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Showing papers on "Noise reduction published in 1975"


Journal ArticleDOI
01 Jul 1975
TL;DR: This paper describes some of the applications to which the omnipresent noise in electronic circuits and devices has been put and the applications where noise is a conceptual or theoretical tool.
Abstract: The omnipresent noise in electronic circuits and devices is generally considered undesirable. This paper describes some of the applications to which it has been put. Short descriptions of a wide variety of applications are given together with references for further details. The applications fall in four categories: applications in which noise is used as a broad-band random signal; measurements in which the random noise is used as a test signal; measurements in which noise is used as a probe into microscopic phenomena; and the applications where noise is a conceptual or theoretical tool. Many examples of applications in each of these categories are given. Some of the applications included are only of historical interest now, and a few are, as yet, only proposals.

44 citations


Journal ArticleDOI
TL;DR: In this paper, the application of homomorphic filtering in marine seismic reflection work is investigated with the aims to achieve the estimation of the basic wavelet, the wavelet deconvolution and the elimination of multiples.
Abstract: The application of homomorphic filtering in marine seismic reflection work is investigated with the aims to achieve the estimation of the basic wavelet, the wavelet deconvolution and the elimination of multiples. Each of these deconvolution problems can be subdivided into two parts: The first problem is the detection of those parts in the cepstrum which ought to be suppressed in processing. The second part includes the actual filtering process and the problem of minimizing the random noise which generally is enhanced during the homomorphic procedure. The application of homomorphic filters to synthetic seismograms and air-gun measurements shows the possibilities for the practical application of the method as well as the critical parameters which determine the quality of the results. These parameters are: a) the signal-to-noise ratio (SNR) of the input data b) the window width and the cepstrum components for the separation of the individual parts c) the time invariance of the signal in the trace. In the presence of random noise the power cepstrum is most efficient for the detection of wavelet arrival times. For wavelet estimation, overlapping signals can be detected with the power cepstrum up to a SNR of three. In comparison with this, the detection of long period multiples is much more complicated. While the exact determination of the water reverberation arrival times can be realized with the power cepstrum up to a multiples-to-primaries ratio of three to five, the detection of the internal multiples is generally not possible, since for these multiples this threshold value of detectibility and arrival time determination is generally not realized. For wavelet estimation, comb filtering of the complex cepstrum is most valuable. The wavelet estimation gives no problems up to a SNR of ten. Even in the presence of larger noise a reasonable estimation can be obtained up to a SNR of five by filtering the phase spectrum during the computation of the complex cepstrum. In contrast to this, the successful application of the method for the multiple reduction is confined to a SNR of ten, since the filtering of the phase spectrum for noise reduction cannot be applied. Even if the threshold results are empirical, they show the limits for the successful application of the method.

33 citations



Journal ArticleDOI
TL;DR: The state of the art for understanding, predicting, and control of airframe noise, levels and spectral content of the noise, correlation with important variables, and noise generation mechansims are discussed.
Abstract: Progress in quieting the commercial aviation fleet has been achieved by reducing the noise generated by jet engines. Recent tests have indicated that noise produced by airflow over aircraft surfaces (lifting surfaces, landing gear, flaps, and cavities) is only 8 to 10 EPNdB below certification requirements for current aircraft and will likely be a design consideration for aircraft of the future as engines become still quieter. This paper reviews the state of the art for understanding, predicting, and control of airframe noise. Levels and spectral content of the noise, correlation with important variables, and noise generation mechansims are discussed. The noise floors for future aircraft, the direction of research projects, and likely impact of this new technology on aircraft design are indicated.

26 citations


01 Jan 1975
TL;DR: In this paper, a background discussion including levels of acoustical sophistication and units of measure is presented as an aid to understand the requirements of an aircraft noise prediction system, which are generally available.
Abstract: Aircraft noise prediction programs which are generally available are described. A background discussion including levels of acoustical sophistication and units of measure is also presented as an aid to understanding the requirements of an aircraft noise prediction system.

19 citations


Journal ArticleDOI
TL;DR: A new and convenient technique for reducing both speckle and coherent noise in holographic imagery and its advantages and disadvantages have been mentioned in the context of other known techniques.
Abstract: A new and convenient technique for reducing both speckle and coherent noise in holographic imagery is described with supporting experimental results. Its advantages and disadvantages have been mentioned in the context of other known techniques.

17 citations


01 Jul 1975
TL;DR: In this paper, the relationship between aircraft operating conditions and interior noise and the degree to which ground testing can be used in lieu of flight testing for performing interior noise research was studied.
Abstract: The relationships between aircraft operating conditions and interior noise and the degree to which ground testing can be used in lieu of flight testing for performing interior noise research were studied. The results show that the noise inside light aircraft is strongly influenced by the rotational speed of the engine and propeller. Both the overall noise and low frequency spectra levels were observed to decrease with increasing high speed rpm operations during flight. This phenomenon and its significance is not presently understood. Comparison of spectra obtained in flight with spectra obtained on the ground suggests that identification of frequency components and relative amplitude of propeller and engine noise sources may be evaluated on stationary aircraft.

14 citations



Journal ArticleDOI
TL;DR: Time-coherent averaging has been implemented on a computer with 12,000 12-bit words of core memory, suitable for on-line use in any clinical laboratory and tested for its efficiency in eliminating ectopic beats from the average.
Abstract: Electrical noise often hampers the interpretation of ECG and other heart-synchronous signals. Time-coherent averaging is a commonly used method to reduce the noise. This method has been implemented on a computer with 12,000 12-bit words of core memory, suitable for on-line use in any clinical laboratory. An analysis was made of which demands apply to the exactness of synchronization of added heart cycles. The method was tested with respect to these demands and also, on a limited scale, for its efficiency in eliminating ectopic beats from the average.

10 citations


Proceedings ArticleDOI
01 Feb 1975
TL;DR: In this article, the sources of diesel engine noise and methods for its abatement are analyzed and described, and various concepts for light weight acoustic engine casings which are closely fitted to the engine without acoustical lining or even integrated into the engine structure are demonstrated, including one concept for future light weight engines featuring a sound attenuating housing.
Abstract: The sources of diesel engine noise and methods for its abatement are analysed and described. Typically, the overall engine noise is determined by the engine surface noise which, however, can hardly be reduced by more than 5 dB(A) with conventional noise abatement techniques. For reductions of 10 dB(A) or more, it is necessary to attenuate the noise of all external engine parts by a complete encasing of the engine. Various concepts for light weight acoustic engine casings which are closely fitted to the engine without acoustical lining or even integrated into the engine structure and which provide a 15 to 20 dB(A) noise reduction are demonstrated, including one concept for future light weight engines featuring a sound attenuating housing.

10 citations


Proceedings ArticleDOI
01 Feb 1975
TL;DR: In this article, several techniques used to define quantitatively the problem of excessive noise through engine structural vibration are presented. But, these techniques include both operating engine tests and bench tests, and analytical techniques are shown which give a better understanding of how the critical components within the engine cause this vibration.
Abstract: This paper presents several techniques used to define quantitatively the problem of excessive noise through engine structural vibration. These techniques include both operating engine tests and bench tests. In addition, analytical techniques are shown which give a better understanding of how the critical components within the engine cause this vibration. Through the use of analytical and experimental techniques, examples illustrate practical solutions for diesel engine noise reduction.


Journal ArticleDOI
01 Jan 1975-Frequenz
TL;DR: It is shown that assigning the poles and zeros "closest to each other" is not always an optimum solution and the dynamic programming method which yields the exact solution is described.
Abstract: Digital filters are often realized by a cascade of second order sections. Each multiplier in the filter generates round-off noise. The noise power at the output is minimized by a suitable selection of the pole-zero pairs, the gain factors and the ordering of the 2nd order sections. These degrees of freedom are optimized simultaneously by dynamic programming which yields an optimal solution. Examples for this method will be presented. It is shown that assigning the poles and zeros \"closest to each other\" is not always an optimum solution. Digitale Filter werden h ufig als Kettenschaltung von Teilfiltern 2. Grades realisiert. Jeder Multiplizierer im Filter erzeugt Rundungsrauschen. Die Rauschleistung am Ausgang kann minimiert werden durch geeignete Wahl der Γοΐ-Nullstellen-Zuofdnung, der Verst rkungsfaktoren und der Reihenfolge in der Kettenschaltung f r die Teilfilter 2. Grades. Diese Freiheitsgrade werden mit dynamischer Programmierung simultan behandelt, was eine optimale L sung liefert. An Hand von Beispielen wird gezeigt, da die Zuordnung von direkt benachbarten Polen und Nullstellen nicht immer eine optimale L sung darstellt. F r die Dokumentation: Digitales Filter / Rundlingsrauschen / Optimierung / Dynamisches Programmieren Introduction Digital filters are often realized by a cascade of 2nd order sections. The multipliers in the filter generate round-off noise. The noise power at the output of the filter should be minimized. To achieve this goal we shall first point out the degrees of freedom for the minimization. Then we shall describe the dynamic programming method which yields the exact solution. Finally we shall give some examples and compare them with previous solutions. The degrees of freedom We start our investigation with a given rational transfer function


PatentDOI
TL;DR: In this paper, a structural discontinuity is introduced at a point along the length of the tool which is a predetermined axial distance from a selected antinode of the bending mode shape to provide an impedance mismatch at that point.
Abstract: A method of modifying a percussion tool to provide noise reduction, and a percussion tool having effective noise damping characteristics. In the method of the present invention, a troublesome noise frequency radiated by the tool is identified, and a resonant bending mode shape for the tool is determined, which bending mode shape corresponds to the identified noise frequency. The tool is then assembled and a structural discontinuity is introduced at a point along the length of the tool which is a predetermined axial distance from a selected antinode of the bending mode shape to provide an impedance mismatch at that point. The tool, which is of predetermined length, is formed of one or more sections, each of which is of a selected length. The lengths of the sections are carefully determined such that when the tool is in assembled condition, impedance mismatches are provided at selected points along the length of the tool, to provide noise reduction of several troublesome noise frequencies.

01 Aug 1975
TL;DR: In this paper, a semi-empirical method for predicting dipole noise radiation from a strut with incident turbulence was proposed, which was in good agreement with data from small-scale models.
Abstract: Methods were developed for predicting externally blown flap (EBF) noise and turbomachinery strut noise. Noise radiated by under-the-wing and upper-surface-blowing EBF configurations is calculated as a sum of lift dipole noise, trailing edge noise, and jet quadrupole noise. Resulting predictions of amplitudes and spectra generally were in good agreement with data from small-scale models. These data cover a range of exhaust velocity, flap deflection, exhaust nozzle position, exhaust nozzle shape, and ratio of exhaust nozzle diameter to wing chord. A semi-empirical method for predicting dipole noise radiation from a strut with incident turbulence was in good agreement with data. Leading-edge regions made of perforated plate backed by a bulk acoustic absorber achieved up to 7 db reduction of strut noise caused by incident turbulence at high frequencies. Radial turbulence in a turbofan exit duct was found to have a relatively high level associated with the mean velocity defect in the rotor blade wakes. Use of these turbulence spectra and a dipole noise radiation equation gave general prediction of measured aft-radiated sound power caused by a splitter ring in a full-scale fan exit duct.

Journal ArticleDOI
TL;DR: In this article, a sidelobe reduction filter was proposed to improve both resolution and clutter performance over standard techniques such as fast time constant (FTC), delay line differentiator (DLD), and pulse length discriminator (PLD).
Abstract: Optimal detection of rectangular pulses in noise is considered, subject to a sidelobe constraint which ensures adequate resolution capabilities, and a new sidelobe reduction filter is derived. Tests in the laboratory and on a Westinghouse AN/TPS-27 search radar system em indicate that use of the new filter substantially improves both resolution and clutter performance over such standard techniques as fast time constant (FTC), delay line differentiator (DLD), and pulse length discriminator (PLD).

01 May 1975
TL;DR: In this article, interior noise levels of existing helicopters are discussed along with an ongoing experimental program directed towards reducing these levels, including measurements taken before and after installation of an acoustically-treated cabin.
Abstract: The interior noise levels of existing helicopters are discussed along with an ongoing experimental program directed towards reducing these levels. Results of several noise and vibration measurements on Langley Research Center's Civil Helicopter Research Aircraft are presented, including measurements taken before and after installation of an acoustically-treated cabin. The predominant noise source in this helicopter is the first stage planetary gear-clash in the main gear box, both before and after installation of the acoustically treated cabin. Noise reductions of up to 20 db in some octave bands may be required in order to obtain interior noise levels comparable to commercial jet transports.


Journal ArticleDOI
TL;DR: Results demonstrate that the Doppler shift can be extracted from automotive noise and related to the vehicle's speed, and may be found useful in large scale traffic speed and density monitoring systems.
Abstract: An alternative method of speed detection is presented based on the Doppler effect in vehicular noise. A procedure is developed to correlate the noise frequency spectrum as the vehicle approaches an observer with the spectrum as it moves away. Limitations of the technique are evaluated using empirical data obtained under typical traffic conditions. These results demonstrate that the Doppler shift can be extracted from automotive noise and related to the vehicle's speed. Although sources of inaccuracy are significant at lower speeds, a resolution of ±5 percent was easily achieved at 60 mph. Such a technique may be found useful in large scale traffic speed and density monitoring systems.

Journal ArticleDOI
17 Apr 1975
TL;DR: The design of nonrecursive and recursive digital filters using linear least squares or linear minimum variance is described, and a polynomial state model is first utilized that results in a completely time-varying filter.
Abstract: The design of nonrecursive and recursive digital filters using linear least squares or linear minimum variance is described. This method of design requires a model, and since polynomial approximations are widely used in laboratory automation, a polynomial state model is first utilized. Then an exact scalar signal model is employed that results in a completely time-varying filter. For both models, the design leads to the recursive Kalman filter. The operation of the filters for noise reduction is first demonstrated for simulated data, and design forms are compared. For the exact scalar signal model, noise reduction and peak separation are demonstrated using simulated data and data obtained from a mass spectrometer. By using the correct model, excellent peak separation and noise reduction can be obtained.


Journal ArticleDOI
TL;DR: In this paper, the authors describe a new electronic circuit, the time domain difference amplifier (TDAM), used to reduce strong drifts which bury weak repetitive signals, where the input, with signal and drift, is sampled at signal on and signal off states, and subtracted from each other.
Abstract: This paper describes a new electronic circuit, the time domain difference amplifier, used to reduce strong drifts which bury weak repetitive signals. The principle is that the input, with signal and drift, is sampled at signal on and signal off states, and subtracted from each other. The random drift is reduced by integrating the subtracted values. Drift suppression is about 35 dB for 3.0 Hz ac drift and 100 repeatings for integration.

01 Aug 1975
TL;DR: In this article, the authors describe the procedures for predicting community noise exposure resulting from aircraft operations using acoustic measurement test conditions, the acoustic data reduction and interpretation procedures, and the type of aircraft performance information needed to obtain a noise data file for predicting the noise produced by aircraft operations.
Abstract: : This report is one of a series describing the research program undertaken by the Aerospace Medical Research Laboratory to develop procedures for predicting community noise exposure resulting from aircraft operations. This report outlines the acoustic measurement test conditions, the acoustic data reduction and interpretation procedures, and the type of aircraft performance information needed to obtain a noise data file for predicting the noise produced by aircraft operations. The procedures result in the descriptions of the noise of an aircraft in terms of the effective perceived noise level, the sound exposure level and several other noise measures. Level flight measurements and static engine noise tests are described, and are applicable to conventional fixed wing aircraft and helicopters.


Journal ArticleDOI
TL;DR: In this paper, the authors characterize flight vehicles according to their manner of operation and type of propulsion system; and their associated sources of noise are identified, including exhaust jets, fans, propellers, rotors, airflow-surface interactions and reciprocating engine exhausts.

01 Aug 1975
TL;DR: In this article, the authors present the theoretical development of a model for the prediction of noise radiated by elevated structures on rail transit lines, in particular, for urban rail transit track and elevated noise and vibration control.
Abstract: The report presents the theoretical development of a model for the prediction of noise radiated by elevated structures on rail transit lines. In particular it deals with noise and vibration control for urban rail transit track and elevated noise and vibration control for urban rail transit track and elevated structures. The model allows for the prediction of both the vibration transmission between elements of the structure and the resulting noise radiation from each major structural element, in terms of design parameters for the different elements. Thus the potential effectiveness of various alternative methods for noise control can be evaluated. Results of a field study of three different types of elevated structure on the MBTA Rapid Transit System are also summarized. These results support the validity of the prediction model. The engineering application of the prediction model is discussed in another report.



01 Jul 1975
TL;DR: In this paper, an analytical model of a typical community has been developed for the purpose of carrying out a cost-effectiveness analysis of various outdoor noise countermeasures, including source reduction, source rerouting, and path-receiver treatments.
Abstract: An analytical model of a typical community has been developed for the purpose of carrying out a cost-effectiveness analysis of various outdoor noise countermeasures. The principal elements of the model are (1) A data base describing the community's geography, population distribution, and typical building construction; (2) a data base describing the location and strength of noise sources; (3) energy summation of noise levels from each source at many locations; and (4) use of noise level versus human response transfer functions to determine the number of people adversely affected by this noise level. Summing these for the whole community and dividing by the total population results in a 'Noise Impact Index (NII)' indicating the quality of the noise environment. Three basic types of countermeasures may be applied: (1) source reduction, (2) source rerouting, and (3) path-receiver treatments.