The international photographer (Jan-Dec 1935)

October, 1935 The INTERNATIONAL PHOTOGRAPHER Seventeen The easiest means of finding out if the sing is from an adjacent circuit carrying a-c current or from an open ground in the microphone is to move the microphone and its cable to a new location at least a dozen feet frim its former position. If the sing still persists, the trouble is then without doubt in the microphone ground; but if the sing stops, a-c induction pick up is probably to blame. When the microphones are not in use, they should be hung in a special drying cabinet built like a desiccator, with several open cans of calcium chloride placed in the bottom to absorb the moisture from the air and so dry out the microphones. Fans to circulate the air within the drying cabinet are an addtional help. To prevent moisture condensation when the microphones are removed, the temperature within the drying cabinet should be kept uniform and as nearly as possible the same as that in the place in which the microphones are to be used. MICROPHONE OUTPUT LEVELS The condenser microphone provides an output of about 0.35 millivolt for a sound-pressure wave of one dyne per square centimeter; as against about five millivolts given by a double-button carbon microphone for the same sound pressure. This output relationship has been slightly changed in later models, however. The greatest movement of a condenser transmitter diaphragm is approximately 0.0001 inch; while the maximum movement of the diaphgram of a carbon microphone is about ten times larger than that, or 0.001 inch. Nevertheless, what the condenser transmitter lacks in sensitiveness it gains in greater fidelity of response. The electrical levels of the outputs of the microphones under discussion and of those which will be described in later chapters is listed below. Average speaking distance is assumed to be about twenty inches from the microphone. Zero level in decibels is 0.006 watts, or six milliwatts. Western Electric Co., single-button carbon, type 337.... — 15 db. Western Electric Co., double-button carbon, type 387W.... — 45 db. Western Electric Co., single-stage condenser — 60 db. Western Electric Co., dynamic, type 618A — 80 db. Radio Corp. of Amer., velocity, type 44A ■ — 80 db. Shure, sound-cell crystal, type 77H — 80 db. Astatic, crystal diagphragm, type D-104 — 60 db. Radio Corp. of Amer., inductor, type 50A — 70 db. MONITORING CONTROLS A high-grade mixing system is employed to combine in the proper proportions the outputs of several microphones. These are variable attenuation networks, either the straight T-type or a modification and simplification of the T-type. These mixers are constructed so that their input impedances match the output impedances of the microphones at all settings of the mixer; and the combined output impedances of the several mixers match at all positions the input impedance of the booster amplifier which follow them in the circuit. A similar mixer is connected between the output of this booster amplifier and the transmission line leading down to the main recording amplifiers in the amplifier room. This last mixer is known as the main volume control, since it regulates the overall volume from the combination of microphones in use. Where several microphones are used for recording a large orchestra, the musical balance is often obtained by setting the several individual mixers, then the overall recording level is regulated by the main BY CHARLES FELSTEAD ASSOCIATE EDITOR volume control. This permits regulation of the recording level without interference with the musical balance of the recording. Ordinary potentiometers are not satisfactory to use as mixers. The input and output impedances of a potentiometer vary continuously throughout its range. This affects the sound quality, due to reflection losses and other factors induced by the unavoidable mismatching of impedance that occurs between the microphone and the potentiometer and between the potentiometer and the booster amplifier. Potentiometers may be used as volume controls only in the grid circuit of a vacuum tube, where theoretically there is no current flow. ADDENDUM It may be well to mention, although it is somewhat aside from our subject, that in public address systems, which closely parallel sound picture recording systems, feedback or acoustic coupling between the speakers and the microphone is often encountered when they are placed in close proximity. The more sensitive the microphone, the more prone it will be to this effect; and for cases where this condition cannot otherwise be avoided, it is well to use a highly-damped and rather insensitive carbon microphone for the pick-up device if the sacrifice in quality is not too high a price. The tendency to acoustic feedback can be reduced by keeping the speakers and microphone as far apart as convenient; and if the speakers or microphone are directional, it is further assistance to place them so that they are not facing each other. The RCA ribbon, or velocity, microphone; the Western Electric electro-dynamic, dynamic, or "moving coil," microphone; and the crystal microphone all represent relatively new developments in the design of sound pick-up devices. As a result, they have not come into as wide use as the condenser microphone for sound recording; although the dynamic microphone is coming into considerable favor with sound recording engineers, particularly for the recording of music because of its accentuation of the bass register. The next chapter of this series will revert to the subject of audio-frequency amplification, and is of a more practical nature than the previous chapters on amplification. SCHNEIDER LENSES Keeping pace with the latest developments in the science of photo-optics, the firm of Jos. Schneider & Company, lens manufacturers of Germany, and represented in the United States by Burleigh Brooks, has lately introduced several new photographic objectives, namely the Aero-Xenar, the Ortho-Angulon and the Componar. Aero-Xenar —As its name indicates, this is a six-element lens of the Xenar type with certain properties which render it the ideal objective for aerial photography. Spherical distortion and chromatic aberration have in this lens been brought down to an irreducible minimum. Aberrations inherent in the oblique rays have likewise been corrected, resulting in an anastigmatically flattened field over a comparatively wide angle. The aperture of the Aero-Xenar is f:4.5 and its focal lengths are 10", 11 3A" and 193/4"'. Ortho-Angulon A wide-angle, special, objective, particularly intended for photogrametric work, that is, land survey, the making of relief maps, etc. It combines in unusually high degree corrections for spherical aberration and astigmatism, and is the last word in an apochromatic lens, thoroughly corrected for the complete gamut of spectral colors. It defines the subject sharply and without the slightest trace of linear distortion to the extreme edge of its angular field. Its aperture, f:4.5. Componar — These f:3.5 and f:4.5 lenses are special flat field enlarging cbjectives for the amateur and professional photographer. They are well-corrected for spherical aberration, color, coma, and astigmatism. They come in several sizes and are adapted for use with instruments with or without enlargers. 1 ^/VARIABLE AREA RECORDERS1 ^T PATENT NO. I9855M. OTHERS PENDING § ALSO ^^ 35 mm to 16 mm ^^ X tjl REDUCTION SOUND PRINTER €jt M O SOUND EQUIPMENT Ijfc * * ^/ Cable address CRSCO ^} O C.R. SKINNER MFG. Co. ' I 125 HVOE STREET PHONE ORDWAY 6909 m k k San Francisco. California U. S. A. , \ ^B ^ A Please mention The International Photographer when corresponding with advertisers.