IGCSE CP7 – Frequency of Sound | Edexcel International GCSE Physics

Theory — Frequency and the Oscilloscope

An oscilloscope connected to a microphone displays sound as a voltage-time graph, allowing frequency to be measured precisely.

Key Quantities

Period T — time for one complete wave cycle (s)
Frequency f — number of complete cycles per second (Hz)
Amplitude — maximum displacement from equilibrium — determines loudness

f = 1 / T

f = frequency (Hz) · T = period (s)

Reading the Oscilloscope

The timebase (ms/div or s/div) tells you how much time each horizontal square represents.

T = (divisions per cycle) × (timebase in s/div) f = 1 / T

Example: 2.5 divisions per cycle, timebase 2 ms/div → T = 2.5 × 0.002 = 0.005 s → f = 200 Hz

The Y-gain (V/div) controls the vertical scale. It affects the apparent amplitude on screen but does not change the frequency.

Tuning Fork vs Signal Generator

Tuning fork — struck once; amplitude decays naturally as the fork loses energy. Frequency stays constant throughout. Freeze the trace to read it.
Signal generator — produces a continuous stable sine wave. Amplitude is constant. Easier to read, but less realistic.

Pitch and Loudness

Higher frequency → higher pitch (period shorter, more cycles per second)
Larger amplitude → louder sound (wave taller on screen, period unchanged)

Procedure

Equipment

Oscilloscope · Microphone · Tuning forks (various frequencies) or signal generator + speaker · Connecting leads · Rubber bung

1

Connect microphone and set Y-gain

Connect microphone to the Y-input. Adjust Y-gain so the wave fills about half the screen height without going off the edges.

2

Adjust the timebase

Set timebase so 2–4 complete cycles are visible. Too fast = flat line; too slow = squashed wave.

💡 For 440 Hz (T≈2.3 ms), try 1 ms/div to see about 4 cycles clearly.

3

Strike tuning fork and freeze trace

Strike the fork on the rubber bung (not a hard surface). Hold near microphone. Press Freeze quickly to capture the trace before amplitude decays.

4

Measure divisions per cycle and calculate f

Count horizontal divisions for one complete cycle (peak to peak). T = divisions × timebase. f = 1/T. Compare with the frequency stamped on the fork.

💡 Measure across several cycles for accuracy: e.g. 3 cycles span 6.9 div with 1 ms/div → T = 6.9/3 × 0.001 = 2.3 ms.

📺 Select Tuning Fork mode — press 🔔 Strike to see the decaying wave. Or use Signal Generator for a continuous stable trace.

Source

Frequency

Strike Force

Timebase

ms / div 1.0

0.25.0

Y-Gain

V / div 0.5

0.11.0

Readings

Timebase1.0 ms/div

Y-gain0.5 V/div

Divs per cycle—

Period T—

True f440 Hz

Calculated Frequency

— Hz

Adjust timebase to see 2–4 cycles

Questions

Question 1

An oscilloscope trace shows a sound wave. The timebase is 2 ms/div and one complete cycle spans 2.5 divisions. Calculate (a) the period T, and (b) the frequency f.

(a) T = 2.5 × 2 ms = 5.0 ms = 0.005 s. (b) f = 1/T = 1/0.005 = 200 Hz.

Question 2

A student strikes a 440 Hz tuning fork and observes the oscilloscope. Describe how the trace changes over the next few seconds, and explain why. What stays the same?

The amplitude of the trace (height of the wave) gradually decreases over time as the tuning fork loses energy to the surroundings — the sound gets quieter. However, the period (and therefore the frequency) stays exactly the same throughout. This is because the frequency of a tuning fork depends only on its physical dimensions and material — these don't change. The fork always vibrates at its natural frequency regardless of how hard it was struck or how much energy remains.

Question 3

Explain why the Y-gain (V/div) setting on an oscilloscope does NOT affect the frequency measurement, but the timebase (ms/div) setting does.

The Y-gain controls the vertical (voltage) scale — it stretches or compresses the wave vertically. This changes how tall the wave appears, but does not change its horizontal spacing (the time between peaks). The period T is measured horizontally, so Y-gain has no effect on the frequency calculation. The timebase, however, controls the horizontal (time) scale — it determines how many milliseconds each division represents. Since T = divisions per cycle × timebase, changing the timebase directly changes the time value assigned to each division. If the timebase is wrong, T will be wrong, and therefore f = 1/T will be wrong.