Propagation of finite‐amplitude ultrasonic waves in air—I. Spherically diverging waves in the free field

An investigation of the free‐field propagation in air of sinusoidal spherical waves of finite amplitude is described. It includes experiments made at 20.4 kHz and a theoretical interpretation using weak shock theory. The acoustic source used was a new high‐power and highly directional ultrasonic transducer which is able to generate a nearly pure sinusoidal wave at sound pressure levels over 160 dB. The experiments were done in an anechoic chamber, using a cw mode. They consisted essentially of measuring the amplitude of the fundamental component of the wave and its first three harmonics for different source levels and at various distances along the axis of the source. In addition, oscillograms and directivity patterns were recorded. In general, the experimental data confirm the predictions of the theoretical model.