How do you say....
BASS A subwoofer covers the range of frequencies below approximately 100Hz, or more frquently, below 60Hz.
You need to have a driver and a resonating chamber. The Bose Wave Cannon is an example. It will kick your ass chip.

Hmm, actually, I am not a big devotee of the subwoofer. I love bass - clear, undistorted bass - but I am not sold on subwoofers, in their common, commercial incarnations.

A bit of background, to show the position I make this argument from: my own system is a home-made 3-way, with reasonably beefy bass drivers. They are 30cm in diameter, with light cones (woven carbon fibre), and 1.4kg magnets. I have them in 70 Litre sealed enclosures made from hardwood... close to 100 kg of thick, solid hardwood.

The drivers and cabinets were selected/built to maximise clarity and transient response. They could go lower in a bass-reflex enclosure, but I decided it was not worth the extra difficulty, enclosure size (and weight!), or degraded transient response, to get that little bit more bass. As it is, they play down reasonably low: the -3db point is 42Hz, and the roll-off from there is quite shallow.

I am quite keen on my hardware, and use an active crossover, so that one amp powers the subs, and a seperate amp powers the bass/mid drivers. Having this active cross gives me the flexibility to fiddle about with levels and cross points, for example to see how my system sounds with a low X-point anywhere between 100 and 1000Hz (it's usually set to 100).

Also, I DIY and read a fair bit, so have a reasonable idea of the tradeoffs involved in putting together audio gear.

Value for money
These bass speakers of mine are self-made, and cost me $400 AU for the pair of drivers, and about another $100 for the timber and other materials. These speakers easily out-perform a Velodyne subwoofer owned by a friend. This sub retails for $2000, for a single box, containing one driver and a small in-built amplifier. The Velodyne is startlingly flimsy - the total weight of the speaker is under 20 kilos... therefore a lot of the sound the speaker generates is due to the vibrations in the walls of the enclosure, resonating with all the acoustic charm of a chipboard guitar. This reduces the intelligibility of music, and is only really good for movie explosions - where distortion doesn't really matter.

Thus, I think the Velodyne is pretty sad value for money... and since this friend sells this sort of electrical gear for a living, I presume that the Velodyne is one of the better value subwoofers available.

Compromise
Bass notes move a lot more air than treble notes. They are more of an engineering challenge to recreate. There is an inverse cube law in effect - a 30Hz note at 100dB represents 8 times as much acoustical energy as a 60Hz note at 100dB. There is also the issue that most systems use two normal speakers for stereo, but only one sub. Therefore, to go an octave lower than your main speakers, the subwoofer will ideally be 16 times larger than the main speakers. The more extension a sub attempts, the larger this number will get.

In the real world, very few people are willing to live with such an enormous speaker. Subs are scaled down, and are therefore a lot less efficient than the main speakers. Thus, the average sub needs to burn a lot more power than a normal speaker. The smaller the sub is, the higher the required power.

Since the surface area of the driver is unlikely to be 16 times the surface area of the main woofers, the sub's cone will also be required to move a lot further to generate equivalent SPL. Again, the smaller the driver is, the greater the excursion requiremed to match the main speakers.

The outcome of all this compromise is that the sub works a lot harder than the main speakers. They work differently, and they *sound* different. This is more or less noticable depending on how much the design is compromised, how it is implemented, and how much money is thrown at the problem.

Integration
A perfect subwoofer only produces very low notes, with no output over 100 Hz, and no distortion. This makes a perfect sub easy to place, since deep bass is not very directional. For real, non-perfect systems, as long as the sub has low-ish distortion, and reasonable efficiency, it should integrate smoothly. But due to the compromised size of the average sub, they are driven hard, compared to the main speakers, thus have high distortion, particularly when playing loudly. This is obviously bad for the system, as intelligibility is reduced, and also prevents the sub from blending in 'invisibly' with the main speakers.

Another issue is that some subwoofers, particularly those for smaller systems, have output higher than 100Hz. This is not a true subwoofer, but is usually sold as such. There is nothing wrong with this approach... if it's done right. Doing it right requires:
a) 2 subwoofers - because higher frequencies are more directional, the subwoofer needs to be near the main channel, or the sound will be funny. Any illusion of reality will be ruined, as sounds will be coming partly from the main speakers (in stereo, arranged in front of the listener) and partly from the sub (in mono, from one location). Simply popping one sub in the corner won't do for realism (Bose systems are guilty of this). So you really need two subs rather than the normal 1, and for the subs to be placed near (or under) the main channels.
b) very clear-sounding subs. Your hearing is much more sensitive in the 300-3000 Hz range than to sounds below 100Hz. Therefore, any difference in sound quality between the main and sub channels becomes more apparent the higher the crossover point between the channels is.

Home Theatre
HT is a pile of crap. Actually, the concept is fine, but a lot of what gets sold as HT is awful. I know this is not a reasoned argument - but let the crap speak for itself. Five little plastic speakers and a "subwoofer" the size of your shoe does NOT make for high fidelity.

If you are considering buying into HT, think about buying separate components rather than a package deal. Instead of dividing your money 6 ways, between 5 flimsy speakers and a receiver, just get a receiver and a pair of *really* good stereo speakers. If they do a decent job, you won't miss the subwoofer. And recycle your old speakers for rear / surround duty. Since their job is just to fill out some of the sound FX, pretty much anything will do.

If you get this lot set up, recover from the financial damage, and are still craving more bass, save up a little and buy a pair of big drivers, and build the subwoofer yourself. Make it big, and solid, the sort of thing that the stores don't sell because the shipping / storage cost is too high, and you will have a real subwoofer. The easiest way to do this is to either keep it very simple (thats what I did), or to use a kit. Jaycar is probably the best retailer in Australia / New Zealand for these. Madisound and Parts Express are good on-line sources.

An explanation of Sub-woofer types

If you take a moment to think about some of the better sound systems you have heard, you may remember that they all had great bass response. The volume, accuracy, and clarity of the low frequencies reproduced by a stereo or home theater system’s woofer is a critical part of the whole acousticpicture”.

In fact, many speakers sold for home use are not capable of reproducing all of the bass that is present in the music, and most of those that can may not reproduce the deep bass notes loud enough to satisfy. That’s when people turn to a subwoofer. A subwoofer is a special kind of speaker that only reproduces the deep bass portion of the music. Some boxes are called subwoofers, but they are only woofers in a separate box, incapable of truly deep bass.

In a speaker, the forward-and-back movement of the cone translates directly into acoustic energy you hear, formed up of pressure waves in the air at a pitch corresponding with the cone’s frequency of motion (measured in hertz.) That is why accuracy is important. If the speaker cone does not move crisply, the resulting sound is muddy. Speaker manufacturers address this need by using very hard and light materials for the cone, so they will move rapidly and easily without losing their shape, and using powerful magnets to control the voice coil with a high level of accuracy with enough power to generate a loud signal. This is especially important with a subwoofer, as bass energy takes more power to generate than any other part of the music spectrum.

Why is power important?
Most of the benefit of a subwoofer is lost if it isn’t driven properly. As mentioned, bass takes more power to reproduce than any other part of the music spectrum. This is because bass frequencies are the large, slow waves, formed by large movements of air. In order to move the speaker cone forcefully and cleanly, sufficient amplifier power is extremely important.

The power rating of a subwoofer represents the amount of power it can handle, just as the lines on a bucket tell you how much you can put into it. The rating is not the amount of power the speaker puts out. Some woofers have their own dedicated amplifiers built in. These are called “active”. If the subwoofer does not have an internal amp, you need a separate channel of amplification to drive it properly.

The advantages of a powered sub are many. Since the amplifier is only there to drive the sub, it can be matched to the speaker’s performance for best operation. Also, this allows for active crossover electronics to be integrated directly into the amplifier circuitry. Not to have to scrounge up an additional channel of amplification is a good reason, as well. The sound quality is also enhanced by the fact that a powered sub takes the signal directly from the pre-amp.

Some subwoofers accept the two- or four-channel output from a standard amplifier. These usually have a crossover built in to separate the low frequencies from the rest of the music for the subwoofer to handle, sending the modified signal onward to the rest of the speakers after using some of the power from the amp for itself. This can be a cost-effective solution, but can be unsatisfying if the satellite speakers aren’t well matched to the subwoofer. In addition, since the sub draws its power from the speaker feed, there is less power for the satellites. Running the signal through the sub’s electronics doesn’t do the quality any good, either. Every extra step in the signal path should be avoided.

Contrary to popular belief, more speakers are destroyed by too little power, not too much. When an amplifier overdrives the music waveform, it is distorted in a way that creates direct current voltage spikes. This is called clipping. These spikes force the speaker cone to move in damaging ways, to the point where the voice coil may destroy itself. A good rule of thumb is if you hear any distortion, turn the sound down. It is better to ride the edge of clarity at a lower volume than broadcast loud distortion until your woofer is blown. This applies to all speakers, but due to the higher power needs of low frequency reproduction, woofers are especially susceptible.

What kinds of subwoofer are there?
The speaker cone in a subwoofer will range in size from as small as 8 inches to as large as 18. The bigger the cone, the better it can move air to create bass. A large cone is harder to drive cleanly, so make sure any subwoofers with a large cone are very light and stiff, with a large enough magnet to move it well in a solid enclosure, with enough power to do the job right. A smaller speaker in a well-made cabinet will sound much better than a big floppy woofer with a tiny magnet in a thin-walled box driven with too little power.

There are four major cabinet types, called enclosures, and each design has its pluses and minuses.

Sealed:
A sealed box is the simplest to make well, and can provide very clean, tight bass. It is the easiest for the hobbyist to make, as no calculations are needed. The design is relatively inefficient, and can use more power than other designs for the same sound volume. Very few subs use this design for that reason.

Vented:
A vented enclosure uses a tuned port to take advantage of the cabinet air volume resonance from the rear-side motion of the speaker cone to create additional sound reinforcement. Properly done, this design creates tight, clean bass, but requires accurate calculations to properly tune the port to the cabinet air space and the speaker driver’s performance. This design is common among speakers of all types, as it can sound up to twice as loud as a sealed enclosure.

Bandpass:
This is a type of vented enclosure where the speaker fires into separate chambers fore and aft, with the front chamber vented. When designed properly, the cabinet and port interaction perform the duty of a bandpass filter, allowing only the desired frequency band to come through. This design is often used in car stereo, as it is very tolerant of the signal given it, and does not require the use of an electronic crossover for bass.

Coupled Cavity:
This design is a combination of the vented and bandpass configurations. The speaker sits between vented chambers fore and aft. Each enclosure/vent combination is another resonant system, creating a high order, multi-tuned resonant system. Generally, these systems have quite complex response and are difficult to design. Properly done, however, they provide the highest efficiency (very loud). This design does not require an electronic crossover, either. This design can sound terrible if not properly made, but when done right, is very hard to beat.

Don’t forget system integration
The most important part of installing a subwoofer is to make sure that the various parts work well with each other. If the sub has a crossover point that is too high or too low for the satellites, there will either be overlap or a gap in the system’s frequency response. You also have to make sure that the volume levels are compatible too. This is especially important with digital audio, since the dynamic range (the difference between the loudest and softest sound) is so wide. For example, in a home theater system, it is too easy to have the bass often drown out the dialog, or in a car system, the bass can overwhelm the rest of the music. Some subwoofers, and all powered units, have crossover and volume adjustment to allow you to make the sub fit to the point where you can’t tell where the sub leaves off and the satellites begin, which is the best way to have it. Like that of a surround speaker, the best subwoofer performance is the one that you don’t notice until it’s turned off.

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