The resonant frequency ought to be a function of frequency, and voids should strongly affect Q. Tuning a tight beam to the desired resonance in the wall should be a good CW indicator of quality. A pair of frequencies, one above and the other below resonance might even indicate thickness.
Jerry
This is principally true, but the realisation with air ultrasound requires a relatively large amount of work. It is necessary to work with
1-2 MHz and a very weak signal. All this is not simple, but your assumption is basically correct....
Fah! Brain now engaged. The resonant frequency ought to be a function of thickness.
Jerry
Thanks for the basis in fact. It trumps conjecture.
Jerry
Yes, you must only know the sound speed in material. But this is something, that can be obtained with other realistic means. Additionally you must know, that 99.9% of sound will be reflected and you must use remaining energy. This is not simple, but realistic.
The aerospace industry regularly use ultrasonic thickness measurement using water as a contact medium - and have done for years. It is a mature technology already and certainly not a new concept.
For example: Composite parts (B777 rudders, A320 wheel doors, etc, etc) are made from a mix of nomex and aluminium honeycomb sandwiched between sheets of prepregnated carbon fibre.
To test for potentially nasty voids in the carbon fibre sandwich, the parts are laid on a moving table. Two water jets fixed to a scanning head are sprayed at the part - one from below, one from above. Ultrasonic sensors mounted in the scanning heads use the water stream as the medium and produce an "ultrasound" image of the completed part - not that dissimilar to a medical ultrasound.
These units are manufactured in the USA, but I don't remember who by. They are ridiculously accurate (and have to be, I suppose) and are not cheap. You could try googling "ultrasonic ndt" or something similar.
HTH, Cameron:-)
We are producing such systems since many years, but in some cases it is good or even necessary to use contactless systems. This is a trend, that will surely bring (in some years) systems, that will replace many existing systems, using water.
After contactless systems will work perfectly, it is to expect, that they will replace systems using water almost completely, because they are simply more convenient and cheaper in use.
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