Improved performance of tonpilz-type transducer elements can result from (1) increasing the area of the radiating face and (2) decreasing the mass of the radiating head. However, such improvement is limited by the introduction of a flexure mode, with the outer ring moving at a different velocity and phase from the inner area of the head mass. The undesirable result is a bi-modal response with, generally, a reduction in output of the design frequency. In order to minimize this effect, the head-mass should have both low density and high stiffness. The design "figure-of-merit" for head mass material is the specific modulus. It is shown that beryllium has the highest specific modulii of metals, being more than five times the specific modulii of such common head-mass materials as stainless steel and aluminum. A simple program is proposed to compare the performance of tonpilz elements with headmasses of beryllium, stainless steel, and aluminum.
