584 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



and located on a line perpendicular to the scanning area and passing through 

 its center. It was sufficiently far away to ensure that its wave front was ap- 

 proximately plane, i.e., of uniform phase and amplitude, over the scanned 

 area. The scanning microwave probe thus picked up a nearly constant 

 phase, constant ampUtude signal from this secondary source in all positions 

 of scan, and in addition sampled the variable intensity, variable phase 

 microwave field coming from the lens. 



Microphone Patterns Using Transposition Techniques 



In the analysis of directional properties of acoustic or microwave radiators 

 such as lenses, reciprocity can be employed, which is the property that equi- 

 valent directional characteristics will be exhibited whether the transducer 

 is used as a transmitter or receiver. Some electro-acoustic transducers are 

 not reversible, however, and the directive properties of a carbon microphone, 

 for example, cannot be easily ascertained unless some means is employed 

 which measures its characteristics while it is receiving acoustic energy, i.e., 

 in the microphone condition. In all of the preceding photographs the scan- 

 ning device probed an actual sound field. In the analysis of a microphone, 

 however, we are interested in its ability to pick up sound coming from vari- 

 ous directions in space. We can therefore replace the sound source in the 

 preceding photographs with the microphone under test, and replace the 

 probe microphone with a scanning sound source. As the source scans the 

 space in front of the microphone, the signal in the microphone will vary 

 depending upon the abihty of the microphone to receive sounds from a partic- 

 ular spot in the scanned area. If this microphone signal is used to control 

 the brilliance of the lamp (still affixed to the scanner), the resulting photo 

 will indicate the directional characteristics of the microphone by itself or in 

 combination with a directional device such as a lens. As in the preceding 

 photos, phase can be added by combining a constant amplitude signal with 

 the microphone signal. The end result of all this is simply to interchange the 

 connections of source and sink. 



Figure 29 shows a microphone pick-up pattern in which the strip lens has 

 been placed in front of the microphone (not shown in the photo). Although 

 this picture is now a representation of the microphone response for sound 

 emanating from various points in the scanned space, it is seen to be nearly 

 identical with the pattern of Fig. 7 which is an analysis of an actual sound 

 field. This fact is simply a consequence of the principle of reciprocity. An 

 interpretation which applies equally well to either of these two phase 

 pictures is that the lines in each pattern are contours of equal phase length 

 between the fixed and scanning transducers. 



With this transposition technique, however, we are now able to examine 



