Determine the velocity of sound in air by measuring the resonant frequencies of an open tube.
Measure the signal distribution in a closed tube, calculate the wavelength and speed of sound from this data.
Compare your results for the speed of sound in air with the results of students working on other experiments (Speed of Ultrasound and Lloyd's Mirror).
Many musical instruments (e.g. organ pipe, trumpet) use vibrations
in a column of air to produce musical tones. These vibrations
resonate (are loudest) for certain frequencies which depend mainly
on the length of the tube containing the column of air. The relative
strength and frequencies of the resonant oscillations differ between
instruments, so that a penny whistle does not sound the same as
trombone. In this experiment a resonance tube is excited at an
open end by a loudspeaker.
Standing waves are set up in the tube when the because the sound
is reflected at both open and closed ends. In figure 1 the displacement
amplitude distribution for resonance states are shown. (Note:
by convention, an open tube has two open ends and a closed tube
has one open end).
The region near an open end is a pressure node (minimum pressure amplitude) and a displacement antinode (maximum displacement amplitude). Conversely a closed end is a displacement node (minimum displacement amplitude) and a pressure antinode (maximum pressure amplitude). The microphone detects the pressure of the sound wave rather than the displacement.
The resonance condition for an open tube of length L and diameter d is:
where n = 1, 2, 3, 4.... and is the wavelength of the resonant sound wave. The tube is effectively lengthened by the end correction (0.4d) for each open end. The resonance condition for a closed tube of length L and diameter d is:
where n = 1, 3, 5, 7.... and is the wavelength of the resonant sound wave.
Resonance Tube (eg perspex tube, about 75 cm long and about 5 cm internal diameter), loudspeaker with similar diameter (0.1 W, higher impedance better for typical oscillator), oscillator (preferably matching loudspeaker power rating/impedance or low impedance audio amplifier may be necessary), tie clip microphone/preamp, oscilloscope, thermometer, two test leads (BNC - 4 mm). The schematic experimental set up is as shown in figure 2.
Setting up the apparatus
Part 1: Open Tube
Part 2: Closed Tube
How do your results for the speed of sound in air compare with
the results of students working on other experiments (Speed of Ultrasound and Lloyd's
© Mark Davison, 1997, give feedback or ask questions about this experiment.Back to the experiment menu