The use of a high impedance acoustical load for the type B diaphone is capable of raising the efficiency of the fog horn system 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 highQresonant loads the mechanical impedance of the diaphone is reduced to permit it to lock in with the horn frequency. The reduction of the mechanical impedance is achieved by reducing the mass of the piston through the use of aluminum and reducing and stabilizing the frictional forces by using a central shaft for supporting the piston instead of allowing it to float in the liner. The resulting frequency stability is such that the efficiency under field conditions equals that obtained in the laboratory. An incidental benefit arises from the fact that the horns are smaller and made of aluminum reducing the weight by a factor of six.