Proposed methods of electronic noise reduction at a listerers ears have been described previously [J. Acoust. Soc. Am. 28, 773 (A) 1956)]. A headset incorporating electronic noise reduction has been termed an active ear defender. One method proposed uses a microphone under each earphone in a negative feedback arrangement. Another method makes use of a microphone located outside the earphone cushion to provide a canceling signal. The canceling arrangement is very sensitive to amplitude and phase errors. Examination of its electrical analog reveals that a surprisingly large band width is necessary because of the critical phase requirements. The band width required for the negative feedback arrangement is also large but amplitude and phase characteristics within the useful band are less critical. In this paper the microphone and earphone characteristics required for effective noise reduction are discussed. A negative feedback system is described which provides approximately 15 db of noise reduction from 100 to 200 cps. Component characteristics which will make possible greater amounts of noise reduction are discussed. (Work supported by U. S. Air Force and U. S. Army Signal Corps.)
Combined feedback-feedforward active noise-reducing headset-The effect of the acoustics on broadband performance
High quality magnetic tape recordings were made of individual bells at the carillon location, including bells cast by Meneely and Company, Watervliet, New York; Van Aerschodt, Louvain, Belgium; John Taylor and Company, Loughborough, England; Franz Schilling Söhne, Apolda, Germany. Measurements made on the tapes with a sound analyzer, Stroboconn and an automatic level recorder give the relative tuning, the relative amplitudes at the microphone position, and the decay rates of the various components in the bell tone.
High quality magnetic tape recordings were made of individual bells at the carillon location, including bells cast by Meneely and Company, Watervliet, New York; Van Aerschodt, Louvain, Belgium; John Taylor and Company, Loughborough, England; Franz Schilling Söhne, Apolda, Germany. Measurements made on the tapes with a sound analyzer, Stroboconn, and an automatic level recorder gave the relative tuning, the relative amplitudes at the microphone position, and the decay rates of the various components in the bell tone. The hum tone, strike tone, minor third, perfect fifth, octave of the strike tone, octave plus a major third and octave plus a perfect fifth, as well as other higher components, will be demonstrated audibly with the aid of a sound analyzer.
Direct-radiator loudspeakers are often mounted with the back of the diaphragm working into a completely enclosed space. Conventional theory states that when the maximum linear dimension of such an enclosure is small compared with the wave-length, the pressure is uniform throughout, and the acoustical impedance presented to the loudspeaker is --j/w(V/od), where V is the enclosed volume. Although it has not been clearly established how small an enclosure must be before it is "small compared with the wave-length," the foregoing expression is generally used, at low audiofrequencies, to calculate the acoustical impedance of closed loudspeaker housings.It is shown here that while the acoustical impedance of a closed rectangular housing is capacitive at very low frequencies, it passes through zero as the frequency increases and becomes that of an inertance as the frequency of the first normal mode is approached. For a typical housing 11 in. X22 in. X22 in., the frequency at
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