| Definition: | a) of an electroacoustic transducer for sound emission, at a specified frequency, ratio of the square of the free-field sound pressure at a fixed point on the principal axis, to the mean-square sound pressure over the surface of a sphere concentric with the effective acoustic centre of the transducer, and passing through the fixed point
b) of an electroacoustic transducer for sound reception, at a specified frequency, ratio of the square of the free-field sensitivity to sound waves that arrive along the principal axis, to the mean-square sensitivity to a succession of sound waves that arrive at the transducer with equal probability from all directions
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