IGCSE Physics 0625 – 3.4 Sound: Determining the Speed of Sound in Air
3.4 Sound
Objective
Describe a practical method that uses measurements of distance and time to determine the speed of sound in air.
Principle
The speed of a wave is given by the relationship
\$v = \frac{d}{t}\$
where
\$v\$ = speed of sound in air (m s⁻¹)
\$d\$ = distance travelled by the sound pulse (m)
\$t\$ = time taken for the pulse to travel that distance (s)
Apparatus
Item
Purpose
Long straight track or hallway
Provides a measurable straight‑line path for the sound
Two identical loudspeakers or a single speaker and a microphone
Generate and detect the sound pulse
Electronic timer or stopwatch with millisecond resolution
Measure the travel time of the pulse
Measuring tape or laser distance measurer
Determine the distance between source and detector
Computer with data‑logging software (optional)
Record multiple readings quickly
Experimental Procedure
Mark two points on the track a known distance apart, \$d_1\$. Typical values are 5 m, 10 m and 15 m.
Place the sound source (speaker) at one end and the detector (microphone connected to the timer) at the other end.
Generate a short, sharp sound pulse (e.g., a click or a balloon pop) and start the timer simultaneously.
Stop the timer when the detector registers the arrival of the pulse.
Record the measured time \$t_1\$.
Repeat steps 1–5 for at least three different distances (\$d2\$, \$d3\$, …) to improve accuracy.
For each distance calculate the speed using \$vi = di / t_i\$.
Obtain the final value of the speed of sound by averaging the individual speeds:
\$\bar{v} = \frac{v1 + v2 + v_3 + \dots}{n}\$
Estimate the uncertainty by considering the least count of the distance measurement (±0.01 m) and the timer (±0.001 s), then propagate these errors to \$\bar{v}\$.
Notes on Reducing Errors
Ensure the line of sight between source and detector is unobstructed; reflections from walls can give false early arrivals.
Use a temperature sensor to record the air temperature, because the speed of sound varies with temperature (\$v \approx 331 + 0.6T\$ m s⁻¹, where \$T\$ is in °C).
Perform the experiment in a quiet environment to minimise background noise.
Take at least five readings for each distance and use the mean of those readings as \$t_i\$.
The method described uses simple distance and time measurements to obtain a reliable value for the speed of sound in air. By repeating the experiment at several distances and averaging the results, random errors are reduced, and the final value can be compared with the theoretical speed at the measured temperature.
Suggested diagram: A straight track showing the speaker at the left end, the microphone at the right end, and the measured distance \$d\$ between them. A timer is triggered by the speaker and stopped by the microphone.