Expressions and tables are presented for computing the shallow-water transmission loss as a function of bottom type, sea state, frequency. water depth, and range. A table of probable errors is also presented as a function of frequency and range.

An expression is presented for sound absorption in sea water as a function of frequency, temperature, salinity, and pressure. It is based on available data reported in the literature and new data taken in the North Atlantic Ocean in the frequency range 2–25 kcps.

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Receivod 25 Fohr]]nry 1963)The change of sound absorption with temperature has less effect on accuracy than do circuit-temperature changes and conventional readout systems.HAT the variation of the absorption coefficient with temperature will change the turn-on time of the triggering circuit in sing-around velocimeters •." has been pointed out by Buchanan s of the U.S. Naval Research Laboratory. Oscilloscope measurements made at the U.S. Naval Ordnance Test Station (NOTS) • at the collector Q3 agree with published absorption data. 4 The 3.5-Mc/sec pulses are attenuated 0.84 dB as the temperature is decreased from 20 ø to 0øC over the 24.7-cm pathlength used in the NOTS sound velocity, temperature, pressure (SVTP) instruments.•. 0 At the nominal reflector setting, a 6-dB attenuation will lower the first significant pulse to just below the triggering level. Therefore, a 0.84-dB amplitude reduction will increase the electrical turn-on time by only 2.7 nsec. This is an order of magnitude less than the 20-nsec increase in electrical time delay found by Lord 7 of Hudson Laboratories, Columbia University, as the circuit temperature of an ACF-TR2E instrument was reduced from 26 ø to 8øC. However, the value found by Lord is greater than the 10-nsec difference in a commercial modification 8 of the NOTS SVTP instrument over the temperature range of 25 ø to 0øC. Two different circuit modules exhibited the same shift. The smaller difference may be attributable to the addition of negative feedback between amplifiers Q• and Qa, which also reduces both the gain and circuit time delays.Observed pulse-amplitude changes at Q, caused by circuit temperature variations are less than those due to changes in absorption.Both types of temperature-related systematic errors may be calibrated out of the final data tabulations. As the error due to changes in absorption is less than the random error of an E/UTX 10 electronic counter, greater benefit would be derivable from a new circuit design using silicon transistors to reduce or eliminate temperature-related circuit delays than would result from a change in transducers. •Jj 3 coo cPS• PH,Fro. 1. Block diagram of readout system to retain accuracy and response of sea-going velocimeter.The extremely fast response of the sing-around velocimeter makes it an ideal tool for measuring sound-velocity microstructure in the oceans, the major limitation usually being imposed by the readout system. If absolute accuracy is not required, a discriminator X-Y recorder system is limited only by the intelligence frequency of the telemetry channel and the response of the electromechanical pen linkage. An E/UTX10 electronic counter will give the required accuracy, but integrates the signal over a 10-sec period, which is intolerable in anything but homogeneous water.