Showing papers on "Noise reduction published in 1971"
TL;DR: In this paper, three analytical techniques for predicting the noise spectrum resulting from non-uniform spacing of events for the purpose of reshaping noise-frequency spectra are presented, focusing on a semigraphical design technique using Bessel functions.
Abstract: This paper presents techniques for determining nonperiodic spacing of events for the purpose of reshaping noise‐frequency spectra. The object of the application is to provide reduced noise levels and the redistribution of the frequencies at which there is noise energy so as to generate fewer perceptible sounds. Three analytical techniques for predicting the noise spectrum resulting from nonuniform spacing are presented. Emphasis is focused on a semigraphical design technique using Bessel functions, which has proven to be of great value in the actual selection of modulated event spacing. Fourier analysis of an impulse approximation and a sinusoidal wave approximation are two other alternative techniques presented. The techniques outlined are applied to the problem of reshaping the noise spectrum of a 22‐blade fan in a 5‐hp induction motor. A comparison of predicted results provides an evaluation of the alternative approaches.
52 citations
TL;DR: In this article, the authors provide a general technical background for the problems of noise due to rotating blades and present a bibliography of some of the more recent work in the areas of rotor and propeller noise, compressor and fan noise, and duct acoustics.
36 citations
TL;DR: In this paper, the authors present noise calculations for Gunn amplifiers with both uniform and non-uniform field distributions, and show that amplifiers having an injection-limited cathode contact, and hence a uniform field distribution, exhibit significantly lower noise figures than those having an ohmic cathode contacts, which yields a non-uneiform field distribution.
Abstract: The letter presents noise calculations for Gunn amplifiers with both uniform and nonuniform field distributions. The results indicate that amplifiers with an injection-limited cathode contact, and hence a uniform field distribution, exhibit significantly lower noise figures than those having an ohmic cathode contact, which yields a nonuniform field distribution. A further reduction of noise could be obtained by using materials with high negative mobility.
32 citations
TL;DR: In this article, the use of spectrum analyzers and X-Y recording techniques with radio noise meters was investigated to reduce time and improve the quality of measurements of EHV power line radio noise.
Abstract: In order to reduce time and improve the quality of measurements* of EHV power line radio noise, the use of spectrum analyzers and X-Y recording techniques with radio noise meters was investigated. Comparisons were made with the usual manual readings taken simultaneously from radio noise meters used as the standard measuring instruments. With the spectrum analyzer, measurements of radio noise from conductor corona both in rain and fair weather conditions were made in the 1 MHz frequency region. In addition, the interference from a gap-type source was successfully measured over a frequency range from 10 kHz to 1 GHz.
18 citations
01 Dec 1971
TL;DR: The work performed on noise in avalanche transit-time devices is reviewed and presented in detail in this paper, where both theoretical and experimental results on the noise mechanisms and performance of these devices when employed as oscillators, amplifiers, and self-oscillating mixers are included.
Abstract: The work performed on noise in avalanche transit-time devices is reviewed and presented in detail. Both theoretical and experimental results on the noise mechanisms and performance of these devices when employed as oscillators, amplifiers, and self-oscillating mixers are included. Suggestions for further studies in this area are also given.
18 citations
Patent•
25 Aug 1971
TL;DR: In this article, an improved band splitter divides the input signals into a plurality of sub-bands wherein all signal components in each sub-band have equal time delays associated therewith.
Abstract: A noise reduction system is provided for reducing the effects of noise on low level signals sent across a transmission path. Input signals to be transmitted are logarithmically amplified such that low level signals are strengthened relative to higher level signals. At the opposite end of the transmission path the signals are logarithmically de-emphasized to reconstitute the original input signals. An improved band splitter divides the input signals into a plurality of sub-bands wherein all signal components in each sub-band have equal time delays associated therewith. Logarithmic amplification is provided for each subband.
13 citations
TL;DR: The results of preliminary subjective experiments with the 3 bits/sample pseudorandom codec are given for a sampling frequency of 12 MHz and indicate the practical limitations of a theory which predicts a high standard of picture quality.
Abstract: The paper describes an experimental system constructed to allow investigation of pseudorandom dither signals for coarse quantization of 625-line monochrome television siginals. The system employs a separate encoder/decoder for the television synchronizing pulses so that the picture signal can be allowed to occupy the full nonsaturating range of the video encoder. Dither samples are added to the video signal before coding and subtracted at the decoder and are negatively correlated to maximize the picture signal-to-noise ratio according to a subjective noise weighting function. The power spectral distribution of the quantizing noise is derived and, assuming this to be picture independent and random, pre- and deemphasis networks are used to increase the signal-to-noise ratio. Empirical methods are used to determine the network parameters. The results of preliminary subjective experiments with the 3 bits/sample pseudorandom codec are given for a sampling frequency of 12 MHz and indicate the practical limitations of a theory which predicts a high standard of picture quality.
7 citations
TL;DR: In this paper, an RC filter was placed between the detector and recorder and the change in the height of a chromatographic peak depending on the ratio of time constant and standard deviation of the peak was used for noise reduction.
5 citations
01 Jan 1971
TL;DR: In this paper, a phototransistor operating in the common-base configuration was used to minimize the switching transients in the video output of a photo-sensor array.
Abstract: Switching transients in the video output of a photo-sensor array can be minimized using a phototransistor operating in the common-base configuration. Experimental results have been found to agree with theoretical calculations of signal and noise.
4 citations
01 Mar 1971
TL;DR: Noise measurement evaluation of takeoff and approach profiles optimized for noise abatement is presented in this paper, where the authors evaluate the performance of a takeoff and an approach profile with respect to noise.
Abstract: Noise measurement evaluation of takeoff and approach profiles optimized for noise abatement
4 citations
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08 Apr 1971
TL;DR: In this paper, the authors present a method for predicting the noise reduction by simple beamforming of an array, which gives an accurate estimate of the known capabilities of LASA, and is applied to the design of possible new arrays.
Abstract: : We present a method for predicting the noise reduction by simple beamforming of an array. The method gives an accurate estimate of the known capabilities of LASA, and is applied to the design of possible new arrays. The method requires field measurement of noise and signal correlations as a function of distance.
Journal Article•
TL;DR: By rotating the lens about the optical axis these diffraction patterns can be essentially eliminated and the technique does not destroy the spatial coherence of the light and permits spatial filtering of the input plane.
Abstract: Optical imaging systems using coherent light introduce objectionable noise into the output image plane. Dust and bubbles on and in lenses cause most of the noise in the output image. This noise usually appears as bull's-eye diffraction patterns in the image. By rotating the lens about the optical axis these diffraction patterns can be essentially eliminated. The technique does not destroy the spatial coherence of the light and permits spatial filtering of the input plane.
TL;DR: Methods of noise reduction for the tractor are outlined along with other approaches to hearing preservation to predict an 'acceptable' noise limit for the variable noise environment on the agricultural tractor.
01 Jan 1971
TL;DR: In this paper, the development of noise measurement units for airport and aircraft noise reduction and psychoacoustic studies is discussed, with the aim of reducing the noise at airports and aircraft.
Abstract: Development of noise measurement units for airport and aircraft noise reduction and psychoacoustic studies
TL;DR: Photographic transparencies were prepared with precisely determined quantities of gaussian and Poisson noise and submitted to observers for subjective ranking of image quality to obtain average signal-to-noise ratio based upon the normalized probability density function.
Abstract: Photographic transparencies were prepared with precisely determined quantities of gaussian (signal-independent) and Poisson (signal-dependent) noise and submitted to observers for subjective ranking of image quality. Average values of noise and signal-to-noise ratio, based upon the relative variance of the signal, were calculated for the imagery: (a) assuming equal a priori probabilities of allowed signal levels; (b) using a normalized probability density function for signal levels derived from scene statistics. Best correlation with the results of subjective quality ranking was obtained with an average signal-to-noise ratio based upon the normalized probability density function.
TL;DR: In this article, a technique to reduce holographic-recording noise is described, which is based on encoding the object wave field with a finely structured, phase-changing screen, and subsequently retrieving the desired image wave field by passing the reconstructed wave field through an identical, or closely matched screen.
Abstract: A technique to reduce holographic-recording noise is described. The technique is based on encoding the object wave field with a finely structured, phase-changing screen, and subsequently retrieving the desired-image wave field by passing the reconstructed wave field through an identical, or closely matched screen. The object-shape-dependent noise, which is generated in the recording process, can be reduced by this technique. As an example, experimental results are presented showing noise suppression to approximately 3%.
01 Jul 1971
TL;DR: Inlet noise suppressors having perforated plate over honeycomb wall construction evaluated over range of passage heights and engine speeds using turbojet engine as noise source as discussed by the authors, inlet noise suppression system was used.
Abstract: Inlet noise suppressors having perforated plate over honeycomb wall construction evaluated over range of passage heights and engine speeds using turbojet engine as noise source