The Speed of Sound Using Standing Waves and a Cathode Ray Oscilloscope


Two Unilab audio-frequency generators, cathode ray oscilloscope, wires, microphone, metre rule, drawing board, loudspeaker, clamp stands, frequency meter (set on 20k scale), thermometer.



1. Connect the loudspeaker to the loudspeaker (LS) terminals of one of the signal generators.

Check that the ‘input switch’ is in the up position. Switch on the signal generator, and set it so that the frequency meter reads about 2 kHz.

2. Place the microphone in front of the loudspeaker. Connect the microphone to the other signal generator as shown in the diagram. With its ‘input switch’ in the down position it will amplify the signal from the microphone. Connect the output of this amplifier to the input of the CRO.

3. Place the loudspeaker on the laboratory bench so that it faces a drawing board which has been clamped vertically so that it will reflect sound back to the loudspeaker. Adjust the settings of both signal generator & CRO until a clear trace is obtained on the CRO’s screen.

4. Move the microphone along a line between the loudspeaker and board. Due to the setting up of standing waves the amplitude of the trace should vary in size. In order to increase the amplitude variation, try adjusting the positions of the loudspeaker and board. Once you have achieved the best effect, do not adjust any further.

5. Measure the distance, d moved by the microphone between adjacent amplitude minima.

Do this by moving through a number of inter-nodal intervals,Measure the total distance moved,and then dividebyin order to obtain the inter-nodal distanceNote also the frequency,of the signal generator, as indicated by the frequency meter.

6. Repeat stage 5 for four other frequencies between 1.0 and 4.0 kHz.

7. Measure the temperature.

8. Tabulate your results under headings:

frequency (/Hz); distance measured (/m); number of inter-nodal intervals (); internodal distance (/m) & the reciprocal of the internodal distance (1//m -1 )

9. Plot a graph of 1/ / m -1 against/Hz. It should be a straight line. Measure its gradient

10. Calculate the speed of sound using the relationship

11. Compare your calculated speed with that expected from the formula:
speed (m/s) = 331 x (temperature in kelvin / 273)

12. Explain why

Remember: wavelength = 2 x internodal distance & wave speed = frequency x wavelength