Direct measurement of ultrasonic velocity of thin elastic layers

This letter reports a simple new technique for the ultrasonic velocity measurement on thin elastic layers of fractional wavelength thickness using broadband transducers. An external trigger to the oscilloscope with continuously variable frequency and an intensity enhancing strobe signal allow direct measurement of the wavefront time shift down to 13 ns. The technique makes it possible to measure the sound velocity of thin layers with the thickness down to 10^-2λ. Using a pair of 2.25-MHz transducers, we have accurately measured the sound velocity of a 26-μm-thick aluminum foil.