The directivity index which is currently used for characterizing the directional properties of transducers, refers primarily to their ability to radiate sound power. When transducers are used in echo ranging, the directivity index is, theoretically at least, of minor interest. Of greater interest is the ability of the transducer to discriminate between the echo from a target at which it is pointed, and the reverberation returned to it from this and other directions. This discrimination is measured by other quantities, called reverberation indices. One of these concerns volume, the other surface or bottom reverberation. The purpose of the present work was to study the relations between the three indices.

Conclusions drawn from a study of typical projector patterns are as follows:

1. The volume reverberation index and the surface reverberation index of a projector are linearly related to the directivity index, provided that the directivity pattern is reasonably similar to that of a circular piston in an infinite baffle. This condition is found in the echo-ranging projectors studied when they are operated at 24 kc without domes. However, the directivity index does not provide a reliable measure of the reverberation indices when the projector pattern has abnormally strong side lobes.

2. Neither projector housing studied has appreciable effect on reverberation indices.

3. The echo: reverberation ratio depends almost entirely on the shape of the main lobe of the composite directivity pattern between zero and -6 db. As a result, the reverberation indices of a transceiver can be determined by measuring the width of its directivity pattern at -6 db. Half of this angle will be termed the half-width of the pattern.

4. Since the reverberation indices can be so readily calculated from the half-width, it is recommended that this quantity be specified in describing a transducer. The directivity index usually, but not always, can be calculated from the half-width to within 3 db.