72 



EXPERIMENTAL PROCEDURES 



distribution ift the ocean during transmission, the 

 depth and nature of the sea bottom, and in many in- 

 stances factors such as condition of the sea surface, 

 wind velocity, the presence of ocean currents, and the 

 presence of saHnity gradients, will affect the trans- 

 mission characteristics of the ocean; these must be 

 recorded along with the geometry of the transmission 

 path itself. In recent experiments, not only the level 

 but also the coherence and the degree of fluctuation 

 of the received signal have been studied. All in all, the 

 number of variables determining a signal is almost 

 overwhelming; also, the characteristics of the result- 

 ing signal are quite complex. In any given investiga- 

 tion, both field procedure and the analysis of data are 

 necessarily concerned with only part of the complete 

 picture. 



Ordinarily, the soimd source in transmission runs 

 is a transmitter driven by a harmonic oscillator 

 through suitable amplifying stages, so that single- 

 frequency sound is put into the water. Frequencies 

 used range from 200 c up to 100 kc and more, but 

 more runs have been carried out at 24 kc than at any 

 other frequency. A second ship carries the receiving 

 gear, hydrophone, amplifiers, and recorders. The 

 hydrophones are usually cable-mounted hydrophones, 

 which can be lowered to any desired depth from a few 

 feet to several hundred feet below the surface. 



In the most common form of run, the depth of the 

 hydrophone or hydrophones is kept constant during 

 one ruii. The range, however, is varied during the run 

 from 100 or 200 yd to several thousand yd, by having 

 the sending ship either approach or recede from the 

 receiving vessel. The run is usually completed in less 

 than half an hour. It is hoped no major changes 

 in temperature distribution or other oceanographic 

 variables will have taken place during that time. A 

 more detailed description of the field procedure will 

 be given in Section 4.3.2. First, however, a brief 

 description of somid-transmitting and sound-receiv- 

 ing equipment will be given. 



4.3.1 Sound Sources and Receivers 



A sound source suitable for transmission runs 

 should satisfy several requirements. It should be 

 easily controlled. It should be capable of being 

 mounted on a ship or towed by a ship. Its output 

 should be stable. Its frequency characteristics should 

 be simple, that is, it should produce either single- 

 frequency sound or wide-band noise with a smooth 



spectrum; and the acoustic power output should be 

 high so that even at long ranges the received signal 

 will usually be above the background. In practice, 

 most results have been achieved -wnith the use of single- 

 frequency sources, such as echo-ranging projectors. 

 Some work has also been done with noise makers of 

 the type used for acoustic mine sweeping. 



Single-frequency sources have been of three kinds, 

 electromagnetic or dynamic speakers for sonic fre- 

 quencies, and magnetostrictive and piezoelectric pro- 

 jectors for supersonic frequencies. Work has been re- 

 ported by UCDWR at 200, 600, and 1,800 c, and 14, 

 16, 20, 24, 40, 45, and 60 kc, and by WHOI at 12 and 

 24 kc. More transmission runs have been carried out 

 at 24 kc than at all the other frequencies combined 

 because echo-ranging gear used by the Navy was de- 

 signed for use at approximately that frequency. Oc- 

 casionally, transmission runs have been made with 

 "chirp" signals; these are frequency-modulated sig- 

 nals in which the frequency rises linearly from 23.5 

 to 24.5 kc or some similar frequency range during' 

 a pulse. 



Other important parameters of the sound source 

 are its directivity and its power output. The direc- 

 tivity may be reported in the form of pattern plots 

 in the horizontal plane and in the vertical plane. Ten 

 times the logarithm of b, the pattern function of the pro- 

 jector, is plotted on a circular graph against the angle 

 from the axis. These plots are incomplete since no in- 

 formation is given concerning the value of b off the 

 two planes plotted. Most echo-ranging projectors, 

 however, approach rotational symmetry with respect 

 to the axis so that a single plot including the axis 

 gives adequate information on the pattern in all 

 directions. Figure 3 shows the directivity pattern of 

 the JK crystal projector which has been used by 

 UCDWR for many transmission runs at 24 kc. 



Frequently, the directivity of a projector is re- 

 ported by means of a single quantity, the directivity 

 index D. The directivity index is defined (see Section 

 2.4.4) by means of the equation 



Z> = 10 log 



(— fbcm) 



(2) 



in which ft denotes the full solid angle. The units are 

 decibels. The directivity index so defined has the 

 value of zero decibel for a spherically symmetric 

 sound source. Since the axis for echo ranging is invar- 

 iably the direction of greatest power output, b no- 

 where exceeds unity, and Z> is a negative quantity. 

 For the standard Navy sound heads QC (magneto- 



