ELIMINATION OF REFLECTIONS 



45 



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Figure 8. Use of screen with orifice to reduce surface and 



I .mi. mi reflections in calibration. 



having dimensions roughly 2x4 feet. The layer of 

 enclosed air acts as an effective reflector, and the felt 

 is intended to damp out resonant frequencies. 17 



Two arrangements of screens have been employed. 

 One consists of hanging the screen vertically with the 

 top edge just breaking the surface midway between 

 the source and receiver, with the plane of the screen 

 perpendicular to the line joining the two instru- 

 ments. (See Figure 9A.) A more effective arrange- 

 ment has been found lo be a pair of screens in the 

 form of a "V" suspended at the surface in the position 

 shown in Figure 9B. In this arrangement, sound re- 

 flected from the screen is thrown off more or less side- 

 wise along the surface. A series of such screens placed 

 end to end along the entire distance between source 

 and receiver (Figure 9C) is still more effective. While 



reflected to the receiver again, if the latter is placed 

 too far away. Finally, one cannot place the source too 

 close to the orifice or else the transmitted beam will 

 have too great an angular divergence and thus strike 

 the surface near enough to give a reflected wave to the 

 receiver. Therefore, an orifice in a screen can be con- 

 sidered only as a method of producing an effective 

 piston source of larger area than would be practical 

 for the diaphragm of a projector. 



A similar analysis of the feasibility of using acous- 

 tic lenses leads to the same conclusions: a lens also 

 acts in principle like a piston source of the same 

 diameter. The construction of suitable acoustic lenses 

 involves various technical difficulties in addition to 

 the theoretical ones outlined above. 



A helpful yet considerably simpler method for 

 reducing reflections is by using, at the surface or 

 bottom, baffles or screens so oriented as to cause re- 

 flections to be directed away from the receiver. Such 

 screens may be considered to reduce the effective 

 value of the reflection coefficient R. While they may 

 be placed so that, according to geometrical acoustics, 

 no reflected sound should reach the receiver, diffrac- 

 tion about the screen and waves on the water surface 

 usually allow still a considerable part of the reflected 

 sound to reach the receiver. For a given size reflector, 

 the diffraction is greater at lower frequencies. This is 

 unfortunate since the low-frequency region is just 

 the region in which directional projectors also be- 

 come difficult to construct. 



The type of screen used by USRL is essentially a 

 watertight sandwich of 14-inch hard green felt en- 

 closed between % 2 -inch galvanized iron sheets, and 



BOTTOM 



Figure 9. Use of screens to reduce surface and bottom 

 reflections. 



