International photographer (Jan-Dec 1935)

May. 1935 The INTERNATIONAL PHOTOGRAPHER Nine a certain permissible maximal value, which is given by Morecroft as 0.0001 inch for an ordinary carbon microphone, the current in the microphone circuit will be an even less faithful copy of the sound-pressure waves producing the vibration of the diaphragm than is normally the case, as both odd and even harmonics will be present in the electric wave thus created. The single-button microphone usually employs a mouthpiece ; and its sensitivity is high although its fidelity of reproduction is poor. It is still used in wire telephony because it operates efficiently in transforming to reasonably faithful speech current just enough of the speech frequency band to make possible intelligible conversation between persons. The construction and functioning of this type of microphone are so simple that it will not be necessary to consider them in more detail. The Hiffh-Quality Carbon Microphone The double-button carbon transmitter is of the stretched-diaphragm, carbon-granular type; and because of its construction, it is often termed the "push-pull" transmitter. The sensitiveness of this type of microphone is extremely low — about one100th that of the single-button transmitter — but the quality of its output is high, comparing favorably with that of the condenser transmitter. The double-cell carbon microphone is less bulky and less troublesome than the condenser microphone, and it requires a smaller amount of amplification to bring the speech current up to a useful value ; but the carbon hiss that is inherent in this type of transmitter somewhat limits its use. The presence of the carbon hiss brings the ground noise in the circuit up to such a high level as to cause it to interfere with the quality of the speech. Sometimes a filter is used with the two-button microphone to reduce the amount of this hiss. The filter is formed of series resistances in each of the three leads from the buttons and small condensers shunted across the buttons. The two-button microphone is widely used in public address systems, where its many advantages for this type of work outweigh the annoying effect of the carbon hiss. Up to the time of the development of the condenser transmitter to its present high standard of excellence, double-button carbon microphones were used exclusively in radio broadcast stations; but, as has been mentioned, they have been replaced almost entirely by condenser 82%: %P 82% 82% 82% 82% 82% 82% 82% 82% 88% By CHARLES FELSTEAD Associate Editor microphones. In the motion picture sound recording installation, this type of microphone is still used with the small public address system that permits the monitor man to address the actors on the stage ; but it is never used for recording. Construction of the Double-Button Microphone The double-button transmitter is flat and circular and approximately the size of a doughnut (about one inch by three inches). To overcome the effect of resonance and the resultant distortion in ontput that would be introduced by a mouthpiece or other form of collecting horn, this microphone is used without such an attachment. It is quite sensitive to mechanical shock.,, and so a simple form of spring suspension is used to insulate it from vibrations and jars. Quite often the whole assembly is enclosed by two metal covers that have several large circular holes cut in them. These openings are covered by a fine-mesh metal screen that serves to protect the delicate diaphragm from injury by pencils, the ends of wires, or other objects. The microphone in its frame may be fitted with a base and used on a desk ; or the high stand type of mounting may be employed when the microphone is to be used by a speaker who is standing. The stretched-diaphragm of this type of transmitter is made of a metal called duralumin, and it has a thickness of 0.0017 inch. A spacing washer on mil (0.001 (Continued on Page 32) ROY DAVIDGE FILM LABORATORIES ••• An Exclusive "Daily" Laboratory ••• 670 1-6715 Quality and Service SANTA MONICA BOULEVARD GRanite 3108 82% 82% as ag M 82% 82% 82% 8i% Please mention The International Photographer when corresponding with advertisers.