Sound File-Size Calculations — Text, Sound and Images | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

ASCII/Unicode, sound sampling and pixel/colour depth appear regularly as 2 to 4 mark questions.

The standard exam formula for the size of an uncompressed sound file is:

file size (bits) = sample rate × sample resolution × duration

where:

sample rate is in hertz (samples per second)
sample resolution is in bits per sample
duration is in seconds

To convert to bytes, divide by 8 (since 1 byte = 8 bits). To convert to kilobytes, divide by 8000 (or 8192 if the question uses the binary definition of a kilobyte).

Example — A 5-second sound clip is recorded with a sample rate of 8000 Hz and a sample resolution of 16 bits. Calculate the file size in bytes.

Step 1: Apply the formula in bits.
- file size = 8000 × 16 × 5 = 640 000 bits
Step 2: Divide by 8 to convert to bytes.
- 640 000 ÷ 8 = 80 000 bytes (about 80 kB)

Example — A 30-second recording at 44 100 Hz and 16 bits is compared with the same recording at 22 050 Hz and 8 bits. How much smaller is the lower-quality version?

High quality: 44 100 × 16 × 30 = 21 168 000 bits = 2 646 000 bytes
Low quality: 22 050 × 8 × 30 = 5 292 000 bits = 661 500 bytes
Ratio: 2 646 000 ÷ 661 500 = 4. The low-quality version is a quarter of the size.

This makes sense: halving the sample rate halves the file size, and halving the sample resolution halves it again, giving a factor of 4 overall.

Common slip: forgetting to divide by 8 at the end. If the question asks for bytes, kilobytes or megabytes, you must divide; if it asks for bits, you do not.