IGCSE CP6 – Speed of Sound | Edexcel International GCSE Physics

Theory — Speed of Sound

Sound is a longitudinal wave that travels through air at approximately 340 m/s at room temperature.

Wave Speed Equation

The speed of any wave is related to its frequency and wavelength:

v = f λ

v = wave speed (m/s) · f = frequency (Hz) · λ = wavelength (m)

For this experiment, we measure speed directly using distance and time:

v = d / t

d = distance between microphones (m) · t = time delay between signals (s)

The Two-Microphone Method

Two microphones are placed in a line, separated by a distance d. A sharp sound (clap, starter pistol) is made near microphone 1. Sound reaches microphone 1 first, then travels to microphone 2. A datalogger records the time delay t between the two signals. Since v = d/t, measuring d and t gives the speed of sound.

By varying the separation d and measuring t each time, then plotting d (y-axis) against t (x-axis), the gradient of the straight line through the origin equals the speed of sound.

d = v × t → Plot d against t → gradient = v (speed of sound)

Straight line through origin confirms v is constant (speed doesn't depend on distance).

Factors Affecting Speed of Sound

Temperature — speed increases with temperature (~0.6 m/s per °C). At 0°C: ~331 m/s, at 20°C: ~343 m/s
Medium — sound travels faster in denser/stiffer materials. In water: ~1500 m/s. In steel: ~5000 m/s
Frequency and amplitude do NOT affect the speed of sound in a given medium

Procedure

Measuring the speed of sound using two microphones and a datalogger.

Equipment

Two microphones · Datalogger or oscilloscope · Metre ruler or measuring tape · Sharp sound source (clap boards or starter pistol) · Computer with timing software

1

Set up the microphones in a line

Place both microphones on a bench facing the sound source, separated by a measured distance d (start with d = 0.5 m). Connect both to the datalogger. Set the datalogger to record the time delay between the first signal (mic 1) and second signal (mic 2).

💡 Make sure both microphones face the same direction and the sound source is in line with them, on the mic 1 side.

2

Make a sharp sound and record the time delay

Clap two boards together sharply, about 0.5 m in front of microphone 1. The datalogger records the time delay t between the two signals. Record d and t.

💡 Repeat 3 times and take the mean t. Reject any anomalous readings where the delay is clearly wrong.

3

Repeat for 6 different separations

Move microphone 2 to different positions: d = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 m from microphone 1. At each separation, record the time delay t.

4

Plot d against t

Plot d (y-axis, metres) against t (x-axis, milliseconds or seconds). Draw best-fit line through the origin. Gradient = speed of sound v in m/s.

💡 The line must pass through the origin — when d = 0 there is no delay. If your line doesn't pass through the origin, check for a systematic error in your timing.

      🔊 Set the microphone separation, then press ▶ Make Sound. The datalogger shows the time delay between microphone signals. Record 6+ readings then check the Graph tab.
    

Microphone Separation

Distance d 1.0 m

0.5 m3.5 m

Datalogger Reading

Separation d1.000 m

Time delay t—

v = d/t—

Readings: 0 / 6 target

Data Table

Plot d (y-axis) against t (x-axis). Straight line through origin — gradient = speed of sound.

#	Separation d / m	Time delay t / ms	Time delay t / s	v = d/t / m/s
No readings yet. Use the Simulation tab.

Graph — d vs t

Distance (y-axis) against time delay (x-axis). Gradient = speed of sound in m/s.

Speed of sound

—m/s — from graph gradient

Results

Gradient (= v)—

R²—

Expected (~20°C)343 m/s

% error—

Interpretation

Record at least 5 readings to see analysis.

Questions

Question 1

Two microphones are separated by 2.0 m. A datalogger records a time delay of 5.8 ms between the two signals. Calculate the speed of sound from this single measurement. Why is it better to use the gradient of the d vs t graph rather than this single calculation?

v = d/t = 2.0 / 0.0058 = 345 m/s. Using the graph gradient is better because: (1) it uses all readings simultaneously — random errors in individual readings partially cancel out, giving a more reliable result; (2) the straight line through the origin can be checked — if points are scattered or the line misses the origin, there is a systematic error; (3) a single measurement could have a large random error in timing that would significantly affect the result, whereas the best-fit gradient averages over all measurements.

Question 2

Explain why the d vs t graph should pass through the origin. A student's graph has a positive t-intercept (it cuts the x-axis to the right of zero). Suggest what systematic error this indicates.

The graph should pass through the origin because when the separation d = 0, the time delay t = 0 — sound arrives at both microphones simultaneously when they are at the same position. A positive t-intercept means the datalogger is recording a time delay even when there should be none. This indicates a systematic error: most likely, the datalogger has a built-in time offset (zero error), or microphone 1 is slightly further from the sound source than assumed, so there is always an extra delay regardless of separation. This could be corrected by recalibrating the datalogger or measuring the true starting position of microphone 1.

Question 3

The speed of sound at 0°C is 331 m/s and increases by approximately 0.6 m/s for every 1°C rise in temperature. A student measures the speed of sound as 347 m/s. Estimate the air temperature in the laboratory during the experiment.

Extra speed above 0°C = 347 − 331 = 16 m/s. Temperature = 16 / 0.6 = 26.7°C ≈ 27°C. The laboratory temperature was approximately 27°C. This is a reasonable room temperature, slightly warmer than a typical UK classroom (which might be ~20°C giving ~343 m/s), perhaps reflecting a warm day or a heated room. Note: the accepted value at 20°C is 343 m/s, so 347 m/s corresponds to about 27°C, which is physically plausible.