The use of a high impedance acoustical load for the typeBdiaphone is capable of raising the efficiency of the fog horn system from an often encountered 0.2% to values of 10 to 16% for normal operating pressures. The high impedance is provided by a resonant exponential or catenoidal horn. To provide the high degree of frequency stability required by the highQresonant load the mechanical impedance of the diaphone piston is reduced to permit it to lock in with the horn frequency. The piston impedance is reduced by reducing its mass through the use of an aluminum alloy and by supporting it on a central shaft thus reducing (and stabilizing) the frictional forces. The stability thus achieved is such that the frequency varies only 3% for an air supply pressure change from 5 to 35 psi.