The main speaker in a phone is a sealed plastic box roughly the size of a fingernail and a few millimetres thick. It fills a room with sound, and a single drop of water can silence it. Both facts follow from how it is built.
The four parts that make sound
- Magnet. A small, strong permanent magnet fixed in place.
- Voice coil. A tiny coil of hair-thin wire sitting in the magnet's field. When the phone sends an electrical audio signal through it, the coil becomes an electromagnet and is pushed and pulled by the fixed magnet.
- Diaphragm. A thin, light membrane attached to the coil. As the coil moves, the diaphragm moves with it and pushes the air in front of it.
- Enclosure. The sealed box behind the diaphragm. Its trapped air acts as a spring and shapes the bass.
Sound is air being pushed back and forth. To play a 200 Hz note, the diaphragm moves in and out 200 times a second. Louder means it moves further. Lower notes also need it to move further, which is why small speakers struggle with bass.
The path out of the phone
The speaker does not sit right at the holes you see on the bottom edge. Sound travels from the diaphragm through a short channel, then through a fine mesh, and finally out through the grille holes. The mesh has two jobs: it keeps dust out, and on water-resistant phones it is treated to repel water so that liquid stops there and goes no further.
Most phones have a second speaker in the earpiece at the top of the screen. It is smaller still, cannot play low notes well, and has its own mesh.
Why a little water has a big effect
- It blocks the mesh. The mesh openings are a fraction of a millimetre wide. Water bridges them and surface tension holds it in place like a film across a bubble wand. Air, and so sound, cannot pass freely.
- It loads the diaphragm. If water sits in the channel against the diaphragm, the membrane has to push that weight as well. It moves less, and the sound is quieter and duller.
- It moves. Droplets slapping against the mesh and diaphragm cause the crackle people notice on bass notes.
High notes suffer most, because they are small, fast movements that are easily absorbed. That is why a wet speaker sounds muffled instead of merely quiet.
Why sound can fix it
The same diaphragm that is being blocked can be used to clear the blockage. A loud, low tone makes it travel as far as it can. Each outward stroke pushes a pulse of air through the mesh, and the repeated shaking breaks the surface tension holding the droplets. They are flung out through the grille. This is what the water eject sound does, and it is the same method smartwatch makers build in. For the physics in more detail, see how it works.
Why sound cannot fix everything
- Microphones have a membrane but no motor. They cannot push water out. See muffled microphones.
- Water inside the phone, past the seals, is nowhere near the speaker's air path.
- Dried residue from salt or sugar is stuck to the mesh and needs wiping.
- A torn diaphragm or burnt coil is physical damage. It buzzes at every volume and needs a new speaker.
How phones protect such small speakers
Modern phones use a smart amplifier, a chip that monitors the coil's temperature and the diaphragm's movement in real time and reduces power before either goes too far. It is the reason a phone can play at full volume for hours without burning out its speaker, and why a short eject tone is safe to use.