A subwoofer is a speaker that only does one job: reproducing the lowest bass, the notes you feel as much as hear. Here’s how it works, in plain language, with no maths required.
The basic idea: moving air
All sound is air pressure changing quickly. To make a deep bass note, a speaker has to push and pull a lot of air slowly, perhaps 30 times a second for a 30 Hz note. Small speakers can’t move enough air to do that loudly. A subwoofer uses a big cone that travels a long way to do exactly that.
The key parts
The driver
The driver is the moving part. An electrical signal flows through a coil of wire (the voice coil) sitting in a magnetic field. The coil is pushed and pulled, and because it’s attached to the cone, the cone moves air. The surround and spider hold the cone in place and let it move back and forth.
The enclosure
The box stops the sound from the back of the cone cancelling the sound from the front, and its design (sealed or ported) shapes the bass. See enclosure types.
The amplifier
Home subwoofers are almost always powered: they have a built-in amplifier matched to the driver. It boosts the small signal from your receiver into the power needed to move the cone. See passive vs active.
Why you feel bass as much as hear it
Below roughly 40 Hz, our ears become less sensitive, and we start to sense bass through our bodies and the floor. That’s why a good sub adds physical impact to films and live music recordings.
Why bass seems to come from everywhere
Low-frequency sound waves are very long (over 14 feet at 80 Hz), and our brains locate sounds largely using the timing and level differences between our ears. At long wavelengths those differences are tiny, so deep bass is hard to locate. That’s why you can usually put a sub off to one side and still hear bass coming from the speakers in front of you, as long as the crossover is set low enough.
What the crossover does
The crossover splits the sound: bass below a set frequency goes to the sub; everything above goes to your speakers. It lets each do what it’s best at. See what Hz is best for bass.