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J277 1.2.4 Sound

Part 3 of 5 · J277 1.2.4-1.2.5 · Data representation

Sampling an analogue wave, and the three factors that decide the size and quality of the result.

Students will:

  • explain how an analogue sound is sampled
  • define sample rate, bit depth and duration
  • explain how sample rate and bit depth affect quality, and how all three factors affect file size
  • calculate the size of a simple uncompressed sound file
  • correct the misconception that frequency is what gets sampled

Inside: 5 text sections, 2 multiple-choice activities, 3 completion activities, 5 written answers. 19 marks. Suggested teaching time: 40–60 minutes. 1 diagram.

Includes a teacher guide with suggested sections, timings and common misconceptions.

Series: J277 1.2.4-1.2.5 · Data representation, part 3 of 5.

Shared by Coding PathwayVerified teacher

  • 15 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 27 Oct 2026

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The whole resource, exactly as a class sees it. Answers and marking are held back.

Sound

An analogue sound wave changes continuously. A computer creates a digital recording by measuring its amplitude, or height, at regular time intervals. Each measurement is a sample, stored as a binary value.

Amplitude sampled at equally spaced times A continuous curved wave is shown with four measurement points at equally spaced times. Each vertical line measures the wave's height at that time. The resulting amplitude samples are stored as binary values. Measure the wave's height Height Time 1 2 3 4 Equal time gaps between samples Store each amplitude as a binary value.
Fill in the blanks3 marks
A sound sample measures Answer 1 at regular time Answer 2. The measurement is stored as a Answer 3.
  • amplitude
  • intervals
  • binary value
  • frequency

1. Three factors

FactorMeaningEffect when increased, with other factors unchanged
Sample rateSamples each second, measured in Hz.More frequent measurements can follow the wave more closely; more samples need storage.
Bit depthBits for each sample.More possible amplitude levels allow a closer value for each measurement; each sample needs more bits.
DurationLength of the recording in seconds.More seconds mean more samples and a larger file, without improving the quality of each second.

A measured height must be represented using the available amplitude levels. More levels can give a closer match. Adding bits per sample does not add more samples per second.

Multiple choice1 mark

A recorder takes measurements more often while keeping the bits per sample unchanged. Which factor has increased?

  • ADuration
  • BSample rate
  • CBit depth
  • DThe frequency of the original sound
Written answer2 marks

Explain why using a higher bit depth when capturing an analogue sound can improve its representation.

Students type their answer here.

2. Amplitude is not frequency

The sampled measurement is amplitude. Sample rate is sometimes called sampling frequency: this still means measurements per second. It does not mean that each sample stores the original wave's frequency.

Written answer3 marks

Describe how an analogue sound wave is represented in a digital recording.

Students type their answer here.

3. Uncompressed sound-file size

For a mono recording, with one sound channel:

Bits = sample rate × duration in seconds × bit depth. Divide by 8 for bytes.

At 8,000 samples each second for 10 seconds, using 8 bits per sample: 8,000 × 10 × 8 = 640,000 bits, then ÷ 8 = 80,000 bytes. Divide bytes by 1,000 for KB.

Now calculate a mono recording at 20,000 Hz for 30 seconds, using 8 bits per sample. Ignore headers; use 1 KB = 1,000 bytes.

Written answer2 marks

Show your method for calculating this 30-second recording's sound-data size in KB.

Students type their answer here.

Fill in the blanks1 mark
The final size of this recording's uncompressed sound data is Answer 1 KB.

4. Apply the sample-rate idea

A student lowers the sample rate of a podcast before uploading it. Bit depth and duration stay the same.

Written answer2 marks

(a) Explain one effect on the podcast's file size.

Students type their answer here.

Written answer2 marks

(b) Explain one possible effect on playback quality.

Students type their answer here.

Multiple choice1 mark

A recording is extended from 20 to 40 seconds. Sample rate and bit depth stay the same. What happens?

  • AFile size doubles, with representation quality per second unchanged
  • BBoth file size and quality double
  • CBit depth doubles
  • DFile size stays the same because sample rate is unchanged
Fill in the blanks2 marks
Answer from memory. (a) Measurements per second: Answer 1 (b) Bits per measurement: Answer 2