The directivity of gradient-type loudspeakers depends upon the difference in phase, or powers of the difference in phase, of the generated sound pressures between two or more points in space. The directivity of the conventional wave-type loudspeakers depends in some manner upon wave interference of the sound emanating from the elements of the radiating surface. To obtain any semblance of directivity from the wave-type loudspeaker, the radiating surface must be comparable to the wavelength. Therefore, the dimensions of the wave-type loudspeakers become relatively large in the low audio frequency range. In contrast, gradient-type loudspeakers are small compared to the wavelength. For sound reproduction requiring directional loudspeakers, the gradient-type loudspeaker provides the features of relatively small size and uniform directivity with respect to frequency.
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