44 



TESTING TECHNIQUE 



or hydrophone, when the receiving directivity pat- 

 tern is being obtained. If the transmitter is a direc- 

 tional one. it should be so oriented that the principal 

 part of its acoustic energy is directed away from 

 shore. If the projector is oriented in the opposite 

 direction, a large part of the energy is reflected from 

 shore and reaches the receiver coming from that 

 direction. When the receiver is oriented to measure 

 rear response, it is most sensitive to sound arriving 

 from shore. In such a case, the reflected signal voltage 

 may greatly exceed the direct signal voltage. With 

 the other orientation this difficulty is avoided. This 

 is shown epiite clearly in Figure 6. 



Another scheme which often may be of value in 

 calibrating a source is to use a velocity or pressure- 

 gradient sensitive receiver. Such a receiver is theoret- 

 ically insensitive to signals coming in from a direction 

 at right angles to its axis. By orienting the source and 

 receiver as shown in Figure 7, the receiver may be 

 made almost entirely insensitive to reflected signals 

 from the surface. The difficulties of rigging the in- 

 struments in the proper orientations as shown have 

 essentially prevented this method from being used by 

 USRL. 



534 Use of Screens, Lenses, and Orifices 



Another method of eliminating reflections which 

 readily suggests itself is the use of screens or baffles 

 to shield the receiver from any but the direct waves 

 from the source, much as is done in optics for similar 

 reasons. This method is not as effective as first 

 thought might indicate because the acoustic wave 

 lengths in which one is interested are generally of 

 the same order of magnitude as the dimensions of 

 suitable baffles, unlike the corresponding situation 

 in optics. Therefore, in the range of frequencies of 

 usual interest, geometrical acoustics is a far from 

 valid representation of acoustic phenomena. How- 

 ever, in spite of the importance of diffraction in such 

 cases, some useful results are obtainable by the proper 

 use of screens or baffles. 



The collimation of light by lenses or pinholes is a 

 well-known optical method for preventing stray re- 

 flected light from interfering with a measurement. 

 Would it then be possible to use the same methods in 

 acoustics? Consider first the analogue of a pinhole, 

 that is, a large screen in which an orifice is cut, and 

 the source and receiver placed on opposite sides. (See 

 Figure 8.) If the source is placed far enough behind 



BOTTOM 



Figure 6. Dependence of interference from shore reflec- 

 tions on arrangement of instruments in measuring rear 

 response. 



the screen so that the waves reaching the orifice are 

 essentially plane, it is found by a calculation using 

 the wave equation that the resultant sound field on 

 the receiver side of the screen is the same as though 

 the orifice were considered a piston source of the 

 same diameter. The same considerations apply to 

 such orifices as to directional sources of the piston 

 type which have been discussed in the previous sec- 

 tion. One is again limited by proximity effects if the 

 receiver is put too close to the orifice. On the other- 

 hand, the beam through the orifice has a finite angu- 

 lar divergence and strikes the water surface and is 



IMAGE OF SOURCE 



SOURCE - 



Figure 7. Orientation of pressure-gradient receiver to 

 eliminate interference from surface reflections. 



