Subwoofer Q&A
What's the difference between a
woofer and a subwoofer?
A - These terms have
become almost interchangeable, and there is definitely a gray area between the
two. The difference can be in the woofer itself, or how the woofer is being
used. A raw speaker or "driver" that we call a subwoofer generally has a limited
frequency response range, often not extending above about 400 Hz. A standard
"woofer" can have frequency response easily reaching 2500 Hz or higher. This
upper limit is a function of electrical and mechanical characteristics; often
the large voice coil inductances on high-excursion subwoofers limit their
high-frequency capabilities. It is a matter of compromise in the design of the
woofer: trying to achieve good high frequency performance generally will cause
poor low frequency and power handling abilities, while producing a powerful
subwoofer with ultra-low frequency abilities and high power handling will not be
able to play well at higher frequencies. However, if a wider-range woofer is
used only below 80 Hz or so it could be called a subwoofer due to how it is
being implemented.
What does a subwoofer do?
A - A subwoofer is used
to reproduce the lowest frequencies that other speakers in the system can't
reach at full volume. In the most general application, the subwoofer will work
with a pair of smaller speakers to produce the bass that the smaller drivers are
not capable of reproducing faithfully. While some media may not contain much low
bass information, many types of rock, hip-hop, jazz, classical, or electronica
rely heavily on low frequency content. In these cases, using a subwoofer will
help fill out the sound and produce a more realistic experience. In home theater
systems, there is a specific channel of sound specifically designed for the
subwoofer, including most bass effects such as explosions, gunshots, and
rumbles. Most home theater processors also contain settings to divert all low
frequency content to the subwoofer, which frees the satellite speakers to play
much louder and with less distortion.
What is the difference between an
active and a passive subwoofer?
A - A passive subwoofer
contains only a woofer in an enclosure with no amplification. An active
subwoofer contains an on-board amplifier that will accept a low-level input and
usually contains electronic crossovers. A passive subwoofer must be powered by
an external amplifier and connected via speaker-level connection. Many times
this passive subwoofer contains a built-in passive crossover that sends the bass
to the subwoofer driver and passes the higher frequencies to the satellite
speakers. This methodology is inherently difficult to implement and will usually
result in very poor integration between the woofer and the satellites. Using an
active subwoofer system will almost always provide superior results due to the
greater control in matching output levels and matching the crossover point
between the subwoofer and satellites.
What is a
subwoofer amplifier and why would I use it?
A - subwoofer amplifier is a type of amplifier that is usually used in
making active powered subwoofers. They are an aluminum plate with inputs,
controls, and heat sinks on one side and the amplifier section and other
electronics on the other. They're intended to be mounted into a cabinet with the
subwoofer driver, and have features to optimize them for subwoofer duty. By
using a plate amplifier in the subwoofer cabinet, the need for an extra external
amplifier can be eliminated, which is very useful in home theater situations.
Other benefits of using a plate amplifier are the ability to have independent
volume control from the other speakers, a built in low-pass crossover, and the
ability to adjust the phase of the subwoofer.
Why is adjusting the frequency
that comes out of the subwoofer so important?
A - We want the subwoofer to be a natural extension of our left, right,
or center speakers in both volume and frequency. For example, let's say the
subwoofer plays from 150 Hz and down and the main speakers in the system work
from 40 Hz and up. Between 40 Hz and 150 Hz, both the main speaker and the
active subwoofer are reproducing sound. This will cause these frequencies to
stand out as a "peak" in the response of the system. These overlapping
frequencies will create "boom" in this region that will detract from the
performance of the entire system. Likewise, if the main speakers play from 150
Hz on up and the subwoofer plays only below 50 Hz, there will be a large "hole"
in the response that will reduce the impact and accuracy of the system.
How is the crossover properly
adjusted?
A - The lowpass filter on most subwoofer amplifiers can be adjusted
between roughly 40 and 160 Hz. As an example of what it is doing, if we set the
filter to 80 Hz, it will produce everything lower than 80 Hz. It is called a
"lowpass" crossover because it allows all frequencies lower than the crossover
point to pass. Most home stereo speakers can work at their best down to 60-100
Hz, so we would like our subwoofer to begin making sound right about where the
main speakers stop. To find this setting, get the system up and playing music
that has a good bass component. Adjust the subwoofer's volume so you can hear
its output clearly. Adjust the crossover knob back and forth through its full
range. As you increase the cutoff frequency to the point where it begins to
overlap the main speakers, you'll hear the system begin to "boom". (If you have
trouble hearing this change while standing very close to the subwoofer, go to
the area where you would normally listen and have someone else adjust the knob
for you.) Turn the knob back until the boom just falls away. Leave the knob set
there. Optimize the volume of the subwoofer so it matches the main speakers, and
you're done. Once optimally set, your active subwoofer will require no further
adjustment if used exclusively for either music or home theater. You may find
that different settings work better for each situation, so take note of these.
Because of this, often a remote controlled plate amplifier is used, or the
enthusiast will have a separate system for music and home theatre.
What is the best way to get the
audio signal to the subwoofer amplifier?
A - If your system is a relatively new multi-channel home theatre
receiver, it will have an "LFE" (low frequency effect) or subwoofer output. This
is a single or dual-mono RCA jack output and is the best way to get the signal
from the processor to the subwoofer. The output level of this jack will change
in unison with the main volume control of the receiver, meaning that once you
set the relative level of the subwoofer it should always match the main
speakers. This jack also usually has an adjustable output level that can provide
more or less signal to the subwoofer, useful in "fine tuning" the bass levels.
Usually the default setting of 0 dB will work well with most subwoofers, but in
some cases raising or lowering this may be necessary. Generally we want the
subwoofer's volume control to be set near 50%.
I don't have a subwoofer out
jack, what other connection can I use?
A - The next best connection possibility is using the speaker, or
high-level, connections. This input on the plate amplifier receives the signal
that is normally sent to speakers and converts it internally into a smaller
signal that it can use. This can be implemented either as a loop-through or as a
straight feed. When used with small main speakers, it may be beneficial to route
the speaker signal through the high-level inputs, and then connect the
high-level outputs to the satellites. This provides a 6 dB/octave highpass
crossover to the main speakers which will help protect them from receiving too
much bass information. The other possibility is to "parallel" the speaker input
connection with the feed going to your main speakers. Because the input
impedance is very high on the high-level inputs, this method usually will not
strain the main amplifier. This connection method can be used with main speakers
that are relatively robust on their own, and if they have a steep low-frequency
rolloff, decent integration between the subwoofer and the mains is possible.
Many people try to use a "tape monitor" loop to feed the subwoofer amplifier,
which will work, but the level will not adjust as the main level is adjusted.
Since you have to re-set the relative subwoofer level every time you use your
speakers, it becomes a very annoying prospect.
Can I connect another subwoofer
amp to the low level output?
A - No you can't. There is an active highpass filter in the sub amp that
rolls off everything below 150 Hz from the signal output here. With this
high-pass output, a second sub amp will produce very little if any bass from
this connection. This low level output is designed to be connected to another
amplifier or receiver with full-range speakers. If you need to connect a second
subwoofer amplifier, simply use a "y adaptor" before the inputs to provide
multiple low-level signals.
How are the high level inputs
used?
A - The high level inputs are generally used in stereo systems having
small speakers that product little or inadequate bass. The speaker wire from the
receiver connects to the input binding posts, left and right channels. The
subamp takes its signal through a 1Kohm resistor on each channel and sums the
two. The high level output is then connected to the full-range speakers and has
a shallow highpass crossover at roughly 150 Hz. The lowpass active filter on the
subamp will generally need to be set relatively high, though this will vary
depending on the main speakers.
Can I connect speakers to the
high level output, but use the low level input?
A - No. If you don't use the high level input, there is no high level
output. Similarly, if there is no low level input, there is no low level output.
Do I need two powered subwoofers
for my Home Theatre?
A - Unless the listening room is exceptionally large, you should not. The
average listening room is about 1500 cubic feet. That is a room roughly 14' by
14' with an 8' ceiling. A good quality 10" or 12" subwoofer will generally
produce sufficient levels in this size room for most listeners. However, if more
extreme output levels are desired, or if the room is very large, multiple
woofers can be used to achieve the desired output. Also, often a single
subwoofer will sound good in some locations within the room, but lacking in
other locations. Using two subwoofers may help "even out" the bass response
throughout the room.
Can I shield my powered sub so it
does not affect my TV set?
A - Shielding the very large and powerful voice coil in a powered
subwoofer is very difficult and often impossible. If the driver itself is not
fully shielded, it is very difficult to shield the subwoofer as a whole. There
are "bucking magnets" available that you can attach to the back of the
subwoofer's magnet to help reduce the stray magnetic field. While this does not
eliminate the magnetic field, it may reduce it enough to prevent the field from
bothering a nearby TV set. Slight changes in location or orientation will often
help greatly, as the field is somewhat directional in nature and tends to extend
perpendicularly to the axis of the driver.
When I hooked up the RCA cable to
my receiver from my subamp, it began to make an audible hum. Is it defective?
A - Likely it is NOT defective. What you are hearing is called a "ground
loop" and is caused by uneven ground potentials at various locations in your
audio system. These potentials cause small levels of electricity to flow through
the ground paths, which will often be amplified as a 60 Hz hum.
How do I stop it from humming?
A - One of the first things to try is changing the outlet into which the
subwoofer power is plugged. Since often a subwoofer is located away from the
rest of the equipment, many times the outlet will be on a different circuit or
have a different grounding point. Try connecting the subwoofer to the same
outlet as the rest of your equipment via an extension cord or power strip. The
next thing to check is the cable TV feed going into your system. While this at
first seems like a silly idea, if you consider the web of connections in your
A/V system, it begins to make sense. Temporarily unhook the main cable
connection and see if the hum stops or is reduced. If it does, the easiest
solution is to purchase a coax isolation transformer such as our #180-075. If
this does not completely solve the problem, try unhooking the connections of
different components in the system and see if the problem stops. If it does,
consider using a line-level ground loop isolator in that location. Our #265-012
works well. If nothing seems to quite eliminate all hum, the #265-012 can be
used directly on the subwoofer line-level feed and generally will solve most
problems.
What if the subwoofer hums when
it is not plugged into anything but the wall outlet?
A - If there are mechanical hums or consistent loud hums coming from the
speaker when nothing is connected, then it is likely defective. Contact customer
service for assistance.
What if I hear a buzzing noise?
A -
This is usually from external
sources such a fluorescent lights and light dimmers. Fluorescent lights radiate
electro-magnetic interference (EMI) that can get into a bad or cheap RCA patch
cable. Low voltage light dimmers often put noise directly onto the house
electrical wiring. Test by turning these types of lighting off, making sure that
the dimmer has a complete "off" position. Many of the "slider" or "rotary"
dimmers do not have a completely off position even when at their lowest setting.
If this is determined to be the source of the problem, try changing the circuit
into which the subwoofer is plugged. As a last resort a line level ground loop
isolator has been seen to improve this problem on occasion.
What if I hear a radio station?
A - This is almost always a bad patch cable with leaks in the shield.
Replace with a new or known good cable.
| 
