Musical frequencies shown on a C.R.O.

Class practical

To show the similarities between S.H.M. and other oscillating systems.

Apparatus and materials

For each pair of students

Microphone

Tuning fork

Small rubber hammer or large rubber bung

For a several related optional demonstrations

Power supply, low voltage, AC

Bicycle dynamo

Simple AC generator

See the experiment The motor as a dynamo.

Health & Safety and Technical notes

Students unfamiliar with oscilloscopes may need to refer to the Apparatus note Oscilloscope.

Procedure

a Connect the microphone to the input terminals of the oscilloscope. Set the oscilloscope to maximum gain. Set the time-base to a middle value.

b Hit the tuning fork gently with a small rubber hammer or strike it on a large rubber bung. [Striking the tuning fork on the bench will damage both the bench and the tuning fork.]

c Touch the base of the vibrating tuning fork to the microphone case. Adjust the time-base control to give a good display.

d Try singing and whistling across the microphone.

Teaching notes

1 Students should previously have seen the time trace produced by an oscillating pendulum in the experiment Broomstick pendulum, sinusoidal motion.

Students may be able to conclude, from the fact that the waveform in step c is a sine wave, that the oscillation of the tuning fork is a S.H.M. However, there are two intervening processes: first the sound wave is converted into an equivalent electrical signal, then that signal is amplified and displayed on the oscilloscope screen. Those who find this analysis difficult to follow may prefer a direct demonstration of the tuning fork motion, in the experiment Vibrating tuning fork and rotating mirror

Singing and whistling are likely to produce harmonics, so the waveform will not be so 'clean', though there will be discernable main frequency.

2 You could go on to show that several devices produce a similar trace on the oscilloscope: the alternating voltage from a transformer (AC power supply), a bicycle generator, and simple AC generator. In each case, the driver follows the pattern of a S.H.M.