Showing papers on "Noise (radio) published in 1969"

Quadrupole correlations governing the pattern of jet noise

Abstract: The effects of convection and refraction dominate the heart-shaped pattern of jet noise. These can be corrected out to yield the small ‘basic directivity’ of the eddy noise generators. The observed quasi-ellipsoidal pattern was predicted by Ribner (1963, 1964) in a variant of the Lighthill theory postulating isotropic turbulence superposed on a mean shear flow. This had the feature of dealing with the joint effects of the quadrupoles without displaying them individually. The present paper reformulates the theory so as to calculate the relative contributions of the different quadrupole self and cross-correlations to the sound emitted in a given direction. Some minor errors are corrected.Of the thirty-six possible quadrupole correlations only nine yield distinct non-vanishing contributions to the axisymmetric noise pattern of a round jet. The correlations contribute either cos4θ, cos2θ sin2θ or sin4θ directional patterns, where θ is the angle with the jet axis. A separation into parts called ‘self noise’ (from turbulence alone) and ‘shear noise’ (jointly from turbulence and mean flow) may be made.The nine self-noise patterns combine as $A\; cos^4\theta(1)+A\; cos^2sin^2\theta(\frac{7}{8}+\frac{7}{8}+\frac{1}{8}+\frac{1}{8})+A sin^2 \theta (\frac {12}{32} & + & \frac{12}{32}+\frac{7}{32}+\frac{1}{32})\\ & = & A(cos^2\theta+sin^\theta)^2 = A;$ this is uniform in all directions as it must be, arising from isotropic turbulence. The two non-vanishing shear-noise correlation patterns combine as The overall ‘basic’ pattern (self noise plus shear noise) thus has the form A + B(cos2θ + cos4θ)/2; this is a slight change from the previous result. The dimensional constants A and B are of comparable magnitude; the pattern in any plane through the jet axis thus resembles an ellipse of modest eccentricity.Frequency spectra are also discussed, following the earlier work. Since the self noise depends quadratically on turbulent velocity components and the shear noise only linearly, there is a relative shift of the self noise to higher frequencies. This in conjunction with refraction figures in the explanation of the deeper pitch of jet noise radiated at small angles to the axis.Finally, the predictions are shown to be compatible with recent experimental results.

Low-frequency waves in the magnetosphere

Abstract: LF wave propagation and emission in magnetosphere, discussing steady noise and discrete emissions

Noise and Flow Characteristics of Coaxial Jets

Abstract: The noise from cold subsonic coaxial jets issuing from concentric nozzles with external mixing has been examined and an attempt made to relate the results to the flow patterns of the jets. As the velocity of the annular jet was increased in relation to a given central jet velocity, an initial attenuation of the jet noise was observed. On further increasing the annular jet velocity until the velocities of the two streams were nearly equal, the noise was in excess of that of the central jet alone. An attempt has been made to predict the curve relating the attenuation attained to the coaxial jet velocity.In comparing the attenuation produced by a coaxial jet combination with that of a single jet, it could be argued that the total thrust in each case should be the same and experimental results are presented on this basis. Results are also available for a nozzle configuration in which the jets are mixed internally in a tube before discharge, and the paper is completed with the results obtained from an annular ...

Magnetic emissions in the magnetosheath at frequencies near 100 Hz

Abstract: Noise signals in earth magnetosheath interpreted as electromagnetic waves propagating in whistler mode

Sound Generation by Turbulent Two-Phase Flow,

Abstract: Sound generation by turbulent two-phase flow is considered by the methods of Lighthill's theory of aerodynamic noise. An inhomogeneous wave equation is derived, in which the effects of one phase on the other are represented by monopole, dipole and quadrupole distributions. The resulting power outputs are obtained for the case of a distribution of small air bubbles in water. The monopole radiation resulting from volumetric response of the bubbles to the turbulent pressure field overwhelms that from the quadrupoles equivalent to the turbulent flow, the increase in acoustic power output being about 70 dB for a volume concentration of 10%. The monopole radiation occurs through the forced response of the bubbles at the turbulence frequency; resonant response is shown to be impossible when the excitation is due to turbulence alone. Surface radiation arises from the edge of a cloud of bubbles. This radiation is important when the region containing bubbles is in the form of a sheet with thickness smaller than the length scale of the turbulent motion. Dipole radiation is also considered, and found to be negligible whenever monopole sources are present. In the case of a dusty gas, only dipole and quadrupole sources are present, and here it is shown that the dipole radiation is equivalent to an increase in the usual quadrupole radiation. The increase depends upon the mass concentration of dust, and is significant for mass concentrations in excess of unity.

Noise of ferromagnetic materials

Abstract: The main types of noise in ferromagnetic materials are thermal or Nyquist noise, excess noise, and Barkhausen noise. They differ in magnitude as well as in the frequency dependence of their associated power spectra. The comparatively weak Nyquist noise is generated by the thermal energy of the conduction electrons and spin waves acting on the domain configuration in the ferromagnetic materials. Excess noise occurs during temperature changes of the sample. It is generated by magnetization processes caused by a changing mechanical strain inside the sample. Strongest by far can be the Barkhausen noise. This is due to a pronounced tendency to form clusters of Barkhausen jumps. The temporal and spatial behavior of such clusters can be well described as a diffusion process; a simple model of this process is given.

Spatial Growth of Waves in a Beam‐Plasma System

Abstract: An experiment on spatially growing electron plasma waves in a beam‐plasma system is described. The properties of the electron beam and the plasma separately are known from accurate, independent measurements. Using a “test wave” technique, the dispersion of the waves in the presence and absence of the beam and their linear growth rate as a function of frequency were measured. The growth of a spectrum of waves from broad‐band noise and the termination of the growth due to nonlinear effects were observed.

Radio noise levels within and above the ionosphere

Abstract: The principal results obtained from the background noise experiment in the Alouette satellites are summarized. The various natural emissions, most of which can reach intensity levels as high as 80 dB above receiver threshold, are identified in the recordings with reference to the characteristic frequencies of the ionosphere local to the spacecraft and are described in turn. Galactic noise is the predominant signal above f T , with some occasional solar noise contributions; emissions are described and values are presented for the galactic spectrum between 0.6 and 12 MHz. Below f T , four distinct ionospheric emissions are recognized and tentatively associated with plasma processes. They are 1) a noise band below f H which is thought to result from cyclotron emission from energetic electrons, 2) a band that occurs mainly between f H and f z S and which probably results from a spacecraft interaction with the local plasma, 3) a noise band between f z S and f T which is believed to result from transverse electrostatic oscillations, and 4) a band at frequencies above f N which probably results from longitudinal electrostatic oscillations in the ionosphere. These ionospheric emissions show an association with ionization irregularities; at times, also, overtones and intermodulation products of such emissions can be clearly recognized at frequencies above f T .

Deep‐Ocean Ambient‐Noise Spectra in the Northwest Atlantic

Abstract: Ambient sea noise was measured from an omnidirectional hydrophone located in the deep‐ocean environment. The hydrophone is installed in about 2400 f and is suspended 400 ft above the ocean bottom. The results illustrate the time variability and the correlation of deep open‐ocean ambient‐noise spectra with simultaneous wind‐speed and wave‐height measurements recorded during typical winter conditions. Broad‐band ambient noise levels were automatically recorded on magnetic tape for 2 min every 2 h. The average value of ambient‐noise in 25 logit‐filter bands of frequencies ranging from 11 to 2816 Hz was measured for each 2‐min sample. Autocorrelations of ambient‐noise levels, wind speed, and wave height, as well as cross‐correlation of ambient‐noise levels with wind speed and wave heights, are presented. The ambient‐noise spectra show the superposition of two major noise sources. One noise source is suspected to be distant shipping. This noise source predominates in the frequency range between 17 and 112 Hz and shows noise maxima occurring at 20 and 60 Hz. The other noise source is wind dependent and is observed below 17 and above 112 Hz. The variation in the standard deviation as a function of frequency at low wind speeds is suspected to be the result of the superposition of the two noise sources. The correlation coefficient of wind speed or wave height to sea noise varies as a function of frequency and wind‐speed group.

The Physics of Flow Noise

Abstract: The nearfield flow noise that is generated in the boundary layer of a vehicle in water turns out to be practically, independent of the kind of vehicle that generates the turbulence. Recorded levels depend on the size and shape of the recording hydrophone, even at very low frequencies when the linear dimensions of the hydrophone are small as compared to the convection wavelength of the turbulence. At high frequencies, because of their finite size, all hydrophones become nearfield insensitive and record the radiated noise. No significant difference was observed in the radiated‐noise curve measured at a distance of about 100 ft from a rising buoyant unit, not even at frequencies as low as 150 Hz, whether the unit was of solid wood or had a thin metal shell. The radiation field depends on the second time derivative of the turbulent velocity fluctuations and, consequently, on the unsteadiness and intermittancy of the turbulence in the boundary layer. For a rotating cylinder, the measured radiation field levels approach those predicted by the recently improved Lighthill formula. For a ship, they are higher; for a buoyant unit, which is a small and a very unstable vehicle, the radiated noise becomes particularly great.

Auditory Masking and the Critical Band in Atlantic Cod (Gadus morhua)

Abstract: Auditory masking was determined for cod by using five half-octave bands as masking stimuli and as signals. For each of 15 cod, masking noise was kept constant at 10 db re 1 microbar in one band while thresholds were determined for signals in all five bands. Results indicate masking to be most pronounced when noise and signal coincide in frequency, and to drop off as frequency separation between noise and signal increases. Masking is calculated in terms of threshold in relation to masking noise level, and varies from about 11 db when signal and noise are in the same band to about −19 db when they are in bands furthest removed from each other. Estimates of critical bands are made from the results.

VHF Technique for Locating Lightning

Abstract: The principles of a lightning location method using the VHF signals emitted by the discharge are outlined. Consideration of the source characteristics (notably signal strength and intervals between pulses; of the noise background; of the differences between the signals incident at spatially separated receivers; and of the electronic equipment now available commercially) indicate that the method is entirely feasible. For a lightning flash at a distance of 20 km, a spatial resolution of 200 meters, and a temporal resolution of 20 μsec could be readily obtained; consequently, the development of the successive VHF radiating events in the discharge could be traced.

The generation and accumulation of timing noise in PCM systems-an experimental and theoretical study

Abstract: Three sources of timing noise in a self-timed regenerative PCM repeater, namely, tank circuit mistuning, amplitude to phase conversion, and pulse shape, were studied both experimentally and theoretically. We discuss how these noises accumulate and combine along a chain of repeaters. The theoretical work is from the viewpoint of frequency analysis which leads easily to the spectrum of the timing noise. We first give a simple form of this theory applicable in a number of cases, and then a more general form useful in other cases, which shows the approximations and limitations of the simple theory. We found that the spectrum of timing noise caused by tank circuit mistuning has no energy at zero frequency and because of this fact, timing noise from this source does not build up indefinitely along a chain of repeaters but soon reaches a limit. On the other hand, the spectrum of timing noise caused by amplitude to phase conversion does have energy at zero frequency; thus, timing noise from this source increases indefinitely along a repeater chain. Some of the timing noise is attributable to pulse shape alone and in some cases may include a very low frequency part. This latter comes about through the small energy near the harmonics of the pulse rate in the tuned circuit response and the aliasing of this energy down to very low frequencies by the sampling process used in measuring the phase deviation or in generating the retiming pulses.

Effect of Rain on Underwater Noise Level

Abstract: The underwater noise due to precipitation has been measured in a small, shallow lake. The local wind speed, rain rate, atmospheric pressure, and noise level were recorded. If was found that the noise due to wind and the contribution from bottom and boundary reflections over the period during which the data were recorded had a negligible effect. The data in the interval 300–9600 Hz have been analyzed in octave bands, and an estimate of the average noise level versus rain rate has been obtained. In comparison with the laboratory work done by Franz on splashes as a source of sound, our results show significantly higher noise levels and a difference in the shape of the noise level versus frequency curve. However, the average rate of change in noise level with change in rain rate is in agreement.

Noise Spectrum Received by an Antenna in a Plasma

Abstract: Antenna noise spectrum in collisionless isotropic plasma, considering plasma fluctuation theory and reciprocity theorem

Digital Search Methods for Pulsars

Abstract: How to search for weak pulsars in the radio noise picked up by telescopes. Two kinds of analysis are described; both have been used successfully at Arecibo.

Radio emission from auroral electrons.

Abstract: IN recent years a number of rocket and satellite observations have been made of intense radio noise in the topside ionosphere (refs. 1–5, 12, 15 and personal communication from D. Walsh). The noise occurs in frequency bands in the neighbourhood of the plasma and gyro frequency. Data received from the British satellite Ariel III show that some of the noise bands occur only when the satellite is within the range of invariant latitude which corresponds to the auroral zones. Until recently very little has been known about how noise bands are generated. Our results show that the noise bands observed by Ariel III are probably generated by Cerenkov radiation from auroral electrons with energies of a few keV.

Ionospheric observation of VLF electrostatic noise related to harmonics of the proton gyrofrequency.

Abstract: Ionospheric VLF electrostatic noise observed by sounding rocket, noting proton gyrofrequency harmonics effects within emission attenuation bands

Observations of lower hybrid resonance phenomena on the OGO 2 spacecraft

Abstract: Continuous and triggered audio frequency noise bands associated with ionospheric lower hybrid resonance frequency observed on OGO 2

An experimental investigation of the composition of jet noise

Abstract: Noise intensity measurements per octave band have been carried out in the far field of a small circular nozzle, at angles 0-150° from the jet axis, with and without vortex generators. The results, interpreted in the light of recent theoretical conclusions, confirm Lighthill's original suggestion that the dominant noise radiator is the pressure-shear x-r quadrupole at all but the lowest frequencies. At very low frequencies Reynolds stress-shear x-x and x-r quadrupoles contribute comparable amounts of radiation. When the product of Strouhal number (based on nozzle diameter and jet velocity) and jet Mach number exceeds unity the geometrical acoustics approximation becomes valid, but even at such high frequencies the shear noise contribution dominates. Vortex generators, while reducing noise intensity, do not appear to modify drastically the above composition of noise radiators.

PHOTOMIXING AT 10.6μ WITH STRONTIUM BARIUM NIOBATE PYROELECTRIC DETECTORS

Abstract: Theoretical limitations of optical heterodyne detection with pyroelectric detectors are discussed. It is shown that the minimum detectable power Pm of Sr1−xBaxNb2O6 pyroelectric detectors at 10.6μ, with an intermediate frequency of 1 MHz and a local oscillator power PL = 1 W is Pm = 8×10−16 W/Hz. An experimental heterodyne receiver with the above conditions is shown to have a value Pm = 1.5×10−11 W/Hz where amplifier noise is much larger than detector noise. The theoretical and experimental dependence of Pm on PL are compared in the amplifier noise limit.

Current noise in thin gold films

Abstract: Measurements have been made of the current noise in gold films deposited on glass substrates with surface resistivities in the range 10 to 104 Ω/sq. The experimental results have shown that the spectral noise power density may be expressed by where I is the direct current flowing through a gold film of surface resistance R at a temperature T, f is the noise-channel frequency, F(T) is a function of T and G(S) is a function of film structure. Observations have also been made on the phenomenon of noise bursts. The experimental data are interpreted in terms of a trap-assisted tunnelling conduction mechanism with a range of barrier widths. Further support is given to the proposed model from current-noise measurements made on cermet films.