ELIMINATION OF REFLECTIONS 



43 



RECEIVER 



LINE SOURCE 



RECEIVER 



Figure 5. Effect ot orientation of line source on magni- 

 tude o£ surface reflections. 



and a piston become ineffective, but a dipole, whose 

 pattern is independent of the frequency, retains its 

 effectiveness. However, the dipole is inferior to a 

 piston or a line at high frequencies. 



At high frequencies, a piston is the most effective 

 source for eliminating interfering reflections. How- 

 ever, before final conclusions as to its usefulness can 

 be drawn, account must be taken of the fact that at 

 relatively close distances the sound field of a piston, 

 even in the absence of reflecting surfaces, is not a free 

 progressive plane wave. This limits the minimum 

 value of the testing distance that may be used. Simi- 

 lar considerations apply to other sources. A discus- 

 sion of the choice of both testing distance and source 

 will be taken up in Section 5.4 when reflections and 

 so-called proximity effects will be considered. 



Another fact should be pointed out. ft is difficult 

 to construct a dipole source which develops aij ap- 

 preciable radiation field. This is a result of the fact 

 that the radiation impedance of a dipole source is 

 largely reactive, so that the pressure and particle 

 velocity in the field are almost in quadrature at short 

 distances from the source, and therefore little energy 

 is radiated. 



Finally, the characteristics of the receiver must be 

 weighed in considering the elimination of reflections. 

 It has been assumed that the receiver in the preceding 



discussion was essentially a pressure-sensitive device. 

 Some receivers are essentially velocity or pressure- 

 gradient actuated. The problem is somewhat simpli- 

 fied here, since the velocity components of the re- 

 lict ted wave are not parallel to the axis of the receiver 

 when it is oriented toward the source. Such receivers 

 thus discriminate against surface reflections. How- 

 ever, most receivers have more complex behavior and 

 their discrimination against surface reflections can be 

 judged by their directional patterns. Proximity ef- 

 fects in the receiver are also a consideration limiting 

 the shortness of testing distances. Thus the effective- 

 ness of directional sources in discriminating against 

 surface reflections can be properly judged only when 

 considered with other limitations on testing distances. 

 At this point one tan only point out the potential 

 value of directional sources in testing. 



5.3.3 The c^oit-g f Instrument 



Orientation 



The preceding discussion regarding the use of di- 

 rectional sources to reduce reflections suggests that 

 the choice of orientation of the instruments being 

 tested may make a difference in the prominence of 

 reflection interference. When the instruments have 

 their own directional characteristics, these often can 

 be used to advantage. One prominent example is 

 shown in measuring the frequency response of a line. 

 The directionality of a line hydrophone is such that 

 most of the acoustic energy is contained in the region 

 between two cones having a common vertex and axis, 

 the latter coincident with the line itself. The pattern 

 is roughly like a pancake. If the line is suspended 

 horizontally, the acoustic power incident on the 

 hydrophone from the point on the surface from 

 which reflections are received generates nearly as 

 much voltage as the acoustic energy coming directly 

 from the source (if the latter is nondirectional), as is 

 shown in Figure 5A. However, if the line is suspended 

 vertically, this is no longer the case, for the hydro- 

 phone is then relatively insensitive to signals coming 

 from the surface. (See Figure 5B.) Thus considerable 

 reduction in reflection interference can be effected 

 by choosing the vertical orientation for the line 

 rather than the horizontal one. Unfortunately, it is 

 not usually possible to use the same scheme when 

 directivity patterns for the line are being measured. 

 A second important case occurs in connection with 

 the measurement of the rear response of a projector 



