How a speaker works

The air does not go anywhere

This is the part almost everyone gets wrong, and the model is built to show it. Sound is not air travelling from the speaker to you. If it were, a concert would be a gale.

What travels is a pattern. The cone shoves the layer of air touching it; that layer crowds into the next one and shoves that; and the shove keeps handing itself forward at about 343 metres a second. Each individual parcel of air moves back and forth by a tiny fraction of a millimetre and ends the night exactly where it started. Watch the rings in the model: they bunch together and spread apart, but each ring stays in its own place. Only the bunching moves outwards.

Two details in the model are worth pausing on. The rings crowd together exactly where they are brightest, because the crowding is the pressure — they are not two effects, they are one quantity drawn two ways. And the brightest band is not where the cone is furthest forward, but a quarter wavelength ahead of it, where the air is moving forward fastest. Pressure follows velocity, not position.

A microphone is a speaker running backwards

Not “similar to”. The same mechanism, read in the other direction.

A dynamic microphone is a diaphragm glued to a coil of wire hanging in the gap of a permanent magnet. Pressure waves arrive, the diaphragm moves, the coil moves with it, and a coil moving through a magnetic field has a voltage induced across its ends. Current in, motion out becomes motion in, current out.

This is not a thought experiment: wire a small speaker into a microphone input and it works, badly but unmistakably. Old intercoms used one driver as both, switching it between the amplifier’s output and its input. It is also why a speaker connected to nothing is hard to push by hand, and much harder if you short its terminals — you are generating a current in the coil, and that current fights the motion that made it. Amplifier designers call that damping, and it is the reason a bass note stops when the signal does instead of ringing on.

Why woofers are big and tweeters are tiny

The loudness you hear depends on how much air the cone displaces, and displaced air is area multiplied by excursion. Neither is free.

A low note swings slowly. To keep shoving air at 40 Hz the cone has to travel a long way on every stroke, and even then a small cone runs out of room: to match the output of a 12-inch driver, a 4-inch one would need roughly nine times the excursion, which its suspension will not survive. So bass drivers get area — more surface, more air moved per millimetre of travel.

High notes have the opposite problem. At 15 kHz the cone reverses direction thirty thousand times a second, and accelerating a heavy cone that hard takes force nobody has. Worse, a large surface stops moving as a single piece at high frequencies and starts flexing into its own standing patterns, which sounds like neither the signal nor anything else. So tweeters get small and light: a one-inch dome, barely moving, is easy to accelerate and stiff enough to stay in one shape. No single cone is good at both ends, which is why a speaker box usually holds two or three of them and a crossover to decide which gets what.

What the model leaves out

The cone does not really follow the current. Above the driver’s resonant frequency — which is most of its useful range — the cone’s motion is limited by its own mass, and a mass driven by a force lags it by half a cycle. So the real cone is moving backwards while the current says forwards. The model keeps them in step because the point being made is that force follows current, and the phase relationship only muddles it.

The wavelengths are not to scale. In the cable the signal moves at a significant fraction of the speed of light; in the air it moves at 343 m/s. A 440 Hz note is about 780 millimetres long in air and hundreds of kilometres long in the wire. Drawing both to scale would leave one of them invisible.

Real radiation is not a beam. The model sends the sound out as a narrowing cone of rings because that is readable. A woofer whose cone is small compared to the wavelength radiates almost equally in every direction, including backwards — which is exactly why the box exists, to stop the rear output from arriving out of phase and cancelling the front.

One frequency at a time. The model runs a clean sine wave. Music is hundreds of frequencies at once, and the cone traces their sum: a single complicated squiggle that happens to contain all of them. Your ear takes the sum apart again.