How Do Speakers Make Sound?
Speakers make sound by moving a coil in a magnetic field to push air. Here is the mechanism, why bass needs size, and what the cabinet is for.
Speakers make sound by pushing air back and forth. An electrical signal passes through a coil of wire sitting inside a permanent magnet’s field, which forces the coil to move. The coil is glued to a paper or plastic cone, so the cone moves with it, and the pressure waves that creates are what your ears hear as sound.
That is the whole mechanism. Everything else in speaker design, the crossovers and cabinets and cone materials, exists to control how accurately that one movement follows the electrical signal it was given.
The parts that do the work
A conventional speaker driver has five components that matter.
The permanent magnet is a heavy ring, usually ferrite or neodymium, sitting at the back. It creates a fixed magnetic field across a narrow circular gap.
The voice coil is a cylinder of fine copper or aluminum wire wound onto a former, hanging in that gap. It is the only part connected to your amplifier.
The cone, or diaphragm, is attached to the top of the voice coil. Paper, treated paper, polypropylene, aluminum or woven fabric, depending on what the designer wanted.
The surround is the flexible ring joining the outer edge of the cone to the metal frame, and the spider is a corrugated fabric disc doing the same job at the base of the coil. Together they hold the coil centered in the gap and pull the cone back to rest.
The basket, or frame, holds all of it in alignment. That alignment matters more than it sounds: the gap the coil moves in is often under a millimeter wider than the coil itself.
How electricity becomes movement
Your amplifier sends alternating current through the voice coil. Current flowing through a coil creates its own magnetic field, and the direction of that field flips every time the current reverses.
The coil’s field pushes against the permanent magnet’s field. Same poles repel and opposite poles attract, so the coil is driven out of the gap when the current runs one way and pulled back in when it reverses. This is the motor principle, and a speaker is genuinely a small linear motor.
Because the cone is glued to the coil, it moves with it. A 100 Hz signal moves the cone out and back 100 times a second. A 5,000 Hz signal does it 5,000 times a second, over a much smaller distance.
The amount of movement follows the voltage. Louder means a larger swing, which is why a woofer playing bass at volume visibly pumps and a tweeter playing the same track looks completely still.
How movement becomes sound you can hear
When the cone pushes forward it compresses the air in front of it. When it pulls back it leaves the air slightly rarefied. Those alternating bands of high and low pressure travel outward at roughly 1,125 feet per second, and when they reach your eardrum they push and pull it in the same pattern.
Frequency, how many times per second the cycle repeats, is what you perceive as pitch. Amplitude, how far the cone travels, is what you perceive as volume.
This is also why speaker size and bass are linked. Producing a 40 Hz note at a useful level means moving a large volume of air slowly, and there are only two ways to do that: a big cone, or a small cone traveling a long way. Both need physical space, which is why a phone speaker cannot produce real bass no matter how the software is tuned.
Why one driver cannot do everything
A cone light enough to start and stop 20,000 times a second is far too flimsy to move the air needed for a bass note. A cone stiff and heavy enough for bass cannot change direction fast enough for treble.
So speakers split the job. A woofer handles low frequencies, a tweeter handles highs, and a three way design adds a midrange driver in between. A crossover filters the signal so each driver only receives the band it can handle.
The design goal is that all of them together produce output that is even across the audible range. Our explainer on what a good frequency response for speakers looks like covers how to read whether a manufacturer achieved it.
What the cabinet is actually for
A bare driver sounds thin, and the reason is not obvious. When the cone pushes forward it creates pressure at the front and an equal rarefaction at the back. At low frequencies those two waves wrap around the edge of the driver and cancel each other out.
The cabinet exists to keep the rear wave away from the front. A sealed box traps it entirely. A ported box uses a tuned tube to delay it by half a cycle so it emerges in phase and reinforces the bass instead of canceling it, which is why ported speakers go lower for their size and why they sound boomy when placed too close to a wall.
Cabinet walls are braced and damped for a simpler reason: a panel free to vibrate becomes a second, badly designed speaker.
Other ways to make sound
The moving coil design above covers almost everything you will own, but it is not the only approach.
Planar magnetic and ribbon drivers replace the coil and cone with a thin conductive film or foil suspended between magnets. The whole surface is driven at once, so there is far less mass to move, and the treble is exceptionally clean. They struggle to move enough air for bass.
Electrostatic panels suspend a charged membrane between two perforated plates and vary the voltage across them. Very low distortion, very large panels, and they need a mains power supply.
Piezoelectric elements flex when voltage is applied, which is how a smoke alarm or a greeting card makes noise. Cheap, tiny, and only good for a narrow band of high frequencies.
Frequently asked questions
How does a speaker turn electricity into sound?
An alternating current from the amplifier passes through a voice coil sitting in a magnetic field. The current creates its own magnetic field that pushes and pulls against the permanent magnet, moving the coil back and forth. A cone attached to the coil moves with it, and the pressure waves it creates in the air reach your ears as sound.
Why do speakers need magnets?
The permanent magnet provides the fixed field that the voice coil pushes against. Without it there is nothing for the coil’s own magnetic field to react with, so no force is produced and the cone does not move. The strength of that magnet is a large part of how efficient a speaker is, which is why higher sensitivity speakers tend to have heavier magnets.
What makes a speaker produce bass?
Moving a large volume of air relatively slowly. That requires either a big cone or a long excursion, plus a cabinet large enough to control the rear wave from the driver. This is why small speakers cannot produce deep bass regardless of processing, and why a subwoofer is physically large. Bass is a question of air displacement, not electronics.
Why are there several drivers in one speaker?
Because no single cone can be both light enough for treble and large enough for bass. A tweeter’s diaphragm has to reverse direction up to 20,000 times a second, which demands almost no mass. A woofer needs stiffness and area. Splitting the range between two or three drivers, with a crossover filtering the signal, is how a speaker covers the whole audible band.