Determining the Speed of Sound in Air

Equation linking wave speed, frequency and wavelength

v = fλ; Speed (ms⁻¹) = Frequency (Hz) x Wavelength (m)

Measuring frequency from oscilloscope

The time between two identical points on adjacent waves can be measured from the time-axis. This is the time period of the signal. The frequency is given by the inverse of the time period.

Traces displayed on oscilloscope

Both the signal sent to the loudspeaker and the signal taken in by the microphone should be displayed on the oscilloscope screen.

Effect of distance on traces

The two traces will move past each other and the phase between the traces will change.

Apparatus connected to loudspeaker

A signal generator should be connected to the loudspeaker. This generator should also be connected to the oscilloscope so that its signal trace is displayed.

Positioning traces on oscilloscope

The spacing of the two signals inputted into the oscilloscope should be changed so that one is directly above the other. A peak of the lower trace should be inline with a trough of the upper trace.

Setting initial separation of speaker and microphone

The separation should be adjusted so that a trough on the upper trace touches the peak of the lower trace. This distance should then be measured using a metre ruler.

Determining wavelength by moving speaker

The speaker should be moved away from the microphone until the trace has moved to a point where the peak of the lower trace once again touches a trough of the upper trace. The distance moved to achieve this is the wavelength.

Uncertainties in the experiment

When determining the frequency, there is an uncertainty produced by the resolution of the oscilloscope scale. The traces also have a thickness and so there will be an uncertainty in positioning the speaker exactly at a complete cycle each time.

Choosing frequency values for experiment

The frequency values need to be chosen so that the waves have a wavelength easily measurable to a good resolution with a metre ruler. Frequencies around 4kHz are suitable choices.

Setting scales on oscilloscope

The scales should be set so that each trace displays around 3 complete wave cycles on the screen.

Significance of traces moving past each other

One complete cycle on the oscilloscope represents the wave's time period. The distance that the wave travels in this time is the wavelength.

Advantage of measuring frequency from oscilloscope

A greater resolution will be obtained by measuring from the oscilloscope.