How a Subwoofer Works

Background

While a subwoofer may sound like a fancy peripheral, it's actually just a large speaker, often with its own built-in power supply. The subwoofer uses a large woofer to replicate only the lowest frequencies from an audio source.

Signal

Like any speaker, the subwoofer receives a signal from a stereo receiver or amplifier (or preamplifier). There are two ways a subwoofer 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 subwoofer, which is all it is capable of reproducing. The audio is prefiltered by the receiver. In a stereo-level connection, the subwoofer is connected using speaker cable and receives the full audio signal. In this case, the crossover circuitry in the subwoofer 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 subwoofer, the signal is amplified. Just like an external amplifier, the amplifier within a subwoofer adds the power necessary to drive the large, flat, low-frequency sound waves. The signal is then sent on to the subwoofer's driver. In a passive subwoofer, 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 subwoofer driver works. If not, it's actually pretty simple. The audio signal sent from your receiver and passed through your subwoofer's crossover and amplifier is alternating current (AC). This means the direction in which the current flows is constantly changing. The subwoofer'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 subwoofer'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.