IGCSE Physics / Sound
Practical: measuring the speed of sound in air
Edexcel International GCSE Physics (4PH1), chapter 13: Sound, specification point 3.25P. Free lesson.
Read the revision notes · IGCSE Physics past papers
The speed of sound in air can be measured by timing sound over a known distance. Using a long distance, repeated measurements or microphones helps reduce timing errors.
How two people measure the speed of sound
In a large open space, one person makes a sound while another measures how long it takes to arrive. The light from the action reaches the observer almost instantly, so the measured time is the sound's travel time. Use a trundle wheel to measure a distance of at least 100 m. Person A bangs two wooden blocks together above their head, so Person B can see the blocks hit. Person B starts a stopwatch when they see the blocks hit and stops it when they hear the bang. Repeat several times and calculate the mean time. Then divide the distance by the mean time.
How to calculate the speed
The equation for speed is v = d/t. Here, d is the distance between the people and t is the mean time for the sound to travel that distance. v is the speed of sound, measured in metres per second (m/s). d is the distance, measured in metres (m). t is the mean time, measured in seconds (s). A result should be close to 340 m/s.
How to reduce timing error
When you time the sound by hand, the main source of error is the timer user's reaction time. Measuring over a long distance makes the sound take longer to arrive, so the reaction time has less effect on the result. Use a distance of over 100 m. A distance of 20 m is too short: sound travels it in about 0.06 s, which is shorter than a person's reaction time. Repeating the timing does not fix the problem caused by such a short travel time.
How two microphones remove reaction time
Place two microphones a measured distance apart, in line with the sound. Connect them to an electronic timer or to the two inputs of an oscilloscope. A clap beside the first microphone starts the timer when its sound arrives. The sound reaching the second microphone stops the timer, so no person has to time the interval. Calculate speed using the distance between the microphones divided by the recorded time. An echo can also be used: clap facing a wall, time the echo and use the distance travelled there and back.