The observations and results posted on this web site have been achieved using a small mass on a flat vibrating spring.
Although giving good and constant results this method does not allow continuous tuning and by necessity has to be made in small increments.

In the following method the vibrating mass is replaced by the cone of a loud speaker, the speaker being driven by a variable frequency audio tone generator.
A piece of stiff card is glued to the centre cone of the speaker and this supports a razor blade which serves as a knife edge.
The knife edge interrupts a laser beam to a photo electric cell, the output from this cell is then viewed on an oscilloscope.
The output of the tone generator should be set so that the output of the speaker is at a minimum and only just audible, the position of the photo cell and laser is adjusted to give a good shaped sine wave signal.
The frequency of the tone generator can now be swept over the frequency range say from 20-100 Hz. and a plot made of the response curve viewed on the oscilloscope.
The setup should be securely fixed, as in the manner of an optical bench.

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