SOUND WAVE AND MICROWAVE SPACE PATTERNS 587 



built up with about 300 lines or strokes corresponding to a 10-minute scan- 

 ning time. However, pictures up to twice this length are occasionally taken. 



To make a picture (see Fig. 1) the loud speaker is turned on to provide a 

 steady sound field. A filter in the microphone ampHfier is tuned to the signal 

 frequency to reduce the interference from external noises. Absorbing blan- 

 kets are sometimes desirable on nearly reflecting surfaces. The scanner is 

 first moved manually to sample the sound level at various points so as to 

 determine the proper gain settings and then placed in a starting position 

 close to the acoustic lens whose pattern is to be taken. The room is darkened, 

 the camera shutter opened, and the scanner started. Because the scanning 

 process is relatively slow, the observer sees only the individual strokes of 

 the flickering lamp. However, all the strokes are recorded on the camera 

 film and form the desired pattern. When the scanning is completed, the 

 shutter is closed and the room lights turned on. The film is then re-exposed 

 for a few seconds to add the image of the acoustic lens. A dark background 

 is provided so that the sound pattern will not be obliterated. 



The miniature glow lamps used for these pictures have been neon and 

 argon types having no base resistances, e.g., the NE17, NE51, or AR4 and 

 AR7. Neon seems to produce smoother gradations in intensity but argon is 

 sometimes desirable because the film is blue sensitive. The lamps operate 

 only over a voltage range from 70 to 120 volts, so that compression must be 

 used in the amplifier circuit if the pattern is to show the maximum ampli- 

 tude variations encountered which are of the order of 10 to 30 db. Many of 

 the patterns were taken by just connecting the lamp to a high impedance 

 circuit in the output of a power amplifier, the compression being obtained 

 by the decreasing resistance characteristic of the lamp with applied voltage. 



In this as in any photographic process, the operator can control the effects 

 pictured, intensifying or subduing images as desired by adjusting the film 

 exposure and the circuit compression. When directivity is to be displayed 

 and the major lobe is of main interest, the minor lobes may not even be 

 exposed; when the phase fronts are desired in weak regions, the maximum 

 range of the film and largest circuit compression may be used. 



When quantitative information is desired, the relative intensity of the 

 sound field in the amplitude patterns can be ascertained by a calibration 

 test performed before or after each run. This is illustrated in Fig. 31. A 

 signal equal to the maximum signal the lamp receives during the scanning 

 run is fed at constant level to the lamp while it is scanning the unused part 

 of the photograph at the right. Successive reductions of the signal by a 

 known amount of attenuation (3 db steps in this case) give a series of arcs 

 of decreasing brightness. These can be compared to the various scanned 

 portions of the photograph to get a measure of the amplitude. 



