An electrical system is described which permits the use of sound film, with its limited signal-to-noise ratio, as a recording medium for wide-range stereophonic reproduction of symphonic music. Noise reduction is accomplished both by pre-equalization, rising to 18 db above 8000 cycles, and by automatic signal compression and expansion of 30 db. To secure maximum suppression of noise and freedom from distortion, a pilot-operated, fiat-top compandor system was selected. In each channel low level signals are recorded on a separate track with constant gain 30 db above normal, which places them above the film noise. Higher-level signals cause automatic gain reductions and are recorded at substantially full modulation. These signals vary the intensity of a pilot tone, which in turn controls the compressor gain. There is a pilot frequency for each of the three channels, and the three are combined and recorded together on the fourth film track. During reproduction they are separated by filters, and operate expandors which restore the signals to their original forms but reduce the noise to inaudible levels. The compressor and expandor gains are made proportional to pilot level in db, and the expandor range over which this relation holds is 45 db. Therefore a 15-db variation in average pilot level during reproduction causes a corresponding average level change but no distortion. This is used to allow expansion of the original signal intensity range during recording or re-recording by simple gain controls in the pilot circuits. The paper describes the apparatus and circuits developed to accomplish these results, and discusses the frequency, load, distortion, noise, and dynamic characteristics of both constant and variable gain elements. Also included are considerations of microphone and loudspeaker arrangement and equalization to secure high fidelity of reproduction.
Acoustic pressure fields generated by large-caliber rapid-fire weapons are sufficiently intense to constitute sources of concern to airplane designers. The operation of such weapons could result in greatly increased equipment and structure vibration and danger of hearing damage to flight personnel. Acoustic pressures from both fuselage and wing-mounted M-61-A1 guns have been measured on the fuselage and wing of an A-7 airplane. The measurements covered an area extending several feet fore and aft of the gun muzzles. Both static and flight firings were measured. Recordings of acoustic pressure were analyzed for waveform, instantaneous amplitude distributions, spectra, and spacial distribution. The waveform for a burst of fire is approximately a series of decaying sinusoids at the gunfire repetition rate. Peak-to-rms ratios as high as 5 to 6 are not uncommon. On the fuselage near the gun muzzle, peak pressures can reach 2 to 3 psi (177–181 dB SPL). The possible effect of these high-pressure fields on airplane design and operation is discussed in relation to structure and equipment vibration and personnel safety.
Wente's impact on engineering acoustics commenced in 1916 with his invention of a practical condenser microphone. This and his following contributions to early motion picture sound recording, high fidelity sound reproduction, stereophony, room acoustics, instruments and facilities for acoustic measurement, plus other items are recalled by the authors. Both were witness to many of the later developments while involved in operation and experiment with associated equipment at Bell Laboratories, beginning in the late twenties and continuing for two decades.
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