How does one voice fill a whole hall?

Put your fingers gently on your throat and hum. Feel that buzz? Your vocal cords are vibrating, which means they are moving backwards and forwards very fast. Every time they move, they push the air next to them. That push travels through the air like a ripple on a pond, until it reaches someone's ear and makes their eardrum wobble too. That is sound. It is just air moving in waves.

Two things describe every sound you have ever heard:

• How fast it wobbles. Fast wobbles sound high, like a whistle. Slow wobbles sound low, like a drum. We measure this in hertz (Hz), which means wobbles per second. A bass drum is around 60 Hz. A whistle can be 3,000 Hz. Your ears can hear from about 20 Hz to 20,000 Hz.
• How big the wobble is. Big wobbles are loud, small wobbles are quiet. We measure loudness in decibels (dB). A whisper is about 30 dB, talking is 60 dB, and a rock concert is over 100 dB.

When a sound engineer sets up for an event, every piece of equipment is just one stop on a journey. The sound always travels in the same order. Tap each stop to follow the signal.

If you can remember "source, mic, mixer, amp, speaker", you already know more than most adults about how a sound system works.

The most common microphone at events is the handheld one you see singers use. It is called a dynamic mic. It is tough, you can drop it, and it mostly hears what is right in front of it. That shape of hearing is called cardioid, because if you drew it, it would look like a heart. The mic hears the singer in front and ignores the speakers behind. That matters, and here is why.

If a microphone hears the speaker, and the speaker is playing what the microphone hears, you get a loop. The sound goes round and round, getting louder every time, until it screams. You have heard it: that horrible squeal at assembly when someone walks too close to the speaker. It is called feedback, and the sound engineer's biggest job is making sure it never happens.

The fix is simple: keep the mics in front of the speakers, never behind them, and keep the mic close to your mouth.

There are also tiny clip-on mics (lapel mics), mics on a bendy stalk on a lectern (gooseneck mics), and mics that live over your ear (headset mics). They all do the same job: turn wobbling air into electricity.

A mixer looks complicated, but it is the same small set of controls repeated over and over. Each microphone or instrument gets its own column of knobs, called a channel. Here is what the knobs in one channel do, from top to bottom:

• Gain. How much to boost the tiny signal from the microphone. The engineer sets this once so the signal is strong but not so strong that it crackles. Then they leave it alone.
• EQ. Short for equaliser. These knobs make a sound more bassy or more trebly. If a voice sounds boomy, the engineer turns down the low knob. If it sounds muffled, they turn up the high knob a little. EQ is like the bass and treble on a car radio, but for one voice at a time.
• The fader. The long slider at the bottom. This is the volume for that one channel. Push it up, that person gets louder in the mix. The engineer spends most of the show with their fingers on the faders.
• Mute. A button that switches the channel off. Every mic that is not being used gets muted, so it cannot feed back.

All the channels flow into one big slider on the right, called the master. That is the volume for the whole room.

So a mixer does three things: it makes each sound strong enough (gain), it makes each sound nice (EQ), and it blends them together at the right volumes (faders). That is it. Everything else on the desk is a fancier version of one of those three.

The sound engineer's job starts hours before anyone arrives. They put the speakers on tall stands, high above people's heads, so the sound reaches the back row. They run cables along the walls and tape them down so nobody trips. They plug every mic into the mixer and label each channel. Then they do a soundcheck: someone talks into each mic while the engineer sets the gain, fixes the EQ, and makes sure nothing feeds back.

During the event they listen all the time. When the speaker is too quiet, the fader goes up. When the music needs to start, a different fader goes up and the mic fader goes down. When the CEO wanders in front of the speakers, the engineer is already reaching for the mute button.

It is a job that mixes science (waves, electricity, decibels), craft (listening, taste, timing) and calm under pressure. Good sound engineers are the people nobody notices, because when sound is done well, it just sounds like the room.