The properties of a sound wave determine what we hear.
Pitch and frequency
- Higher frequency = higher pitch (squeaky).
- Lower frequency = lower pitch (deep/bass).
The audible range for humans is approximately 20 Hz to 20 000 Hz.
On an oscilloscope trace, a higher-frequency sound shows more cycles in the same time span (waves are closer together).
Loudness and amplitude
- Greater amplitude = louder sound.
- Smaller amplitude = quieter sound.
On an oscilloscope trace, a louder sound has taller waves (greater displacement from the centre line).
Quality (timbre)
Different instruments playing the same note at the same loudness sound different because of their waveform shape (timbre).
- A pure tone (tuning fork) produces a smooth sinusoidal wave.
- Musical instruments produce complex waveforms (a fundamental frequency plus overtones/harmonics).
- The pattern of harmonics gives each instrument its characteristic sound.
Reading an oscilloscope trace
| Feature on trace | Wave property |
|---|---|
| Waves closer together | Higher frequency (higher pitch) |
| Taller waves | Greater amplitude (louder) |
| Different wave shape | Different quality/timbre |
Common errors and how to correct them
- Confusing pitch with loudness. Pitch depends on frequency; loudness depends on amplitude. They are independent.
- Saying louder sounds have higher frequency. A louder sound simply has a larger amplitude at the same frequency.
How to apply this in an exam
Link each property of the sound to the corresponding property of the wave. Describe what you would see on an oscilloscope trace. Use the correct terms (frequency/pitch, amplitude/loudness).
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