The influence of roughness, angle, range, and transducer type on the echo signal from planar interfaces

The received electrical echo signal from a pulse-echo system insonifying a planar interface was measured for varying degrees of rms roughness [0 to 0.29 mm (0 to 1.7 lambda)], angles of incidence, theta, (-7 degrees to 7 degrees), and ranges to a planar or focused transducer. The effect of varying theta is quantified in terms of the energy of the received signal, E(theta), and the normalized spectrum of the received signal. E(theta) is approximately Gaussian when using a planar transducer or a focused transducer with the reflecting interface located at or beyond the focal point. For focused transducers with the interface located closer than the geometrical point of focus, two maxima can sometimes be observed when varying the incident angle. As is generally known, the width of E(theta) is strongly dependent on transducer type, e.g., for a smooth interface, the -3 dB width for a 25.4 mm diameter 5-MHz planar and focused transducer was approximately 0.5 degree and 4 degrees (at the focal point), respectively. E(0 degree) as a function of surface roughness, Rq, was nearly linear on a decibel scale, with a slope of -109 dB/(Rq/lambda) and -61 dB/(Rq/lambda) for planar and focused transducers, respectively. The characteristic nulls present in the normalized spectra of the echo signal at non-normal incidence tend to vanish with increasing Rq when using planar transducers. For focused transducers, the normalized spectra change from relatively flat to monotonically decreasing as Rq increases, and they exhibit reduced amplitude with increased incident angle.