This page provides detailed information to help in chosing a sub-woofer,
positioning it within a room, and calibration to integrate it into your system.

By


Sub-woofer Purchasing, Positioning and Calibration.

 

So what exactly is a sub-woofer?

B&W Matrix 800ASW Sub-wooferSub-woofers are large speakers, specifically designed to produce deep, precise bass. A large driver and enclosure are needed to move the high volume of air required to reproduce these frequencies accurately. The upper frequency limit is usually around 140Hz, and depending on design, can extend to reproduce frequencies below 20Hz, the theoretical limit of human hearing. Because hearing sensitivity varies across the spectrum, and we are not particularly good at hearing low frequencies compared to those in the mid-range, a higher sound pressure level (SPL) is needed to reproduce convincing bass (this is compensated for by loudness circuits in some pre-amplifiers which increase low and high frequencies at low listening levels to compensate for our non-linear hearing).

 

Why do I need one?

This site has been specifically designed to help with home theatre issues, but a sub-woofer can also dramatically improve a music only system, and many audiophiles with large, expensive main speakers still use one or more sub-woofers.

As I described in the opening paragraph, it is very difficult to reproduce deep bass with small speakers, so the addition of a sub-woofer will complete the audio spectrum if you are in this situation. Music, and more so movies, contain a large amount of low frequency information. Why does a movie theatre sound so impressive compared to the average home system? Answer: the theatre is using sub-woofers to reproduce those gut wrenching explosions and T-Rex footsteps. The maximum energy in a movie soundtrack is centred around 60Hz, and can be as loud as 105dB. Many movies have 20Hz content. The good news is that you too can achieve this at home (if you have friendly neighbours).

 

How?

First of all, you have to decide what type of speaker system you want, and how you would like to integrate a sub-woofer into your existing system.

Sub/Sat systems are usually bought as a complete package, the sub-woofer is hidden away in the room as it is purported that bass frequency direction cannot be determined by the brain, and the two satellite speakers reproduce the rest of the spectrum.

More on sub-woofer positioning later, because it's not as simple as that. A sub-woofer can also be added using your existing speakers, but it must be carefully calibrated to integrate smoothly.


Definitive Technology Sub-woofers Types.

There are two main types of sub-woofer - active and passive.

Passive sub-woofers contain no amplification. This type of sub-woofer is driven using your existing amplifier by directly connecting it to the speaker outputs. Bass frequencies consume a large amount of amplifier power, and this can prove detrimental if you don't have oodles to spare. They are also difficult to integrate unless specifically designed for use with another speaker from the same manufacturer.

Active sub-woofers (also known as powered sub-woofers) have built-in amplification, an active cross-over circuit, and other goodies such as distortion reducing circuitry. They can be connected either to the speaker outputs (drawing no power) or to dedicated low frequency line level outputs such as those found on surround receivers. They also offer the option of disconnecting your existing main speakers from the amplifier, and routing the signal through the sub-woofer (which in this case must be connected to the amplifier speaker outputs). In this configuration, the bass frequencies are removed from the signal sent to your existing speakers, which makes their job a good deal easier.

 

Positioning a sub-woofer.

The general misconception as hinted at earlier, is that because the brain finds frequencies below 100Hz difficult to localise, the sub-woofer can be placed anywhere in a room without detrimental effects. This just isn't the case - nothing is simple in audio is it? Correctly positioning a sub-woofer is the hardest part of installing one.

Because the wavelengths in question are long (some as long as 50 feet) the position of a sub-woofer is hard to localise, but it must be stressed that it is still a point source with the soundwaves radiating out from it. Moving a sub-woofer out from it's typical placement in a corner, can change the amplitude of standing waves and either increase or decrease the amount of bass perceived at your listening position. An incorrectly placed sub-woofer can increase specific bass frequencies and completely change the tonal balance of not only your room, but the recording you are listening to.

It must be born in mind that because your sub-woofer will share some frequencies with the Left, Right and Centre speakers, offset problems can occur. The offset is the distance the sub-woofer is placed in front or behind the horizontal axis of the front speakers. Placing a sub-woofer more than three feet in front or behind the main front speakers causes the shared frequency wavelengths to overlap and cause cancellations at those frequencies. Keep the offset to an absolute minimum.

Because of the length of bass frequencies, standing waves can become a problem. Standing waves are caused by low frequency waves reflecting off room surfaces and colliding with themselves out of phase to produce a change in amplitude. Large rooms suffer far less than small rooms because the waves are distributed more randomly. Standing waves cause the bass to sound sluggish, and most often in this case, the sub-woofer mistakenly takes the blame. What you are actually hearing is the time it takes for the room standing waves to build up and decay.

In most cases, the absolute worst location for a sub-woofer is behind the listening position - most commonly behind a couch. Because of the precedence effect of two similar frequencies (the Hass Effect), confusion is caused within the brain and the listening experience can be fatiguing if not completely unrealistic. This is the case with analogue Dolby Pro-logic™ material. The only exception is when a sub-woofer is fed directly by the rear channels, which usually means using the loudspeaker signals rather than a line-level output. It is possible for Dolby Digital™ sources to have full range rear signals, so therefore a rear sub-woofer may help. It's important to note that in this instance the bass produced by the sub-woofer will be specific to it, and not reproduced by other sources, also the sound will be emanating from the intended direction. Unless a system is designed so that the rear sub-woofer is used only with discreet digital material, it is preferable to negate this option which may be more trouble than it's worth. It's interesting to note that most commercial movie theatres have relatively small rear speakers which are not augmented by a sub-woofer - this is the playback system most soundtracks are intended and mixed for.

NHT SW3P Sub-woofer
So how Do I position my sub-woofer?

The easiest method to correctly position your sub-woofer is to stand it at your listening position - don't worry, you're not going to sit on it whilst watching a movie! What you are going to do, is move around the room until you find the spot with the best bass response. Use a pink noise signal to do this - the signals from the Delos 'Surround Spectacular' CD, THX "Wow!" laserdisc or even the calibration noise from your Dolby Pro-logic™ processor (although the latter is not ideal), and make sure that all your other speakers are turned off.

The best position is not necessarily the one with the loudest bass, but the position with the smoothest bass. LucasFilm describe non-flat bass as having a "hooty" or "one-note" quality.

Using multiple sub-woofers can help rooms with problematic standing waves by driving the wave out of phase, but it is also just as easy to drive the wave in phase, making the problem much worse.

 

Calibration.

Now that your sub-woofer is in the correct position (hopefully), it needs to be carefully integrated into your system. This isn't as difficult as you may have thought.

If you have a THX system, things are simplified somewhat as the crossover frequency is 80Hz. This is the point in the audio spectrum where the main Left Centre and Right speakers are cut, and the sub-woofer takes over. The filters that make this possible aren't like stone walls, the sound from the main speakers doesn't just vanish at 80Hz, although the THX crossover filters do have a very narrow overlap range.

Most users will be relying on the crossover built into the sub-woofer. This can vary from 1st to 4th 'order', which corresponds to 6dB per octave to 24dB per octave. All sub-woofers have a calibration control which enables the user to select the desired crossover frequency. If your front speakers are going to continue to be driven full range (without a low-cut filter) then it is important to set the upper limit of the sub-woofer's operating range to within 10-15Hz of their low frequency capability. For instance, if your speaker literature shows they have a flat response to 50Hz, then the sub-woofer should be set with an upper limit of 60Hz. In this way, the response will remain smooth across the entire spectrum. If you have some type of low-cut filter applied to the front speakers, then the sub-woofer can be allowed to reproduce a wider range of frequencies, but no higher than 100Hz. In this case, the response of the main speakers would be reduced so that they do not produce frequencies below 90Hz.

In some instances, even with very careful postioning, the frequencies shared between sub-woofer and main speakers may prove troublesome, characteristically 'boomy' around the crossover point. In this instance, a sub-woofer with a phase reversal switch is advantageous. Most sub-woofer owners won't need this switch, but it is impossible to tell in advance in the store. When purchasing a sub-woofer, bear this in mind. Reversing the phase of the sub-woofer causes partial cancellation of the shared frequencies and a smoother, more pleasing result. The most likely situation in which a reversal of phase can be beneficial, is in a system with very small main speakers (and a high crossover frequency).

 

REL Acoustics 'Storm' Sub-woofer Levels?

When you have selected the correct crossover frequencies, it is important to calibrate the volume of the sub-woofer. This is done by initially turning it off! First calibrate your main Left, Centre, Right and Surround speakers in the normal way using an external test signal such as those found on the LucasFilm THX 'Wow!' laserdisc or the Delos 'Surround Spectacular' compact disc or using the internal calibration noise of your Dolby Pro-logic™ processor. In a THX system, Front Left, Centre, Front Right and Rear Total should each give an SPL of 85dB with a 0dB input using pink noise. Pink noise is a specific type of signal, one which contains equal energy across the entire audio spectrum. A Radio Shack SPL meter (Cat. No. 33-2050) set at C-Weighted / Slow Response mode is ideal for this. You don't have to calibrate to THX specifications, as long as you leave the system volume constant throughout the whole procedure.

Once you have calibrated the other speakers, turn them off and adjust the sub-woofer volume to give a reading that is 6dB lower. The frequencies of audible sound (20Hz to 20kHz) can be divided into ten octaves, the sub-woofer, set with a cross-over frequency of 80Hz will cover the bottom two (20Hz to 40Hz and 40Hz to 80Hz), so the total output energy of the sub-woofer should be lower than that of the other channels. Remember it is important to use a calibration test signal that contains the full spectrum of audio frequencies. Of course, if you have your sub-woofer cross-over set to a figure greater or less than 80Hz, the relative volume should be adjusted fractionally also. On the whole, this is being very exacting and in a real world situation these small adjustments are outweighed by room effects.

Moving from one side of your listening area to the other (for example one end of the couch to the other) and taking note of the SPL readings is also a good idea - these can then be averaged so that bass volume is more constant throughout.

You may at first notice that the sub-woofer doesn't sound as 'loud' as it did in the store - this is because salespeople are notorious for increasing the sub-woofer level to unrealistic proportions, because this impresses customers. We're not going to fall for that are we! Remember also that the amount of bass will vary from movie to movie, and some will impress your friends more than others.

If you really must have bass that knocks the drinks out of people's hands during a party, by all means increase the level of the sub-woofer by a few dB's - but remember to write down the previous level, and return it to that when the revellers have gone home.

 

Conclusion.

Hopefully this page will have helped in purchasing, positioning and calibrating a sub-woofer, now all that remains is for you to have fun watching all those action films (and music)!

- April 1997

 


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You can post sub-woofer questions in the appropriate SMR Forum area.


Sub-woofers featured on this page:

B&W Matrix 800ASW
Definitive Technology Powerfield 15; 1500; 1800
NHT (Now Hear This) SW3P
REL Acoustics - Storm


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