Measuring the sound power of a moving source

The sound power of a moving source was measured with a vector sound-intensity probe, for the simple case of an axisymmetric source moving along a known straight path. The probe tracks the position of the source and, at the same time, measures the component of intensity normal to a hypothetical measurement cylinder enclosing the source, the axis of the cylinder lying along the path of the source. The sound power of the axisymmetric source is determined by integrating sound intensity over the surface of the cylinder. The contributions of the ends of the cylinder can be neglected if the cylinder is long enough. The measured sound power of the moving source was compared with the sound power of the source, (a) estimated from the applied voltage and (b) measured with the source stationary, and it was found to be greater than (a) by 14% and greater than (b) by 18% or 0.7 dB. The difference is believed to occur because the source is not completely axisymmetric due to reflections in the tank. The method can be applied to a nonaxisymmetric source using a circular array of sound-intensity probes instead of a single probe.