586 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



the spatial response of a non-reversible transducer such as a carbon micro- 

 phone. This is shown in Fig. 30 for an F-1 carbon telephone transmitter. 

 It is also the directional characteristic the unit would have if it were capable 

 of radiating. 



The addition of phase to a microphone directional pattern may seem 

 superfluous but a knowledge of phase can often be useful. For example, in 

 highly directional microphone arrays, the wave fronts in the close-in field 



Fig. 31 — A procedure for calibrating the photographic sound patterns. Part of the photo 

 is scanned a second time with a constant signal on the lamp. Successive reductions of 

 the signal (by 3 db steps in this case) produce the caUbration arcs. (A mask between 

 the camera and the scanner prevents the calibration arcs from registering in the pat- 

 tern area.) 



give information on the directional properties in the distant (far-field) 

 region. 



Additional Details 



This concluding section will describe in more detail the scanning mecha- 

 nism, the photographic procedure and methods for calibrating the photo- 

 graph to provide a measure of the relative field intensities. 



The scanning arm rocks up and down over an angle of about 60°, making 

 one stroke every two seconds. The vertical travel of the lamp in the course of 

 a stroke is adjustable up to 40" while the horizontal travel is fixed at ys 

 of an inch per stroke to provide a fine grain picture. The average picture is 



