Subwoofer - Wikipedia

A Loudspeaker device which reproduces audio sub bass frequencies below about 80 Hertz. Subwoofers add the feeling of punch to music or a soundtrack. Often the music which can be felt is the result of powerful subwoofers. They can often be found in high end car stereo installations, cinemas and home cinema, and generally anywhere faithful reproduction or special effects of low bass music/sound are required.

Subwoofers will usually be powered by a high power amplifier. Often an electrical crossover will ensure that frequencies above about 80-150Hz will not be directed to the subwoofer. Subwoofers are obviously not designed to deliver such frequencies and can create unpleasant sounds if otherwise.

There are a multitude of audio designs some of the common ones being:

Sealed: a driver (a speaker) mounted on a baffle with its backside totally enclosed.
Ported: a driver enclosed in a box with a "port" which is basically a hole in the box. This is tuned to a certain frequency so the whole system will naturally resonate at a certain frequency to efficiently reproduce low frequencies.
Isobaric: Two drivers mated together. The key being that the air volume between the two drivers is as small as possible. They drivers can be mounted facing each other or in many other configurations. This pair of drivers can then be mounted in a similar configuration as a single driver be it sealed, ported etc.
Bandpass: A combination of enclosures where the end result which has the natural effect of making a crossover.

Subwoofer drivers vary from 6 inches to 24 inches in diameter. Some modern subwoofers claim to handle 5000 watts of power and are rated by their ability for high excursion, i.e. the ability for the cone to move one and a half inches in or out from a stand still for an overall displacement of three inches. This excursion can generate extremely high (loud) sound pressure levels up to 174 decibels.

In car stereo world championship competitions, up to 64 subwoofers with 100,000 watts are used to generate up to 175 decibels. Naturally, volumes such as this are not safe for humans.

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