Showing papers on "Noise measurement published in 1980"

Suppression of acoustic noise in speech using two microphone adaptive noise cancellation

Abstract: Acoustic noise with energy greater or equal to the speech can be suppressed by adaptively filtering a separately recorded correlated version of the noise signal and subtracting it from the speech waveform. It is shown that for this application of adaptive noise cancellation, large filter lengths are required to account for a highly reverberant recording environment and that there is a direct relation between filter misadjustment and induced echo in the output speech. The second reference noise signal is adaptively filtered using the least mean squares, LMS, and the lattice gradient algorithms. These two approaches are compared in terms of degree of noise power reduction, algorithm convergence time, and degree of speech enhancement. Both methods were shown to reduce ambient noise power by at least 20 dB with minimal speech distortion and thus to be potentially powerful as noise suppression preprocessors for voice communication in severe noise environments.

Low-Noise Cooled GASFET Amplifiers

Abstract: Measurements of the noise characteristics of a variety of gallium-arsenide field-effect transistors at a frequency of 5 GHz and temperatures of 300 K to 20 K are presented. For one transistor type detailed measurements of dc parameters, small-signal parameters, and all noise parameters (T/sub min/, R/sub opt/, X/sub opt/ g/sub n/) are made over this temperature range. The results are compared with the theory of Pucel, Haus and Statz modified to include the temperature variation. Several low-noise ampifiers are described including one with a noise temperature of 20 K over a 500-MHz bandwidth. A theoretical analysis of the thermal conduction at cryogenic temperatures in a typical packaged transistor is included.

Transcutaneous Blood Flow Measurements Using Pseudorandom Noise Doppler System

Abstract: In order to obviate the limitations of the conventional coherent ultrasonic Doppler flowmeters we have developed a new device. In this apparatus the continuous wave is replaced by a pseudorandom signal. We show, both theoretically (Waag et al. [12]) and experimentally, that the pseudorandom system offers advantages. The most important result concerns the improvement of the signal-to-noise ratio which can be as large as 20 dB. Furthermore, it is possible to decrease the frequency and the distance ambiguity by a factor of two. Compared to the "random noise flowmeter" (Newhouse et al. [7], Jethwa et al. [5]) our system measures flow near the wall of vessels or in ultrasonic field in which there are important fixed echoes. To illustrate the feasibility of this new device, we show some results obtained by the transcutaneous method.

Location and propagation of shock associated noise from supersonic jets

Abstract: Shock associated noise from unheated supersonic jets is investigated through acoustic measurements in both the near and far fields. The peak Helmholtz number of broadband shock noise is found to be independent of nozzle pressure ratio when based on the length of the shock cells and the ambient speed of sound. This indicates that the acoustic wavelengths generated in the shock noise process are limited in size by the shock cell spacing. Excellent agreement between power spectral densities measured at various far-field angles is obtained at and above the peak shock noise frequency when source convection effects are included. Results show a directivity of broadband shock noise pointed in the upstream direction, with omnidirectionality being approached only at high pressure ratios. The relative importance of shock noise with respect to jet mixing noise is found to be maximum near the pressure ratio at which a Mach disc begins to form in the jet. Near-field measurements point to the downstream shock cells as the region where the dominant shock noise eminates from the jet.

Is Wiener Filtering an Effective Method of Improving Evoked Potential Estimation

Abstract: In order to obtain better estimates of the true evoked potential signal with fewer stimuli than is possible with classical signal averaging, the Wiener theory [16] for optimum linear filtering, as formulated by Walter [15] and Doyle [5], was implemented in four different versions of the Wiener filter?two a posteriori filters and two recursive filters. The effectiveness of the filters in separating signal from noise was tested in two simulations with known signal, and both simulated noise and real spontaneous brain wave activity. Correlation coefficients between the known test signals and filtered and unfiltered averages of test signal plus noise showed no significant difference between filtered and unfiltered averages.

Master Plan for Prediction of Vehicle Interior Noise

Abstract: Substantial advances have been made in improving the analytical methods available for prediction of interior noise and providing an experimental data base for their assessment and validation. A review of recent accomplishments, including representative results, is presented in the context of a Master Plan. It is suggested that modal representations of structural walls and acoustic cavities provide a useful conceptual and computational theoretical framework for including the mass, stiffness and damping of both the structure and acoustic cavity. Geometrical details and acoustic damping (absorption) are included in a rigorous manner. Future efforts, both near term and long term, are identified which are required to complete the Master Plan and provide the noise control practitioner with access to these improved methods.

Non-Wiener solutions of the adaptive noise canceller with a noisy reference

Abstract: The performance of the adaptive noise canceller is typically analyzed in terms of Wiener filter theory. However, it has recently been shown by Glover that a narrow-band signal in the reference at a frequency for which there is no correlated signal in the primary gives rise to a notch in the output spectrum, a solution which lies outside the scope of the classical theory. In this paper an analytic expression for the output spectrum is derived for the more general case of a reference signal containing a sinusoid in white noise. It is found that although the notch depth is a decreasing function of the reference signal-to-noise ratio, the exact relationship also depends on the primary input; thus, with noise present, there is no longer a time-invariant relationship between the output and primary spectra. These results are shown to reduce to those of Glover and the classical Wiener filter in the appropriate contexts.

A Performance Study of Control Systems with Dead Time

Abstract: This paper discusses the performance of the Smith Dead Time (SDT) controller for systems with appreciable dead time. Tuning, modeling accuracy, and the effects of parameter variations and measurement noise are all studied using a simulated second-order process with dead time. The corresponding performance of the conventional proportional plus integral (PI) controller is used as a reference for comparison. It is shown that the SDT controller yields improved results over the PI controller if the plant is fairly stationary and the noise level is moderate. Otherwise adaptive filtering and control will be needed to maintain the superiority of the SDT controller over the PI controller.

Airframe Noise Component Interaction Studies

Abstract: Acoustic wind-tunnel tests were conducted to examine the noise-generating processes of an airframe during approach flight. The airframe model was a two-dimensional wing section, to which high-lift leading and trailing edge devices and landing gear were added. Far-field conventional microphones were utilized to determine component spectrum levels. An acoustic mirror directional microphone was utilized to examine differences in noise source distributions on airframe components extended separately and in combination. Measured spectra are compared with predictions inferred from aircraft flyover data. Aeroacoustic mechanisms for each airframe component are identified. Component interaction effects on total radiated noise generally were small (within about 2 dB). However, some interactions altered local flow velocities and turbulence levels, causing redistribution of local acoustic source strength. Possibilities for noise reduction exist if trailing edge flaps could be modified to decrease their noise radiation caused by incident turbulent flow.

Nonlinear temporal filter for television picture noise reduction

Abstract: A conventional recursive interframe lowpass filter for 625 line 5.5 MHz monochrome television is modified so that its attenuation of frame differences is instantaneously dependent on their amplitude. This results in improved rendition of motion when compared with the original design. Performance is assessed by objective measurement of noise attenuation for a range of input noise levels and nonlinear characteristics, and by subjective opinion tests on pictures containing noise and movement.

Estimation of the signal‐to‐noise ratio of seismic data with an application to stacking*

Abstract: The amplitude of the signal and the energy of the noise on each of at least three traces can be estimated provided that the signal has the same form (but not necessarily the same amplitude) on these traces and that the noise on any trace is correlated with neither the signal nor the noise on any other trace. This estimation of signal amplitude and noise energy can be achieved by a rather simple algorithm. The accuracy of the estimate depends, of course, on the degree to which the assumption that signal and noise on the different traces are mutually uncorrelated is actually met. The accuracy tends to improve with increasing number of traces.

Simultaneous determination of device noise and gain parameters through noise measurements only

Abstract: A novel method of measuring the noise and gain parameters of a linear two-port is presented. This method is based on the Friis formula to determine all the noise and gain parameters solely from noise figure measurements. This results in a much simplified procedure and improved accuracy over the conventional methods. An example using a low-noise transistor at 2 GHz is shown.

Pseudo-atmospheric noise generator with control of temporal characteristics

Abstract: An improved pseudo-atmospheric noise generator includes the capability forroducing noise in the VLF/LF spectrum with known amplitude probability distributions while controlling the temporal characteristics of the simulated impulses. The outputs from a number of pseudo-random bit generators are selected by a microprocessor in accordance with expected or anticipated atmospheric disturbances from a number of geographical regions. The microprocessor selected outputs are fed to a digital-to-analog converter which feeds the converted signals to a summer. An independent Gaussian noise source is coupled to the summer and the outputs from the Gaussian noise source and the digital-to-analog converter are combined to create a composite pseudo-atmospheric noise signal. The improvement also includes capability for self-testing and set up.

Core noise measurements from a small, general aviation turbofan engine

Abstract: As part of a program to investigate combustor and other core noises, simultaneous measurements of internal fluctuating pressure and far field noise were made with a JT15D turbofan engine. Acoustic waveguide probes, located in the engine at the combustor, at the turbine exit and in the core nozzle wall, were used to measure internal fluctuating pressures. Low frequency acoustic power determined at the core nozzle exit corresponds in level to the far field acoustic power at engine speeds below 65% of maximum, the approach condition. At engine speeds above 65% of maximum, the jet noise dominates in the far field, greatly exceeding that of the core. From coherence measurements, it is shown that the combustor is the dominant source of the low frequency core noise. The results obtained from the JT15D engine were compared with those obtained previously from a YF102 engine, both engines having reverse flow annular combustors and being in the same size class.

Coherence Improvement in White Noise Analysis by the Use of a Repeated Random Sequence Generator

Abstract: A device is described which allows the averaging of the output of an unknown system in response to identical bursts of pseudorandom noise. The use of this technique produces an improvement in the measured coherence function for the system and a reduction in the variance of the estimated system parameters.

Optimum Measurement of Broadband Ultrasonic Data

Abstract: The increasing use of quantitative techniques for characterizing flaws in materials often requires accurate measurement of flaw properties over a wide range of frequencies despite t he presence of noise and despite the variability of the properties of the transducer, electronics, etc. In this paper, we provide a statistical treatment of the problem of extracting the scattering amplitude of a flaw from measured data. A technique is derived for achieving extended bandwidth by combining data from two or more transducers.

Robust sequential detection of weak signals in undefined noise using acoustical arrays

Abstract: In detection problems where the noise distribution may be unknown, employing nonlinear elements that precede the standard detector often significantly improves system performance in non‐Gaussian noise. However, in order to optimize the system to the noise conditions and, thereby, prevent degradation under Gaussian noise, the nonlinear elements should possess an inherent capability for adapting to the changing noise conditions. This paper presents a discrete formulation based on m‐interval partitioning for sequentially detecting weak signals in undefined noise. Partitioning requires knowledge of a small number of quantiles and related functions from the unknown noise distribution. The resultant detector is easily implemented and is adaptable to slowly changing noise conditions. Two examples are given that clearly show improved performance, over a wide class of noise distributions, for an array employing m‐interval partitioning compared with arrays employing linear correlators and sign detectors.

Erratum: Low-noise millimetre-wave mixer diodes: results and evaluation of a test programme

Abstract: The results of extensive measurements on metal-gallium arsenide Schottky-barrier diodes in a waveguide mixer are presented, with emphasis on cooled, low-noise operation. Aspects of diode design and manufacture are reviewed, particularly with a view to the better understanding of noise generation in the diode. It appears that previous current-transport and noise-generation models require modification.

Factorial linear modelling, algorithms and applications

Abstract: The paper emphasizes the importance of normalization of parameters in identification of linear models as now commonly applied to digital signal processing. Classical LPC, Pisarenko, Prony methods are unified and compared. The factorial approach plays a central role when additive noise is considered. The computational requirement is the determination of eigen vectors of correlation and covariance matrices. Various algorithms are then given including sequential estimation procedures in the covariance case. The methods are compared on close sinewaves merged in noise in terms of resolution, windowing, signal-to-noise ratio.

Acoustic test and analyses of three advanced turboprop models

Abstract: Results of acoustic tests of three 62.2 cm (24.5 inch) diameter models of the prop-fan (a small diameter, highly loaded. Multi-bladed variable pitch advanced turboprop) are presented. Results show that there is little difference in the noise produced by unswept and slightly swept designs. However, the model designed for noise reduction produces substantially less noise at test conditions simulating 0.8 Mach number cruise speed or at conditions simulating takeoff and landing. In the near field at cruise conditions the acoustically designed. In the far field at takeoff and landing conditions the acoustically designed model is 5 db quieter than unswept or slightly swept designs. Correlation between noise measurement and theoretical predictions as well as comparisons between measured and predicted acoustic pressure pulses generated by the prop-fan blades are discussed. The general characteristics of the pulses are predicted. Shadowgraph measurements were obtained which showed the location of bow and trailing waves.

Relations between geometrical factors for noise magnitudes in resistors

Abstract: A relation between the spatial weighting function for voltage fluctuations in the presence of a magnetic field and weighting functions determined in the absence of a magnetic field on four‐probe resistors is demonstrated. The tensor character of conductivity fluctuations is shown to have a significant effect on the ratios of measurable fluctuations. The results are intended for use in determining the sources of 1/f noise in semiconductors.

Quarter micron gate low noise GaAsFET's operable up to 30 GHz

Abstract: Quarter micron gate low noise GaAs MESFET's have been developed by delineating gate electrodes with an electron beam lithography technique and by reducing parasitic source and gate resistances. At 18GHz, a noise figure of 1.9dB with an associated gain of 7dB and a maximum available gain of 11dB were obtained at drain currents of 10mA and 30mA, respectively. At 30GHz, a noise figure of 4dB with an associated gain of 5dB and a maximum available gain of 8dB were obtained. The measured noise figures are the best values reported so far, and this work has demonstrated the feasibility of utilizing GaAs MESFET's up to millimeter-wave regions.

A comparison between an existing propeller noise theory and wind tunnel data

Abstract: The noise of three supersonic helical tip speed propellers was compared with the noise predicted by an existing noise theory Comparisons of the peak blade passage tones showed fairly good agreement beween theory and experiment at the lowest helical tip Mach numbers tested, 086 and 100, while at higher numbers, the theory predicted higher noise levels than measured When the differences among the propellers were considered the theory and measurement showed fairly good agreement Directivity measurements in general showed that the measured blade passage tone data peaked further downstream than the theory predicted At the cruise design condition the harmonics appeared to fall off faster in the data than the theory indicated

Measurement of Impulsive Noise on Electric Distribution Systems

Abstract: Impulsive noise measurements were made on two electric distribution systems as part of Electric Power Research Institute Project RP1024-1, Study of Distribution System Surge and Harmonic Characteristics. This paper describes the instrumentation system used to measure impulsive noise conducted along, and emanated from, distribution lines in the 3-300-kHz frequency band. A significant source of this noise was identified as synchronously switched power converters. The concern is that such noise on distribution systems might cause interference to digital data carrier systems proposed for distribution systems. Test data from one of the utility systems are presented; potential problems caused by interference are discussed. Additional work utilizing the same or similar instrumentation on a relatively broad spectrum of distribution systems is recommended to identify and characterize impulsive noise.