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Acoustic standing waves: results

Acoustic standing waves results graph

Once you have finished logging select an XY graph to give a graph of voltage (distance) against sound intensity. Use the readings that you took at the setup stage to scale your voltage readings to distance. The graph should look something like pictured, the nodes and anti–nodes are clearly visible.

Download the PicoLog for Windows file (.plw).

Answers to questions

If the frequency of the source is increased then the wavelength is decreased; the distance between the nodes would decrease and vice versa.

The distance between adjacent nodes (or antinodes) in a standing wave is equal to half a wavelength; so the wavelength is double the distance between two successive nodes (or antinodes).

The intensity decreases because the sound dissipates and loses energy as it travels through air — this is called attenuation.
The ‘depth’ or ‘quietness’ of the nodes is dependent on the intensity of the reflected and transmitted wave, these are more closely matched nearer the reflector, giving a greater ‘cancellation’.

If the reflector were removed then the standing waves would disappear because there would be no more interference; the graph would simply be how sound intensity decays with distance - is this attenuation frequency dependent? Try it!

Credits, comments and further info

Experiment devised by Paul Fernihough at Kingswood School, Bath.

Please contact us if you have any comments about this experiment or suggestions for improvements.