A Photographic Method for Displaying Sound Wave and 

 Microwave Space Patterns 



By W. E. KOCK and F. K. HARVEY 



{Manuscript Received Oct. 27, 1950) 



A photographic method using mechanical scanning for displaying the space 

 patterns of sound and microwaves is described. A probe pick-up scans the sound 

 or microwave field and the amplified probe output controls the brilliance of a small 

 lamp affixed to the probe. A camera set at time exposure records the light intensity 

 variations of the lamp as it moves across the scanned field, forming a pattern on 

 the film of the amplitude distribution. Phase fronts can be delineated by adding 

 a constant amplitude signal to the probe output. Photographs are included 

 which show : sound and microwave patterns of lenses, diffraction at a straight edge 

 and disk, refraction by a prism, diffusion of sound by a divergent lens, and radia- 

 tion from loud speakers. Also, by transposition of source and receiver, directional 

 patterns of transducers acting as microphones are obtained which (by reciprocity) 

 appear identical with their radiation patterns. This provides a means for examin- 

 ing the directional characteristics of non-reversible transducers such as a carbon 

 microphone. A calibration method is described which allows the relative value 

 of the field intensities to be determined. 



Introduction 



In analyzing the performance of an acoustic or microv^ave radiator it is 

 helpful to know the v^^ay in which the waves proceed as they emerge from 

 the source. It is desirable in some cases to haye a photographic record of the 

 distribution of intensity in the field generated by the radiator. This paper 

 describes a simple mechanical scanning method for accomplishing this result. 

 For acoustic analysis, a probe microphone is moved back and forth through 

 the sound field to be explored and a small lamp is afhxed to the microphone. 

 When the output of the microphone is connected to the lamp through an 

 amplifier, the intensity of the light varies in accordance with the sound level 

 encountered by the probe microphone. A camera set at time exposure records 

 the variations of light intensity. In this way the desired picture is buih up 

 by scanning the sound field somewhat after the manner in which a television 

 image is formed.^ For analyzing microwave fields the microphone is re- 

 placed with a microwave pickup probe. 



Experimental 



The scanning device is shown in Fig. 1.^ The microphone is located at the 

 end of the rocking arm. Attached to it (at the left) is a small neon lamp 



' A similar procedure in which the probe output, instead of lighting a lamp and bein- 

 photographically recorded, is traced on spark paper (Teledeltos paper) has been emg 

 ployed for microwave presentations by H. lams, "A Phase Front Plotter for Centimeter 

 Waves," R.C.A. Review, 8, 270 (1947); also Proc. I.R.E., 38, 543 (1950). 



' This device was designed by Mr. T. Aamodt of these Laboratories. 



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