Signaling along elastic plates with wideband acoustic pulses

Abstract

High data-rate signaling with acoustic pulses along an elastic plate is made difficult by the conversion of a transmitted pulse into many component pulses that arrive separately during a time span that can exceed the travel time of the first component pulse to arrive. In addition, the amplitude of the received signals fluctuates widely with changes in pulse center frequency. But to obtain high data-rate signaling, many pulses with different center frequencies must be transmitted at closely spaced time intervals. It is important in these circumstances to understand the principal features of the component pulses that originate from a single transmitted pulse. This paper derives analytical expressions for the arrival times, the peak envelope amplitudes, and the center frequencies of the arriving component pulses that are transmitted by wideband line forces acting normal to the plate surface. Each component pulse is the manifestation of a different mode of propagation. By carefully picking the bandwidth and center frequency of the launching pulse, the number of its component pulses (or, alternatively, excited modes) that have significant amplitude can be minimized, if the pulse center frequency is low enough. If the frequency is high enough, so that more than about six propagating modes can exist, then the number of arrivals cannot be reduced significantly. At higher frequencies, several of the modes merge to form the Rayleigh wave, which is centered within both earlier and later modes. A series of photographs show how transmitted Gaussian pulses look when the pulse center frequency is picked to illustrate three regimes; where the Rayleigh wave is not formed; where it is partially formed; and where it is fully formed.