Sound

Aim

• Measure the speed of sound in air using two different methods, and compare their accuracy
• The accepted value for the speed of sound in air at ordinary room temperature is about 340 m/s

Method 1: direct timing over a long distance

• Independent variable: the distance d between the source and the listener (m)
• Dependent variable: the time t taken for the sound to travel from source to listener (s)
• Control variables: the same location with no large reflecting walls nearby; the same person doing the clapping; the same person operating the stopwatch

Method:

1. On open, flat ground with no large walls behind either person, mark out a measured distance (start with about 80 m) between the signaller and the timekeeper
2. The signaller raises the two wooden blocks above their head and strikes them together to make a sharp click
3. The timekeeper starts the stopwatch the instant they see the blocks meet (light covers 80 m in well under a microsecond, so the sight is effectively immediate) and stops it the moment they hear the click
4. Record the time, then repeat the measurement at least three times at the same distance; take the mean
5. Move the signaller and timekeeper further apart and repeat the whole procedure for distances of around 110 m, 140 m, 170 m and 200 m

Analysis:

• Calculate the speed of sound from the well-known equation:

speed of sound = distance / mean time

• Compare each distance's result with the accepted value of 340 m/s

Sources of error:

• The main source of inaccuracy is human reaction time, which can be as large as 0.2 s. For sound covering 80 m the travel time is only about 0.24 s, so the reaction time is a serious fraction of the result. Lengthening the distance to 200 m more than triples the travel time and pushes the reaction error down to a smaller proportion of the total

Method 2: oscilloscope and two microphones

• This method removes the reaction-time error by letting the oscilloscope do the timing
• Independent variable: the distance between the two microphones (m)
• Dependent variable: the time delay between the two pulses on the oscilloscope screen (s)
• Control variables: the same two microphones, the same oscilloscope settings, the same clap-source position

Method:

1. Connect microphone 1 to channel 1 of the oscilloscope and microphone 2 to channel 2
2. Place the two microphones in a straight line, starting about 1.2 m apart on a level surface; measure the separation with the tape
3. Set the oscilloscope to trigger from channel 1 and adjust the time base until a single short clap can be seen on both channels with the second pulse a clear distance to the right of the first
4. Stand close to microphone 1 (so the time delay starts from a known position) and clap the wooden blocks once
5. Freeze the trace, then read off the time gap between the two pulses from the oscilloscope's time base
6. Repeat steps 3–5 a further two times at the same separation and take the mean
7. Increase the microphone separation to about 1.7, 2.2, 2.7 and 3.2 m and repeat

Analysis:

• For each separation, calculate the speed of sound from:

speed of sound = microphone separation / mean time delay

• Results should cluster around 340 m/s