Showing papers on "Noise reduction published in 1977"

Minimum uncorrelated unit noise in state-space digital filtering

Abstract: For the class of digital filters described by the state equations, new expressions for the roundoff noise and the dynamic-range constraint equation have been established. Then, minimization of the roundoff noise subject to the l 2 norm dynamic-range constraint is considered. Using the polar decomposition of a nonsingular matrix, the effects of an equivalent transformation on the noise are analyzed, then, a lower bound and the global minimum of the unit noise, i.e., the noise generated-under the assumption that each state equation contains exactly one noise source, is obtained. A method of realizing the minimum unit noise filter is given, and possible further optimization of the filter is discussed. A numerical example is given to illustrate the computational procedure.

Airframe Noise Prediction Method

Abstract: : A noise component method is presented for calculating airframe noise. Noise from clean wing and tail surface is represented as trailing edge noise caused by the turbulent boundary layer. Landing gear noise is given by an empirical representation of model data. Trailing edge flap noise is modeled as a lift dipole normal to the deflected flap, with amplitude and spectrum given by a correlation of flyover data. Measured flyover data for the Prue-2 sailplane, Aero Commander Shirke general aviation airplane, Lockheed JetStar business jet, Boeing 747 commerical jet, and Convair F-106B delta wing airplane are used as test cases. These spectra are compared with predictions by this method and the NASA ANOPP total aircraft method and drag element method. Methods for reducing various components of airframe noise are examined and evaluated.

Noise reduction by radiographic magnification.

Abstract: Effective noise in the rare-earth system can be substantially reduced by use of radiographic magnification technique. The basic imaging properties of the rare-earth system (Alpha 8-XM) and the medium-speed system (Par-RP) were evaluated by the Wiener spectra, modulation transfer functions, and H&D curves. The visibility of radiographic details was determined with simple test objects. Results indicate that, as the magnification increases, the effective noise in the rare-earth system is reduced, and that the visibility of small structures can be improved.

Bidimensional noise reduction system for television

Abstract: A bidimensional noise reduction system for processing a television video signal includes a vertical dimension noise reduction system having a first adjustable threshold coring circuit and a horizontal noise reduction system having a second adjustable threshold coring circuit.

GE Core Engine Noise Investigation - Low Emission Engines.

Abstract: : This program was directed towards elements of combustor and turbine noise; the latter including turbine tone interaction with jet stream turbulence Combustor (Core) Noise - Investigations were conducted to determine the variables affecting source strength, spectrum shape, and farfield directivity This investigation include scale model tests to evaluate the effects of exhaust nozzle geometry on radiation patterns of low frequency noise A full-scale combustor rig test was used to identify the controlling variables of combustor noise at the source Two engine tests were run to validate the findings from the scale model tests and add to the overall data base of core noise measurements The relationship between combustor source noise and emissions was studied and qualitative trends identified for advanced low emissions combustors Turbine Noise - Studies were made of the attenuation of high frequency turbine noise by downstream blade rows, the broadband noise generation by turbines, and the controlling parameters for turbine tone/jet stream interaction This included a turbine rig test in single and multistage configurations, along with a unique data acquisition system Scale model tests were used to define the effect of the pertinent aero-acoustic parameters on turbine tone scattering by jet stream turbulence The results of these investigations were used to improve the component prediction techniques derived under the Core Engine Noise Control Program (FAA-RD-74-125, III) These improved prediction techniques were used to predict the noise contribution of each source for high bypass turbofan engines representative of current and advanced technology (Author)

Nozzle Exit Flow Profile Shaping for Jet Noise Reduction

Abstract: Mixing of primary and secondary flows in a conventional turbofan engine provides a means of reducing jet noise. By shaping the nozzle exit velocity profile, noise reduction greater than that resulting from fully mixed flow has been achieved. In a static jet noise experiment, five primary flow nozzles were used with a common secondary nozzle to simulate exhaust flows of turbofan engines with bypass ratios from 1 to 5. Data are shown which relate jet noise to the location, extent, and magnitude of the peak velocity region. In general, minimum noise is obtained for inverted profiles where the outer area peak velocity is 5. to 15% greater than the reference uniformly mixed velocity, and the area of the peak velocity region is 40 to 50% of the total flow area. The inverted flow profiles produce noise characteristics similar to multielement jet suppressor nozzles, i.e., low frequencies are reduced and high frequencies are increased. It is shown that these spectral effects can be used to obtain a balanced noise signature.

Combustion Noise Investigation

Abstract: : Improved methods for predicting both direct and indirect combustion noise from aircraft engines are developed and experimentally evaluated by conducting rig experiments and by comparing with data from several full scale engines. Comparison of predictions with full scale engine data indicated that direct combustion noise is the dominant source for the P and WA engines investigated. The direct combustion noise prediction system includes expressions for acoustic power level, peak frequency and full-scale engine acoustic transmission loss due to combustor/duct coupling and turbine attenuation. These expressions are derived in terms of readily available performance and geometry parameters from the burner and turbine. New parameters introduced by the prediction system include the effects of fuel nozzle number and burner length. Predictions are shown to be in good agreement with data obtained from component rig tests on several JT8D type burner configurations (including single and multiple fuel nozzle, conventional and low emission designs). In addition, when transmission losses are accounted for, the predictions are also shown to be in good agreement with observed combustion noise levels and peak frequencies from four P and WA turbofan engines (i.e. the JT8D-109, JT9D-7A, JT9D-70 and the prototype JT10D). Predicted combustion noise directivity patterns and spectra shapes are determined empirically, using the data from both the rig tests and these four engines. Results from the analytical and experimental combustion noise investigations are used to identify combustion noise reduction methods obtainable through modifications in burner design and/or performance parameters.

Summary of forward velocity effects on fan noise

Abstract: Available experimental data comparing the in-flight and static behavior of fan noise are reviewed. These results are then compared with recent data obtained for a fan stage tested with forward velocity in a low speed wind tunnel. Tentative conclusions are presented about the significance and nature of the changes in noise observed when a forward velocity is imposed. Finally, the implications of the emerging picture of in-flight fan source noise for suppressor design are discussed.

Loudspeaker system equalization

Abstract: Variations in the amplitude-frequency response characteristic of a loudspeaker are compensated by a drive circuit in which an audio input signal is summed with a plurality of delayed replicas of the input signal to provide a drive signal for the loudspeaker. The delayed replicas are derived by discrete time filtering. Noise reduction techniques applied to the least delayed of the plurality of delay signals substantially improves the S/N ratio in the drive signal.

Improving linear prediction analysis of noisy speech by predictive noise cancellation

Abstract: The analysis of speech using Linear Prediction is reformulated to account for the presence of acoustically added noise and a technique is presented for reducing its effect on parameter estimation. The method, called Predictive Noise Cancellation (PNC), modifies the noisy speech autocorrelations using an estimate of present background noise which is adaptively updated from an average all-pole noise spectrum. The all-pole noise spectrum is calculated by averaging autocorrelations during non-speech activity. The method uses procedures which are already available to the LPC analyzer, and thus is well suited for real time analysis of noisy speech. Preliminary results show signal to noise improvements on the order of 10 to 20 db.

Noise emission measurements for regulatory purposes

Abstract: A review is given of the measurement needs attendant to regulation of the noise generated and emitted by commercial products. The emphasis is primarily on measurement procedures for use in conjunction with point-of-sale regulations as opposed to regulations on the noise which a source actually emits when in operation. The report is divided into three major parts. Part I is a discussion of overall measurement requirements and the type of data and information which are needed in order to promulgate regulations based on appropriate measurement techniques. Part II is designed as a checklist for the evaluation of the suitability of a noise measurement standard for a particular class of products or, in the absence of a suitable standard, as a framework for development of one. The intent is to identify and discuss in some detail those factors which can impact on the accuracy, precision, and applicability of a noise measurement process. Part III consists of a series of flow charts depicting the development of appropriate procedures for the measurement of product noise emission.

Interior noise analysis and control for light aircraft

Abstract: This paper describes experimental and analytical studies of the interior noise of twin-engine, propeller-driven, light aircraft Experimental results indicate that interior noise levels due to propeller noise can be reduced by reduction of engine rpm at constant airspeed (about 3 dB), by synchronization of the twin engines/propellers (up to 12 dB), and by increasing the distances from propeller tip to fuselage The analytical model described uses modal methods and incorporates the flat-sided geometrical and skin-stringer structural features of light aircraft Initial results show good agreement with measured noise transmitted into a rectangular box through a flat panel

Noise-free contrast improvement with a low frequency polarizing filter: a practical evaluation.

Abstract: Contrast improvement in a coherent image enhancement system by very low spatial frequency attenuation is shown, demonstrating possible errors incurred by incorrect attenuation. A novel way to achieve a continuous range of low frequency attenuation is introduced. The variable attenuation is produced by constructing the spatial filter from polarizing material and altering the relative polarization between the object transparency illumination and the spatial filter. This technique gives the observer control over a continuous change in contrast enhancement and greatly increases the versatility of low frequency spatial filtering. Coherent imaging systems are usually contaminated by noise, and a simple way to reduce the noise content in the image is shown.

Interior noise reduction in a large civil helicopter

Abstract: The results of an evaluation of the effectiveness of current noise reduction technology in attaining acceptable levels of interior noise in a large (about 20,000 kg) passenger-carrying helicopter are presented. The helicopter studied is a modified CH-53A with a specially designed, acoustically treated passenger cabin. The acoustic treatment reduced the average A-weighted interior noise levels from 115 db to 87 db. The study suggests selected improvements in the acoustic treatment which could result in additional reduction in cabin noise levels. The resulting levels would be only slightly greater than the interior noise levels of current narrow-body jet transports.

Airframe noise measurements by acoustic imaging

Abstract: Studies of the noise produced by flow past wind tunnel models are presented. The central objective of these is to find the specific locations within a flow which are noisy, and to identify the fluid dynamic processes responsible, with the expectation that noise reduction principles will be discovered. The models tested are mostly simple shapes which result in types of flow that are similar to those occurring on, for example, aircraft landing gear and wheel cavities. A model landing gear and a flap were also tested. Turbulence has been intentionally induced as appropriate in order to simulate full-scale effects more closely. The principal technique involves use of a highly directional microphone system which is scanned about the flow field to be analyzed. The data so acquired are presented as a pictorial image of the noise source distribution. An important finding is that the noise production is highly variable within a flow field and that sources can be attributed to various fluid dynamic features of the flow. Flow separation was not noisy, but separation closure usually was.

Steerable steel wheel systems and wheel noise suppression

Abstract: Steel wheel/steel rail vehicles have a long and successful history in transportation; however, the irritating screech noise generated while negotiating sharp curves is a characteristic of these systems. This paper shows how steerable wheel systems can reduce curving noise, determines the necessary accuracy for effective noise reduction. The most effective method to reduce squeal is to provide accurate steering, and some methods of improving steering systems are suggested.

Noise reduction circuit for multiple gap magnetic head for automatic typewriter

Abstract: A noise reductive circuit for use with an automatic typewriter of the type that uses a multiple inductance, multiple magnetic gap head for translating information to and from a magnetic recording media is disclosed which includes an electric circuit between one of the inductances to the signal handling channel of the other of the inductances so as to couple to it a noise reducing signal.

Noise filter for TV signals - attenuates low amplitude components and adds to own delayed output

Abstract: The noise filter, for TV signals, removes noise whilst leaving subtle changes in picture unaffected. The output (18) of the filter is delayed (20) by one frame (2 fields) and passed to an adder and a subtractor. The subtractor (12) deducts it from the input signal (10) and the adder (16) adds it to the output of a filter (14). The filter input is connected to the output of the subtractor. The filter attenuates components with low amplitudes whilst leaving those with larger amplitudes relatively unattenuated.