Representing Sound

Spec & Goals 3 min

AQA Spec 3.3.7 · Representing sound

By the end of this lesson you can:

1. Explain how an analogue sound is made digital by sampling.
2. Define sample rate and sample resolution (bit depth) and their effect on quality and file size.
3. Calculate the size of a sound file using sample rate × bit depth × seconds.

Warm-Up 5 min

Last lesson covered images — resolution and colour depth set quality and file size. Sound works the same way.

Quick starter

A microphone in a Penang studio picks up a singer's voice. The voice is a smooth wave, but a computer can only store numbers. How might it turn the wave into numbers?

Key Concept — sampling an analogue wave 14 min

Real sound is analogue — a continuous, smoothly changing wave. A computer is digital, so it must convert the wave into numbers.

To digitise sound the computer takes samples — it measures the wave's height at regular time intervals and records each height as a binary number.

Sample rate

The sample rate is how many samples are taken each second. It is measured in hertz (Hz). A common music rate is 44100 Hz — 44,100 samples a second.

Sample resolution (bit depth)

The sample resolution, or bit depth, is the number of bits used to store each sample. More bits means each height is recorded more precisely.

A higher sample rate and a higher bit depth give a more faithful, higher-quality recording — but also a larger file.

Worked Example — size of a music clip 12 min

Problem: A 10-second mono clip is sampled at 44100 Hz with a bit depth of 16. Calculate its size in bits, then in bytes.

Write the formula, then substitute the numbers:

So the file is 7,056,000 bits, which is 882,000 bytes (mono). If it were stereo you would multiply by 2 again.

Try It Yourself 12 min

📝 Exam Practice 10 min

Answer the way the examiner expects — the command word and the marks tell you how much to write.

Recap & Key Terms 3 min

Sound is analogue. A computer digitises it by sampling — measuring the wave at regular intervals. The sample rate (Hz) and bit depth set the quality and the file size. Size in bits = sample rate × bit depth × seconds.

Analogue

A continuously varying signal, such as a real sound wave, with no fixed steps.

Sampling

Measuring the height of an analogue wave at regular intervals and storing each value digitally.

Sample rate

The number of samples taken per second, measured in hertz (Hz).

Sample resolution (bit depth)

The number of bits used to store each sample.

Homework 1 min

Task (≤ 15 min): Calculate the size in bits of a 30-second mono voice note sampled at 11025 Hz with a bit depth of 8, then convert your answer to bytes.