In a typical embodiment: the horn sections 12A, 13 and flange sections 19, 20 are preferably of good acoustic conducting material such as aluminum, titanium or magnesium; or alloys thereof such as Ti--6Al--4V titanium-aluminum alloy, 6061-T6 aluminum alloy, 7075 high strength aluminum alloy, AZ 61 magnesium alloy and the like; the discs 15, 16 are of lead-zirconate-titanate such as those manufactu