Radiated sound and the geometric nearfield size

The sound field radiating from a finite-sized body has two distinct regions of acoustic pressure variation. The geometric farfield is the region far from a finite-sized radiator, where the sound pressure level increases at a rate of 6 d with every halving of distance from the body (spherical spreading). The geometric nearfield is the region close to the radiator where the 6 dB relation does not hold. The nearfield size depends on the frequency and the radiator size. In most cases, the geometric nearfield is small enough that the measurement point is in the farfield. When the radiation surface is large and the frequency high, the geometric nearfield is large and a measurement position may be within the nearfield. Because of the nearfield pressure characteristics, nearfield pressure measurements should not be extrapolated to the farfield. This paper contains a discussion of the different estimates of the nearfield size of coherent radiators and, through examples of pressure measurements illustrating the nearfields of variously-shaped radiators, shows that a simple equation may be used to calculate the on-axis, nearfield size of a coherent radiator.