Why Subwoofers?

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How low can you go?

Whether you have a complete home cinema system or just a more basic music system, a subwoofer will make your CDs, DVDs and video tapes sound much bigger and more lifelike.

By adding a low-frequency speaker - a sub-woofer - you can dramatically improve the impact of your music sources. However, a word of caution - if you have neighbours, it would be best to reconsider your purchase and buy a good pair of earphones or headphones instead!

In order to generate low frequency sound waves, a speaker needs a large surface area and/or a long excursion coupled with high power. Bass frequencies are highly taxing on an amplifier and use a large portion of the power available to a speaker to generate sound across the entire audible spectrum. By shunting the bass frequencies to a separate speaker and then powering that speaker with its own amplifier, the other speakers and their amplifiers are better able to produce clean sound in the mid to high frequencies as more power is available to these areas.

Since subwoofers offer the greatest benefit when powered by their own amplifiers, many subwoofers include the amplifier in the package. Thus an active subwoofer need only be plugged into the wall (the amplifier inside the sub has a power cord that must be plugged in) and attached to a preamplifier with an interconnect cable.

So what is a subwoofer?

Subwoofers are specially-designed loudspeakers used to reproduce only the lowest booms, bangs and roars in music or movie soundtracks - typically from 80Hz down. They are generally composed of a large speaker or 'driver' (speaker cone, magnet assembly and frame) inside a cabinet. They used to be larger than otherwise is the situation today - Bang & Olufsen has made them much more compact but still enormously effective. Circuitry inside the subwoofer called a 'crossover' limits the incoming audio signals allowing the subwoofer to play only low frequencies.

There are two basic kinds of subwoofers: active and passive. Active subwoofers - such as BeoLab 2 - have a built-in amplifier driving the speaker. Passive subwoofers such as B&O's former Beovox Cona have no built-in power source; they use amplification from your amplifier as do other passive speakers. It's easy to tell the difference: look for a power lead!

What kind of subwoofer to buy?

Subwoofers work very hard and need solid power to work well. In almost all situations an active subwoofer is the best way to go. This is especially true with regards to home cinema systems. In very small rooms and apartments and with computer-oriented multimedia speaker systems, a passive subwoofer would be quite adequate.

You will also want to consider size and aesthetics. Basically, the bigger the subwoofer, the better the performance. For some, a subwoofer cabinet is a little hard to hide in the living room and they don't make very good end tables. The good news is that they can be placed anywhere in a room - behind furniture or in a corner. Pick out the biggest subwoofer cabinet your room can handle. Then look for high power from the built-in amplifier.


There are a number of different kinds of subwoofer enclosures. The two most frequently used are 'sealed cabinet' and 'vented cabinet' designs. Most subwoofers have an outward facing speaker (or speakers). Sealed box enclosures will have no other opening. Vented boxes will have a hole in the cabinet, a few centimetres in radius which allows to air escape from the cabinet.

Each cabinet design has advantages and disadvantages

A sealed box has more accurate bass response and will handle super-low frequencies better. The disadvantage is that they require much more power, a larger cabinet, and a larger speaker/magnet assembly than a vented subwoofer. This means that in effect they cost more

A vented enclosure requires much less energy to create powerful bass response. This means smaller amplifiers, smaller cabinets and smaller speaker assemblies. That makes them less expensive. The down side of a vented subwoofer is that it's less accurate. It is louder but less refined - particularly in the lowest frequencies

A 'band-pass' is a third kind of subwoofer cabinet, and is essentially a sealed cabinet subwoofer inside a vented box. This design requires very little power and is used for small, passive subwoofers the kind frequently used with compact multimedia speaker systems. These are not high performance home cinema or audio systems.

Adjustable Subwoofers

Individuals who are really serious about their music and movie soundtracks might want to consider a subwoofer with a 'variable crossover'. This allows the user to adjust up and down the high frequency cut-off point. If you have small main, centre and surround speakers, you may want an adjustable subwoofer that reaches higher up in the frequency spectrum than if you have large main speakers.

Better subwoofers have adjustable level controls. This is helpful because different music sources may often have different optimal bass levels.


Some people can be initially thrown off by the size of a subwoofer and where to place it in a room. The good thing is that subwoofers, unlike all other speakers, are non-directional so they can be placed anywhere. This is because they work on the principle that the human ear cannot detect from which direction deep tones are coming. As long as they are on the floor, you can hide them behind or under furniture, built-in to cabinetry, even in a vented cupboard in or near the listening room.

Generally if you do plan to hide your subwoofer away in a cabinet or closet, you will want a proportionally larger and more powerful subwoofer.

On a practical note: remember an active subwoofer needs to be plugged into the mains electricity. So best plan ahead before you buy and come to install your new toy!

A good starting point would be to place your subwoofer in front of, and to the left or right side of your main listening position. Close to, or touching a wall is less preferable than a position set away from a wall. Pick a place for your sub that you can live with - out of the way, but accessible. From there check the sound from your listening position. If it sounds pretty good, fine tune the crossover and levels on the sub.

Hybrid Speakers

A variation on the active speaker theme is the hybrid speaker design becoming popular that mates a powered subwoofer with a traditional, passive array of midrange and high frequency drivers. Such designs enable a listener to enjoy the benefits of a powered subwoofer coupled with traditional speakers without needing to provide space for separate subwoofers. Most full-range speakers featuring powered subwoofer sections are fairly large, floor standing models. They combine three or four speakers into two. However, they eliminate some degree of choice in subwoofers and subwoofer placement since the subwoofers are integral parts of the speakers themselves. The subwoofer cannot be chosen for its own merits and cannot be placed in a room according to where it sounds the best since it is part of the complete full-range speaker. These hybrid designs are also more costly than traditional, passive full-range speakers, but they often cost less than separate subwoofers and full-range speakers and provide bass superior to most passive, full-range designs.

So why should I buy a Bang & Olufsen sub-woofer?

The benefit of active subwoofers is that they can take the low-bass burden off other speakers in an audio system allowing those speakers and the amplifiers connected to them to concentrate on the easier to reproduce frequencies above 80 Hz or so. And the benefit of buying a subwoofer from Bang & Olufsen is that not only do they perform extremely well but they look strikingly good too!

Distortion Reduction: Servo Technology

Servo technology is used with some active speakers (those speakers powered by their own internal amplifier) to reduce distortion. Distortion occurs whenever the output signal of a speaker (or any electronic device) does not exactly match the intended output based on the input signal. If a speaker is supposed to reproduce a loud sonic boom, the sound should be reproduced as an exact duplicate of the input signal. Any deviations from the input are distortions.

Most solid state electronic components feature extremely low distortion ratings. Speakers, however, are prone to heavy distortion, especially in the difficult bass frequencies. Woofers used to reproduce bass are often called upon to move long distances back and forth while pushing large volumes of air. The woofers themselves are also generally quite large possessing a large mass compared to the air that must be moved. These features add up to make it difficult for low frequency drivers to produce accurate, distortion-free sound.

One common method for reducing distortion by controlling more tightly the movement of the speaker drivers and the woofers in particular is to use large magnets and large amplifiers. Subwoofers used to reproduce low bass exclusively often feature built-in amplifiers rated at hundreds of watts with some such speakers featuring amplifiers rated at over 1,000 watts of power. Additionally, these speakers use increasingly large magnet structures as the size of the drivers increases. The larger magnets generate more powerful magnetic fields that in turn are better able to control the movement of the voice coil as current passes through it. However, these methods are not always enough, and some subwoofers suffer from distortion of 10 percent or higher.

Servo technology works with active speakers to measure distortion in a speaker and correct for it. An electronic device known as an accelerometer is attached to a speaker driver's diaphragm. This chip measures the movement of the driver. The measured driver movement is sent from the accelerometer to another computer chip. The second chip compares the movement of the driver as recorded by the accelerometer to the intended movement of the driver from the input signal.

The comparator chip compares the actual versus intended movement of the driver several thousand times each second. If the actual driver movement does not match the ideal movement (the input signal), then the comparator chip adjusts the input signal on its way to the amplifier. The adjusted signal reaching the amplifier corrects for driver movements that do not follow the input signal - distortions. This process, which occurs in mere milliseconds, corrects the speaker driver to reduce distortion. Using properly implemented servo technology, named for the servo loop that measures and corrects driver movement, a speaker may enjoy distortion ratings of less than one percent.

While servo technology can greatly decrease distortion, it is not a cure-all for speaker distortion. Good servo systems require powerful amplifiers and magnet assemblies in addition to their servo loops. The servo loop must operate quickly taking many samples each second. Additionally, some amplifier power is lost to the servo loop. Amplifier power is used to correct driver movements. While this reduces distortion, it also takes away power from reproducing sound as some of the power must be used to fix distortion. Effective servo technology tends to be costly and difficult to implement, however, it can result in powerful, highly accurate sound reproduction when done properly.

Servo technology is fairly rare and is used almost exclusively in subwoofers. It requires active speakers and increases the cost of a comparable non-servo speaker. However, servo speakers are able to produce some of the most distortion-free sound available, especially in the lowest frequencies where distortion tends to be quite high.

Created: 9th January 2007
Modified: 2nd April 2007

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