New type of an acoustic plate mode: quasi-longitudinal normal wave.

A new type of an acoustic wave belonging to the family of the normal plate modes is discovered numerically and verified experimentally in ST, x-quartz plates with free surfaces. It exists within restricted bands of the normalized plate thickness H/lambda, where H is the thickness, lambda is the wavelength, separated by forbidden H/lambda-bands attributed to the common plate modes. Polarization of the wave over the whole plate thickness is almost longitudinal. Velocity of the wave is close to or equal to the velocity of the longitudinal bulk wave, propagating in the same direction. The proximity or/and equality of the velocities is a necessary, but not sufficient condition of the existence of the new wave.