Showing papers on "Noise reduction published in 1976"

Angle Estimation with Adaptive Arrays in External Noise Fields

Abstract: A method of estimating the angle of arrival of a signal at an array of sensors in an external noise environment is outlined. The development is based on a maximum likelihood estimator and leads naturally to adaptive sum and difference beams which null the external noise sources. An algorithm for estimating angle of arrival, based on the outputs of adaptively distorted sum and differnce beams, is shown to perform well in the presence of sidelobe and/or main beam interference.

Noise reduction system

Abstract: A method of and apparatus for reducing the noise of an input signal, such as a composite video signal. An orthogonal transform of the input signal is derived, this orthogonal transform having n transformed signal components. At least some of these signal components are suppressed if their respective signal levels are less than a threshold level. Preferably, the n transformed signal components correspond to different portions of the input signal; and those components representing the higher frequency portions are suppressed if their signal levels are less than the threshold level. Following this signal suppression, the n transformed signal components are reconverted into substantially the input signal, less the suppressed components.

Noise reduction in television signals

Abstract: Reduction of noise in a television signal is achieved by comparing signals from successive frame or field scans and noting the differences. Low amplitude differences are assumed to represent noise and are attenuated. Differences above a predetermined level are assumed to represent movement and are not so attenuated by a factor K. Where there are inherent differences between the signals compared, for example they are different fields of an interlaced scan or are color signals with different subcarrier phases, the signal from the earlier scan is modified to compensate for these differences and provide a signal for comparison which is a better estimate of the expected signal on the later scan.

Noise reduction apparatus

Abstract: Improvement of the signal-to-noise ratio for a wideband video signal is effected by subtraction from the wideband video signal of a signal which follows low-amplitude excursions of the video signal's high frequency component. The latter signal is formed by a circuit arrangement which includes a high pass filter, of a C-R differentiator form, coupled to the input of an amplifier. A negative feedback path, coupled between the amplifier output and the junction of capacitive and resistive elements of the differentiator includes threshold devices which permit activation of the negative feedback path only during input signal excursions which exceed selected close-to-axis limits. The threshold device disposition relative to the high pass filter elements is such that negative feedback path activation also serves to introduce an appreciable reduction in the effective time constant of the differentiator, whereby a rapid return from a large input signal excursion beyond one of the selected limits may be followed at the amplifier input. In an illustrative use, the noise reduction circuitry is advantageously employed to process a luminance signal component separated by comb filtering from a composite signal recovered during playback of a video disc, with the subtraction step being effected in apparatus also serving to recombine separated luminance and chrominance components for formation of the disc player's output composite signal.

The Generation and Radiation of Supersonic Jet Noise. Volume 4. Shock-Associated Noise Data

Abstract: : The characteristics of the sound field of shock-containing under- expanded jets are studied by measuring the noise from a two-inch diameter convergent nozzle over an extensive envelope of supercritical jet operating conditions. The measurements, which are accurate and comprehensive, were conducted in the Lockheed anechoic facility. The results are presented in this Volume in a systematic manner in the form of narrowband spectra. The details pertinent to the experimental program and the data presentation format are summarized below. The total noise spectrum from an incorrectly expanded jet flow contains discrete components (or screech) in addition to the basic turbulent mixing noise and the broadband shock-associated noise. In order to study the trends and dependencies of the broadband component accurately, it is vital to keep the contamination by screech to a minimum in the experimental program. In the present experiments, screech suppression was successfully achieved by wrapping all surfaces surrounding the nozzle exit with sound absorbing material, and incorporating a small projection inside the nozzle lip. This projection interrupts the feedback loop between the first shock and the nozzle exit plane, which has been previously proposed and verified as the physical mechanism of screech generation. The detailed calibration tests, conducted prior to the main shock-associated noise experiments, established that all results are essentially uncontaminated by the presence of screech.

The Generation and Radiation of Supersonic Jet Noise. Volume 2. Studies of Jet Noise, Turbulence Structure and Laser Velocimetry

Abstract: : This volume discusses (1) turbulent mixing noise tests and observations relative to effects of temperature and Mach number on intensity, directivity, and spectra; (2) numerical solutions of the Lilley theory for sound radiated from point sources simulating small-scale turbulence noise sources, and comparisons of these solutions with experimental data relating to temperature and velocity effects on directivity and spectra; (3) the theory describing the development of the large-scale coherent motion of the jet structure and the far- field noise radiated from this turbulence; (4) a detailed discussion and interpretation of the jet turbulence and mean velocity data; and (5) a comprehensive description of the shock-associated noise tests, a preliminary description of the broadband shock-associated noise model, and a discussion of shock-associated noise in the overall jet noise picture

Lip noise generated by flow separation from nozzle surfaces

Abstract: The results of a series of experiments, performed to investigate flow separation and classic lip noise and to aid in understanding aeroacoustic noise generation are presented. Several types of nozzle-lip configurations were used to study the high frequency noise generated by small regions of flow separation at the nozzle lip. These included coaxial nozzles, and circular and slot nozzles with splitter plates. The jet flow velocity was varied and far field noise was measured for all nozzle-lip geometries (coaxial and splitter plate). The effect of a velocity difference across the lip of the coaxial nozzle and the splitter plate on the far field noise was also measured. Finally, an effort was made to find means to reduce the high frequency noise caused by flow separation at the lip.

Automatic noise reduction system of FM stereo receiver

Abstract: Disclosed is an automatic noise reduction system of FM stereo receiver wherein the input level and carrier frequency deviation of an FM stereo transmission signal received by an antenna are detected and, when at least either one of the input level and carrier frequency deviation decreases, the separation between first and second audio outputs of a stereo demodulator for demodulating the FM stereo transmission signal is decreased and simultaneously high frequency components on the first and second audio outputs are attenuated.

Airframe self-noise: Four years of research

Abstract: A critical assessment of the state of the art in airframe self-noise is presented. Full-scale data on the intensity, spectra and directivity of this noise source are evaluated in the light of the comprehensive theory developed by Ffowcs-Williams and Hawkins. Vibration of panels on commercial aircraft is identified as a possible additional source of airframe noise. The present understanding and methods for prediction of other component sources - airfoils, struts, and cavities - are discussed, and areas for further research as well as potential methods for airframe noise reduction are identified. Finally, the various experimental methods which have been developed for airframe noise research are discussed and sample results are presented.

Stereo noise reduction circuit

Abstract: A stereo noise reduction circuit is disclosed having circuitry responsive to the automatic gain control voltage generated within an FM stereo receiver for reducing the noise associated with low levels of the RF carrier. A first portion of the noise reduction circuit provides for out-of-phase noise cancellation when the automatic gain control voltage reaches a first level and a second portion provides for rolling-off the higher frequencies of the output audio signals when the automatic gain control voltage reaches a second level.

Skid control system having a switched low speed noise filter

Abstract: A switched noise filter circuit for a skid control system which is adapted to filter out noise signals picked up by the wheel sensors as a result of brake squeal which often occurs at low vehicle speeds. Unfiltered, the sudden termination of a noise signal of this type could be interpreted as a rapid wheel deceleration by the skid control system causing a dumping of the brake pressure, resulting in an inadvertent release of the brakes.

Measurement, analysis, and prediction of aircraft interior noise

Abstract: Considerations of comfort of passengers and crew in light aircraft and helicopters indicate substantial benefits may be obtained by the reduction of interior noise levels. This paper discusses an ongoing research effort to reduce interior noise in such vehicles. Data from both field and laboratory studies for a light aircraft are presented. The laboratory data indicate that structural vibration is an efficient source of interior noise and should be considered in the reduction of interior noise. Flight data taken on a helicopter before and after installation of acoustic treatment demonstrate that over 30 dB of noise reduction can be obtained in certain portions of the spectra. However, subjective evaluations of the treated vehicle indicate that further reductions in interior noise are desirable. An existing interior noise prediction method which was developed for large jet transports was applied to study low-frequency noise in a light aircraft fuselage. The results indicate that improvements in the analytical model may be necessary for the prediction of interior noise of light aircraft.

Economic welfare impacts of urban noise

Abstract: The basic purpose of this project was to develop a conceptual framework for estimating the social welfare gains or benefits of reducing current noise levels in urban environments. The project has concentrated on developing economic welfare theory and empirical techniques to assess willingness-to-pay by individuals for noise avoidance. Particular attention was paid to noise produced by motor vehicles and noise produced by operations at construction sites. The theoretical effect of the localized nature of noise on people's willingness-to-pay to control noise was investigated. An efficient pricing scheme for aggregate noise disturbance was devised, based on people's willingness-to-pay for noise reduction. A questionnaire was developed to elicit responses on the physical and psychic costs of noise in urban areas. The attempts to assign dollar values to the costs of noise pollution by determining people's willingness-to-pay to control or reduce noise.

Low-frequency noise reduction of lightweight airframe structures

Abstract: The results of an experimental study to determine the noise attenuation characteristics of aircraft type fuselage structural panels were presented. Of particular interest was noise attenuation at low frequencies, below the fundamental resonances of the panels. All panels were flightweight structures for transport type aircraft in the 34,050 to 45,400 kg (75,000 to 100,000 pounds) gross weight range. Test data include the results of vibration and acoustic transmission loss tests on seven types of isotropic and orthotropically stiffened, flat and curved panels. The results show that stiffness controlled acoustically integrated structures can provide very high noise reductions at low frequencies without significantly affecting their high frequency noise reduction capabilities.

Sources and characteristics of interior noise in general aviation aircraft

Abstract: A field study has been conducted to examine the interior noise characteristics of a general aviation aircraft. The purposes of the study were to identify the major noise sources and their relative contribution and to establish the noise transmission paths and their relative importance. Tests were performed on an aircraft operating under stationary conditions on the ground. The results show that the interior noise level of light aircraft is dominated by broadband, low frequencies (below 1,000 Hz). Both the propeller and the engine are dominant sources; however, the contribution from the propeller is significantly more than the engine at its fundamental blade passage frequency. The data suggests that the airborne path is more dominant than the structure-borne path in the transmission of broadband, low-frequency noise which apparently results from the exhaust.

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.

A Correlation Method for Measurement of Variable Magnetic Fields

Abstract: A correlation technique is described, which allows a sensible reduction of the noise in power spectrum measurements of the interplanetary magnetic field by means of the search coil magnetometer. Two sensors are required that detect the same signal. If the noises of the two amplifying channels are uncorrelated, the timeaverage of the product of the two channel outputs depends on the power of the magnetic field signal, but is not affected by the noises of the two channels. The application of the proposed technique to space-borne instrumentation leads to a relevant reduction in weight and bulk because of the lower sensitivity required from the sensor. A search coil magnetometer has been developed that is based on the use of sensors of small length (9.5 cm) and that exhibits an equivalent input noise smaller than 10-4/f2 expressed in ?2/Hz as magnetic noise power spectrum.

Noise elimination technique in holography.

Abstract: A method has been described for reducing speckling and the noise arising due to cosmetic defects present in the elements of the hologram recording geometry by using a multifrequency grating–diffuser–lens combination for object illumination at the time of recording the hologram. The averaging of the speckling is simulated at the plane of the object. The experimental results show that this method of object illumination is quite effective for noise reduction and may be applied to recording noise-free holograms of shortlived events. It may also be useful in holographic microscopy.

Core Engine Noise Program. Volume III. Prediction Methods -- Supplement I. - Extension of Prediction Methods

Abstract: The core noise prediction technique described in Volume III was validated using several additional sets of engine data The data included discernible core noise at high power settings and were derived from both General Electric and external tests, on engines by GE and other manufacturers The three line power level prediction method was collapsed to single unified line prediction through addition of a turbine work extraction term to account for the low frequency noise attenuation due to turbine blading Data from combustor component tests were compared to engine noise levels and found to indicate significant attenuation of low frequency noise in propagation through turbine stages An analytical method for predicting this low frequency noise attenuation is provided (Author)