directivity factor

6.66  directivity factor. (a) Of an electroacoustic transducer for sound emission, at a specified frequency, ratio of the time-mean-square free-field sound pressure at a fixed point on the principal axis, to the time-mean-square sound pressure over the surface of a sphere concentric with the effective center of the transducer and passing through the fixed point. (b) Of an electroacoustic transducer for sound reception, at a specified frequency, ratio of the time-mean-square free-field sensitivity to sound waves that arrive along the principal axis, to the time-mean-square sensitivity to a succession of sound waves that arrive at the transducer with equal probability from all directions.