40 



TESTING TECHNIQUE 



sibility, and difficulties in satisfactory rigging, which 

 are increased by water currents. In general, the ad- 

 vantages of a river, without the disadvantages, may 

 be realized just as satisfactorily in a lake of proper 

 size. 



525 The Use of Acoustic Tanks 



for Testing 



A disadvantage presented by all natural testing 

 sites is that the temperature of the water cannot be 

 controlled. The hydrostatic pressure can be con- 

 trolled somewhat by the choice of testing depth but 

 is limited by the depth of water at the location and by 

 other factors, such as rigging problems. In the cali- 

 bration of many devices, the determination of the 

 temperature and pressure dependance of certain of 

 their characteristics is both desirable and necessary, 

 since many of these devices must be operated in water 

 temperatures ranging from near-freezing to tropical 

 and at hydrostatic pressures corresponding to depths 

 of several hundred feet of water. The only satisfactory 

 method of making such measurements seems to be 

 by the use of an acoustic tank. The principal dis- 

 advantage of tanks is their small size, which causes 

 interfering reflections and limited testing distances. 

 However, if the inner surfaces of the tank can be 

 coated with some sound-absorbing material, or if a 

 pulsing technique in measurement can be employed, 

 a tank may form a satisfactory testing site. It has the 

 advantage of having low ambient noise, accessibility 

 independent of weather, and simplified rigging, in 

 addition to temperature and pressure control. The 

 characteristics of particular tanks that have been con- 

 structed are treated elsewhere in this volume. 



5-3 ELIMINATION OF REFLECTIONS 



gradient. The superposition of the direct wave and 

 reflected waves produces in the medium a compli- 

 cated standing wave pattern, whose configuration 

 changes as the frequency is varied. Thus, if a nondi- 

 rectional (pressure-sensitive) receiver is used to meas- 

 ure the pressure at a point in the resultant field, the 

 measured pressure oscillates about the pressure in 

 the direct wave as the frequency is varied. Assuming 

 the surface to be perfectly reflecting, which is very 

 closely true in many tests, the expression for the pres- 

 sure at a horizontal distance d from a point source 

 at a depth h and with an operating frequency / is 



5.3.1 



General Considerations 



It was pointed out previously that acoustic reflec- 

 tions from the surface, bottom, or shores of a body 

 of water, or from other reflecting objects such as pil- 

 ings used to support a pier or dock from which tests 

 are made, interfere with the calibration of instru- 

 ments since they make it difficult to establish a plane 

 progressive sound wave. Receivers in such cases meas- 

 ure not only the pressure or pressure gradient due to 

 the desired wave from the source, but also the contri- 

 butions of reflected waves to the pressure or pressure 



[po a 



+ 



d 2 + 4/? 2 

 2p 2 d 



(d* + 4h--y 



cos 



2 ^f(\/d? + iW - d)J (3) 



where p„ is the pressure which would exist at the 

 point in question if the water surface were not pres- 

 ent, and c is the velocity of sound. Thus, as / varies, p 

 oscillates between the values 



*»o[ 



1 + 



(rf 2 + 4/j 2 ) 



2TH] and Po|j 



(d 2 + 4/i 2 ) 



J (4) 



because of the cosine term. It will be noted that the 

 oscillation of p with / becomes more violent as d be- 

 comes greater compared to h. These phenomena are 

 well illustrated in Figure 2, where the voltage devel- 

 oped by an essentially nondirectional hydrophone 

 in the field produced by an essentially nondirectional 

 source is shown for several testing distances. For any 

 fixed depth the interference minima are increasingly 

 prominent as the testing distance is increased. The 

 characteristic appearance of these interference maxi- 

 ma and minima is quite helpful in indicating when 

 reflections are interfering in a test. 



If the reflecting surface, which may be the bottom 

 or any other surface, has a pressure reflection coeffi- 

 cient R, then equation (3) is modified to 



[W 



R 2 p,rd 

 d 2 + 4/? 2 



2R-p„ 



(d 2 + 4/i 2 )' 



cos 



gfr/* + W-d)-a\]* 



(5) 



where a is the phase shift on reflection. Hence, by re- 

 ducing the reflection coefficient of the surface, one 



