Showing papers on "Background noise published in 1976"

Least - squares frequency analysis of unequally spaced data

Abstract: The statistical properties of least-squares frequency analysis of unequally spaced data are examined. It is shown that, in the least-squares spectrum of gaussian noise, the reduction in the sum of squares at a particular frequency is aX22 variable. The reductions at different frequencies are not independent, as there is a correlation between the height of the spectrum at any two frequencies,f1 andf2, which is equal to the mean height of the spectrum due to a sinusoidal signal of frequencyf1, at the frequencyf2. These correlations reduce the distortion in the spectrum of a signal affected by noise. Some numerical illustrations of the properties of least-squares frequency spectra are also given.

The Detection of Signals in Impulsive Noise Modeled as a Mixture Process

Abstract: A first-order mixture noise density is considered as a model for impulsive noise channels It consists of a mixture of a small variance, probably Gaussian, background noise pdf and a large variance impulsive pdf Optimal nonlinear detector structures for known discretetime signals in such noise are derived Their large-sample performance compared to that of a linear detector is studied using asymptotic relative efficiency

A digital filter bank for spectral matching

Abstract: A new digital filter bank design is proposed for the processing of speech waveforms where spectral pattern matching techniques are applicable. Outputs in decibels from the 30 channels of the filter bank are computed every 12 ms. Care has been taken to select a time window and filter center frequency and bandwidth values that take into account the acoustic characteristics of speech. A distance metric is proposed for comparing a spectral frame with previously derived reference patterns. The metric incorporates procedures for crude speaker/microphone normalization, signal level normalization, background noise normalization, and procedures for emphasizing differences in the region of spectral peaks.

In-Flight Far-Field Measurement of Helicopter Impulsive Noise

Abstract: : An in-flight technique for measuring UH-1H helicopter impulsive noise (sometimes called 'blade slap') by stationkeeping with a quiet instrumented lead aircraft was found to be highly successful. Far-field quantitative acoustic waveforms and radiation patterns were easily obtained over a wide, continuous range of UH-1H flight conditions, including several areas known to produce annoying acoustic radiation. The data collected using this technique were not (to any significant degree) contaminated by transmission path distortions that have hindered measurement efforts in the past. The two major finding of this initial measurement program were: (1) Judging the occurrence and severity of a helicopter's radiated impulsive noise signature from cabin-based noise measurements can be misleading. For the UH-1H helicopter, reduction in cabin audible impulsive noise levels may constitute a necessary but certainly not sufficient to indicate that far-field impulsive noise radiation has been reduced. The following three distinct types of impulsive noise are radiated by the Un-1H helicopter while flying between 80 and 115 knots at descent rates from zero to 1000 ft/min: (a) A series of positive pressure pulses believed to be related to blade-tip vortex interaction. These pulses are responsible for the crisp popping sound of the radiated noise; (b) A negative pressure disturbance that rapidly increases in amplitude with forward velocity, becoming quite intense and sawtoothed in shape at 115 knots IAS; and (c) A narrow positive pressure spike that closely follows that sawtooth-shaped negative pressure pulse at high airspeeds (115 knots).

Subharmonic pitches of a pure tone at low S/N ratio

Abstract: 2AFC scores were obtained on 50 untrained subjects who had to respond to a 3% increment or decrement of the fundamental frequency of two successive signals. The first signal in each trial was a reference, containing six harmonics of 200 Hz. Main variables were the number of harmonics of the second signal in a trial (either 3, 2, or 1) and the presence or absence of continuous background noise. Low scores were observed in the absence of the noise (typically 65% for the 3‐ and 2‐component signals, and virtually 50% for the 1‐component signals), whereas in the presence of the noise much higher scores were observed (typically 90% and 80%, respectively). The results indicate that background noise has great impact on the perceptibility of a ’’low pitch’’ related to the fundamental frequency, even more so than the number of components presented (within the range of three, two, or one components): at a low S/N ratio, even a single harmonic may give rise to the perception of a subharmonic low pitch. Subsequent exp...

A probabilistic analysis of time delay extraction by the cepstrum in stationary Gaussian noise

Abstract: Often, the cepstrum has been used when estimating the time delay r between elements of a composite signal embedded in noise, n(t) . Here a probabilistic analysis is conducted dealing with the effect of stationary Gaussian noise on the characteristics of such a nonlinear processor. The input noise, n(t) is reflected at the cepstrum's output as: 1) modulation M_{1}(\omega) of sinusoidal information carrier with subsequent reduction in the height of echo peak at \tau ; and 2) generation of background noises \hat{phi}(\omega) and \hat{psi}(\omega) with tendency to obscure that echo peak. The resulting probabilities of these terms are derived. The expected mean and standard deviation of reduction in the peak level at \tau due to noise are analytically described. Similar statistical measures on the background noise are also obtained. The results point out the dependence of the-statistical measures upon the pointwise variation of input signal to noise spectra. Thus the total signal power to total noise power (S/N) alone is an insufficient measure for setting the cepstrum performance, and relative bandwidths of signal and noise are also needed. The detection probability P_{D} of a peak corresponding to \tau is calculated for exponential spectra with equal bandwidths in signal and noise and with echo strength a = 0.4; P_{D} is found to be close to one for an SIN greater than -4 dB; deterioration in detection is found to be rapid with thresholding near an S/N \sim -6 db where P_{D} falls to 0.3.

System for reproducing sound information

Abstract: In an environment of acoustic noise, such as a car radio playing in traffic or engine noise, both the volume (AGC) and dynamic range (Compression) of the audio amplifier are controlled by a local ambient acoustic noise sensor, such that gain is increased and dynamic range decreased simultaneously with increased ambient noise.

Detectability of auditory signals presented without defined observation intervals

Abstract: Ability to detect tones in noise was measured without defined observation intervals. Latency density functions were estimated for the first response following a signal and, separately, for the first response following randomly distributed instances of background noise. Detection performance was measured by the maximum separation between the cumulative latency density functions for signal-plus-noise and for noise alone. Values of the index of detectability, estimated by this procedure, were approximately those obtained with a 2-dB weaker signal and defined observation intervals. Simulation of defined- and non-defined-interval tasks with an energy detector showed that this device performs very similarly to the human listener in both cases.

An electro-acoustical technique for the detection of knee joint noise.

Abstract: Distinguishing acoustical signatures of sound emitted by normal and pathological knee joints are picked up using a double microphone-differential amplifier setup. Extraneous background noise is minimized using the principle of "noise cancellation". Two identical sensitive condenser microphones and an F.M. recorder with flat responses in the audio range were used. Preliminary studies covering normal and diseased knee joints showed that their respective waveforms and spectral patterns are unique and proved to be a promising nondestructive diagnostic tool for early detection of knee joint cartilage damage.

A Microchannel Plate with Curved Channels: An Improvement in Gain, Relative Variance and Ion Noise for Channel Plate Tubes

Abstract: Publisher Summary This chapter describes an improvement in gain, relative variance, and ion noise for channel plate tubes in an microchannel plate (MCP) with curved channels. MCPs have reached a rather high level of quality and they are used in second-generation image tubes as well as in some sophisticated devices, such as high speed cathode ray tubes and photomultipliers. A means of limiting ion feedback has been devised. This involves curving all the channels of the MCP and has been made possible by the development of a new technology. The channel curvature overcomes ion feedback limitations and offers some distinct improvements over the straight MCP in electron gain, the statistical fluctuation of gain, the background noise level, and the lifetime of the tube. The very sharp resolution in gain is demonstrated by the multielectron distribution curves. These were obtained by exposing the photocathode to very faint light pulses. It is found that the background noise of the curved MCP has been measured for the experimental photomultiplier by removing the photocathode voltage.

Highway noise. a design guide for prediction and control

Abstract: A description of motor vehicle noise sources and the potential for their control is provided. The report presents an overview of the suggested noise prediction methodology, which is a four step process that includes approximating noise impacts using a nomograph procedure to determine noise levels associated with given traffic and site characteristics; refining these predictions by more accurate representation of design parameters and use of a computer program that is able to plot as well as calculate noise impact; introducing appropriate noise-control options and evaluate them, using a noise barrier nomograph solution; lastly, selected options are fully evaluated using the computer program. A comprehensive review of noise control measures, including not only barriers but also other alternatives, such as grade separation, decks, and right-of-way modifications. Procedures for measuring background noise levels, an evaluation of an environmental impact assessment procedure, and a user's manual describing and listing the computer programs for predicting and plotting highway noise levels are also included.

Noise generated by axisymmetric turbulent boundary layer flow

Abstract: The noise generated by hydrodynamic flow over an axisymmetric body with a blunt nose is described quantitatively. Flush‐mounted piezoceramic hydrophones were used to measure that part of the turbulent boundary‐layer pressure fluctuations that propagates as true sound. Power spectra of the sound pressure were measured in a 3–50‐kHz frequency range over a wide range of Reynolds numbers (U∞D/ν ⩽ 3.03 × 106, where D is the diameter of the body) for the model operating in the Garfield Thomas Water Tunnel. The use of flush‐mounted hot‐film probes to locate turbulence transition is also described. The power spectra of the noise measured in the laminar flow regions correspond closely to those measured in the transition and fully developed turbulent regions of the flow, The exceptions were those spectra measured on the flat part of the nose, but correction for diffraction loss effects suggests that the noise measured there is due to the noise generated by the turbulent part of the flow. Nondimensionalization of th...

Magnetoresistance variation and noise properties of chevron stretcher detectors

Abstract: The angular variation of the magnetoresistance of one‐ and two‐level chevron stretcher detectors has been determined and related to the underlying microscopic domain configuration. The origin of the 1ω‐2ω transition (where ω is the angular frequency of the rotating field) in one‐level detectors is shown to be the result of incomplete magnetization reversal along the detector. The characteristics of this transition and the associated output are discussed for 28‐ and 20‐μm‐period devices. Also discussed are the amplitude‐ and phase‐shift modes of operation of these detectors for normal drive fields. Noise measurements made on zero‐ and one‐state detector outputs show that both states exhibit a √N noise dependence with the number of chevrons N and that the drive‐field dependence of the noise is approximately 1/Hxy at large drive fields. Comparison of the noise output from different detector geometries is made.

Automated high-precision amplitude and phase calibration of seismic systems

Abstract: Automated amplitude and phase response of the complete seismometer-recording system is obtained from step inputs to the calibration coil. High accuracy is achieved by summing as many pulses as desired (to eliminate background noise) by a correlation technique and subsequent Fourier analysis. The only parameters required are the seismometer mass, the Cal-coil constant (referred to the center of mass if appropriate) and current, and the precise onset time of one reference calibration current, which are all very stable over long time periods. Application to the High-Gain Long-Period system at KIP yields the magnification curve from only six pulses with less scatter (

Alternative test method for evaluating impact noise

Abstract: The current method of measuring impact noise transmission involves the use of a standard hammer machine to produce a series of impact on the floor‐ceiling structure, and the measurement of the resulting noise produced in the room below. The method has been criticized on the ground that ratings based on the test data correlate poorly with the subjective judgments of people listening to real‐life impacts on the same floors. An alternative test method is proposed that uses a modified hammer machine whose internal impedance, intensity of impact, and striking frequency simulate those of real footfalls. The new method involves several changes from the present standard: short‐term rms impact sound levels are measured instead of long‐term rms levels; no normalization for the sound absorption of the receiving room is required; since the short‐term levels are higher than the long‐term levels usually measured, background noise is less of a problem than for the existing method. These proposed changes based on recent ...

Radio source parameter estimation by maximum likelihood processing of variable baseline correlation interferometer data

Abstract: The accurate joint determination of the direction and strength of a point noise source when the mutual coherence function of its radiated field is spatially sampled at M baselines by a correlation interferometer is considered. The measurements are corrupted by the combined effects of a) the additive background and receiver noises at the interferometer antennas and b) the finite integration time of a practical correlator. The problem is approached from a statistical point of view (as contrasted with beam forming techniques). First the probability density function of the measurements is derived. The source's two parameters (direction and strength) are then jointly estimated using the maximum likelihood (ML) method. Investigation of the estimates' properties shows that they are virtually unbiased with variances that effectively attain the standard Cramer-Rao (C-R) lower bound when the number of measurements exceeds a "threshold" which is a decreasing function of the measurements' signal-to-noise ratio (SNR). The empirically observed fact that such a threshold is quite small, even at low SNR's, as well as the unbiasedness of the estimates, makes the performance of these (ML) estimates optimum for most practical applications.

Noise limitations of multiplier phototubes in the radiation environment of space

Abstract: The contributions of Cerenkov emission, luminescence, secondary electron emission, and bremsstrahlung to radiation-induced data current and noise of multiplier phototubes were analyzed quantitatively. Fluorescence and Cerenkov emission in the tube window are the major contributors and can quantitatively account for dark count levels observed in orbit. Radiation-induced noise can be minimized by shielding, tube selection, and mode of operation. Optical decoupling of windows and cathode (side-window tubes) leads to further reduction of radiation-induced dark counts, as does reducing the window thickness and effective cathode area, and selection of window/cathode combinations of low fluorescence efficiency. In trapped radiation-free regions of near-earth orbits and in free space, Cerenkov emission by relativistic particles contributes predominantly to the photoelectron yield per event. Operating multiplier phototubes in the photon (pulse) counting mode will discriminate against these large pulses and substantially reduce the dark count and noise to levels determined by fluorescence.

Magnetoresistance and noise properties of chevron stretcher detectors for field access bubble domain devices

Abstract: Experiments were devised to study the angular variation of the resistance and noise properties of one- and two-level chevron stretcher magnetoresistive detectors for use in field access bubble memory devices. All measurements, made with an electronic system, were performed on glass or garnet samples upon which 1 micron of SiO2 was sputter-deposited, followed by 4000 A of Permalloy for the 28-micron-period devices and 0.8 microns of SiO2, followed by 3000 A of Permalloy for the 20-micron-period devices. The geometrical and drive-state dependence of the zero-state noise were studied, as was its frequency dependence. It is found that both types of detectors operate primarily in the amplitude-shift mode for drive fields of interest and that the presence of a bubble in a detector causes a magnetoresistance change equal to that produced by increasing the in-plane drive field about 8 Oe in the absence of a bubble.

Staring Mosaic Sensor Noise Sources

Abstract: Four noise sources occurring in an earth-viewing staring mosaic sensor mounted in a satellite are discussed. The four noise sources are (1) the internal system noise (detector and CCD), (2) the photon noise due to the background radiance level, (3) the noise due to background spatial characteristics scanned by an instability of the satellite, and (4) the noise due to the background temporal variations caused by cloud movement, solar movement, and scintillation. These four noise sources are affected by the particular sensor and their effect at the output of a sensor normally requires extensive calculations. A method of estimating bounds for each noise is demonstrated. This is done by approximating the actual functions by means of log-amplitude (Bode) plots.

Loudspeaker automatic volume control - adjusts to background noise level by eliminating loudspeaker component detected by microphone

Abstract: The automatic volume control for loudspeakers automatically adjusts to the level of background noise by modulating the signal voltage (US) to the loudspeaker according to the ambient noise level. The background noise is monitored e.g. by a microphone whose output is processed and used to control the gain of an amplifier etc., in the main signal path. Sound detected by the microphone from the loudspeaker is eliminated from the amplifier gain control signal. the gain control voltage is derived from the time average of the background noise.

Audible noise reduction

Abstract: A practical approach for prevention or correction of electric power plant substation noise problems is presented. (PCS)

Prediction of noise levels in ships

Abstract: An empirical method for prediction of noise levels in ships is presented and the problems involved in the development of the method are summarised. The main problem is that different ships show large differences in bending-wave attenuation figures per deck. Problems involved in getting proper input data to the calculation steps concerning structureborne sound transmission between steel structure and cabin linings as well as radiation of sound into the cabins are discussed briefly. Some results of preliminary calculations with SEA-models using up to 25 subsystems are reported. Although the results are encouraging, more work needs to be done to confirm the validity of the method, particularly with respect to larger systems and other reference ships. Order from BSRA as No. 48,246.

Designing a national study of railway noise in great britain

Abstract: Phase I of a two-phase project to investigate railway noise in Great Britain is now in the data collection stage. Two major problems, subjects of this paper, have been site selection procedure and noise measurement methodology. Using a statistically drawn sample, a map-based noise level prediction method and an efficient noise measurement strategy, it has been possible to simultaneously estimate the proportion of residences in such proximity to rail lines that they may be annoyed by train noise, test all reasonable hypotheses about the causes and correlations of railway noise annoyance, and derive a mathematical model of physical characteristics of railway noise annoyance.