Showing papers on "Noise reduction published in 1978"

High Velocity Jet Noise Source Location and Reduction. Task 2. Theoretical Developments and Basic Experiments.

Abstract: : This report presents the results of Task 2 conducted under the subject program over a period of 45 months. Task 2 was formulated as a fundamental theoretical and experimental study aimed at an understanding of the noise generation and suppression mechanisms of high velocity jets. The mechanisms investigated included changes in turbulence structure, fluid shielding, and alteration of convective amplification of jet noise sources. Several other areas such as physical shielding, shock associated noise, lip noise, effect of fluid/particle additives on jet noise, orderly structure in jets, ejector aeroacoustics, and flight effects on jet noise were also investigated. The most significant achievements of Task 2 were as follows. A unified aeroacoustic theory composed of three ingredients were developed: (1) the prediction of mean properties of the jet plume, (2) deduction of turbulence properties relevant to jet noise by similarity arguments, and (3) the prediction of far-field noise including the effects of fluid shielding. A semiempirical shock noise prediction procedure was also developed. Exhaustive theory-data comparisons for a wide range of nozzle configurations and velocity/temperature combinations were conducted and have confirmed the essential validity of this model as a prediction tool. A comprehensive series of experiments with simple suppressor elements (such as a single rectangular tube, twin jets, linear arrays of jets, circular arrays of jets) was carried out and revealed the importance of acoustic shielding by adjacent jets. A fundamental series of experiments, specifically tailored to reveal fluid shielding as a jet noise suppression mechanism, was successfully conducted.

A noise reduction

Abstract: A noise reduction. A hollow body 1 is traversed by several passages 2, juxtaposed and separated, oriented in the direction of noise propagation, and length decreasing as one moves away from a vertical area. Noise can pass freely beyond the body 1 and intersects the noise that has passed through the passages 2 and is out of phase, resulting in an attenuation zone in rear of the device. Application protection against trains and road noise.

Digital Removal of Random Media Image Degradations by Solving the Diffusion Equation Backwards in Time

Abstract: We consider the image restoration problem with Gaussian-like point spread functions and reformulate it as an initial value problem for the backwards diffusion equation. This approach leads to rigorous bounds on the reliability of the restoration, as a function of the noise variance, without any assumptions on the spectral characteristics of either signal or noise.In the latter half of the paper, we then describe a powerful algorithm, based on the above reformulation, and successfully use it to restore a turbulence degraded image. Typically, a complete restoration and display requires 10 seconds of CDC 7600 computing time, for a $128 \times 128$ image. We also describe a restoration experiment where Gaussian blur was simulated, and multiplicative noise added according to Huang’s model. The algorithm performs competently even at low signal to noise ratios.

A Digital Noise Reducer for Encoded NTSC Signals

Abstract: Until recently, most of the random noise problems that have long plagued television engineers could only be attacked at the source. The recent advent of practical digital television technology, however, now makes it possible to process a television signal on an element-by-element basis. The noise reduction system described in this paper uses a digital frame store operating as an adaptive recursive filter, under the control of a motion detector employing comb filtering and signal modification. Using these techniques, an improvement of 15 dB can readily be achieved without introducing objectionable artifacts into the picture. Among the many areas of application of the digital noise reducer are: improvement of ENG material (both live and taped, especially at low light levels); processing of multigeneration 1-in or 2-in videotape; microwave reception; CATV satellite reception; telecine film-grain reduction (monochrome and color); and off-air reception.

Jet noise source modification due to forward flight

Abstract: XTENSIVE use of noise-absorbi ng materials in the inlet and exhaust ducting of modern high bypass ratio turbo fan engines has reduced the noise generated by fans, compressors, turbines, and burners to the level that the jet noise (noise generated in the exhaust plume by mixing the high velocity jet with the ambient air) is an important part of the total noise signature of current aircraft. The characteristics of jet noise from turbojet and turbofan engines have been well documented under static conditions. However, aircraft noise certification limits must be satisfied under actual aircraft flyover conditions during takeoff and approach operations. Thus, it is important that methods be developed to more accurately predict the jet noise under flight conditions. The effect of flight on jet noise of a circular jet exhaust has been simulated by testing in wind tunnels.1'2 Noise measurements obtained in wind tunnel tests show that the jet exhaust noise of nozzles operating subsonically is reduced in flight from the static levels at all measurement angles. However, noise measurements obtained from some flyover tests3 have shown less noise reduction than the results of the wind tunnei tests. In order to help resolve the differences in wind tunnel and flyover results, it is necessary to understand the effects of flight on the fundamental mechanisms of jet noise generation. Measuring the changes caused by flight in the aerodynamic parameters responsible for noise generation and relating these changes to the measured noise reductions caused by flight will provide a more basic understanding of the effect of flight on jet noise than is currently available. This understanding will allow eventual improvements in our ability to develop more accurate in-flight predictions. The noise reduction in flight is generally attributed to changes in the strength of the acoustic sources distributed throughout the jet shear layer. The acoustic source strength at a point in the jet shear layer is determined from the local aerodynamic flow properties. The mean and turbulence flow quantitites needed to determine the acoustic source strength are mean velocity, mixing layer growth, turbulence intensity, integral length scale, eddy convection velocity, and integral

Automobile Interior Noise Reduction Using Finite Element Methods

Abstract: Low-frequency interior noise in the automobile passenger compartment can be significantly affected by the vibration behavior of the body panels surrounding the enclosed cavity. An acoustic finite element method for computing panel-excited interior noise is reviewed and an approach outlined for identifying potentially noisy panels adjacent to the passenger compartment. To illustrate its potential, the analytical method is applied to a production automobile. A structural modification suggested by the procedure is shown to reduce significantly the low-frequency interior noise to which the occupant is exposed. Experimental verification of the method is presented.

A new technique for reducing test section noise in supersonic wind tunnels

Abstract: A new technique is presented for obtaining a low-noise test environment in a high-speed wind tunnel. This technique utilizes the fact that the primary noise source for high supersonic/hypersonic wind tunnels is radiated noise from the turbulent, tunnel-wall boundary layer. Because of the high directionality of sound in supersonic flows this test section noise originates far upstream on the walls of the nozzle at the 'acoustic origin'. It is shown that tailoring the nozzle contour to reduce the acoustic origin Mach number significantly reduces the noise level in the upstream half of the nozzle test rhombus. Experimental noise measurements are presented from a conventional, Mach 5 nozzle and are compared with measurements from a rapid-expansion, Mach 5 nozzle.

Digital Image Processing by Use of Local Statistics.

Abstract: : Computational techniques involving contrast enhancement and noise filtering on two-dimensional image arrays were developed, based on their local mean and variance. These algorithms are nonrecursive and do not require a transform. They share the same characteristics in that each pixel is processed independently. Consequently, this approach has an obvious advantage when used in real-time digital image processing applications and where parallel processors can be used. For both the additive and multiplicative cases, the a priori mean and variance of each pixel is derived from its local mean and variance. Then, the minimum mean-square-error estimator in its simplest form is applied to obtain the noise-filtering algorithms. For multiplicative noise a statistical optimal linear approximation is made. Experimental results show that such an assumption yields a very effective filtering algorithm. Examples on images containing 256 x 256 pixels are given. Results show that in most cases the techniques developed in this report are readily adaptable to real-time image processing. (Author)

Noise Control Handbook of Principles and Practices

Abstract: NOISE CONTROL HANDBOOK OF PRINCIPLES AND PRACTICES Engineering, law, health care, architecture and design, transportation, government administration—each in its own way is concerned with the control and elimination of noise pollution from the environment. This handbook provides a basic guide for preparing, enforcing, and meeting noise control requirements. General enough to contain all the essentials of noise control, it is sufficiently detailed to meet specialized needs in areas such as aircraft noise reduction and housing insulation. You need no prior knowledge of the noise control field for a full understanding of this material. Each chapter is written by a recognized expert and concentrates on a particular aspect of noise abatment. The first sections focus on a basic explanation of sound and noise. Here you will discover the theoretical basis for the commonly used measuring techniques, as well as how to judge which instrumentation is best suited to your particular needs and situation. Practical applications of the important and widely used Equivalent Sound Level system of measuring noise are thoroughly explained. Using common industrial and construction situations, the book demonstrates the basic principles of noise control and acoustical treatment in the conservation of human hearing. This coverage includes necessary technical formulas, advance design methods, and explanations of sound test codes and legislation. There is a full discussion of both sides of the airport'aircraft noise controversy. The authors examine the measurement of noise levels, legislative and regulatory efforts to keep these levels with:n safe limits, and the impact of lonj terrfi exposure to noise on the human population. The Handbook deals in realistic teims with the problems of vehicular engine and tire noise levels. And vou will find a complete section on noise leveis In tn« home—how they can adversely effect people and how to design and insulate against them. In addition, the authors fully explain how to go about writing an acceptable Environmental Impact Statement. You learn how to write, word and properly define the limits in noise control ordinances. To round out the comprehensive coverage, you also find a thorough evaluation of the noise levels of recreational activities and vehicles, such as power boats, off-road motorcycles and snowmobiles. Whether you are a layman concerned with the ecological impact of noise or an engineer concerned with machine design, Noise Control will serve as your basic reference to today's critical problems in noise reduction and abatmer. l.

Systems for reducing noise in video signals using amplitude averaging of undelayed and time delayed signals

Abstract: Noise reduction of a signal such as a video signal is effected by averaging a plurality of relatively delayed replica signals, e.g. with line or field delays therebetween, in a decoding processor, the highly redundant information content of the signal being reinforced by the averaging while random noise undergoes a partial cancellation. In order to overcome the problem of signal changes caused by the decoding processor, the signal is encoded prior to application to the noise-introducing signal channel by a complementary encoding processor in which one or more replica signals are subtractively combined with one or more other replica signals. In both processors a variable combining means is employed to exclude from the combination replica signals which have a non-small difference from the replica signals in the combination, thereby to avoid smearing of the information content (with consequent picture smearing in the case of a video signal) when the signal is changing, e.g. because of movement in a picture.

Linear predictive coding of speech signals in a high ambient noise environment

Abstract: Preservation of both the spectral distribution and the periodicity of speech signals are essential in speech processing. This paper describes a method of speech coding in a high ambient noise environment and shows that the spectral envelope of speech signal is a most reliable information when the noise reduction method proposed in this paper is used. Also reported in this paper comparisons of several pitch extraction methods with extensive experimental data, based on which a pitch extraction method suited for noisy speech signals is proposed.

Noise transmission through flat rectangular panels into a closed cavity

Abstract: Five panels backed by a closed cavity were studied experimentally and analytically to determine the noise transmission characteristics of the coupled panel-cavity system. The closed cavity was studied both with and without fiber-glass lining to provide either an absorbent or a reverberant acoustic space. The effects on noise reduction of cavity absorption, measurement location within the cavity, panel mass, and panel stiffness were examined. Results indicated that both measurement location and absorption in the cavity have significant effects on the noise reduction. Increasing panel mass improves the noise reduction at almost all frequencies, and increasing panel stiffness improves nose reduction below the fundamental resonance frequency. A simple, one dimensional analytical model was developed which provided good agreement with the experimental results.

Noise reduction circuit for stereo signals

Abstract: Noise reduction circuit for stereo signals comprising a control input terminal, first and second outputs as well as a controllable coupling circuit for a controllable mutual signal coupling between the two outputs, the two outputs being decoupled from one another only during those sound passages wherein an effective after-effect of a stereo impression is produced to obtain an optimal combination of noise level and stereo effect.

Backlash filter apparatus

Abstract: A backlash filter apparatus for the removal of spurious electrical components of a pre-established amplitude level from a voltage signal. The filter apparatus provide a backlash window to which the input signal comprising a desired signal and its spurious noise component is applied. The spurious noise component is removed in the backlash window, thus providing a jitter-free filter output.

Acoustic evaluation of a novel swept-rotor fan. [noise reduction in turbofan engines

Abstract: Inlet noise and aerodynamic performance are presented for a high tip speed fan designed with rotor blade leading edge sweep that gives a subsonic component of inlet Mach number normal to the edge at all radii. The intent of the design was to minimize the generation of rotor leading edge shock waves thereby minimizing multiple pure tone noise. Sound power level and spectral comparisons are made with several high-speed fans of conventional design. Results showed multiple pure tone noise at levels below those of some of the other fans and this noise was initiated at a higher tip speed. Aerodynamic performance of the fan did not meet design goals for this first build which applied conventional design procedures to the swept fan geometry.

Noise reduction in optical measurement systems

Abstract: The cost of conventional current and voltage measurement equipment has led to an increasing interest in systems using a minimum of expensive insulation. At CERL, several optical systems for the measurement of current and voltage have been produced. These devices are found to suffer from noise due to intensity fluctuations of the light source and the wandering of the light beam across the photodetector surface caused by vibration and atmospheric turbulence. In the paper several possible techniques to reduce this noise are outlined, and a novel modulation scheme which has produced an improvement in the signal/noise ratio of over 20 dB is described. A device for the simultaneous measurement of voltage and current, making use of this system, is soon to go on trial at a generating station.

Helicopter cabin noise: Methods of source and path identification and characterization

Abstract: Internal noise sources in a helicopter are considered These include propulsion machinery, comprising engine and transmission, and turbulent boundary layer effects It is shown that by using relatively simple concepts together with careful experimental work it is possible to generate reliable data on which to base the design of high performance noise control treatments

Noise minimization in RC-active filters using generalized impedance converters

Abstract: A procedure has been given for minimizing the total output noise of a Generalized Impedance Converter (GIC), subject to constraints dictated by signal handling capability of the Operational Amplifiers and ease of microcircuit fabrication. The noise reduction is achieved only by the adjustment of RC elements of the GIC, and the total output noise after optimization in the example cited is close to the theoretical lower limit. The output noise of a higher-order filter can be reduced by RC-optimizing the individual GIC's of the active realization. Experimental results on a 20–24 kHz channel bank band-pass filter demonstrate the effectiveness of the above procedure.

A Composite Grip Design for Elimination of Extraneous Noise During Acoustic Emission Testing

Abstract: A major problem in acoustic emission analysis during dynamic loading is the noise generated by the test system itself. A composite grip assembly, designed on the principle of rapid attenuation of sound waves when they pass through multiple interfaces, can reduce the background noise by almost two orders of magnitude. This allows a maximum amount of information to be obtained through acoustic emission analysis.

Insulating buildings from aircraft noise

Abstract: Field measurements of insulation against aircraft noise were made for a wide range of building components. Single‐figure ratings of noise insulation were derived, and a design procedure for use of these ratings is presented. The dependence of the effective sound insulation on the spectral balance of aircraft noise is examined.

A theoretical investigation of noise reduction through the cylindrical fuselage of a twin-engine, propeller-driven aircraft

Abstract: Interior noise in the fuselage of a twin-engine, propeller-driven aircraft with two propellers rotating in opposite directions is studied analytically. The fuselage was modeled as a stiffened cylindrical shell with simply supported ends, and the effects of stringers and frames were averaged over the shell surface. An approximate mathematical model of the propeller noise excitation was formulated which includes some of the propeller noise characteristics such as sweeping pressure waves around the sidewalls due to propeller rotation and the localized nature of the excitation with the highest levels near the propeller plane. Results are presented in the form of noise reduction, which is the difference between the levels of external and interior noise. The influence of propeller noise characteristics on the noise reduction was studied. The results indicate that the sweep velocity of the excitation around the fuselage sidewalls is critical to noise reduction.

Reducing noise in recursive digital filters by residue retention

Abstract: This letter describes the application of residue-retention techniques, normally associated with digital differential analysers, to reduce round-off noise within short-word-length implementations of recursive digital filters. Simulations show significant improvements over single- and partial double-precision realisations, especially if high sampling rates are used.

Comparison of measured and calculated helicopter rotor impulsive noise

Abstract: The thickness noise theory is discussed. Two full-scale rotors were tested in a wind tunnel with several tips involving changes in chord, thickness, and sweep. Impulsive noise data reduction procedures used are described. The calculated and measured impulsive noise peak pressures as a function of advancing tip Mach number are compared, showing good correlation for all rotors considered.