Displaying a sound wave with a microphone and oscilloscope revision notes

Edexcel International GCSE Physics (4PH1), chapter 13: Sound, specification point 3.26P, 3.27P. Free lesson.

Watch the video lesson · IGCSE Physics past papers

A microphone changes sound into a changing voltage, which an oscilloscope displays as a trace against time. You can use the trace's time-base to find the period, then calculate the sound's frequency.

How sound becomes a trace

A sound wave is a pattern of compressions and rarefactions in the air. These make the microphone's thin diaphragm vibrate. The microphone turns that movement into a voltage that rises and falls with the sound. The oscilloscope displays the voltage against time as a wavy line called a trace. The trace looks like a transverse wave, but it is a graph of a longitudinal sound wave, not a picture of the wave in the air. The microphone detects the sound. The oscilloscope cannot hear it. Only the microphone connects to the oscilloscope.

How to find the frequency

Use a sound with a controlled frequency, then read the time for one complete wave from the trace. The time-base tells you the time represented by each square across the screen. It is usually given in milliseconds per square.

What the trace tells you

The period, T, is the time for one complete wave. Frequency is the number of complete waves each second, so a shorter period means a higher frequency. The equation is on the equation list. f is frequency, measured in hertz (Hz). T is the time period, measured in seconds (s). For example, 2.5 ms = 0.0025 s. Always use T in seconds in f = 1/T.

Reading a 400 Hz trace

For the 400 Hz note, the time-base is 0.5 ms per square. One complete wave covers five squares, so the period is 2.5 ms, or 0.0025 s. This gives a frequency of 400 Hz, matching the signal generator. The peak is three squares above the centre line, so the amplitude is 3 V with a Y-gain of 1 V per square. For a more accurate period, measure across several waves and divide by the number of waves.

Avoid the milliseconds mistake

Do not put a period in milliseconds straight into f = 1/T. Convert it to seconds first. For example, using 2.5 instead of 0.0025 would give 0.4 Hz, which is incorrect. The correct calculation is 1/0.0025 = 400 Hz.