126 Lecture 7 
retical and experimental work involving a great many measurements. The number 
of measurements is likely to be great, and the calculations many and extensive, 
but modern instrumentation and computers should make the problem more 
attractive. A few researchers are attacking this problem, but it will require the 
efforts of the ablest people in the field to obtain the knowledge required for 
reliable design of open arrays. 
For some applications it seems desirable to operate transducers at great 
depth; thus, there is a need to know the effects of high pressures on various 
types of transducer elements. 
Calibration facilities for large low-frequency arrays is another major 
problem. Because of the long wavelengths involved, free-field measurements 
are not possible at most present-day calibration facilities. Pachner and others 
have done theoretical work onthe extrapolation of many near-field measurements 
to the far-field pattern. Another approach is the design of a stable platform for 
use in deep water, so that free-field measurements can be made. This approach 
is certain to be very costly and cumbersome. Pachner's work must be carefully 
evaluated and subjected to experimental test. 
7.2, PROPAGATION 
In the past ten years the use of low-frequency sound which propagates to 
great distances with little attenuation has stimulated investigations of numerous 
propagation paths, namely: propagation in surface-bounded ducts, propagation by 
bottom-reflected paths, by refracted paths to convergence zones in deep water— 
and in recent years some investigation has been made of propagation paths from 
deep sound sources. Much of the propagation data is not susceptible to rigid 
analysis because the conditions under which it was taken could not be rigidly 
controlled, and it is impossible to simulate the ocean adequately in laboratory 
tanks. Some facilities and techniques recently developed make it possible to 
remedy the situation in part. These facilities are fixed, high-power, low-fre- 
quency sound sources, sensitive hydrophone arrays, and vastly improved 
signal processing equipment. This equipment, combined with modern recording 
techniques and electronic computers, makes it possible to collect and process 
data on a continuous basis. All of this, however, is expensive and will require 
the cooperation of national governments, navies, and scientists of many nations. 
For example, underwater sound sources and receiving arrays set up on either 
side of the English Channel and cabled to shore could be operated on a year- 
round basis. Properly instrumented, these stations could continuously record 
signal levels, reverberation levels, and noise background levels, and could 
correlate them with weather and oceanographic conditions. Data in quantity will 
make valid statistical analysis possible. Atthe sametime, such a pair of stations 
could provide a continuous surveillance for the passage of submarines through 
these narrow waters. Other sets of sound stations in restricted shallow water 
areas, such as the Danish Straits, the Bosphorous, the Straits of Gibraltar, etc., 
could provide surveillance of strategic areas andatthe same time yield valuable 
propagation data to sound physicists. The stations described, of course, could 
only provide data for shallow water situations. 
The political and physical geography of Europe is an asset to the NATO 
nations in the matter of submarine surveillance. To capitalize on this asset, it is 
