Besides the microphone, you should definitely take the measurements with a decibel meter. Your measurements may vary depending on the quality of the microphone. I recommend using a decibel meter to check the accuracy of this. The microphone and decibel meter should be at the same distance from the metal plate in each coordinate. The acoustics of the environment you are experimenting with is also important. If the room is small, sound waves hitting the wall and returning may affect your test results. When you make an article, what I wrote by the referees will come to you as a question. Good luck with.

Hello Silvia.

Yes, it will work. It's quite convenient also. Just keep microphone close to the surface, in so called very near field, approximately few millimeters above the surface. You may find this useful:

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Filip Pantelic

Your title: " How to measure vibration on a steel plate with a microphone? " and following text "the surface sound pressure" looks a little ambiguous. The vibration of the plate itself should be sensed by some very little acceleration sensors (piezo?) but the sound pressure, correctly, by a small microphone at small distance from the surface. And it should be performed in echoless environment.

No need for echoless environment when recording in very near field. Sound radiated from plate is locally dominant comparing to all sound reflections.

Filip has rightly introduced the notion of "near-field" which is important to understand the physics of structure-borne sound: "sound" is only the fraction of the air disturbance created by the plate vibration that propagate without attenuation far away - the so-called far-field (mathematically, a singular solution of the air equilibrium equation). When you get close to the plate, you measure also all the air disturbances that reorganize along the plate without creating sound (this defines the near-field), so you better speak of "dynamic pressure" or "total pressure", not sound! It will turn into sound at a distance which depends of the plate size, thickness and frequency but typically at a distance exceeding 10 wavelengths (20cm at 20kHz). Effectively the near-field levels dominate the sound so you don't care much of the ambient sound level (still the fixture of your mike shall remain thin to not create a local reflector). The good thing for you is that near-field is better spatially correlated to your plate vibration velocity distribution than sound, this validates physically your experimental approach! (There is no way to find accelerometers light enough to carry valid measurements at this frequency - the added weight is modifying the vibrational field - the ideal sensor would be a laser velocity-meter (or vibro-meter) but it is far more expensive and complex to operate).

Your proposed set-up is thus fine however under some conditions:

• For your experimentation, you have to set a miniature microphone as close at possible (say 1mm max as you want record up to 40kHz) to the plate as it will measure almost a copy of the plate radial vibration,
• Your mike fixture shall remain the least obtrusive you can,
• The microphone membrane must remain a fraction of the wavelength (at 40kHz, the wavelength is around 1cm, so your mike membrane diameter cannot exceed 2mm, otherwise it would already cancel some components by spatial averaging).
• You should effectively obtain a qualitative measure of the plate vibration distribution by mapping simply the RMS pressure you get, but it would be very difficult to express it in absolute units.

Hoping this helps!

Bernard

If your intention is to map vibrational modes---the maxima and nulls in the normal velocity of plate modes--- then the microphone will not work for you.

The microphone will measure the acoustic pressure, which, according to the equations of motion, will be proportional to the divergence of the air particle velocity vector. And, if the air moves with negligible friction across the face of the plate (as it does in linear acoustics), then the tangential component of the air particle velocity can be large relative to the normal velocity near the plate, which means that the very near-field pressure is not proportional to the local normal velocity of the plate.

You will no doubt measure interesting pressure patterns in your experiment, but it will be very difficult to relate your measurements of acoustic pressure to the distribution of vibrations in the modes of the plate (location of its maxima and minima of vibration). You might for instance have pressure maxima where plate normal velocity is in fact a minimum for some frequencies, but there is no simple heuristic or equation relating the two for the vibrating plate.

The connection between the pressure you measure and the vibration of the plate is determined very exactly by the Helmholtz Integral Equation (HIE), or approximately by the Kirchhoff approximation to it. See Morse and Ingard's "Acoustics". Or better, see Baker and Copson's fabulous classic "The mathematical formulation of Huygens' principle" -- where the acoustic pressure is the superposition of wavelets distributed over the plate surface, excited in this case by its normal velocity.

For the experiment, it's better to use some other sensor for directly sensing the plate motion, rather than a microphone sensing the acoustic pressure it imparts to the air.

Its easy :)

Just folow two papers:

Mechanical Systems and Signal Processing

Volume 40, Issue 1, October 2013, Pages 194-207

On acoustic very near field measurements

And

Applied Acoustics

Volume 186, 15 January 2022, 108486

Using sound in the very near field of vibrating plates for determination of their mechanical properties.

BR, Jurij

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