Lesson focus: amplitudeThe height of a wave from the middle line to the top of a peak; bigger amplitude means a louder sound. and frequencyHow many times something happens each second; for a wave, how many waves pass each second, measured in hertz (Hz). describe different features of a sound traceA wavy line on a screen that shows the shape of a sound wave. and correspond broadly to loudnessHow loud a sound seems; a bigger amplitude usually sounds louder. and pitchHow high or low a sound is; a higher frequency gives a higher pitch..
Interpret microphoneA device that changes sound into an electrical signal. traces and calculate frequency while keeping amplitude, frequency, loudness and pitch distinct.
Build on earlier learning
Sound and energy transferThe previous lesson modelled sound as changing pressure. A microphone trace now makes those variations measurable.
Practical
Seeing sound on an oscilloscope
- Connect a microphone to an oscilloscope.
- Play quiet and loud notes: the trace becomes taller (greater amplitude).
- Play low and high notes: more waves fit on the screen (higher frequency).
Record: Sketches of each trace with amplitude and frequency labelled.
Sound waves and the oscilloscope
A loudspeaker makes air particles vibrate backwards and forwards, in the same direction the sound travels. A microphone turns the vibrations into a trace on an oscilloscope.
Controls
Taller trace: louder. More waves on the screen: higher pitch. Speed of sound in air ≈ 340 m/s.
You will be successful when you can…
- Measure amplitude from the centre line.
- Calculate frequency from cycles and time.
- Link amplitude with loudness under comparable conditions.
- Link frequency with pitch without treating the trace as a particle path.
Words that carry the science
- microphone
- A detector that converts sound-induced vibrations into an electrical signal.
- trace
- A graph of signal value against time.
- amplitude
- The maximum signal displacement from the equilibrium line.
- frequency
- The number of complete cycles per second, measured in hertz.
- pitch
- The perceptual quality mainly related to frequency.
- loudness
- A perception affected by sound intensity, frequency and the listener; larger comparable amplitudes usually sound louder.
Read a sound trace without confusing its axes
01Amplitude is measured from equilibrium
On a centred microphoneA device that changes sound into an electrical signal. traceA wavy line on a screen that shows the shape of a sound wave., amplitudeThe height of a wave from the middle line to the top of a peak; bigger amplitude means a louder sound. is the maximum vertical displacement from the middle line. Crest-to-trough height equals twice the amplitudeThe height of a wave from the middle line to the top of a peak; bigger amplitude means a louder sound..
For the same equipment and scale, a larger signal amplitude represents a larger pressure variation and usually a louder sound. Gain settings must be held constant for a fair comparison.
02Frequency counts cycles in a stated time
Frequency = number of cycles ÷ time. One hertz means one complete cycle per second. A shorter period means more cycles each second and therefore a higher frequencyHow many times something happens each second; for a wave, how many waves pass each second, measured in hertz (Hz)..
Humans usually perceive higher frequency as higher pitchHow high or low a sound is; a higher frequency gives a higher pitch.. Frequency is an objective measurement; pitchHow high or low a sound is; a higher frequency gives a higher pitch. is a perception and can be influenced by context.
03Traces depend on settings and sampling
Comparisons require the same time scale and vertical sensitivity. Changing an oscilloscope or app scale can make the same signal look taller or more spread out without changing the sound.
Real sounds are often complex rather than smooth sine waves. The repeating time pattern can still reveal frequency, while amplitude must be interpreted using the calibrated scale.
Calculate frequency from a trace
A trace shows 8 complete cycles in 0.020 s. Calculate the frequency.
- Write frequency = number of cycles ÷ time.
- Use 8 complete cycles and 0.020 s.
- Calculate 8 ÷ 0.020 = 400.
- Attach the unit hertz because frequency counts cycles per second.
How to make the reasoning visible: The frequency is 400 Hz.
Compare four microphone traces
All traces were collected with identical time and voltage scales.
| Trace | Cycles in 0.010 s | Amplitude / divisions | Interpretation |
|---|---|---|---|
| A | 2 | 1 | 200 Hz, smaller amplitude |
| B | 2 | 3 | 200 Hz, larger amplitude |
| C | 5 | 1 | 500 Hz, smaller amplitude |
| D | 5 | 3 | 500 Hz, larger amplitude |
Which pair isolates the effect of amplitude, and what audible difference is expected?
Compare B and D.
Check your understanding
Answer from memory, then check against the suggested answer.
How is amplitude measured on a centred trace?
What does frequency measure?
Which wave feature mainly determines pitch?
A sound completes 12 cycles in 0.030 s. Calculate its frequency.
Two screenshots show traces of different heights. Explain why this alone does not prove one original sound was louder.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Trace X has twice the amplitude of trace Y. Both show six cycles in the same time.
Compare the loudness of the two sounds.
(2)Compare the pitch of the two sounds.
(2)Total for this question: 4 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., treat amplitude and frequency as two independent comparisons.