Projection engineering (Sept 1929-Nov 1930)

Page 20 Projection Engineering, May, 1930 Public-Address and Centralized Radio Equipment Directly Useful Information on This Timely Subject By E. W. D'Arcy THE author shall confine himself in this article to types of microphones used commercially for public-address, talking pictures aud broadcast work. There is room for improvement in even the best microphones, and it is rather peculiar that instead of having several different principles used commercially, the field has been limited to resistance, and capacitative types. Possibly an understanding of the limitations of present microphones can be had by considering that, for perfection, a microphone should possess no inertia, and should produce an output directly proportional to the air pressure applied. This supposition has not been fulfilled practically, however, although there have been types developed which do not rely on any mechanical agency for transposition of sound energy to electrical energy. The glow microphone is one example of a type which offers greater theoretical perfection compared with the stretched diaphragm type. Unfortunately, this type has been quite unreliable. Research will, undoubtedly afford some basically new design of commercial microphone which will replace existing types with their non-linear characteristics. Carbon Microphones An illustration of a high quality microphone is shown, in Fig. 28. This type is representative of the best carbon microphones as it is very well built mechanically and possesses excellent frequency response characteristics. The circuit for operation of this microphone is displayed in Fig. 29. It is easily seen that as the diaphragm moves in either direction, the current through one side of the trans * Chief Engineer D'Arcy Laboratories, Chicago, III. CARBON / Hill TO AMPLIFIER V. Microphones former will increase while the current through the reverse side will decrease. This effect contributes to the elimination of microphone hiss caused by the normal current flowing through the carbon as well as eliminating distortion caused by even harmonics. It also eliminates any effect of the microphone current upon the transformer, since the battery current flowing through the microphone transformer creates opposing magnetic fields and therefore is balanced out. Mechanical Construction One of the most important factors in a successful microphone is that of the material used in the diaphragm. It must be of uniform thickness in order to avoid having resonant points when stretched. It must be sensitive to sound waves even when placed under extreme tension. It also must be tough enough to allow its being Fig. 29. Microphone Circuit. Fig. 28. Carbon microphone A. Rear plate assembly. B. Damping chamber. C. Front ring. D. Button support spider. E. Diaphragm tension ring. F. Rubber button bushing. G. Carbon granule button assem bly. placed under sufficient tension to raise its natural period to the upper end of the used audible frequencies. Practice indicates that successful results can be obtained if the diaphragm is stretched to a natural period of 5,000 cycles. Copper aluminum alloys seem to possess the best requisites for diaphragm material when properly heat treated. Thickness of the diaphragm unstretched as a rule varies between one and two mils, depending upon the product of the manufacturer. The copper aluminum diaphragm is, by the best manufacturers, plated to insure a contact affected as little as possible by oxidation. Damping Required for Good Response If no damping were applied to the microphone diaphragm, considerable difficulty would be experienced clue to the resonance points caused by stretching the diaphragm. Therefore, damping is applied by an enclosed air chamber between the back plate and diaphragm as well as by the carbon granules which are held under compression by the carbon containers. Mechanical Construction It can be seen that the physical dimensions of the supporting assembly have to be of considerable size to insure uniform response unaffected by mechanical resonance of any part of the microphone other than the diaphragm. For that reason the supporting rings, tension ring, and back plate are made of steel of sufficient mass both to insure rigidity and a very low natural period. Effective Impedance of Carbon Microphones The alternating-current impedance of a carbon microphone is not, as one would expect, its apparent talking resistance but rather the ratio of the power absorbed by it to the square of the current flowing through it. It is generally assumed that the a-c. resistance of a carbon microphone is 80 per cent of its apparent talking resistance. Measurement of Effective A-C. Resistance A reasonably accurate system of determining the a-c. resistance of a carbon microphone is shown in Fig. 30. The system operates on the supposition