Keele has shown that the low-frequency response of a loudspeaker can be measured in a reverberant room if the microphone is placed in the nearfield of the driver. To predict the on-axis farfield pressures from nearfield measurements, Keele developed a simple nearfield-to-farfield transformation which neglects diffraction of the sound field by the loudspeaker cabinet. In this paper the low-frequency diffractive effects of a loudspeaker cabinet are investigated using the Helmholtz Integral Equation. It is found that the diffractive effects on the farfield on-axis pressures depend on the area of the front baffle and the average radius of the front baffle as seen by the driver. A new approximate nearfield-to-farfield transformation is derived which includes the effects of diffraction. The new transformation is found to be suitable for realistic loudspeakers having a conical driver and spherical dust cap.
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