EQUIPMENT AT MOUNTAIN LAKES 



111 



could be placed anywhere in the tank as long as it 

 was beamed at the hydrophone and absorbers. 



Each absorber unit 57 is a hollow cylinder of acous- 

 tic rubber 4 inches in diameter and 46.4 inches long, 

 packed with 29 ounces of No. 00 mesh steel wool and 

 filled with deaerated castor oil. 



The absorber units are sufficiently rigid to stand 

 unsupported in water. With the particular stacking 

 arrangements used, the sound must travel through 

 the equivalent of four such units before reradiating 

 into the tank. The characteristics of one absorber are 

 shown in Figure 45. The insertion loss for frequencies 

 above 1 mc was beyond the sensitivity of the measur- 

 ing system and transducers. 



Two types of standard transducers are used with 

 this system. One set, with the active face 3 cm in 

 diameter, has an exceptionally narrow beam, and as 

 a result will go down to 80 kc before reverberation 

 affects the results. This type of unit is used up to 800 

 kc. The diameter of the second type is 1 cm and it is 

 satisfactorily used as far as 2,200 kc. Its directivity is 

 such that reverberation interferes below 300 kc. 



One method of minimizing the effects of reverbera- 

 tion is pulsing, w4iich is used with success on the in- 

 termediate-frequency systems but has not been tried 

 with this system. The only reason was lack of time. 

 The required modification is not difficult and the 

 benefits to be derived should be as great as in the 

 other systems. Another method that was tried utilizes 

 acoustic lenses. (See reference 57.) The idea behind 

 the use of such lenses was that, if the beam from a 

 small projector could be focused by the lens on the 

 hydrophone, the ratio of signal-to-reverberation in- 

 tensity should be raised considerably— high enough 

 in most cases to give an accurate evaluation of the 

 sound field. 



A lens 10 centimeters in diameter was constructed 

 of polystyrene for operation at 150 kc. A projector 

 was set at 25 cm from the lens center and a hydro- 

 phone at the conjugate focus 125 cm on the opposite 

 side. Corrections were applied for spherical aberra- 

 tion and thick-lens effects, and the coincidence of 

 center of curvature and focus was avoided to prevent 

 standing waves. The theory of geometrical optics can 

 be used in designing sonic lenses and if due care is 

 taken the actual lenses will perform according to the 

 theory. For the lens described the increase in signal 

 strength at the focus was calculated to be 1 1 db for the 

 I -cm projector and 8 db for the 3-cm projector. These 

 values were obtained experimentally. It should be 



200 400 600 



FREQUENCY IN KC 



800 1000 



Figure 45. Insertion loss of one sound-absorbing unit. 



mentioned that lenses can be used in making reci- 

 procity calibrations, if a properly modified parameter 

 is used. 



One objection to the use of lenses is that the sound 

 velocity in them changes considerably with frequen- 

 cy. This is to be expected from the analogous optical 

 dispersion, but it adds to testing the difficulty that 

 the instruments w4iich are in focus at one frequency 

 are not at another. To eliminate dispersion, various 

 shapes of reflectors may be used. These behave as 

 expected, but the problem of preparing and main- 

 taining surfaces sufficiently smooth for the high fre- 

 quencies is too difficult. Another objection to lenses 

 is that the reflections are excessive in some cases. In 

 addition, the lenses are inconvenient to mount and 

 modified parameters must be used in the calculations. 

 All in all, the use of more directive beams and sound 

 absorbers is found to be the most effective. 



Standing Waves. Another problem in all acoustical 

 measurements is the presence of standing waves. Even 

 with no reflections from the boundaries of the me- 

 dium, standing waves may still occur between projec- 

 tor and hydrophone, or simultaneously between each 

 instrument and a third object in the sound field. In 



