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234 IX. SOUND-EECOEDING EQUIPMENT AND AEEANGEMENT
plified designs of this general nature available, improved performance and a reduction in manufacturing costs should be possible. 16-mm-equipment users would do well to watch the development trend very closely, since the microphone often holds the key to the excellence of the recorded sound.
Almost all up-to-date microphones are of low connection impedance, either 50 ohms or 250 ohms, and are characterized by a conductor moving in a magnetic field of high flux density provided by a light yet efficient magnet. (Alnico 5 is a typical magnet material.) The trend toward smaller and lighter microphones will demand improved materials and still closer tolerances in manufacture. Considerable improvement in ruggedness should likewise be expected as a result of the experience gained during World War II with intercommunication microphones and
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Fig. 49D. Response-frequency characteristics of RCA MI-10,001 microphone.
electrical instruments, such as photoelectric exposure meters. It should be mentioned that there is a standardization movement in the direction of making the connection impedance 150 ohms for all microphones. This impedance has also been proposed as the standard connection impedance for other low and medium-level circuits. At the time of writing, however, few commercial microphones with the proposed connection impedance are being supplied regularly.
Highly Directional Microphones
Although not ordinarily suited to sound-film recording, this group of microphones is entitled to passing mention. Typical of the group are the parabolic reflector microphone and the line microphone.
A typical parabolic reflector microphone consists of a shallow, dishlike, approximately parabolic reflector about 3 feet in diameter with a pressure-type microphone located at the approximate focus of the para