228 © ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
buoys. Due to reverberation, multiple reflections, and other causes, a 
radio signal in such cases may be prolonged as much as 7 seconds. 
Moreover, defects occurring in the electric circuits or unwanted noises 
may tend to make a particular sono-radio buoy transmit almost con- 
stantly. The so-called shortening circuit limits the length of radio 
transmission to a half second or less, after which the sono-radio buoy 
is rendered inactive for a period of 8 to 5 seconds. There are certain 
disadvantages in using these circuits. When all the radio signals 
transmitted are of equal length, signals caused by bombs cannot be 
distinguished from other signals, as, for example, those caused by 
water noises. Moreover, if a sono-radio buoy is actuated by an extra- 
neous cause just before the bomb signal arrives, the silencing circuit 
prevents the bomb signal from operating it. Shortening-and-silencing 
circuits, therefore, are not used where prolonged signals are not par- 
ticularly bothersome. 
To obtain constancy of radio frequency, a quartz crystal is incorpo- 
rated in the transmitter. Several radio frequencies between 2492 and 
4160 kilocycles are authorized for use in sono-radio buoys, but those 
most frequently used are 4135 and 4160 kilocycles. Using these latter 
frequencies, the minimum radio frequency power required for satis- 
factory results under normal operating conditions is about 3 watts, 
although up to 26 watts has been used. 
A hydrophone is a subaqueous sound-detecting device. It is used in 
R. A. R. to receive the sound energy from a distant underwater bomb 
explosion and to convert it to electric energy. Most hydrophones con- 
sist of a watertight housing containing an electromagnetic, piezoelec- 
tric, or other electroacoustic device, which is coupled to the housing in 
such a way that the sound impinging on the housing, or on its dia- 
phragm, is transmitted mechanically to the electroacoustic device, 
which in turn converts this mechanical energy into electric energy. 
As sound passes through an elastic medium, such as water, there is 
an alternate condensation and rarefaction of the medium at a given 
point, resulting in a corresponding increase and decrease of the pres- 
sure at this point. In addition, at any point the particles of the me- 
dium undergo regional displacement forward and backward along the 
direction of sound propagation. Hydrophones are operated by this 
pressure variation and particle displacement. Several different types 
of hydrophones have been designed especially for use in R. A. R. The 
hydrophone itself does not have to be extremely sensitive, but the 
hydrophone and the audio amplifier must be designed so that together 
they will have the required sensitivity. A hydrophone must respond 
well to the frequency of a sound caused by a subaqueous explosion. 
The hydrophone must not be directive to a marked degree, for in hy- 
drographic surveying the sound which is to actuate it may come from 
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