58 



BELL SYSTEM TECHNICAL JOURNAL 



A spacing washer .001 inch in thickness separates the diaphragm 

 from the damping plate. A single concentric groove is provided in 

 the damping plate. 



The buttons are of the con\entional cylindrical type but are provided 

 with a novel form of closure to jirevent carbon leakage at the point 

 where they make contact with the diaphragm. The closure consists 

 of twenty-seven rings of .0004 inch paper clamped firmly together at 

 the outer edge and spreading apart at the inner edge to form a structure 

 which effectively seals the junction between the diaphragm and the 

 buttons without adding materially to the mechanical impedance. 



As has already been pointed out the granular carbon is made from 

 selected anthracite coal. The size of the granules is such that they 

 will pass through a screen having 60 meshes per inch but will be re- 

 tained on a screen having 80 meshes per inch. Before heat treatment 

 the raw material is treated with hydrofluoric and hydrochloric acids 

 to reduce the ash content. Each button contains .060 cc. of carbon, 

 i.e., about 3000 granules. 



The bridge which supports the button on the front of the diaphragm 

 partially closes the acoustic cavity on that side. It is essential, 

 therefore, that it be so proportioned as to have a minimum reaction 

 on the response of the microphone and yet provide the required degree 

 of rigidity. It was this consideration that led to the smooth stream 

 line contour now employed. 



-40 



-45 



(0-50 



(0 -55 



50 



10.000 



FREQUENCY IN CYCLES PER SECOND 

 Fig. 9 — -Pressure calibration of the 387-type carbon microphone. 



Referring to Fig. 9 it will be observed that the adoption of an air 

 damped stretched duralumin diaphragm for the carbon microphone 

 has resulted in an instrument having a substantially uniform response 

 over a wide range of frequencies. The arrangement of the apparatus 

 employed in securing the data from which this curve was plotted is 

 shown in the photograph, Fig. 10. The microphone under test was 

 mounted in a highly damped room at a distance of six to eight feet 

 from a source of sound which consisted of two loud speaking receivers. 



