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Showing papers on "Noise measurement published in 1983"


Journal ArticleDOI
TL;DR: In this paper, an acoustic elliptic mirror was used to observe the noise sources along the jet, which revealed local noise source characteristics in different shear layer regions and noise source location changes from unexcited to excited jets.
Abstract: Coaxial model jets, including those of high bypass ratio engine exhaust hot gas conditions, were excited internally by tone and broadband noise. Acoustic excitation in the secondary (outer) duct was found to be most effective in jet noise amplification due to the sensitivity of the outer shear layer. Jet noise amplification at the subharmonic of the excitation frequency occurred in a number of cases. An acoustic elliptic mirror was used to observe the noise sources along the jet. It revealed local noise source characteristics in different shear layer regions and noise source location changes from unexcited to excited jets.

58 citations


Journal ArticleDOI
TL;DR: In this article, the 1/f noise from a forward biased dark solar cell is a non-destructive reliability estimation and the experimentally observed 1 /f noise is compared with Kleinpenning's one-dimensional calculations for p-n diodes.
Abstract: The 1/f noise from a forward biased dark solar cell is a non-destructive reliability estimation. The experimentally observed 1/f noise is compared with Kleinpenning's one-dimensional calculations for p-n diodes. At medium and low currents the 1/f noise of n+-p solar cells is about 50 times as large as predicted. Such deviations can be caused by non-uniformities in the large junction area. Local areas with lower built-in potentials at the junction lead to hot spots and reduced reliability. At large currents, reliability problems due to possible poor contacts can be studied from the proportionality between the noise and the square of the current.

58 citations


Journal ArticleDOI
A. Macovski1
01 Mar 1983
TL;DR: In this article, a cross-sectional image of an object can be accurately reconstructed if its projections or line integrals are known at all angles, and this property has been applied to a variety of applications, primarily in the area of medical imaging.
Abstract: A cross-sectional image of an object can be accurately reconstructed if its projections or line integrals are known at all angles. This fundamental and exciting property has been applied to a variety of applications, primarily in the area of medical imaging. In many cases, however, the physical measurements fail to accurately define the complete set of line integrals. This leads to inaccuracies and distortions in the resultant reconstruction. The physical measurements can be inadequate in a number of ways. These include nonlinearities, noise, and insufficient data. The nonlinearities can arise from a nonlinear detector process, or the inability to accurately extract the information in the exponent by taking logs. The noise can be the usual statistical uncertainty of the measurement or an interfering component such as scatter. The data can be insufficient in a number of ways including inadequate sampling or regions of missing data. Also, the measurements of a source distribution can be distorted by an unknown attenuation distribution, resulting in errors in the reconstruction.

47 citations


Journal ArticleDOI
TL;DR: It is proven that for large data records the detection performance is identical to that of an optimal prewhitener and matched filter, and therefore the detector itself is optimal.
Abstract: The problem of detecting a known signal in colored Gaussian noise of unknown covariance is addressed. The noise is modeled as an autoregressive process of known order but unknown coefficients. By employing the theory of generalized likelihood ratio testing, a detector structure is derived and then analyzed for performance. It is proven that for large data records the detection performance is identical to that of an optimal prewhitener and matched filter, and therefore the detector itself is optimal. Simulation results indicate that the data record length necessary for the asymptotic results to apply can be quite small. Thus, the proposed detector is well suited for practical applications.

45 citations


Journal ArticleDOI
TL;DR: A derivation of the method from the principle of minimum cross entropy is given, and the method is compared to minimum cross-entropy spectral analysis, of which it is a generalization.
Abstract: This paper presents a new information-theoretic method for simultaneously estimating a number of power spectra when a prior estimate of each is available and new information is obtained in the form of values of the autocorrelation function of their sum One application of this method is the separate estimation of the spectra of a signal and additive noise, based on autocorrelations of the signal plus noise A derivation of the method from the principle of minimum cross entropy is given, and the method is compared to minimum cross-entropy spectral analysis, of which it is a generalization Some basic mathematical properties are discussed Three numerical examples are included, two based oil synthetic spectra, and one based on actual speech data

41 citations


Journal ArticleDOI
TL;DR: The table-lookup method is a convenient and very flexible way to generate high-quality sinusoidal test signals for measurements in psychoacoustics, speech perception, etc.
Abstract: The table-lookup method is a convenient and very flexible way to generate high-quality sinusoidal test signals for measurements in psychoacoustics, speech perception, etc. Exact spectra of such signals, which are of great interest in experiments, are derived, and the relation between spectral shape, table length, and sine frequency is shown.

35 citations


Journal ArticleDOI
TL;DR: In this article, two approaches for noise source identification based on theory for multiple-input systems have been investigated, and the concepts of the frequency response function and the coherent residual spectral density function were used to estimate the spectra of the noise sources that were accounted for in the multiple−input model.
Abstract: Two approaches for noise source identification based on theory for multiple‐input systems have been investigated. The concepts of the frequency response function and the coherent residual spectral density function were used to estimate the spectra of the noise sources that were accounted for in the multiple‐input model. The key factors which determine the applicability of the techniques for noise source identification are found to be the degrees of coherence between the measured inputs. The experimental results showed that, for a noise system with multiple sources which were slightly coherent to each other, the techniques presented might be able to estimate the spectra of the noise sources, provided there was only modest measurement contamination.

32 citations


Journal ArticleDOI
TL;DR: In this article, two new methods using the concepts of cross correlation and cross variance are proposed for comparing three oscillators, which allow one to characterize individually each oscillator and reduce the influence of the measurement system noise if the noise sources on each channel are independent.
Abstract: Two new methods using the concepts of cross correlation and cross variance are proposed for comparing three oscillators. They allow one to characterize individually each oscillator and they reduce the influence of the measurement system noise if the noise sources on each channel are independent. Therefore, better resolution can be obtained in the spectral purity and frequency stability measurements.

31 citations


01 Mar 1983
TL;DR: It is shown that this noise model can itself be closely approximated by a computationally much simpler noise model, and the simplest form of this approximation yields nearly optimal (asymptotic) performance for locally optimum detection.
Abstract: : The Middleton Class A narrowband non-Gaussian noise model is examined. It is shown that this noise model (which is known to fit closely a variety of non-Gaussian noises) can itself be closely approximated by a computationally much simpler noise model. It is then shown by numerical examples that, for the problem of locally optimum detection, the simplest form of this approximation yields nearly optimal (asymptotic) performance. The performance of other simple suboptimal threshold detectors in Class A noise is also examined. Finally, a useful relationship between the Class A model and the epsilon-mixture model is developed. (Author)

28 citations


Journal ArticleDOI
TL;DR: In this paper, the modal-noise probability density function (PDF) can be generated from several individual readings, which provide information on the probability of any noise level by extrapolation.
Abstract: The need to predict modal-noise performance limitations in optical-fiber systems has led to noise characterization techniques based on probability density functions. These have been used for evaluation of both components and systems, and are found to give meaningful and repeatable data. The modal-noise probability density function (PDF) can be generated from several individual readings which provide information on the probability of any noise level by extrapolation.

27 citations



Journal ArticleDOI
TL;DR: The approach has been to generalize the homomorphic transformation, which is a point transformation that transforms the output of a system into a space where the noise becomes independent of the signal.
Abstract: Techniques for processing of images in signal-independent additive noise are well developed. However, in practice the noise is often dependent on the signal. Some attempts to take into account the dependence of the noise on the signal have been made: our approach has been to generalize the homomorphic transformation, which is a point transformation that transforms the output of a system into a space where the noise becomes independent of the signal. Once this has been accomplished, standard techniques such as Wiener filtering should be applicable with results that are predictable. A rigorous proof of the general homomorphic transformation is given, and its accuracy is discussed. Applications to speckle noise and to film-grain noise are presented.

Patent
28 Jun 1983
TL;DR: In this paper, a method for discriminating against non-Gaussian noise is presented. But the method is not suitable for non-linear elements. And it is not applicable to nonlinear elements with nonlinear components.
Abstract: Apparatus and method for discriminating against non-Gaussian noise. Analog signals from an array of sensors are converted to real and imaginary digital representations and processed such that non-Gaussian noise is separated from signals of interest. The processor uses estimates of Kurtosis and quantiles from either past or adjacent frequency components to construct non-linear elements, which are then used to process remaining signal data to improve the signal-to-noise ratio thereof by removing non-Gaussian noise therefrom.

Journal ArticleDOI
TL;DR: An optimal filter (a Kalman filter) has been developed which is intended to capture the disturbances of the electromyographic noise on the basis of an a priori modelling which considers a series of impulses with a temporal occurrence according to a Poisson distribution as a noise generating mechanism.

Journal ArticleDOI
TL;DR: Wiener filtering techniques using a Markovian covariance model for the image signal are applied to the transformed data followed by an inverse transformation to restore the degraded image.
Abstract: A transformation to convert signal-dependent noise corrupting an image to additive Gaussian signal-independent noise is derived in this paper. Wiener filtering techniques using a Markovian covariance model for the image signal are applied to the transformed data followed by an inverse transformation to restore the degraded image. An ad hoc technique using contrast manipulation to adaptively convert signal-dependent noise to signal-independent noise is also described. The results of the computer simulations designed to evaluate the performance of these techniques are also presented.

Journal ArticleDOI
TL;DR: The sensitivity of interpolation of the pth derivative of a band-limited signal directly from the signal's samples in the presence of additive stationary noise is considered and a lower bound on the interpolation noise level is found.
Abstract: The sensitivity of interpolation of the pth derivative of a band-limited signal directly from the signal's samples in the presence of additive stationary noise is considered. Oversampling and filtering generally decrease the interpolation noise level when the data noise is not band-limited. A lower bound on the interpolation noise level can be approached arbitrarily closely by increasing the sampling rate. The lower bound is equivalent to the noise level obtained by low-pass filtering and pth-order differentiation of the unsampled additive input noise.

Journal ArticleDOI
TL;DR: In this paper, Farassat and Nystrom found that neglecting the sources on the airfoil-shaped cut at the very tip of the blade causes serious errors in the predicted thickness noise.


Journal ArticleDOI
TL;DR: It is shown that the matched filter as a prefilter in the generalized cross correlator gives the optimum correlator configuration for the low signal-to-noise ratio case.
Abstract: The evoked response signal-to-noise ratio in peripheral sensory nerves is of the order of one or less. To reduce noise induced errors in the nerve conduction velocity measurement, signal averaging is employed. The number of responses required depends upon the signal-to-noise ratio and the acceptable error. It is proposed to use maximum likelihood estimators to improve the signal-to-noise ratio and hence, reduce the number of responses required. The matched filter and generalized cross correlator are studied and noise performance equations obtained. It is shown that the matched filter offers a 6.8 dB improvement over the conventional 5 kHz band-limiting filter. It is further shown that the matched filter as a prefilter in the generalized cross correlator gives the optimum correlator configuration for the low signal-to-noise ratio case. These results are verified with experiments.

Proceedings ArticleDOI
01 Jan 1983
TL;DR: In this paper, a method for determining transfer functions across turbofan engine components and from the engine to the far-field is developed based on the three-signal coherence technique used previously to obtain far field core noise levels.
Abstract: A method for determining transfer functions across turbofan engine components and from the engine to the far-field is developed. The method is based on the three-signal coherence technique used previously to obtain far-field core noise levels. This method eliminates the bias error in transfer function measurements due to contamination of measured pressures by nonpropagating pressure fluctuations. Measured transfer functions from the engine to the far-field, across the tailpipe, and across the turbine are presented for three turbofan engines.

G. J. Healy1
01 Feb 1983
TL;DR: In this paper, the Xwing circulation controlled rotor system model was tested for hover performance, and the rotor and subsystem noise measurements were made of both the rotor 'bowl' and the loudspeaker system used in the "bowl" calibration measurements.
Abstract: The X-wing circulation controlled rotor system model was tested for hover performance. During these performance tests, noise data from 12 microphones was recorded on magnetic tape for subsequent data reduction. The rotor system was operated at 4 tip speeds ranging from 529 to 650 ft./sec. (404 to 497 rpm), collective angles of attack fro 0 deg to 8.5 deg (maximum), and blade pressure ratios from 1.0 (no blowing) to a maximum of 2.1. The 12 microphones included 11 in the far field, and one in the transmission area. Following completion of the rotor and subsystem noise measurements, sound field calibration measurements were made of both the rotor 'bowl' and the loudspeaker system used in the 'bowl' calibration measurements. The location of 10 far field microphones was measured by a surveyor. Additionally, detailed tape logs were prepared for the six reels of tape used for the program.

Proceedings ArticleDOI
01 Apr 1983
TL;DR: In this paper the so called 'locking' phenomenon is described and a measure is given to the locking sensitivity of a certain algorithm.
Abstract: Recursive computation can be used to generate sine waveforms. The waveforms are degraded by the round-off noise caused by rounding the results of multiplications. The variance of the round-off noise will increase during the computation if white noise model is assumed. However, in certain cases this will not happen. In this paper the so called 'locking' phenomenon is described. A measure is given to the locking sensitivity of a certain algorithm. Some results of computer simulations and hardware measurements are also given.

Patent
07 Apr 1983
TL;DR: In this paper, a digital system for removing, from a television signal, the electrical noise generated by faulty isolators from power lines is presented, where a thin horizontal segment noise detector locates possible noisy pixels (picture elements), and an image horizontal line verifier prevents false alarms given by the noise detector.
Abstract: A digital system for removing, from a television signal, the electrical noise generated by faulty isolators from power lines A thin horizontal segment noise detector locates possible noisy pixels (picture elements), and an image horizontal line verifier prevents false alarms given by the noise detector An image corrector is provided to remove detected noise The real time algorithm described in the invention is designed to provide a robust detector pseudo independent of noise waveforms, a memory window of three video delay lines, and a low cost digital hardware system

Journal ArticleDOI
TL;DR: In this article, the authors reviewed the National Bureau of Standards (NBS) preision noise measurements program for antenna systems which have been made using Cassiopeia A and the moon.
Abstract: This paper reviews the National Bureau of Standards (NBS) preision noise measurements program for antenna systems which have been made using Cassiopeia A and the moon. The Earth Terminal Measurement System (ETMS) was developed by NBS to make measurements of figure of merit (G/T), and the noise equivalent flux (NEF). The accuracy of the noise measurements are, typically, between 5 and 15 percent for systems with antenna gains between 51 and 65 dB and frequencies between 1 and 10 GHz. Key words?antenna gain; antenna half-power beamwidth; atmospheric loss; Cassiopeia A; earth terminal measurement system; figure of merit; moon; noise equivalent flux; noise measurement; radio stars; satellite communication.

Journal ArticleDOI
TL;DR: In this paper, low frequency noise measurements are reported in modulation-doped GaAs field effect transistors, where the noise spectrum is 1/f and relatively large, and the equivalent saturated diode current I eq varies as V 2, as expected for a fluctuating resistor and saturates when the characteristic saturates.
Abstract: Low frequency noise measurements are reported in modulation-doped GaAs field effect transistors. The noise spectrum is 1/f and is relatively large. At a given frequency the equivalent saturated diode current I eq varies as V 2 , as expected for a fluctuating resistor, and saturates when the characteristic saturates.

Journal Article
TL;DR: A noise-reduction approach that estimates signal and noise levels in each of several frequency bands and removes the appropriate amount of noise with little effect on the signal in each band is developed.
Abstract: Common methods of reducing random noise in nuclear medicine use lowpass filtering, which has the disadvantage that it affects high-frequency components of the image. We developed a noise-reduction approach that estimates signal and noise levels in each of several frequency bands and removes the appropriate amount of noise with little effect on the signal in each band.

Proceedings ArticleDOI
01 Apr 1983
TL;DR: In this paper, the noise transmission loss characteristics of the sidewall treatment in the propeller plane of a twin-engine turboprop aircraft are experimentally investigated in the NASA Langley Research Center Transmission Loss Facility.
Abstract: The noise transmission loss characteristics of the sidewall treatment in the propeller plane of a twin-engine turboprop aircraft are experimentally investigated in the NASA Langley Research Center Transmission Loss Facility. The sound attenuation properties of the individual elements of this treatment are evaluated showing least noise transmission loss in the low frequencies (below 500 Hz) where the excitation levels at the propeller blade passage frequency and the first few harmonics are highest. It is shown that single and double wall resonances play an important role in the noise transmission loss values of the treatment at these low frequencies suggesting that a limp mass with a very low resonance frequency serves better as a trim panel than a trim panel having a high structural stiffness. It is indicated that the window structures might be a potential noise control problem.

Journal ArticleDOI
TL;DR: An iterative impulse noise cleaning technique based on non-linear prediction is presented and the filtered image visual quality is considerably improved by the use of a 'soft' decision rule in the noise detection process.

Journal ArticleDOI
TL;DR: In this article, a special muffler was devised in accordance with the results of the analysis, and a sound source simulation test and a noise measurement in the experimental compressor refrigeration system were performed.
Abstract: Of the noises radiated from home refrigerators, freezers, and air conditioners, the noise due to cavity resonance in the compressor shell is of the highest level. For clarifying the nature of this noise and developing an effective method of reducing it, some theoretical and experimental analyses were performed. As a rst step, the resonant frequency and sound field characteristic within the shell were analyzed by simulating the cavity, defined by the outer surface of a compressor mechanism, and the inner surface of its shell structure to a simple concentric double cylinder model. Then a special muffler was devised in accordance with the results of the analyses. As a second step, a sound source simulation test and a noise measurement in the experimental compressor refrigeration system were performed. In the sound source simulation test, a muffler was mounted on the mechanism over the discharge port of the 3/4-hp Rolling piston-type refrigerant compressor and a speaker driven with a pure-tone oscillator was connected to the discharge port, and the transfer functions were measured with three microphones. The test demonstated that the noise in the compressor shell is derived from cavity resonance and that the transfer function, and hence the magnitude of noise reduction, depends on the relative position of the muffler outlet and the discharge port.