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 /spl lambda/)], angles of incidence, /spl theta/, (-7/spl deg/ to 7/spl deg/), and ranges to a planar or focused transducer. The effect of varying a is quantified in terms of the energy of the received signal, E(/spl theta/), and the normalized spectrum of the received signal. E(/spl 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(/spl 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/spl deg/ and 4/spl deg/ (at the focal point), respectively. E(0/spl deg/) as a function of surface roughness, R/sub q/, was nearly linear on a decibel scale, with a slope of -109 dB/(R/sub q///spl lambda/) and -61 dB/(Rq//spl 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 R/sub q/ when using planar transducers. For focused transducers, the normalized spectra change from relatively flat to monotonically decreasing as R/sub q/ increases, and they exhibit reduced amplitude with increased incident angle.