Search NASA⌕ Search

SEARCH · Search NASA

Results for “NOISE SPECTRUM”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

A revised low-frequency cosmic noise spectrum.

Cosmic radio noise spectra obtained from an Astrobee sounding rocket and from more detailed analysis of RAE-1 data are presented. The flux levels measured by the two spacecraft are consistent and fall off more rapidly below 1 MHz than previous data.

Weber, R. R.↗
NASA NTRS1983

Measurement of macroscopic plasma parameters with a radio experiment: Interpretation of the quasi-thermal noise spectrum observed in the solar wind

The ISEE-3 SBH radio receiver has provided the first systematic observations of the quasi-thermal (plasma waves) noise in the solar wind plasma. The theoretical interpretation of that noise involves the particle distribution function so that electric noise measurements with long antennas provide a fast and independent method of measuring plasma parameters: densities and temperatures of a two component (core and halo) electron distribution function have been obtained in that way. The polarization of that noise is frequency dependent and sensitive to the drift velocity of the electron population. Below the plasma frequency, there is evidence of a weak noise spectrum with spectral index -1 which is not yet accounted for by the theory. The theoretical treatment of the noise associated with the low energy (thermal) proton population shows that the moving electrical antenna radiates in the surrounding plasma by Carenkov emission which becomes predominant at the low frequencies, below about 0.1 F sub P.

Couturier, P.↗
NASA NTRS1984

Growth rate of loudness, annoyance, and noisiness as a function of tone location within the noise spectrum

Absolute magnitude estimation methods were used to investigate the relation between the overall perceived magnitude of noise tone complexes and the location of the tone within the spectrum. In contrast to 'noisiness', loudness and annoyance growth behavior depends on the relationship of the single tones added to low- and high-pass noises in these tests, and to the spectral shape of the noise. Tones centered in noise produce nonmonotonic loudness and annoyance growth functions, while those added to the noise skirt produce power functions. Although a tone correction for annoyance is warranted for certain noise-tone configurations, none of the calculation procedures proposed can take all of the variables relevant to the perceived annoyance of tonal components into account. Complex auditory interactions generated by the simultaneous presentation of noise and tone can substantially account for the effects observed.

Hellman, R. P.↗
NASA NTRS1967

Quadrupole mass filter with means to generate a noise spectrum exclusive of the resonant frequency of the desired ions to deflect stable ions

A mass spectrometer for the separation or separate indication of ions of different specific electric charges is reported. The instrument uses a periodically varying electric field in its analyzing section to excite the injected ions into oscillations while traveling along their trajectories which are either stable or unstable depending on specific parameters. Only stable trajectory ions pass through the electric field to the collector for indication.

Langmuir, R. V.↗
NASA NTRS2004

Measurements of Man-Made Spectrum Noise Floor

This report consolidates research carried out at Clemson University and Stanford University where a series of measurements were undertaken to identify the man-made radiation present in four bands used by rather different services, namely, L1 Band (1563.42 1587.42 MHz), the Unified S-Band (2025 2110 MHz), the 2.4 GHz Industrial, Scientific and Medical (ISM) Band (2400 2482.50 MHz), and the 23.6-24.0 GHz Passive Sensing Band. Results show that there were distinctive differences in the measurement data in the frequency bands, which should be expected based on the function/regulation associated with each. The GPS L1 Band had little to none terrestrial man-made sources, but the ISM 2.4 GHz Band had a large number of man-made sources regardless of the site and the time. The Unified S Band showed mixed results depending on the sites. The Passive Sensing Band does not contain appreciable man-made radiation.

FROM↗
NASA NTRS1974

Further analysis of broadband noise measurements for a rotating blade operating with and without its shed wake blown downstream

An experimental investigation has been conducted to investigate the broadband noise generated by a rotating-blade system. Tests were made with circular and NACA 0012 rotor-blade sections. The blades were operated only with zero lift at each radial station. Tests were made both with zero axial velocity, so that the blades operated in their own turbulent wake, and with a small axial velocity imposed by the wind tunnel to blow the wake of one blade away before the passage of the next blade. The rotor with cylindrical blades generally radiated more noise throughout the noise spectrum than did the rotor with airfoil blades. Blowing the blade wake away from the rotor with cylindrical blades did not have any appreciable effect on the amplitude frequency spectrum, and the predominant noise was broadband, either with tunnel wind on or off. For the rotor with airfoil blades, however, blowing the blade wake away changed the character of the noise spectrum completely in that broadband noise was eliminated or diminished to such an extent as to be indistinguishable. The broadband noise of the airfoil-bladed rotor with zero axial velocity is apparently caused by lift fluctuations due to velocity components of the turbulence normal to the plane of rotation.

Scheiman, J.↗
NASA NTRS1996

On the Two Components of Turbulent Mixing Noise from Supersonic Jets

It is argued that because of the lack of intrinsic length and time scales in the core part of the jet flow, the radiated noise spectrum of a high-speed jet should exhibit similarity. A careful analysis of all the axisymmetric supersonic jet noise spectra in the data-bank of the Jet Noise Laboratory of the NASA Langley Research Center has been carried out. Two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the noise from the fine-scale turbulence, are identified. The two similarity spectra appear to be universal spectra for axisymmetric jets. They fit all the measured data including those from subsonic jets. Experimental evidence are presented showing that regardless of whether a jet is supersonic or subsonic the noise characteristics and generation mechanisms are the same. There is large turbulence structures/instability waves noise from subsonic jets. This noise component can be seen prominently inside the cone of silence of the fine-scale turbulence noise near the jet axis. For imperfectly expanded supersonic jets, a shock cell structure is formed inside the jet plume. Measured spectra are provided to demonstrate that the presence of a shock cell structure has little effect on the radiated turbulent mixing noise. The shape of the noise spectrum as well as the noise intensity remain practically the same as those of a fully expanded jet. However, for jets undergoing strong screeching, there is broadband noise amplification for both turbulent mixing noise components. It is discovered through a pilot study of the noise spectrum of rectangular and elliptic supersonic jets that the turbulent mixing noise of these jets is also made up of the same two noise components found in axisymmetric jets. The spectrum of each individual noise component also fits the corresponding similarity spectrum of axisymmetric jets.

Tam, Christopher K. W.↗
NASA NTRS1998

Jet Aeroacoustics: Noise Generation Mechanism and Prediction

This report covers the third year research effort of the project. The research work focussed on the fine scale mixing noise of both subsonic and supersonic jets and the effects of nozzle geometry and tabs on subsonic jet noise. In publication 1, a new semi-empirical theory of jet mixing noise from fine scale turbulence is developed. By an analogy to gas kinetic theory, it is shown that the source of noise is related to the time fluctuations of the turbulence kinetic theory. On starting with the Reynolds Averaged Navier-Stokes equations, a formula for the radiated noise is derived. An empirical model of the space-time correlation function of the turbulence kinetic energy is adopted. The form of the model is in good agreement with the space-time two-point velocity correlation function measured by Davies and coworkers. The parameters of the correlation are related to the parameters of the k-epsilon turbulence model. Thus the theory is self-contained. Extensive comparisons between the computed noise spectrum of the theory and experimental measured have been carried out. The parameters include jet Mach number from 0.3 to 2.0 and temperature ratio from 1.0 to 4.8. Excellent agreements are found in the spectrum shape, noise intensity and directivity. It is envisaged that the theory would supercede all semi-empirical and totally empirical jet noise prediction methods in current use.

Tam, Christopher↗
NASA NTRS2017

A Comparative Study of Simulated and Measured Main Landing Gear Noise for Large Civil Transports

Computational results for the NASA 26%-scale model of a six-wheel main landing gear with and without a toboggan-shaped noise reduction fairing are presented. The model is a high-fidelity representation of a Boeing 777-200 aircraft main landing gear. A lattice Boltzmann method was used to simulate the unsteady flow around the model in isolation. The computations were conducted in free-air at a Mach number of 0.17, matching a recent acoustic test of the same gear model in the Virginia Tech Stability Wind Tunnel in its anechoic configuration. Results obtained on a set of grids with successively finer spatial resolution demonstrate the challenge in resolving/capturing the flow field for the smaller components of the gear and their associated interactions, and the resulting effects on the high-frequency segment of the farfield noise spectrum. Farfield noise spectra were computed based on an FWH integral approach, with simulated pressures on the model solid surfaces or flow-field data extracted on a set of permeable surfaces enclosing the model as input. Comparison of these spectra with microphone array measurements obtained in the tunnel indicated that, for the present complex gear model, the permeable surfaces provide a more accurate representation of farfield noise, suggesting that volumetric effects are not negligible. The present study also demonstrates that good agreement between simulated and measured farfield noise can be achieved if consistent post-processing is applied to both physical and synthetic pressure records at array microphone locations.

Konig, Benedikt↗
NASA NTRS1971

The relationship between phase stability and frequency stability and a method of converting between them

A method is presented for obtaining the value of the phase stability from time domain frequency stability measurements. Definition of frequency and phase stability are presented. The various types of noise sources in an oscillator and how their location in the oscillator circuitry determines the resultant phase and frequency noise spectrum are described. With this knowledge, the type of noise spectrum from time domain frequency stability measurements is determined. Using certain conversions, the total phase noise spectrum is obtained, which is integrated to obtain the phase stability. Examples of the conversion process are also presented.

Bohn, P. P.↗
NASA NTRS2003

A Computational and Experimental Study of Slit Resonators

Computational and experimental studies are carried out to offer validation of the results obtained from direct numerical simulation (DNS) of the flow and acoustic fields of slit resonators. The test cases include slits with 90-degree corners and slits with 45-degree bevel angle housed inside an acoustic impedance tube. Three slit widths are used. Six frequencies from 0.5 to 3.0 kHz are chosen. Good agreement is found between computed and measured reflection factors. In addition, incident sound waves having white noise spectrum and a prescribed pseudo-random noise spectrum are used in subsequent series of tests. The computed broadband results are again found to agree well with experimental data. It is believed the present results provide strong support that DNS can eventually be a useful and accurate prediction tool for liner aeroacoustics. The usage of DNS as a design tool is discussed and illustrated by a simple example.

Tam, C. K. W.↗
NASA NTRS1973

Rotating-blade vortex noise

An experimental investigation has been made of the Langley full-scale tunnel and outdoors to investigate some of the characteristics of vortex noise generated on a rotating-blade system. Acoustic measurements were made at several microphone positions for two different blade sections with several tip shapes and spoiler configurations. The blades were operated only at zero lift at each radial station, both for operating in their own wake and for operating with the wake blown downstream. Rotors with cylindrical blades generally created more noise throughout the noise spectrum than the rotor with NACA 0012 blades. Blowing the shed wake from the rotor with cylindrical blades did not have an appreciable effect on the frequency-amplitude spectrum. The tip shape changes had very little effect on the frequency-amplitude spectrum of the noise. Spoilers applied to the rotor with NACA 0012 blades increased the amplitude of the spectrum and decreased the number of harmonics of blade passage frequency.

Scheiman, J.↗