East Doncaster Secondary College – Unit 3 Physics
Measuring the speed of sound using a standing wave
Aim
To use a sound tube to create a standing wave.
To measure the distance between successive nodes at a set frequency and hence calculate
the speed of sound in air.
Apparatus
Sound tube – an air column closed at one end and with a speaker at the other end. The
reflected sound wave from the closed end superimposes with the incoming sound wave to
produce a standing wave in the tube.
Frequency signal generator, power supply, CRO, meter rule.
microphone
moveable rod
sound tube
speaker
power supply
CRO
signal generator
Method
 Set up the equipment as shown in the diagram below.
 Set the frequency signal generator to 2000 Hz.
 Estimate the absolute uncertainty of this frequency setting. Be careful of the scaling
factor.
 Move the microphone inside the sound tube to about 10cm from the speaker. Pull the rod
holding the microphone towards the fixed end of the tube whilst observing the signal on
the CRO screen. A node will appear as an amplitude minimum on the CRO screen.
 When the first minimum (node) is located record the position on the meter rule fixed
next to the tube. Estimate the absolute uncertainty of your measurement.
 Continue moving the microphone towards the fixed end locating and recording the
positions of at least 3 successive nodes (if possible).
 Repeat the above steps for two other frequencies: 3000 & 4000 Hz.
Analysis
 Determine the distance between successive nodes. The absolute uncertainty in the
distance is found by adding the absolute uncertainties of the two readings used in the
calculation.
 Calculate the average distance and average absolute uncertainty for each frequency.
Mr Bowen & Mr Lewis - 687315832
East Doncaster Secondary College – Unit 3 Physics
 Using your knowledge of the distance between adjacent nodes in a standing wave
determine the wavelength of the sound for each frequency.
 Calculate the speed of sound for each frequency using the wave equation v = f.
 The uncertainty in the speed of sound can be calculated using:
% in speed = % in freq. + % in wavelength
(% = the percentage uncertainty in a measured value)
(Note: % = absolute uncertainty  measured value x 100).
 Complete the following table.
Frequency
(Hz)
2000
3000
4000
 %
Wavelength
(m)
 %
Speed of sound
(m/s)
 %
 Calculate the average value and uncertainty for the speed of sound.
Questions
1. John and his prac. partner Mary, were discussing whether a point existed in the tube
where either a node or antinode would always be found. John felt that the fixed end of
the tube might be such a point. Using appropriate physics principles explain what you
would expect at the fixed end – use diagrams where appropriate.
2. Discuss the sources of uncertainty in this experiment and how they could be minimised.
Conclusion (write a conclusion for the prac)
Mr Bowen & Mr Lewis - 687315832