Speakers & microphones

Headphones, phone speakers, the big boxes in a cinema: everywhere, sound is made from electric current. And when you talk into a microphone, exactly the opposite happens, sound becomes current. The astonishing part: almost every speaker and the classic microphone need just three parts for this, a membrane, a coil and a magnet. Let us look at how they work together.

Sound is trembling air

Everything you hear is air swinging back and forth very quickly. How often it swings per second is called the frequency and is measured in hertz (Hz). Fast vibrations sound high to you, slow ones sound low. How strongly the air swings determines the volume. The vibration itself travels through the air, at room temperature at about 340 metres per second. Divide that speed by the frequency and you get the wavelength, the distance from one wave crest to the next. To create sound, all you need is one thing: something that pushes the air in exactly the right rhythm.

The speaker: current moves the membrane

Inside a speaker sits a light membrane, often a kind of cone made of paper or plastic. Glued to its back is a coil of wire, and that coil sits in the field of a strong permanent magnet. When current flows through the coil, the coil becomes a magnet itself and is attracted or repelled by the permanent magnet. The music arrives as an alternating current that constantly changes direction and strength. So the coil twitches back and forth exactly in the rhythm of the music, drags the membrane along, and the membrane pushes the air. An electrical signal has become sound.

The microphone: the same trick in reverse

Here comes the aha moment: a dynamic microphone is essentially the same construction, it just runs in reverse. Your voice hits a membrane and makes it vibrate. Here too, a coil is attached to the membrane and moves in the field of a magnet. And when a coil moves in a magnetic field, an electrical voltage appears in it. This effect is called induction. The vibration of your voice is translated point by point into an electrical signal. By the way, that is why in an emergency you can use a headphone as a tiny microphone, it really is the same machine.

From signal back to sound

This closes the loop: vibration becomes signal becomes vibration. When you make a call, your microphone turns your voice into a signal that travels halfway around the world as data. At the other end, a tiny speaker turns it back into exactly the vibration your vocal cords produced. The microphone signal is very weak though, so between microphone and speaker there is always an amplifier that makes the signal stronger without changing its shape.