Acoustic Impedance Matching by Means of Screens

The acoustic impedance of a transducer may be matched to that of the surrounding medium by means of a metal screen. When operating in air, transmission increases of nine decibels have been obtained by placing correctly selected and positioned screens in front of a crystal transducer. The device narrows the response curve of a sharply resonant transducer only slightly, while the directional characteristics are not measurably altered. The situation is somewhat analogous to the non-reflection coating of optics. For purposes of analysis, a more useful analogue is that of a transmission line containing discontinuity. Experiments with woven and perforated screens of different geometric form have been performed and the results evaluated.