Musical frequencies shown on a C.R.O.
Practical Activity for 14-16
To show the similarities between S.H.M. and other oscillating systems.
Apparatus and Materials
For each pair of students
- 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:
- Connect the microphone to the input terminals of the oscilloscope. Set the oscilloscope to maximum gain. Set the time-base to a middle value.
- 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.]
- Touch the base of the vibrating tuning fork to the microphone case. Adjust the time-base control to give a good display.
- Try singing and whistling across the microphone.
- Students should previously have seen the time trace produced by an oscillating pendulum in the experiment:
- Students may be able to conclude, from the fact that the waveform in step 3 s 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:
- Singing and whistling are likely to produce harmonics, so the waveform will not be so
clean, though there will be discernable main frequency.
- 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.