Non-Destructive Testing By UT

Actually you need several wavelengths to be easily detectable. I do not know what "defect invisibility" is. Sometimes its not a defect but just different medium the wave reflects from. Remember a wave reflects according to acoustic impedance mismatch.

Re: "is the law below correct"... It is not a law, but rather, a formula / guideline that we adhere to. It reminds us that the maximum wavelength that we should use for any given interrogation, should be no longer than two times the diameter of the critical flaw.

Thus, knowing what the smallest flaw that you must detect is, allows you to determine the optimum frequency to use, for any given material, in order to assure detectability, while avoiding too much "grass", or "trash" on the CRT screen (or other type of display), which will result from using too high of a frequency, and thus seeing echoes from itty-bitty tiny stuff with which you are not concerned.

Velocity = Frequency X Wavelength, so

Wavelength = Velocity / Frequency, and

Frequency = Velocity / Wavelength

These things, above, irrespective of any defects.

Once (either) Code or Specification gives you a critical defect size, THEN you employ the other formula, to determine your optimal frequency for that interrogation.

Hope that "clarifies things".

Cheers, and Best Wishes for your success!