Subwoofers

__What it is __ A sub woofer is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a sub woofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below 80 Hz in THX-approved systems. Sub woofers are intended to augment the low frequency range of loudspeakers covering higher frequency bands. Subs are usually enclosed in a box able to withstand

__How it works __

Signal
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 * Like any speaker, the sub woofer receives a signal from a stereo receiver or amplifier (or preamplifier). There are two ways a sub woofer receives the audio information it is responsible for line-level connection and stereo-level connection. In the former, the receiver sends only the lowest frequencies to the sub woofer, which is all it is capable of reproducing. The audio is pre-filtered by the receiver. In a stereo-level connection, the sub woofer is connected using speaker cable and receives the full audio signal. In this case, the crossover circuitry in the sub woofer is responsible for directing only the lowest frequencies to the sub and sending all higher frequencies to the speakers. On many subs, you can set the crossover frequency that serves as a threshold, generally around 100 Hz or so.
 * ==Amplification==
 * Once the low frequencies have been sent to the sub woofer, the signal is amplified. Just like an external amplifier, the amplifier within a sub woofer adds the power necessary to drive the large, flat, low-frequency sound waves. The signal is then sent on to the sub woofer's driver. In a passive sub woofer, there is no internal amplifier; the sub relies solely on power from an external amp.
 * ==Driver Operation==
 * If you know how a speaker works, you know how a sub woofer driver works. If not, it's actually pretty simple. The audio signal sent from your receiver and passed through your sub woofer's crossover and amplifier is alternating current (AC). This means the direction in which the current flows is constantly changing. The sub woofer's driver has an electromagnetic coil through which the current flows. The flow of current through the coil creates a magnetic field. Because the current is constantly changing directions, the magnetic field is constantly changing polarity. The change in polarity interacts with a stationary magnet, which is mounted around the electromagnetic voice coil. Due to a constant fluctuation of attraction and repulsion with the stationary magnet, the sub woofer's voice coil is constantly moving back and forth in tune to the frequency of the signal (the music or audio you're listening to).
 * ==Sound==
 * Attached to the voice coil is a speaker cone. This cone moves with the voice coil, and the movement pressurizes the air directly in front of your subwoofer, sending out sound waves that travel via moving air molecules. You then hear the bass that was recorded on your original source, such as a CD, DVD or TV. In this way, the subwoofer takes an electrical signal and transforms it into audible sound.

bibliography http://en.wikipedia.org/wiki/Subwoofer http://www.ehow.com/how-does_4885502_how-subwoofer-works.html