Page 613 radio acoustic ranging 656 



(h) The hydrophone may be fouled with other parts of the buoy, such as the anchor or reheving 

 buoy cable. Noises may be caused by shackles, counterweight fastenings, or other gear. 



(z) Vessels passing near the sono-radio buoy. Noise from such a source may last from a few 

 minutes to as long as an hour. 



656. Hydrophones 



A hydrophone is a subaqueous sound-detecting device used in Radio Acoustic 

 Ranging to receive the sound energy of a distant underwater bomb explosion and to 

 convert the sound energy to electric energy. Most hydrophones consist of a watertight 

 housing containing an electromagnetic, piezoelectric, or other electroacoustic device, 

 which is coupled to the housing in such a way that the sound impinging on the housing, 

 or its diaphragm, is transmitted mechanically to the electroacoustic device which 

 converts this mechanical energy to electric energy. For Radio Acoustic Ranging the 

 hydrophone is submerged in water at a suitable depth and connected to an audio 

 amplifier by means of a waterproof electric cable. 



As sound passes through an elastic medium there is an alternate condensation 

 and rarefaction of the medium at a given point, resulting in a corresponding increase 

 and decrease of the pressure at this point. In addition, at any point, the particles 

 of the medium undergo regional displacement forward and backward along the direc- 

 tion of sound propagation, but without any progress from this cause. (See 621.) 



Sound-detecting devices, such as hydrophones, are operated by- this pressure 

 variation and particle displacement. Sound detectors designed to use the differential 

 pressure of the medium caused by the sound wave are known as pressure-operated 

 instruments, and those designed to be operated by particle displacement are known as 

 displacement or velocity types. No instrument is exclusively operated by either 

 pressure or displacement — each instrument is typed according to its predominant 

 mode of operation, i. e., either pressure or displacement. Hydrophones, like all other 

 sound detectors, can be classified either as pressure or displacement instruments. 

 Most hydrophones are pressure types, for this type is more sensitive because of the 

 high radiation resistance of water, but displacement types are also used where the lack 

 of sensitivity is compensated for by other advantages. 



Hydrophones used on sono-radio buoys must be designed to have certain charac- 

 teristics favorable for use in Radio Acoustic Ranging, which are: 



(a) The hydrophone must be reasonably sensitive. Extreme sensitivity is not necessary, how- 

 ever, for the audio-amplifier gain can be increased to compensate for some lack of hydrophone sensi- 

 tivity. The hydrophone and audio amplifier must be designed so that together they will furnish the 

 required sensitivity. 



(6) The hydrophone must respond favorably to the frequencies of the sound from a subaqueous 

 explosion. There are little quantitative data on the frequency spectrum that includes the maximum 

 amount of sound energy from subaqueous explosions, or the variation in the frequency spectrum 

 caused by various conditions of the medium, depth, distance, bottom, and other factors such as the 

 sizes and kinds of bombs used. However, there is sufficient evidence to show that, under average 

 conditions of R.A.R., the frequency of most of the sound energy is below 300 cycles. The hydro- 

 phone should be designed, therefore, to favor frequencies below 300 cycles. The frequency charac- 

 teristics of the hydrophone need not be flat — in fact there may be some advantage in having resonant 

 characteristics. 



(c) The hydrophone must not be directive to a marked extent. An advantage might be gained 

 by the use of a directive hydrophone with a sensitivity null in a vertical direction to exclude noises 

 originating on or near the water surface, but most hydrophones used by the Coast and Geodetic Survey 

 in R.A.R. are not intentionally made directive. 



