CHOICE OF TESTING GEOMETRY 



55 



L/A 

 Figure 15. Spherical wave collection for a uniform line. Correction to be added to measured response. 



of 1.5 feet and 9 feet are compared. The critical dis- 

 tance for the projector tested in this example is 

 about 5 feet. 



When measuring axial response, the first criterion, 

 d > L' 2 /\, is the more important practical one, but 

 for directivity patterns the second criterion, d > 10L, 

 is about equally important. This results from the 

 fact that the directivity pattern of a device is due 

 largely to cancellation at certain angles of in-phase 

 and out-of-phase pressures on the active surface. Close 

 in, the inverse-square-law effect makes the amplitude 

 smaller on the more distant parts of the transducer, 

 so that cancellation is not so effective. This effect is 

 particularly troublesome for a long-line hydrophone, 

 since the testing distance must be very great to keep 

 the amplitude at the two ends of the line approxi- 

 mately equal when the line has a radial orientation 

 with respect to the source or receiver. 



5.4.6 



Other Proximity Effects 



Another proximity effect which occurs with direc- 

 tional transducers is connected with their beam pat- 

 terns. If the beam pattern of one is quite sharp, it 

 may allow an appreciable variation of amplitude 

 over the face of the other, independent of the inverse- 

 square-law effect. When a directional source is used, 

 the variation in pressure over the area of the oppos- 



ing transducer should certainly be less than 1 db if 

 the proper response is to be obtained. 



When two transducers face each other during a 

 test, the sound field at the receiver includes the 

 doubly diffracted (or reflected) field produced by the 

 original wave from the source being diffracted by 

 the receiver and then rediffracted by the source back 

 to the receiver. This effect is usually negligible, ex- 

 cept when two transducers of large area oppose each 

 other at a short distance, in which case a severe stand- 

 ing wave pattern may be set up between them. The 

 standing waves may even be of sufficient magnitude 

 to change the apparent acoustic impedance of the 

 medium as viewed by the source. While this factor 

 is rarely a cause of trouble, it should be kept in mind 

 when working with transducers of large area. 



Finally, it should be remembered that in a test the 

 proximity effects for both instruments must be con- 

 sidered and allowance made for the nature of each 

 in selecting the testing distance. 



5.4.7 



Correction for Proximity Effects 



The fact that the error due to a spherical wave 

 front can be calculated for many commonly occurring 

 cases, such as the pressure-gradient transducer, the 

 circular piston, and the line, suggests that correc- 

 tions can be made for this effect. Such is indeed the 



