Light and Sound » Practical: Speed of Sound in Air

The idea behind every method

This practical is examined in Paper 2 only.

Sound in air travels at roughly 340 m/s. To measure that yourself you only need two things: a distance that the sound travels and the time it takes. Then you use the equation you already know:

speed = distance ÷ time

Distance is easy to measure well. Time is the hard part. Sound crosses a room in about a hundredth of a second, far too quick for a person with a stopwatch. So every method either makes the time longer (a big distance, or many trips added together) or uses electronics to do the timing.

Key terms:

• Trundle wheel: a wheel on a handle that clicks once for every metre it rolls, used to measure long distances along the ground.
• Microphone: a device that turns a sound into an electrical signal.
• Electronic timer: a timer that is started and stopped by electrical signals, not by a person, so it can measure times as short as a thousandth of a second.

Method 1: see it, then hear it

Two people stand a long way apart on a large, flat, open field, 100 m or more. One has two wooden blocks. The other has a stopwatch.

1. Measure the distance between the two people with a trundle wheel or a long tape measure. Record it in metres.
2. Person A holds the two blocks high in the air and bangs them together.
3. Person B starts the stopwatch when they see the blocks meet.
4. Person B stops the stopwatch when they hear the bang.
5. Record the time. Repeat at least five times and work out the mean time.
6. Work out speed = distance ÷ mean time.

This works because light is so much faster than sound. Light crosses the field in less than a millionth of a second, so person B sees the blocks meet at almost the exact moment the sound is made. The wait before the bang is the time the sound took to arrive.

Distance in m	Time 1 in s	Time 2 in s	Time 3 in s	Time 4 in s	Time 5 in s	Mean time in s
200	0.61	0.58	0.64	0.57	0.60	0.60

The big weakness is reaction time. A person takes roughly 0.2 s to react, and person B has to react twice: once to start the stopwatch and once to stop it. The whole time being measured is only about 0.6 s, so an error of even 0.1 s is a large fraction of the reading. That is why the five times in the table are so spread out, and why this method gives only a rough value.

Wind matters too. Sound arrives sooner when the wind blows from A towards B. To allow for this, the two people swap jobs so the sound travels the other way, and the mean of both directions is used.

Method 2: clapping in time with an echo

This method makes the timed interval much longer. You need a large, flat wall with open ground in front of it.

1. Stand a measured distance from the wall, for example 80 m. Measure it with a trundle wheel or tape.
2. Clap two blocks together and listen for the echo.
3. Keep clapping in a steady rhythm. Adjust the rhythm until each clap lands exactly on the echo of the clap before it.
4. A second person starts a stopwatch on one clap, calls it zero, and counts 20 more claps. They stop the stopwatch on clap 20.
5. Repeat the timing at least three times and work out the mean.

When the rhythm is right, the gap between one clap and the next is the time for the sound to travel to the wall and back. So in each gap the sound travels twice the distance to the wall.

Distance to wall in m	Time for 20 claps in s (three tries)	Mean time for 20 claps in s
80	9.6, 9.8, 9.4	9.6

Why is this better? The stopwatch is still worked by a person, so there is still a reaction time error of a fraction of a second. But now it is spread over 9.6 s, not 0.6 s. An error of 0.2 s in 9.6 s is only about 2% of the reading. Also, the person timing follows a steady beat, so they do not have to react to a surprise.

Method 3: two microphones and an electronic timer

This method takes the person out of the timing altogether, so it can be done on a bench indoors.

1. Place two microphones in a straight line, a measured distance apart. Measure the gap with a metre rule, for example 1.50 m.
2. Connect both microphones to an electronic timer or a data logger.
3. Make a short, sharp sound on the same line, outside the two microphones and nearer to the first one, so the sound passes the first microphone and then the second. Hitting a metal plate with a small hammer works well.
4. The sound reaches the first microphone and its signal starts the timer. The sound then reaches the second microphone and its signal stops the timer.
5. Record the time. Repeat several times and work out the mean.
6. Move the second microphone to change the distance, and repeat.

With the source in line, the extra distance the sound travels to reach the second microphone is exactly the gap you measured.

Distance between microphones in m	Mean time in ms	Mean time in s	Speed in m/s
0.50	1.5	0.0015	333
1.00	2.9	0.0029	345
1.50	4.4	0.0044	341
2.00	5.9	0.0059	339

Here the distance between the microphones is the independent variable and the time is the dependent variable. Plot distance (y-axis) against time in seconds (x-axis). The points lie close to a straight line through the origin, and its gradient is the speed of sound.

Comparing the three methods

Whichever method you use, repeat the readings, leave out any anomalous result and take a mean.

Safety

• Work outdoors only in a safe open space, well away from roads, and stay where the other person can see you.
• Never make a loud sound close to anyone's ears.
• Keep fingers clear when banging blocks together or using a hammer.
• Indoors, keep the microphone leads tidy so that nobody trips over them.

Exam-style question

Two students investigate the speed of sound in air. They stand 180 m apart on a playing field. Student A bangs two saucepan lids together above her head. Student B starts a stopwatch when he sees the lids meet and stops it when he hears the sound. His three times are 0.52 s, 0.55 s and 0.49 s.

(a) State the piece of equipment the students could use to measure the 180 m distance. [1 mark]

(b) Calculate the speed of sound from these results. Use the mean time. [3 marks]

(c) Explain why this method does not give an accurate value for the speed of sound. [2 marks]

(d) Describe how two microphones and an electronic timer could be used to obtain a more accurate value. [3 marks]