Showing papers on "Noise (radio) published in 1977"
TL;DR: In this article, large-scale structures in the form of instability waves are shown to be present in an acoustically and aerodynamically well behaved jet even at high Mach numbers.
Abstract: Large-scale structures in the form of instability waves are an inherent part of a shearlayer mixing process. Such structures are shown to be present in an acoustically and aerodynamically well behaved jet even at high Mach numbers. They do not directly radiate significant acoustic power in a subsonic jet, but do govern the production of the turbulent fluctuations which radiate broad-band jet noise. Over the whole subsonic Mach number range, a significant increase in jet noise can be produced by exciting the shear layer with a fluctuating pressure at the nozzle of only 0·08 % of the jet dynamic head but with the correct Strouhal number. Such excitation by internal acoustic, aerodynamic or thermal fluctuations could explain the variability of jet noise measurements between different rigs and could also be responsible for some components of ‘excess’ noise.
484 citations
TL;DR: In this paper, the characteristics of turbulent mixing noise in the far field from subsonic and fullyexpanded supersonic jet flows have been studied experimentally over an extensive envelope of jet operating conditions (jet exit velocity and temperature).
338 citations
TL;DR: In this article, the authors measured the noise from a convergent nozzle operated over an extensive envelope of supercritical jet operating conditions and compared the results with the spectra predicted by an existing theoretical model, and good agreement was obtained in most cases.
225 citations
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TL;DR: A three-axis short-period seismometer has been operating on the surface of Mars in the Utopia Planitia region since September 4, 1976 as mentioned in this paper, and during the first 5 months of operation, approximately 640 hours of high quality data, uncontaminated by lander or wind noise, have been obtained.
Abstract: A three-axis short-period seismometer has been operating on the surface of Mars in the Utopia Planitia region since September 4, 1976. During the first 5 months of operation, approximately 640 hours of high quality data, uncontaminated by lander or wind noise, have been obtained. The detection threshold is estimated to be magnitude 3 to about 200 km and about 6.5 for the planet as a whole. No large events
have been seen during this period, a result indicating that Mars is less seismically active than earth. Wind is the major source of noise during the day, although the noise level was at or below the sensitivity threshold of the seismometer for most of the night during the early part of the mission. Winds and therefore the seismic background started to intrude into the nighttime hours starting on sol 119 (a sol is a
Martian day). The seismic background correlates well with wind velocity and is proportional to the square of the wind velocity, as is appropriate for turbulent flow. The seismic envelope power spectral density is proportional to frequency to the -0.66 to -0.90 power during windy periods. A possible local seismic event was detected on sol 80. No wind data were obtained at the time, so a wind disturbance
cannot be ruled out. However, this event has some unusual characteristics and is similar to local events
recorded on earth through a Viking seismometer system. If it is interpreted as a natural seismic event, it
has a magnitude of 3 and a distance of 110 km. Preliminary interpretation of later arrivals in the signal
suggest a crustal thickness of 15 km at the Utopia Planitia site which is within the range of crustal models
derived from the gravity field. More events must be recorded before a firm interpretation can be made of
seismicity or crustal structure. One firm conclusion is that the natural background noise on Mars is low
and that the wind is the prime noise source. It will be possible to reduce this noise by a factor of 10^3 on
future missions by removing the seismometer from the lander, operation of an extremely sensitive
seismometer thus being possible on the surface.
175 citations
TL;DR: In this paper, the authors conducted noise and performance tests on three low tip speed, half-stage, axial flow fans to determine the nature of the vortex shedding noise mechanism, which is due to instabilities in the laminar boundary layer on the suction side of the blade where these instabilities are in the form of Tollmien-Schlichting (T-S) waves.
134 citations
TL;DR: In this paper, a new expression is developed to take into account the emitter time constant, and this gives good agreement with measurements of minimum noise figure up to 3 GHz, and over a wide range of emitter current.
Abstract: In many low power microwave bipolar transistors the high frequency performance is largely controlled by the current-dependent time constant of the emitter base junction. This invalidates the use of Nielsen's expression which was derived for the noise figure of transistors controlled by the base transit time, which is substantially independent of emitter current. Values of noise figure obtained from this equation by substituting a current dependent cut-off frequency ƒα, derived from the transition frequency ƒT, are considerably higher than the measured values. The more recent approximation due to Malaviya and van der Ziel gives better agreement, but is still not entirely consistent with the measurements. A new expression is developed to take into account the emitter time constant, and this gives good agreement with measurements of minimum noise figure up to 3 GHz, and over a wide range of emitter current.
114 citations
TL;DR: In this article, a numerical calculation procedure for the radiated far-field, which is based on Lighthill's wave equation approach with a linearized pressure source function, strongly weighted by axial and radial interference functions, is presented.
Abstract: Experimental results of pressure cross-spectra in model jets indicate that large-scale coherent structures do persist at higher Mach numbers up to 0.8 in the relevant range of Strouhat numbers between 0.1 and 2.0. If the pressure field is Fourier-analy zed circumferentially, the lower-order components are seen to dominate. This, along with the high acoustic efficiency of these components, suggests a numerical calculation procedure for the radiated far-field, which is based on Lighthill's wave equation approach with a linearized pressure source function, strongly weighted by axial and radial interference functions.
104 citations
01 Jul 1977
TL;DR: In this article, the characteristics of plasma waves were studied based on data from the Hawkeye 1 spacecraft in the polar-cusp vicinity, and four types of plasma wave were identified: (1) a band of ULF-ELF magnetic noise, (2) broadband electrostatic emissions with maximum intensities at 10-50 Hz, (3) electrostatic electron cyclotron waves near electron gyrofrequency, and (4) whistler mode auroral hiss emissions.
Abstract: Based on data from the Hawkeye 1 spacecraft in the polar-cusp vicinity, the characteristics of plasma waves are studied. Four types of plasma waves are identified: (1) a band of ULF-ELF magnetic noise, (2) broadband electrostatic emissions with maximum intensities at 10-50 Hz, (3) electrostatic electron cyclotron waves near electron gyrofrequency, and (4) whistler mode auroral hiss emissions. Only ULF-ELF noise is a reliable index of the polar cusp region. Since ULF-ELF magnetic noise extends only to the local electron gyrofrequency, it is suggested that the noise consists of whistler-mode electromagnetic waves. Possible mechanisms for this noise include the whistler-mode cyclotron-resonance, Kelvin-Helmholtz, and drift-wave instabilities. It is felt that a current-driven electrostatic instability causes the broadband electrostatic noise.
102 citations
TL;DR: In this paper, the light intensity fluctuations in each longitudinal mode of CW AlGaAs-GaAs stripe-geometry double-heterostructure junction lasers which oscillate in many modes are investigated experimentally, when the operating current is within a few tens of percent above threshold.
Abstract: The light-intensity fluctuations in each longitudinal mode of CW AlGaAs-GaAs stripe-geometry double-heterostructure junction lasers which oscillate in many modes are investigated experimentally, when the operating current is within a few tens of percent above threshold. In addition, experimental results about the noise characteristics for the lasers directly modulated are discussed in relation to the lasing-spectrum broadening. The total intensity output can be well stabilized at currents above threshold by larger than 10 percent, even when the same laser is modulated by large pulse currents at such high speeds as 400 Mbits. However, the relative noise power for each mode is about 30 dB larger than that for total modes within the whole frequency region observed in this work, and the noise spectrum becomes essentially flat when the laser is modulated. The relative fluctuations for each mode of the modulated laser seems to be nearly equal to the low-frequency fluctuations in each mode without modulation for a given normalized pump rate.
92 citations
TL;DR: The Helios search coil experiment provides accurate low background noise measurements of interplanetary magnetic fluctuation spectra from about 4 Hz to 2.2 kHz adjacent to the frequency band from 0 to 4 Hz of the Technical University of Braunschweig flux-gate magnetometer as discussed by the authors.
Abstract: The Helios search coil experiment provides accurate low background noise measurements of interplanetary magnetic fluctuation spectra from about 4 Hz to 2.2 kHz adjacent to the frequency band from 0 to 4 Hz of the Technical University of Braunschweig flux-gate magnetometer. Apart from a slowly varying fluctuation component ranging up to 100 Hz near 1 AU and beyond 500 Hz near 0.3 AU the following superposed ‘events’ can be discerned in the fluctuation spectra which also have a distinct signature in the slowly varying magnetic field: (1) directional discontinuities acting as wave guide boundaries, (2) directional discontinuities producing whistler wave fields because of instability, (3) reversible magnetic field variations, mostly dips of about 1 min duration associated with whistler wave fields, (4) interplanetary shocks, where, for example, the oblique shock of January 8, 1975, has a thickness of about 1 proton gyroradius and produces an increase in whistler wave fields by more than 2 orders o′ magnitude in power spectral density leading to a power spectrum of 1 γ²/Hz f−3.64 in the wake region.
TL;DR: In this article, a laser light-scattering technique was used to study the relative particle concentration field in a round turbulent air jet and the effects of ambiguity noise and noise due to optical path attenuation on the performance of the laser lightscattering system were discussed.
Abstract: A laser light-scattering technique has been used to study the relative particle concentration field in a round turbulent air jet. Measurements were made in the far field of a smoke-marked turbulent jet exhausting into a secondary air stream. Radial distributions of mean particle concentration, concentration fluctuation intensity and intermittency were measured at several streamwise locations. Concentration fluctuation power spectra and the micro- and integral scales of the concentration fluctuations were measured on the jet axis. The effects of ambiguity noise and noise due to optical path attenuation on the performance of the laser light-scattering system are discussed.
TL;DR: In this article, a low noise, high sensitivity, ion detector based on secondary electron emission from a copper dynode has been constructed from a modified Dietz design, and the utility of this type of detector is shown for noise discrimination and suppression on the basis of the different secondary electron pulse height distributions observed for noise and for signal ions.
TL;DR: In this paper, an experimental study of the noise production by high speed jets over a wide range of Reynolds numbers has been performed, and it was shown that at low Reynolds numbers coherent flow instabilities produce a dominant portion of the noises.
Abstract: An experimental study of the noise production by high speed jets over a wide range of Reynolds numbers has been performed. Two jets of nominal Mach numbers 1.5 and 2.3 were run over a Reynolds number range from 5300 to 107,000. Microphone measurements of the radiated noise and hot-wire measurements of the flow fluctuations demonstrate that at low Reynolds numbers coherent flow instabilities produce a dominant portion of the noise. In the nominal Mach number 2.3 jet these instability waves convect downstream supersonically with respect to the ambient air. In the nominal Mach number 1.5 jet the instabilities convect downstream subsonically. In both cases however, sound pressure level amplitude contours show that the low Reynolds number jets radiate noise comparable to intermediate and high Reynolds number jets. These measurements constitute substantial evidence that a flow instability model of the dominant noise generators may be appropriate for conventional high Reynolds number supersonic jets. a0 C D d M m n r Re St u U Nomenclature speed of sound outside jet wavespeed in the downstream direction diameter of the jet effective diameter of the jet Mach number of the jet at the exit normalized mass velocity fluctuations = azimuthal mode number = radial distance from jet centerline = Reynolds number = p Ud/n. = Strouhal number =fd/ (/(/is frequency) = local velocity =mean centerline velocity of the jet at the nozzle exit
TL;DR: In this paper, an extension of Reichardt's theory is utilized to predict the mean velocity, temperature, and axial turbulence intensity distributions throughout the jet plume, where the generic noise intensity spectrum is synthesized by a slice-of-jet approach, wherein each axial location in the plume contributes to the sound generation in one dominant frequency band.
Abstract: The objective of this investigation was to develop a unified prediction method for estimating the aerodynamic and noise characteristics of jets issuing from nozzles of arbitrary geometric shapes. The method has been developed and demonstrated for dual-flow coaxial jets. An extension of Reichardt's theory is utilized to predict the mean velocity, temperature, and axial turbulence intensity distributions throughout the jet plume. The generic noise intensity spectrum is synthesized by a "slice-of-jet" approach, wherein each axial location in the plume contributes to the sound generation in one dominant frequency band. The propagation of the source spectrum to the far field is modeled by means of convected quadrupoles embedded in a parallel slug flow. Extensive predictions of the aeroacoustic characteristics of coaxial jets were made and compared with experiment. The agreement between theory and experiment is quite good, except at high frequencies and shallow angles to the jet axis, where refraction is overestimated. A major conclusion drawn from these results is that the noise reduction attained by a coaxial jet conies primarily from a reduction in turbulence intensity.
TL;DR: In this article, a model of the noise generated by electrochemical reactions and by diffusion is proposed, where the elementary fluctuations are the particle fluxes which are Poisson white noise, and the model is successfully used to describe the experimental stochastic behaviour of two cases of non-equilibrium electrochemical interfaces.
TL;DR: In this paper, a new analytical treatment is proposed for estimating the sound pressure level of turbulent noise radiated from low pressure axial flow fans, and a physical model which is very simple but reasonable is introduced to explain the turbulent noise generation originating from the vortex shedding from rotor blades.
TL;DR: Intracellular voltage was recorded from rods in isolated retinae of the snapping turtle, Chelydra serpentina during darkness or during uniform illumination of a large retinal area of the turtle during which the voltage fluctuated continuously about a mean level.
Abstract: 1. Intracellular voltage was recorded from rods in isolated retinae of the snapping turtle, Chelydra serpentina. The voltage was observed during darkness or during uniform illumination of a large retinal area. During darkness the voltage fluctuated continuously about a mean level. The spontaneous fluctuation is termed ;noise'. During illumination the amplitude of the noise was reduced.2. The noise observed during darkness could also be reduced by injecting a hyperpolarizing current into the impaled rod. The noise could be increased by a depolarizing current. The component of the noise that could be altered by polarizing the rod is termed ;voltage-sensitive noise'.3. When voltage-sensitive noise was first minimized by a continuous hyperpolarizing current, bright light produced an additional decrease in the noise. The component of the noise that was eliminated by light, but not eliminated by the injection of current, is termed ;light-sensitive noise'.4. The power density spectrum of voltage-sensitive noise, G(v)(f), could be described by an equation of the form [Formula: see text] tau(M) was approximately 7 msec, which is in good agreement with an apparent membrane time constant of 5-8 msec. The largest value of alpha(v) was 2.1 x 10(-9) V(2) sec.5. The power density spectrum of light-sensitive noise could be described by an equation of the form [Formula: see text] tau(L) was approximately 200-300 msec. The largest value of alpha(L) was 8.0 x 10(-9) V(2) sec.6. The potential maintained during darkness could be altered by superfusing a retina with artificial media of different compositions. Depolarizing the rods by changing the extracellular calcium concentration from 1 to 5 mM increased the voltage-sensitive noise. A similar effect was observed after adding 2 mM lanthanum.7. In contrast, 5 mM cobalt produced a small hyperpolarization and suppressed the voltage-sensitive noise. Injecting a depolarizing current, after exposure to cobalt, re-initiated the voltage-sensitive noise. The ability to elicit voltage-sensitive noise in the presence of cobalt indicates that it was not of synaptic origin.8. The results are consistent with the noise present during dark being produced by two types of channel in the rod membrane. One is controlled by the phototransduction process; each individual channel of this type may be described as having a mean open time of 200-300 msec and a conductance of approx. 6 x 10(-13) Omega(-1). The absorption of one photon closes approx. 100-300 of these channels. The other type of channel is controlled by membrane potential; each individual channel of this type has a mean open time which is less than the membrane time constant of 8 msec.
TL;DR: In this article, measurements of noise from two-phase flow over hydrofoils were performed in a variable-pressure water tunnel which was acoustically calibrated so that sound power levels could be deduced from the sound measurements.
Abstract: This paper describes measurements of noise from two-phase flow over hydrofoils. The experiments were performed in a variable-pressure water tunnel which was acoustically calibrated so that sound power levels could be deduced from the sound measurements. It is partially reverberant in the frequency range of interest. Cavitation was generated on a hydrofoil in the presence of either a separated laminar boundary layer or a fully turbulent attached boundary layer. The turbulent boundary layer was formed downstream of a trip which was positioned near the leading edge. High-speed photographs show the patterns of cavitation which were obtained in each case. The noise is shown to depend on the type of cavitation produced.
01 Jan 1977
TL;DR: In this article, the authors studied single-interval photon statistics in the generic term "photon correlation", which can be used to analyze fluctuating optical signals of any origin (provided the fluctuation time is ≳10−7 sec).
Abstract: Photomultiplier tubes which perform virtually ideal digital detection of electromagnetic radiation at visible frequencies are now widely available. The only sources of noise are a generally low dark count, typically a few counts per second, and the unavoidable shot noise due to the random (Poisson) nature of the detection process. The output of such detectors consists of a series of discrete pulses of charge, each corresponding to a single photodetection, which constitutes a Poisson process rate-modulated by variations in the incident intensity. Coincident with the development of these detectors has been the development of hardware based on integrated circuits which can perform errorless analysis of digital signals at frequencies up to about 108 Hz. Combination of these two technologies provides the techniques of photon correlation.1 (We will include the study of single-interval photon statistics in the generic term “photon correlation”.) With speed and accuracy close to the theoretical limit these techniques can be used to analyze fluctuating optical signals of any origin (provided the fluctuation time is ≳10−7 sec).
TL;DR: In this article, a model for the large scale coherent structure of subsonic and supersonic axisymmetric jets is developed for a time-averaged component, a periodic wave-like component and a random small scale component.
TL;DR: K+ conduction kinetics in the axon are not adequately described by a single first order process, and significant changes inHH parameter values were required to describe the usual impedance (resonance) feature in Loligo pealei axon data.
Abstract: The form of power spectra of K+ conduction fluctuations in patches of squid axon suggested that K+ conduction kinetics are higher than first order (Fishman, Moore & Poussart, 1975,J. Membrane Biol.24:305). To obtain an alternative description of ion conduction kinetics consistent with spontaneous fluctuations, the complex impedance and admittance of squid (Loligo pealei) axon were measured at low frequencies (1–1000 Hz) with a four electrode system using white Gaussian noise as a stochastic perturbation. As predicted from the spontaneous noise measurements, a low frequency impedance feature is observed between 1 and 30 Hz which is voltage and temperature dependent, disappears after substantial reduction in [K i + ], and is unaffected by the state of Na+ conduction or active transport. These measurements confirm and constitute strong support for the patch noise measurements and interpretations. The linearized Hodgkin-Huxley (HH) equations do not produce the low frequency feature since first order ion conduction kinetics are assumed. Computation of diffusion polarization effects associated with the axon sheath gives a qualitative account of the low frequency feature, but the potential dependence is opposite to that of the data. Thus, K+ conduction kinetics in the axon are not adequately described by a single first order process. In addition, significant changes inHH parameter values were required to describe the usual impedance (resonance) feature inLoligo pealei axon data.
TL;DR: In this paper, a simple model of the noise of a thick-film resistor leads to two limiting cases: the relative noise is proportional to the sheet resistance, R□ and the conduction is mainly dominated by the current constrictions in the contact areas between grains.
Abstract: Our simple model of the noise of a thick-film resistor leads to two limiting cases. For thick-film resistors with a conduction dominated by the glass interface, the relative noise is proportional to the sheet resistance, R□. For thick film resistors, where the conduction is mainly dominated by the current constrictions in the contact areas between grains, the relative noise is proportional to R□3. Both trends have been observed. Some criteria of low-noise thick-film resistors are derived from the developed noise relations.
TL;DR: In this paper, the effects of aircraft forward motion on pure turbulent mixing noise from unheated jets have been examined experimentally in the in-flight simulation mode, and it is shown that the measured reduction in noise at 90° to the jet axis is a pure source alteration effect.
TL;DR: In this article, the dependence of the intrinsic random intensity noise spectrum of the radiation from CW DH GaAIAs diode multimode lasers on optical wavelength has been investigated for the first time in a wide frequency range 30 MHz to 3 GHz.
Abstract: The dependence of the intrinsic random intensity noise spectrum of the radiation from CW DH GaAIAs diode multimode lasers on optical wavelength has been investigated for the first time in a wide frequency range 30 MHz to 3 GHz. It was generally observed, that a lasing mode which is optically isolated from the rest of the radiation, shows much stronger intensity fluctuations as compared to the fluctuations of the total output, particularly in the frequency range below the intrinsic natural resonance frequency of the laser. The magnitude of the fluctuations decreases when the number of modes in the detected radiation increases. Our experimental results are compared to computer solutions which were obtained with a simple, analytical laser noise model. Good qualitative agreement between theory and experiment is demonstrated.
TL;DR: In this paper, the local component of the neutron noise of the Muhleberg BWR is investigated and the validity of some prescriptions used in current calculational models is investigated.
03 Oct 1977
TL;DR: In locust muscle, it is found that the mean lifetime of channels, activated either by the natural transmitter or by externally-applied glutamate, is prolonged as the temperature is lowered, while the conductance of the single channel is decreased.
Abstract: RECENT work has emphasised the important role which the state of the membrane has in the function of its transport and enzyme systems1,2. At various synapses the action of transmitter substances is accompanied by ‘membrane noise’ associated with the opening and closing of ionic channels. Analysis of this noise gives an estimate of the mean conductance and lifetime of the single open channel3–9. In locust muscle we have found that the mean lifetime (τ) of channels, activated either by the natural transmitter or by externally-applied glutamate, is prolonged as the temperature is lowered, while the conductance (γ) of the single channel is decreased. The dependence of γ and τ on temperature shows a change in slope at a temperature which itself depends on the membrane potential. These results are possibly due to a potential-dependent phase transition in the membrane around the channel causing an abrupt change in the operation of the gating molecule and conductance-determining portion of the channel.
TL;DR: In this article, the effect of flight on the noise radiated by the jet exhausting from a J85 engine is studied experimentally on the Bertin Aerotrain, where both convergent and convergent-divergent nozzles have been tested.
TL;DR: The maximum noise area of Los Angeles International Airport shows a 29% increase in admissions over those of a corresponding control area, as like the target area as possible, except that in the control area jet noise is not a dominant factor.
Abstract: We study, in the vicinity of Los Angeles International Airport, the effect of jet noise on mental hospital admissions. The maximum noise area (where the maximum fly-over noise exceeds 90 dBA) shows a 29% increase in admissions over those of a corresponding control area, as like the target area as possible, except that in the control area jet noise is not a dominant factor. The results are similar to those of a previous study carried out near Heathrow Airport.