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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 each affects file size and playback quality
  • calculate the size of a simple uncompressed sound file
  • correct the misconception that frequency is what gets sampled

Inside: 6 explanation cells, 4 multiple-choice questions, 1 fill-in-the-blanks cell and 4 written answers. 33 marks, about 60 minutes.

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

Shared by Coding PathwayVerified teacher

  • 15 cells
  • About 60 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 9 Sept 2026

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

Data representation 3: sound

Sound in the physical world is analogue. Its wave changes continuously. A computer must measure the wave and store those measurements as binary data.

By the end of this worksheet, you should be able to:

  • explain how an analogue sound is sampled
  • define sample rate, bit depth and duration
  • explain how each affects file size
  • explain how sample rate and bit depth affect playback quality
  • calculate the size of a simple uncompressed sound file
  • correct the misconception that the wave's frequency is what gets sampled

Work through the explanation, checks and applications in order. Use the final challenge to connect several ideas.

1. Sampling an analogue wave

To create a digital recording, the computer measures the amplitude, or height, of the analogue sound wave at regular time intervals.

Each measurement is a sample. The sample is assigned a binary value. The sequence of stored samples forms a digital approximation of the original wave.

The wave is continuous, but the samples are separate measurements. More accurate or more frequent measurements can represent the original sound more closely.

Sampling the amplitude of an analogue sound wave A continuous sound wave is plotted against time. Regular vertical sample times meet the wave at sample points. Horizontal levels show that each measured amplitude is stored using one of the available binary values. amplitudetime At each regular time, measure the wave's amplitude.
Fill in the blanks6 marks
An label 1 sound wave is converted into label 2 data. During sampling, the label 3 of the wave is measured at regular label 4. Each measurement is a label 5 and is stored using label 6.
  • digital
  • binary
  • amplitude
  • pixel
  • time intervals
  • character set
  • sample
  • analogue
Multiple choice1 mark

What is measured at regular intervals when an analogue sound wave is sampled?

  • AThe storage device's capacity
  • BThe number of characters in the file
  • CThe file name
  • DThe wave's amplitude

2. Three factors

Sample rate

The sample rate is the number of samples taken each second. It is measured in hertz (Hz). A higher sample rate records the wave more frequently, which can improve playback quality but increases file size.

Bit depth

The bit depth is the number of bits used to store each sample. More bits provide more possible amplitude levels, which can represent each measurement more accurately but increases file size.

Duration

The duration is the number of seconds of audio. A longer recording contains more samples and therefore has a larger file size. Increasing duration does not improve playback quality.

Multiple choice, several answers3 marks

Which three statements are accurate? Select all that apply.

  • AIncreasing sample rate can improve quality and increases file size
  • BIncreasing duration improves the accuracy of every sample
  • CIncreasing bit depth can improve quality and increases file size
  • DIncreasing bit depth reduces the number of possible amplitude values
  • EIncreasing duration increases file size but does not by itself improve quality
Written answer6 marks

Compare sample rate and bit depth. Define each and explain one way that increasing each can improve playback quality.

Sample rate concerns measurements per second. Bit depth concerns bits per measurement.

Students type their answer here.

3. Do not say that frequency is sampled

A common incorrect answer is:

“The frequency of the sound wave is measured at regular intervals.”

The correct property is amplitude. Sample rate is sometimes called sampling frequency, but this means the number of samples taken per second. It does not mean that the original wave's frequency is the measurement stored at each point.

Multiple choice1 mark

Which correction is accurate?

  • AAmplitude is measured; sample rate states how many measurements are taken per second
  • BThe file's duration is measured instead of the wave
  • CColour depth is measured at each point
  • DThe wave's frequency is stored as every individual sample

4. Sound-file size

For a simple uncompressed mono recording:

file size in bits = sample rate × duration in seconds × bit depth

Example:

A recording uses 8,000 samples per second, lasts 10 seconds and uses 8 bits per sample.

8,000 × 10 × 8 = 640,000 bits

640,000 ÷ 8 = 80,000 bytes

Use the values and units stated in the question. This simplified OCR formula does not add extra channels unless the question explicitly supplies them.

Written answer5 marks

An uncompressed sound file has a sample rate of 20,000 Hz, a duration of 30 seconds and a bit depth of 8 bits. Calculate the file size in bits, bytes and kilobytes. Use 1 KB = 1,000 bytes.

Multiply the three values, then divide by 8 and by 1,000.

Students type their answer here.

Written answer4 marks

A student reduces the sample rate of a podcast before uploading it. Explain two likely effects.

Discuss both file size and how closely the digital samples represent the original wave.

Students type their answer here.

Multiple choice1 mark

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

  • AFile size halves and quality improves
  • BFile size stays the same because bit depth is unchanged
  • CFile size doubles, while the representation quality per second is unchanged
  • DThe number of amplitude levels doubles
Written answer6 marks

Challenge: a music archivist can increase either the sample rate or the bit depth of a recording, but not both. Explain what each change would improve, how each affects file size, and why the best choice depends on the weakness of the existing recording.

Distinguish measurements per second from possible amplitude levels. Finish with a recommendation that depends on the needs in the question.

Students type their answer here.

Closed-book review

  1. What property of the analogue wave is sampled?
  2. What is a sample?
  3. Define sample rate and give its unit.
  4. Define bit depth.
  5. Define duration.
  6. How does increasing each factor affect file size?
  7. Which two factors can improve playback quality?
  8. Write the sound-file size formula.

Final check: amplitude is measured. Sample rate tells you how often the measurement is made.