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Let say I have a $500-1000 \ \mathrm{microns}$ thick glass substrate.

How can I experimentally measure the longitudinal and transverse wave speed in this substrate?

I can either attach a piezo disk to one side of the glass, send a pulse and detect the echo and work out the speed or I can set up a standing wave in the material. The fundamental frequency will occur at $1/2$ of a wavelength (the length of your substrate), which should correspond to the first resonance. $v = \mathrm{frequency} \ \times \ \mathrm{wavelength}$ will also give me the velocity in the material.

I think both of the above-mentioned methods will give me the longitudinal sound speed. Any help would be appreciated.

Amir Mokhtare
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1 Answers1

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The approach you are describing will (I think) lead to only one of the wave speeds. The piezo disk expands uniformly over (I assume) a large surface area of the glass. This approach would lead to a nearly-planar compressional wave into the glass, and a nearly-planar compressional wave would return. There would be some shear components near the edges, but these would be negligible.

You need two independent source conditions to get the two wave speeds. To get the shear wave speed you will need to shear the glass at one surface. I know there are ways to do that, but I am not intimately familiar with them.

I am curious why you are trying to get the two wave speeds in such a thin substance. I think the dominant wave motion would be as a plate, which couples the compressional and shear motions together intrinsically. If you are interested in plate motion, you could measure that without too much difficulty in a single measurement; either of the approaches you mentioned would be fine.

Michael M
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