LECTURE 5 
A SING-AROUND VELOCIMETER FOR MEASURING THE SPEED 
OF SOUND IN THE SEA 
M. Greenspan and C.E. Tschiegg 
National Bureau of Standards 
Washington, D.C., U.S.A. 
5.1. INTRODUCTION 
The so-called sing-around velocimeter is by now a well-established instru- 
ment used mostly for the in situ measurement of the speed of sound in the sea 
and other natural waters. It has been applied to a lesser extent to measurement 
problems in the laboratory; there is reasonto believe that with suitable modifica - 
tions the laboratory use of the instrument could be greatly extended. 
The instrument is automatic, has fast response, and is easily adapted to 
recording. The models with which we are here concerned are restricted to use 
with liquids which show no appreciable frequency dispersion except possibly at 
very high frequencies. They must be designed, adjusted, and calibrated for a 
particular class of liquids within which the total variation of the speed and at- 
tenuation of sound is not too large. These changes are commonly caused by 
changes in temperature, pressure, or composition. The velocimeters have high 
stability and are therefore especially adapted to differential measurements, such 
as that of sound—speed gradients in the sea. Other examples of differential 
measurement are the determination of the effect of dissolved air on the speed of 
sound in water [1] and the measurement of the temperature coefficient of the 
speed of sound in water near the turning point (approximately 74°C) [2]. 
5.2. GENERAL CONSIDERATIONS 
A sing-around velocimeter is outwardly similar to the ultrasonic delay line 
employed in digital computers for information storage. It may be thought of as 
a cylindrical tank the ends of which are electroacoustic transducers, and the 
whole filled with the liquid under test. A voltage pulse is applied to the "sender," 
and a corresponding pulse of sound travels through the sample liquid and is 
received and converted to an electrical pulse by the receiver. In order to define 
uniquely the time interval between the pulses and to specify their location in 
time, some characteristic must be selected that will still be recognizable after 
the pulse has been distorted by transmission through the liquid and by the 
bandwidth limitations of the transducers. In the present instrument, the pulse 
position is specified by the instant at which it begins to rise from the noise. 
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