DEMONSTRATION TOPICS

An oscilloscope displays how voltage at its input changes in time. Sound travels as longitudinal pressure waves. As a sound passes, a microphone converts the variations in pressure to variations in voltage which can be displayed on the oscilloscope.

A computer interfaced to a microscope can be used as an oscilloscope to capture sound waves from musical instruments. The computer can analyse the sound waves to extract the frequencies present using a "Fast Fourier Transform" program. Different instruments have different sets of "Harmonics". For example, brass instruments generate even multiples of the lowest or "fundamental" frequency. Two sounds at slightly different frequencies (i.e. two slightly different notes) result in a sound wave that varies in amplitude (beats) with a frequency equal to the difference of the two frequencies.

The speed of sound can be measured using a source of sound that turns off and on periodically (a beeper) and a microphone attached to an oscilloscope that displays the sound after it has travelled a known distance. This picture shows a parabolic reflector behind a beeper along with a square-wave generator used to turn the beeper on and off.

This picture shows the parabolic reflector behind an electret microphone and a battery powered operational amplifier circuit used to detect the sound wave after it has travelled about 4 metres.

This oscilloscope displays the square wave used to generate the beeps and the signal obtained by the microphone. The time delay from the rise in the voltage to the beeper and the arrival of the sound at the microphone (about 12 ms) is the time taken by the sound to travel about 4 metres.