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Subwoofer Questions & Answers

Subwoofer Questions & Answers

What's the difference between a woofer and a subwoofer?

A - These terms have become almost interchangeable, and there is 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 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 onboard 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 a 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 a "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 the music that has a good bass component. Adjust the subwoofer's volume so you can hear its output. 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 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's 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 roll-off, 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, 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 produce little or inadequate bass. The speaker wire from the receiver connects to the input binding posts, left and right channels. The sub amp 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 sub amp 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 and tends to extend perpendicularly to the axis of the driver. 

When I hooked up the RCA cable to my receiver from my sub amp, it began to make an audible hum.

A - 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 quite to 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 mechanical hums or consistent loud hums are 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 electromagnetic 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.