objectives
By the end you'll be able to…
- Describe how an analogue sound wave is converted into digital form.
- Define sample rate and bit depth.
- Describe the effect of sample rate and bit depth on quality and file size.
- Calculate the size of a sound file using the formula.
🔊 what is sound?
Sound is a wave
Sound in the real world is analogue — a smooth, continuous wave of air pressure. Press play to hear a pure 440 Hz sine wave (the note A above middle C). This is what your ears hear before any computer touches it.
ready
A pure sine wave sounds "clean" because it's a single frequency. Real music and voice are many waves added together — but every one of them still has to be digitised the same way.
A2D
How sound becomes binary
Computers can only store binary. So sound has to be converted from analogue to digital — a process called Analogue to Digital conversion (A2D).
- The height (amplitude) of the wave is measured at regular time intervals. Each measurement is called a sample.
- Each sample is stored as a binary value.
- Using all these samples, the computer builds a stepped digital version of the original wave.
- The digital wave is stored on secondary storage and can be played back at any time by reversing the process.
Sampling is like drawing a smooth curve as a dot-to-dot picture. The more dots, and the more accurately each dot is placed, the closer your dot-to-dot looks to the original curve.
🎧 sample rate
Test 1 — change the sample rate
Sample rate is the number of samples taken per second of the analogue wave. It's measured in Hertz (Hz). Here, bit depth is fixed at 16 bits (CD quality) so only the sample rate changes.
| Source | Typical sample rate |
|---|---|
| Telephone | ~8 kHz |
| Audio CD | 44.1 kHz |
| DVD | 48 kHz |
1. Pick a sound to test
2. Adjust the sample rate
at 30 s, 16-bit → about — for the file
3. Listen
ready — try dropping to 2,000 Hz and picking the rising sweep to hear it fall apart
- At 44.1 kHz the sampled version sounds identical to the original.
- Below about 8 kHz the sound gets muffled — high frequencies vanish.
- The rising sweep is the clearest test: at low sample rates the high notes go missing or turn into weird buzzes (aliasing).
- This is exactly why telephone audio (~8 kHz sample rate) sounds hollow.
🎧 bit depth
Test 2 — change the bit depth
Bit depth is the number of bits stored per sample. It decides how accurately each sample's height can be measured. Here, sample rate is fixed at 44.1 kHz (CD quality) so only the bit depth changes.
- 1 bit → 2 possible amplitude levels (very coarse).
- 8 bits → 256 levels.
- 16 bits → 65,536 levels (CD quality).
1. Pick a sound to test
2. Adjust the bit depth
at 30 s, 44.1 kHz → about — for the file
3. Listen
ready — try dropping to 2 bits with the pure tone to hear pure quantisation noise
- At 16 bits the sampled version is indistinguishable from the original.
- By 4 bits the sound has a gritty, chip-tune texture (16 possible amplitudes).
- At 1 bit only up or down can be stored — the sound turns into square-wave buzz.
- Pick the pure tone and drop the bit depth to hear quantisation noise most clearly.
Bit depth in sound and colour depth in images measure the same idea — how many bits per sample/pixel. Higher = more precise.
file size
Calculating sound file size
Then divide by 8 for bytes, by 1024 for kilobytes (KiB), and so on.
A recording is made at 10 samples/second, 4 bits per sample, lasting 30 seconds. Calculate the file size in bytes.
Step 1 — bits: 10 × 30 × 4 = 1,200 bits
Step 2 — bytes: 1,200 ÷ 8 = 150 bytes ✅
🖱️ Try any values
Effect on quality and file size
| Factor | Higher = quality | Higher = file size |
|---|---|---|
| Sample rate | More detail, closer to original | More samples/s → more data |
| Bit depth | More precise amplitude, bigger range | More bits per sample → more data |
quiz
Quick quiz
Answer all five — your score appears at the bottom.
🎉 What you should remember
- Sample rate = samples per second (Hz). Higher = closer to the analogue wave.
- Bit depth = bits per sample. Higher = more accurate amplitude.
- File size (bits) = sample rate × duration × bit depth. Divide by 8 for bytes.
- Low sample rate → hollow, muffled sound. Low bit depth → crunchy, quantised sound.
// end of topic 6 — sound