AN INSTRUMENT FOR THE DIRECT MEASUREMENT 
OF THE SPEED OF SOUND IN THE OCEAN 
by F, J. SUELLENTROP, A.E. BROWN, and ERIC RULE 
Lockheed Missiles and Space Company 
Palo Alto, California 
ABSTRACT 
An instrument has been developed which 
operates on the same principle as the 
National Bureau of Standards velocimeter, 
but in which a considerable reduction in 
size and circuit complexity has been 
achieved. This reduction in size has been 
made possible by selecting electronic com- 
ponents which are insensitive to extreme 
pressures (up to 20,000 psi), so that the 
pressure housings of previous instruments 
have been eliminated. Results of cali- 
bration and stability tests on the instru- 
ment are given. 
INTRODUCTION 
The need for a simple and direct 
method of measuring the speed of sound in 
the ocean has recently been stressed in 
the literature.+ At present, the speed 
of sound is usually derived indirectly by 
considering the speed to be a function of 
the temperature, pressure and salinity in 
the ocean. Knowing or measuring the 
values of these parameters, the speed of 
sound af obtained using any one of a num- 
ver@2394 of empirically-derived formulae 
which contain up to six terms. This pro- 
cess involves considerable computation , 
and controversy exists as to which formula 
is most suitable.- 
Where the need exists to establish 
speed-of-sound profiles rapidly or to 
monitor continuously the speed of sound, 
the advantages of a direct-measuring in- 
strument are obvious. It should also be 
noted that direct measurement is to be 
preferred because the computational 
methods referred to above may not take 
account of all factors influencing the 
speed of sound. 
An instrument capable of measuring the 
speed of sound in fluids has been oaeceed 
at the National Bureau of Standards” and has 
recently been adapted for use in the ocean. / 
These devices use the well-known "sing-around" 
principle in which the frequency of an oscil- 
lator is governed by the transit time of a 
pulse of ultrasonic energy between two trans- 
ducers separated by a fixed distance in the 
liquid in which the speed-of-sound propagation 
is to be measured. 
The instrument described in the present 
paper, developed by Lockheed Missiles and 
Space Company, also makes use of the "sing= 
around" principle but uses different circuitry 
(many fewer components) and a completely dif- 
ferent packaging philosophy (which results in 
a great reduction in size) than earlier in- 
struments. The improvement in packaging was 
made possible by a systematic study of the 
effect of extreme pressure on the character- 
istics of electronic circuit components such 
as transistors, resistors, inductors and cap-= 
acitors. This study made possible the selec- 
tion for use in the velocimeter of components 
known to be stable over the pressure range 
O = 20,000 psie The electronic circuitry of 
the velocimeter can then be encapsulated in 
standard epoxy potting compound, when it is 
effectively exposed to the ambient pressure, 
instead of being contained in the bulky press- 
ure housings which have been an undesirable 
feature of earlier instruments. 
PRINCIPLE OF OPERATION 
The principle of operation of the instru- 
ment and the functions of the major electronic 
circuits are shown in Fig. 1. The pulse gen-= 
erator sends out a pulse to the piezoelectric 
transmitter, causing it to emit a 3-megacycle 
Superior numbers refer to similarly numbered references at the end of this paper. 
186 
