Sound (Years 9–11) — Computer Science

How analogue sound is sampled into binary, and how sample rate and bit depth affect quality and file size.

Sound is a wave — a smooth, continuous ripple of pressure in the air. But a computer can only store numbers, and there are infinitely many points on a smooth wave. So it takes rapid snapshots of the wave's height, thousands of times a second, and stores each as a number. This snapshot process is called sampling.

Why this matters: Sampling explains the settings behind every voice note, song and podcast. Knowing how sample rate and bit depth affect quality and file size lets you understand why a music track is far bigger than a phone call, and choose the right balance yourself.

Sampling measures the wave's height at fixed time intervals.

Sample rate

More samples per second (higher Hz) capture the wave truer.

Sample rate is how many samples are taken each second, measured in hertz (Hz). CD-quality audio uses 44 100 Hz — 44 100 snapshots every second. A higher sample rate captures more of the wave's shape, so the playback sounds truer to the original. Think of it like frames in a film: more frames per second give smoother motion.

Bit depth

Bit depth sets how many height levels a sample can snap to.

Bit depth (or sample resolution) is how many bits store each sample's height. More bits mean more possible height values, so the wave is measured more precisely and quiet detail isn't lost. 8-bit gives 256 levels; CD audio uses 16-bit for 65 536 levels.

Quantity	Meaning	Typical value
Sample rate	samples per second (Hz)	44 100 Hz
Bit depth	bits per sample	16 bits
Duration	length in seconds	varies

Working out file size

For a single (mono) channel the raw size is:

file size (bits) = sample rate × bit depth × seconds

Divide by 8 for bytes. For stereo (two channels) multiply by 2.

Worked example — 10 seconds of mono CD-quality audio.
Sample rate 44 100 Hz, bit depth 16, duration 10 s.
Bits = 44 100 × 16 × 10 = 7 056 000 bits. Bytes = 7 056 000 ÷ 8 = 882 000 bytes (about 861 KB).
In stereo this doubles to 1 764 000 bytes.

Size = rate × depth × seconds ÷ 8; stereo doubles it.

Common mistake: mixing up sample rate and bit depth. Sample rate = how often you measure (across time); bit depth = how precisely you measure each one (the height). Both raise quality and both raise file size, but they are different levers.

This is why a music track dwarfs a phone call: the call may use just 8 000 Hz and 8-bit samples, while the song uses 44 100 Hz and 16-bit stereo. The same few seconds of audio can differ in size by a factor of well over a hundred, purely because of these three settings.

Tip: if a question gives a rate in kHz, convert first: 44.1 kHz = 44 100 Hz. And always check whether the audio is mono or stereo before you finish — stereo doubles the size.