbers typically
called RMS and Program or Peak. Because voice coils in traditional
drivers are inherently resistors, any amount of voltage generates some
amount of heat which then adversely changes the resistance and
properties of the speaker. This is the principle of thermal compression:
As the voice coil heats up, the resistance changes and the efficiency
and performance of the driver decrease until the point of maximum
thermal compression. There are some unique types of materials that have a
close to zero temperature coefficient and of course there is also
superconducting metals that operate at subzero temperatures with no
indications of any sort of resistance. In theory, only these types of
materials would have no thermal compression, but they are not employed
or very practical yet. Copper and Aluminum are still the two most widely
used materials for voice coils. Both copper and aluminum heat up
considerably and the resistance changes as a function temperature, and
there lies the problem, therefore a discrete RMS scalar value is
entirely inappropriate.

Under heavy use, the TSP parameters can
shift as much as 35% and in a generally un-favored direction. (higher
Qts, lower sensitivity). The common ultra high RMS ratings we see of
large and expensive subwoofers are at best marketing ploys to make the
driver seem far more worthy than it is, or in fact they are really
intended to give the customer an idea of what type of amplifier to buy.
The fact is, even the highest “RMS” rated subwoofers in the world in
excess of 5 digit figures will begin to compress with far less power
than you would ever image, try only a few hundred watts! (no joke!). Now
this doesn’t mean you still don’t need lots of power to reach the
maximum potential of the driver. As a rule of thumb, the amplifier
should be much more capable than what the driver needs on average. For
example, quick short bursts will produce huge SPL’s and the voice coil
will not have time to heat up as much, but longer term high power use
will result in considerable performance regression if not failure from
glues giving way due to heat or differences in the thermal expansion of
materials around the glues. Under heavy use thermal compression limits
begin to play a large part in SPL but most people are oblivious to this
concept. It is true that woofers can be used well into their thermal
compression state, and typically that is what occurs. As the power
increases linearity, the SPL does not increase linearly. This is some
form of compression, usually thermally related unless the woofer is
beyond or close to xmax. In an ideal non-compression circumstance of
either power, BL or otherwise, you can expect a 3dB increase every time
the power is doubled. Rarely does this ever occur, in extremely
compressed and dangerous states it can be less than 1dB!

As a
woofers reaches its very limits, unless failure occurs there will become
a point where the resistance of the voice coil is rising faster than
the power going into the subwoofer. When the resistance doubles as the
power doubles then absolute thermal compression has set in. In practice
you can’t actually increase the power from the amplifier because most
amplifiers start to produce less power as the resistance increases
because almost every car, home and pro audio amplifier is a constant
voltage source rather than a constant current source. So in a way this
phenomenon is a self limited occurrence that accidentally works to
protect the driver. However, running the driver at or near the maximum
thermal compression limit will likely result in rapid failure.
Ultimately, thermal compression is a very large but unavoidable
shortcoming of mass controlled transducers. Likely, compliance
controlled transducers, or rather subsonic transducers are not limited
by their thermal properties as much, but rather their compliance or
linear limits (xmax). It is believed by a few experts in the field that
thermal compression plays a much greater role in linearity and
distortion than we know of, but it’s rarely discussed.

# 2 More xmax means more SPL

Subwoofer
drivers really can be broken down in two categories: “Mass” controlled
drivers and “compliance” controlled drivers. Mass controlled drivers
tend to have low xmax and high sensitivity. These tend to be punchy and
very loud and mostly used in live concerts for sound reinforcement or
even car SPL competitions. Compliance controlled subwoofers which tend
to be the majority of car audio subwoofers have high xmax, more weight,
lower sensitivity, but more SPL in the lower frequency spectrum. Then
there are of course hybrid drivers which are basically mixes of the two.
Any driver in these categories can sound good or bad, but more
important is being able to use the woofer where it performs the best.
Using a low xmax woofer for subsonic content is probably not wise,
likewise using a high xmax low sensitivity driver for sound
reinforcement is not going to be very effective. In truth, there is no
best driver and most drivers can overlap these zones with good results.
We are not really used to the idea of a two way subwoofer, but as we
demand more and more SPL and deeper bass, we may some day find that two
different types of subwoofers used together are required to get the full
reference SPL effect we all hunger for!

So yes, more specified
xmax does mean more SPL but only for lower frequencies. Generally
speaking, during lower frequencies, the driver tends to run out of
usable throw (beyond xmax) before high thermal compression states occur
and mechanical failure is a greater risk. 0-40Hz is primarily
mechanical, 40-60 is in between) 60 and up is going to be more thermally
limited. 0-20Hz is the subsonic content and in fact there are more
efficient methods of producing bass in this spectrum rather than a
regular piston based transducer. Surprisingly, even the largest drivers
with high xmax and big voice coils can be bottomed out or run past a
safe mechanical state with only a few hundred watts if the frequencies
are low enough. Without a high pass (subsonic) filter, or in a low tuned
system, bottoming out or breaking a driver could be a very real
possibility without careful modeling and testing. The difference in
displacement from 40Hz to 20Hz or rather half the frequency, or one
octave, is quadruple! In the simple large sealed box example, that means
if your woofer displacement is 1” peak to peak at 40Hz, you’ll bottom
out just about anything in existence by the time you dip below 20Hz
without protection.

Often times when people want more SPL, they
really need higher sensitivity in the form of higher BL product or less
moving mass, rather than more xmax because 50-60Hz is really what they
are after. This is a very sensual frequency range for humans and much of
the bass in music content exists in that frequency domain.

# 3 Subwoofers are fast / slow

More
appropriately labeled Damping or Ringing, these concepts are really
reciprocals of one another have nothing to do with speed, tightness,
“boomieness” or any other misused and inappropriate term for subwoofers.
Subwoofers, or rather bass drivers, all move at the same frequency when
instructed to via an input single. The difference is really about the Q
alignment of the system. There are many famous Q alignments which
produce various frequency responses, but beyond the complex mathematics
is a fundamental principal of force and acceleration and the driver will
respond to a sinusoidal wave at various accelerations depending on the
moving mass and force that the voice coil and motor generate on the
cone. Therefore any driver can be faster or slower depending simply on
the voltage! It makes little sense to call any driver faster or slower.

Damping
or Ringing is really what we’re after and the amount of either is
really a function of system volume along with the electro-mechanical
damping factor of the driver. For example, in a sealed box system, as
the volume of the cabinet becomes small, the internal pressures increase
when the driver pushes in and out. This pressure is a force which, not
nearly as strong as the electromotive damping force, works in the
opposite direction. Contrary to intuition, higher internal pressure
(which we tend to associate with tightness or stiffness) decreases
damping and promotes ringing at one particular frequency (Fc in the case
of a sealed box). The pressure from the air inside the box works
against the driver’s natural damping factor of 1/(Qts). When the
pressure becomes large relative to the motor’s damping factor, the
driver will ring more and cause a peak in SPL at the given resonate
frequency (Fc). This tends to be somewhere around 40-60Hz in a given
sealed box, but could be outside that range under abnormal
circumstances. This peak is ill desired and is accountable to the
proclaimed “boomy” sounding subwoofers which tend to lack clarity, good
transit response and dynamics. However some people prefer some ringing
because it provides a natural boost in a very audible frequency band.
Likewise, in a larger box, the Q will decrease and the ringing and SPL
around that frequency will too, but the low end will open up and you’ll
have more deep bass. This tends to sound better and more controlled.

On
the flip side, over dampened drivers tend to have poor low frequency
response and require equalization to boost the low frequencies. They
tend to work better in vented boxes where their larger motor force
factor (BL^2/Re) is put to good use with a resonator which then makes
the low end much more efficient with its increased displacement.
Likewise, drivers with high Qts will work better in sealed boxes and
should be exempt from being used in a ported system without careful
consideration. When high Q drivers are used in a vented system they will
ring at the tuning frequency of the box (Fb in this case) and the
“boomy” problem is considerably worse.

# 4 Ported boxes don’t sound as good as sealed

In
most cases this is strictly a result of linear response vs non-linear
response and it could go both ways. 4th order systems or “vented” boxes
tend to be far more particular to volume, port size and length and the
driver TPS’s rather than sealed systems. Misalignments are therefore
amplified and greatly affect the frequency response. Often times in car
audio, ported boxes are not tuned low enough, or the volume is too large
and there is a large peak in the frequency response from literately too
much sensitivity or SPL at a very narrow frequency band. The other
issue is if the driver does not have enough BL or has too high of a Qts
and becomes under damped at resonance. This again leads to drastic peaks
at the resonating frequency; however in this case, the driver will be
peaky there regardless of content and it will sound ultimately less
dynamic and very bottom heavy. However, a well designed vented box may
have considerably lower distortion and higher dynamics than a sealed box
because of the added SPL gained from the port without increasing the
active driver displacement requirements. Sealed systems evoke the most
non-linear driver behavior to reach any given SPL, so in fact, they
could be the worst sounding system if your SPL demands are considerable.
It is important to model a ported design or ask the manufacture for a
recommendation. It is also critical to include a high pass filter on the
active driver in a ported box for protection.

# 5 Subwoofers care what they play

Your
subwoofer driver does not have a conscience, and it does not perform
better with one type of music over another. It’s just a driver. Good
subwoofer systems will play all types of music or movie material very
well. A bad subwoofer system may have a null or peak in the frequency
response that may benefit some material over others but essentially this
non-linear behavior is not ideal. It is true that movies have lower
frequency content and perhaps more dynamic bass than music, especially
with the recent compressed CD’s of the last 10 or so years, but a good
system can be used for movies and music alike if it is indeed a “good”
system.

It is also true that it tends to be more important to
emphasize subsonic frequencies in the home theater environment versus
the music environment where there is simply less emphasis on subsonic
inaudible material. As a tradeoff, you can align a system to be more
efficient above 30Hz or so. This trade off reduces the bandwidth but
increases the SPL. Careful consideration should be taken to insure
linear response is still maintained. It is very easy to have peaky bass
with low Q drivers in high tuned ported systems. This is approaching the
concept of basic SPL vehicles which use low Q, highly sensitive drivers
tuned very high for very narrow but ferociously peaky response. Such
systems are not very ideal for listing to music material of any kind. If
you want your system louder, then it is better to add a second driver,
more volume and more amplification, rather than tuning higher. It is
important to understand that getting more SPL without compromise is
never very cheap!

# 6 Sealed box can take more power than ported

There
is some truth to this, and some myth, but as far as the thermal limits
of the driver are concerned, it can’t take more power one way or
another. However, in a sealed box the driver will require more power to
reach the same SPL as the frequency range lowers. A ported system is
simply more efficient so it wont need as much power to reach the same
SPL. Based on the mechanical limits of a driver, different frequencies
can take different power loads. At higher frequencies, driver can be
pushed hard and won’t necessarily be in a mechanical-risk state. However
the driver tends to be in a higher thermal compression state and could
be thermally at risk. This is true for both ported and sealed boxes.
However, for lower frequencies, the sealed box also acts as a filter in a
way because the internal air pressure prevents the driver from over
excursion. In a sealed box, the compliance of the suspension system
almost always forgoes that of the air spring system unless the box is
very large. In a vented box, there is no pressure to protect the driver
and furthermore, when the system unloads below resonance, the active
driver’s excursion increases exponentially and a high pass (subsonic)
filter is critical to prevent mechanical failure.

# 7 Sensitivity does not matter for subwoofers

Sensitivity
is indeed very important for subwoofers. Not all frequencies are
limited by xmax. In fact, most of the bass frequencies for music are
really limited by sensitivity or more accurately BL product and moving
mass, but not by maximum driver displacement. Higher sensitivity means
more SPL and ultimately better performance especially for upper bass
punch or kick such as a “kick drum” which resonates at 63Hz. In fact,
all good SPL competition drivers need to have high sensitivity not xmax!

There
are several standards for sensitivity. SPL at 2.83 volts or SPL at one
watt. The SPL at one watt is the more accurate number as 2.83 volts
could correlate to more than 1 watt which would not be relatively
appropriate to go by. Also sensitivity is a function of, in part, the
driver’s cone area which is never quite explicit and could be
exaggerated slightly. Ultimately as engineers, we do strive for high
sensitivity because not all bass resides in subsonic domain and many
good sounding subwoofers are in fact good because they have great
sensitivity and not necessarily high xmax.

# 8 Smaller drivers sound better than bigger drivers

One
of the biggest myths about woofers is that 8’s and 10’s are “tighter”
and “cleaner” than 15’s or 18’s. Nothing is further from the truth. What
tends to happen is that the smaller drivers have lower Q’s because
manufactures tend to put large cones on smaller motors to increase SPL
and sensitivity but not BL product. Well unless the motor can compensate
for the extra mass it has to push, then the Qts will not be the same as
the smaller drivers and ultimately the driver may not be suited for the
same kinds of alignments and could ring too much and compromise the
perceived sound quality. Having said that, high Qts drivers are not any
less “tight” or “musical” than well dampened drivers, it’s just they
require larger boxes and less internal pressure to prevent ringing.
Ultimately there becomes a point where a driver really should be used in
an infinite baffle where its actual Qts and Fs becomes the system Qtc
and Fc. As enclosure volume decreases, Qtc increases and it will take a
driver with a low Qts to make for an average Q system. So in conclusion,
the only reason to use a smaller bass driver is for space, weight and
potentially power considerations, but likewise, it is inappropriate to
try and fit a larger driver into a space smaller than it is ideal for.

# 9 I can compare two drivers using the same box

What
you will find is primarily how different TSP’s work in different boxes.
And the differences usually observed are of course differences in TPS’s
with a given system, rather than performance. The best way to compare
two drivers is to make two different systems based on the driver itself
and ensure that the frequency responses are linear to the range you
desire, and then compare those two systems in terms of dynamic headroom,
SPL and distortion. Simply saying one system is “louder” or “deeper” in
the same box is inappropriate. In one case it could be a something as
simple as an under dampened driver ringing a lot more than an over
dampened one at resonance causing a larger peak in low frequencies
throughout. It does not mean it’s louder or deeper or better outright,
it is simply non-linear, and all bets are off. Proper enclosure deigns
and/or EQ should be used for any system.

#10 cone material affects the sound

For
low frequencies, the cone on a driver makes no difference in the sound
whatsoever. The only possible affect it could have is in the case of a
metal cone or very stiff composite cone that resonates at a high
frequencies and buzzes. However this frequency would be up around 1000
to 2000Hz: Well beyond a bass driver’s usable limits.

Various
cone materials are used for various purposes. Some cones, such as
composite core with fiberglass or carbon fiber skins are extremely light
and very stiff, especially when pressed with epoxy. Other cones such as
aluminum provide excellent thermal cooling to decrease voice coil
operating temperatures when the heat is conducted though the (if
possible) conductive former. The cones job is to push air, not break,
and ideally not be too heavy (easier said that done). But they don’t
change the tone, pitch or timbre of a subwoofer system whatsoever.
Anyone who tells you otherwise is probably hearing differences in the
motor distortion, likely related to BL, compliance or other non-linear
distortions not relating to the cone.

#11 bigger magnet means more magnetic force

The
motor is essentially the steel and magnets on the bottom of the driver.
Its job to create a magnetic circuit that has an air gap where flux
lines cross in one direction so that a coil can rest in this field and
carry current which then produces a force up and down and moves the
piston to create SPL. The force that this motor creates is dependent on
the amount of power or rather current inside the conductor F = B*L*I. So
we need a more intuitive understanding of how a motor affects a
driver’s performance without considering how much current it receives.
This is the simple concept of “force factor”. Larger motors will ideally
have higher force factors, but this number not only affected by the
motor, its affected by the voice coil size, length, distance to the
motor (gap) and conductive martial used too. The end result is in fact
the BL squared divided by Re (resistance of the vc). This is literally
Newtons squared per watt and is called the force factor. The higher the
number, the more efficient the motor voice coil combination is and the
more performance you get out of the motor.

BL, one of the many TS
parameters you are probably somewhat familiar with. It is literally the
magnetic field “B” crossed with the conductor length “L.” L does not in
fact depend on the number of turns on the voice coil, but rather the
actual cross section area of the coil itself which is inside the gap.
While force factor is entirely important for any high performance
driver, one should also consider the moving mass. A 600 horse power
engine in a semi truck is pretty typical, but in a sports car it’s
certainly something to gloat about. Together, the force factor, moving
mass and the piston area account for sensitivity. This number is very
important even for subwoofers, especially for frequencies above ~60Hz.

#12 Double bass kick, only good sounding drivers can do it

We
have all heard that only good “SQ” drivers can do double bass kick
because they have good transient response or something to that extent.
This is really nothing more than linear frequency response and lack of
ring. If high Q subwoofers are in small boxes or if low Q subwoofers are
in large ported boxes, the frequency response of the system will likely
be greatly non-linear. This non-linear response compromises relative
SPL and can drown out certain sounds and frequencies. Room acoustics can
also do the same thing. The same subwoofer may sound completely
different in another room simply because there could be poor coupling
and non-linear frequency response as a result of standing waves and
peaks in the response curve. A peak at 80Hz may make for a rather anemic
60Hz response, and while 60Hz appears to be the problem, it’s actually
from the nonlinear response else where! The bottom line is “double bass
kicks” are usually not a function of the driver or driver’s performance
but rather the system design, linear frequency and room equalization.

Often
times people associate double bass as something to do with speed and
only good drivers are fast. Believe it or not, even the largest and
heaviest drivers, have no problem producing low frequencies, even 300Hz
is a relatively slow long wavelength with a slow impulse time.
Subwoofers are in fact MUCH faster than you would expect. Bottom line
is, the lack of double bass, within the working limits of a driver, is
not a problem with the driver so much as it is probably a problem with
the system design, room and/or EQ settings.

#13 Transient response is better with sealed boxes

The
fact is “transient response” is truly misleading and probably entirely
unimportant at least for low frequency response. What people hear is
really a function of the linear frequency response and distortion. It is
often accepted that transient is a function of timing, but our ability
to hear differences of a few milliseconds of low frequencies is quite
negligible which is why the low frequency group delay of a 4th order
system is quite unimportant next to the sensitivity advantages provided.
Transient does not exclusively depend on sealed or ported designs, high
Q, low Q, in fact, even drivers with high inductance don’t outright
suffer from “transient response” insofar as we can physically
distinguish certain sporadic behaviors because within their working
range, they may be very efficient and dynamic. The fact is, what makes
bass indeed bass, are long wavelengths that take considerable time to
pass our ears. The perception of transient is really a function of
perceived sound quality and there is really not appropriate example for
good “transient response”. We as humans hear two things, distortion and
SPL, and in the end that’s really want matters. What does improve
“transient” response or perceived quality is usually more headroom, more
drivers (usually larger boxes depending on the Qts of the driver),
better efficiency and ultra low distortion within the prescribed limits
of the system or drivers within the system. Sealed systems in fact don’t
offer better transient response no more than ported even with their
lower group delay tendencies, at least to human ears!

#14 It’s a bigger driver, then I need a bigger amp

Often
times larger drivers require less amplification, that’s sort of the
idea. The concept of bigger woofers need more power is not always true
and plays right into the ever progressing misconception of car audio.
What you should consider is the efficiency of the subwoofer. Efficiency
will literally tell you how much acoustic output you will get given an
amount of power (assuming linear limits of course). If the driver is
bigger, has a larger motor and has a higher sensitivity, there is no
mystery about it, you are going to get more SPL with the same amplifier
provided the impedance is similar and the amplifier can produce high
voltage at impedance peaks when the driver naturally draws very little
current for a narrow range. If a driver is more efficient and has a
larger voice coil, well you just got your cake and you can now eat it.
Not only will it be louder, but it will have less thermal compression
and ultimately more sound provided all else is equal (but such is not
usually not the case). It’s often difficult to make voice coils larger
and increase sensitivity too. This usually requires very large motors
and expense. Sensitivity is most easily achieved by weight reduction
usually from the cone surround and voice coil. Sensitivity is often a
trade off of xmax and thermal compression limits.

However there
are many larger drivers that don’t have ultra high sensitivity. A good
pro audio subwoofer may have 6 to 10dB higher sensitivity over an
average high excursion car audio subwoofer. That advantage makes them
very capable with quite a bit less power at least for their frequency
range which is usually above 40Hz. Likewise, SPL drivers ironically
enough don’t need much power either! Let me repeat. True SPL drivers
ironically enough don’t need much power! That’s because they are used in
the higher frequency domain not limited by displacement and generally
have great sensitivity numbers. They need this in order to get the
excursion and ultimately SPL they need to win contests. High sensitivity
and lots of power means lots of SPL provided the driver is still
reasonably linear and does not physically break of course. Note:
Strictly for SPL contest, drivers are normally burped at Fc (system
resonance) which is the point of maximum current draw and minimum active
driver displacement which is why excessive power must be used. Do not
confuse that requirement with the much lower power requirements for
sound reproduction outside that single SPL frequency. It’s important you
know the TSP’s of the driver you buy, otherwise it could be the wrong
driver for you! Who buys a car without knowing the horsepower? Just
because a driver big and the manufacture claims pie in the sky RMS
numbers doesn’t mean a thing!

#15 Neodymium will lose its strength with heat

Of
course it will, and so will ceramic motors too, but the fact is, under
even extreme operating conditions, it’s not likely the motor will ever
reach these temperatures. There is just too much steel to absorb the
heat from the voice coil in almost any practical case. In practice,
gradual demagnetization due to use simply does not occur. We have been
making high power neodymium based drivers for many years now and we have
never once measured a discernible number from heat.

While
Neodymium is nearly 10 times as strong as a similar sized ceramic
magnets, it can cost up to 50 times too which is almost exclusively why
it is not used often. Also, traditional overhung motors, which account
for more than 95% of all car audio designs, can get everything they need
out of a ceramic magnet assembly and stronger neodymium would be
perhaps unnecessary. If we could use neo more, we would, but because
it’s a patented martial, it’s just not economically practical for most
designs. Furthermore, in order the magnetize neodymium, A magnetizer
with over twice as much power and energy needs to be used. Many
manufactures lack the capabilities of even magnetizing neodymium, so it
becomes impractical to not only use it, but to manufacture.

#16 Its all about maximum displacement

A
DIY’er favorite statistic, displacement / dollars. If you’re
considering any bass above 40Hz then throw it out the door right now.
Often times people assume that simply because one or more drivers have
more maximum displacement over another type of woofer, than they will
ultimately be the better performer(s). In many cases this is true, but
it’s not true in general. Displacement alone does not guarantee SPL. In
fact, SPL depends on not only displacement, but frequency range,
sensitivity, box size, and BL product too. This is simply a matter of
converting energy into acoustic sound pressure level and different
devices work more efficiently than others for different frequency
ranges. For subwoofers, it is generally accepted that BL product is the
dominate factor that accounts for much of the performance or rather
system efficiency, especially in a bass reflex or more complex system
where there is a lot of air mass to displace. But keep in mind,
depending on the type of system, size, frequency range, power and
thermal limits, there may be even more critical and dependent variables
that determine the overall performance of a system. None the less, high
displacement is usually a good indicator that the subwoofer can excel in
deep bass SPL. Of course there are other factors to consider depending
on the system of system.

Great read!!!!

Great read!!!!

Not really

Don't all seem right to me

Great read!!!!

Not really

exactly why I didn't

Not bad at all. 

Some of those seem a little off, well they are a little off lol. 

hmmm, interesting...I couldn't help but notice all the grammatical errors in such a "definitave" and alledgedly "informative" article!

Edited March 9, 201411 yr by THX_Elite

i found it an interesting read even with the spelling mistakes. that said it seems that M5 gave it a thumbs down so id take its conclusions with a full shaker of salt.

'It is also true that it tends to be more important to
emphasize subsonic frequencies in the home theater environment versus
the music environment where there is simply less emphasis on subsonic
inaudible material.'

 

Atually there are quite a few EDM genres that use infrasound, epsecially dubstep, trap and street bass. So if you're going for a sub for that kind of music you would need a good sub to reproduce those low tones. I did catch some spelling errors but I guess spelling doesn't really count if you can get the point. It's not enligh class.

Atually there are quite a few EDM genres that use infrasound, epsecially dubstep, trap and street bass. So if you're going for a sub for that kind of music you would need a good sub to reproduce those low tones.

The article isn't worth reading, but that has something similar with your post. Those may be "genres" but they aren't of music. Really odd you joined to bump a 3 year old crap post.

I just came across a yes, I will gain more flexibility if I use a subwoofer instead of the placement-sensitive speakers. I can move the subwoofer all over the room depending on its layout. I may try putting the subwoofer in different positions to find the right location and enjoy the better sound quality.

Read an article, Sealed vs. Ported Subwoofer, it mat helpful, share it here

https://curateview.com/sealed-or-ported-subwoofer/