UC San Diego Library – Scripps Digital Collection

Crangon (Alpheus) and Synalpheus are the two principal genera of noise-producing shrimp. These animals (not to be confused with commercial shrimp, which are noiseless) are about 3/4 inch to 1-1/2 inches long. They have one enlarged claw which produces a vigorous snap when closed. Over a large colony there is a continuous succession of snaps which causes an intense crackling noise resembling the burning of dry twigs. With increasing distance from the shrimp bed, the crackle merges into a sizzle or a hiss.

UCDWR No. U146. A relationship has been established between crackling ambientnoise a~d the habits and distribution of snapping shrimp* in Americanwaters, and unusually high noise levels at the higher frequencies canbe expected in the areas where these animals are known to .live. There- ·fore, it is considered important to know their geographic distributionin the inaccessible areas of the Pacific ln order that the generalsound conditions in coastal waters and areas of shoal water can beantlcipated as an aid in the use of Sonar cqutpm.ent.These an~~s live under rocks and shells on the bottom, or incoral. Evon when bottoo. condi tiona are favorable, they do not oftenlive at depths greater than 30 fathons. The report covers a survey or ·all biological literature dealing with the distribution of the noiseproducinggenera Crangon (Alphous) and Synalpheus of the Pacific Ocean,the east and west coasts of North .America and sone contiguous areas.Three charts are presented giving the geographic distribution. Theseindlcate that the animals occur virtuelly throughout the whole tropicaland subtropical areas of the Pacific with the northern and souhhernmargins delimited approximately by the 52~ surface winter isotherm.In examining the charts, it nust be renembered that the absence of apositive indication nny menn only· that no observation has been made.Tn.e noise-producing anll:lals undoubtedly have a wide . distribution withinthe limits indicated. Within this area, their distribution is probablylimited only by unfavorable water depths and bottom characters. Referenceis :oade to recent survey work which verifies that serious shrimp crackledoes occur in the central and southwest Pacific areas.

UCDWR No. U100. This investigation was undertaken to obtain information onthe northward extension of crackling underwater noises characteristicof the coastal waters of Southern California and for the purpose ofobserving if there were any seasonal changes in background noises inthe Puget Sound area which had been visited previously in November1942. The regions visited include Santa Barbar.a, San Luis Obispo,Monterey and Eureka to Crescent City in California, and AdmiraltyInlet and the San Juan Archipelago in Puget Sound.The characteristic crackling noises produced by snappingshrimps were found only at Santa Barbara, San Luis Obispo, andMonterey. In the northern California and Puget Sound areas wherethese animals have not been reported the waters were relatively freefrom biological noises, only very weak crackling and piping soundswere detected. There was no noticeable seasonal change .in noiseconditions in the Puget Sound area.

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.

This "Bibliography of Publications" includes general background information on UCDWR, followed by the Completion Report Covering Operations of the University of California Division of War Research at the U.S. Navy Electronics Laboratory.  Under Contract Nobs-2074 from 26 April 1941 to 30 June 1946, n.d.