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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.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Measured acoustic properties of variable and low density bulk absorbers

Experimental data were taken to determine the acoustic absorbing properties of uniform low density and layered variable density samples using a bulk absober with a perforated plate facing to hold the material in place. In the layered variable density case, the bulk absorber was packed such that the lowest density layer began at the surface of the sample and progressed to higher density layers deeper inside. The samples were placed in a rectangular duct and measurements were taken using the two microphone method. The data were used to calculate specific acoustic impedances and normal incidence absorption coefficients. Results showed that for uniform density samples the absorption coefficient at low frequencies decreased with increasing density and resonances occurred in the absorption coefficient curve at lower densities. These results were confirmed by a model for uniform density bulk absorbers. Results from layered variable density samples showed that low frequency absorption was the highest when the lowest density possible was packed in the first layer near the exposed surface. The layers of increasing density within the sample had the effect of damping the resonances.

Dahl, M. D.↗
NASA NTRS1982

Acoustical measurements of vibrational relaxation in moist N2 at elevated temperatures

Sound absorption measurements are conducted by the resonant tube technique to study vibrational relaxation in moist N2 at 301, 343, and 387 K. Analysis of the data shows that improved measurement accuracy at the higher temperatures establishes vibrational-vibrational energy transfer as the operative relaxation path for the de-excitation of N2 by H2O beyond the uncertainty of experimental error. The best-fitted temperature dependence of the vibrational-vibrational rate constant is given, noting that the expression is consistent with seven independent sets of both acoustical and nonacoustical data.

Zuckerwar, A. J.↗
NASA NTRS1972

A method for reducing ground reflection effects from acoustic measurements

The method involved placing foam blocks on the ground between sound source and receiver in an approximation of the wedges in an anechoic chamber. The tests were performed out of doors as a function of the receiver height and source-receiver separation distance. The spacing between blocks and the extent of ground covered were varied to estimate the optimum placement and minimum amount of foam treatment needed. Base-line tests without foam were also performed. It was found that the foam treatment reduced the amplitude of the peaks and valleys in the sound pressure spectra substantially. The foam was least effective at low frequency, especially for the low receiver height and for large source-receiver distances. Results from the base-line tests were compared with theoretically predicted results. These base-line test results were in reasonable agreement with those from theory.

Noerager, J. A.↗
NASA NTRS1973

Study of porous surface microphones for acoustic measurements in wind tunnels

Porous surface sensors acting as directional microphones in subsonic airflow were investigated. The first part of the report deals with the design of a porous strip sensor set in an aerofoil. The second part presents the experimental results of frequency response, directivity, and flow noise of a porous pipe sensor and a porous strip sensor. For flow noise, these sensors were compared with the Bruel and Kjaer half-inch condenser microphone with a nose cone. The flow noise was examined under two conditions of flow: in a very quiet flow where the turbulence was approximately 0.3% and in a spoiled flow where the turbulence was approximately 5%.

Noiseux, D. U.↗
NASA NTRS1980

Acoustical measurement separates core noise and jet noise

Measuring technique discriminates between jet noise and core noise of jet engine. Results of experimentation confirmed that core noise and jet noise can be separated by examining cross-correlation of far-field microphone signals and that crossover point between core noise and jet noise moves toward higher velocities at higher angles with respect to jet axis.

Parthasarathy, S. P.↗
NASA NTRS1984

Acoustic measurements of a full-scale rotor with four tip shapes. Volume 1: Text, appendices A and B

A full-scale helicopter with four different blade-tip geometries was tested in the 40- by 80-foot wind tunnel at Ames Research Center. Performance, loads, and noise were measured. The four tip shapes tested were rectangular, tapered, swept, and swept-tapered. Noise measurements from that test are presented in the form of tables and plots. The noise data include measurements of the sound pressure level in dB, dBA, and tone-corrected PNdB, for all of the conditions tested. Detailed measurements, 1/3-octave spectra and time-histories for some selected data are included as well as plots of dBA as function of test condition. Some performance measurements are given to aid interpretation of the noise data.

Mosher, M.↗
NASA NTRS1984

Acoustic measurements of a full-scale rotor with four tip shapes. Volume 2: Appendices C, D, E and F

A full scale helicopter with four different blade tip geometries is tested in a 40- by 80-foot wind tunnel. Performance, loads, and noise are measured. The four tip shapes tested were rectangular, tapered, swept, and swept/tapered. The noise data include measurements of the sound pressure levels in decibels dB, decibels adjusted dBA, and tone-corrected PNdB, for all of the conditions tested. Also included are the detailed measurements, 1/3 octave spectra and time histories for some selected data, and plots of dBA as function of test condition. Some performance measurements are included to aid interpretation of the noise data.

Mosher, M.↗
NASA NTRS1985

In-flight acoustic measurements on a light twin-engined turboprop airplane

Four series of flight tests were conducted to measure sound pressure levels inside and outside the cabin of a twin-engined turboprop airplane. Particular emphasis was placed on harmonics of the propeller blade passage frequency. The cabin was unfurnished for the first three flights, when the main objective was to investigate the repeatability of the data. For the fourth flight, the cabin was treated with fiberglass batts. Typically, the exterior sound pressure levels were found to vary 3 to 5 dB for a given harmonic, but variations as high as 8 dB were observed. The variability of harmonic levels within the cabin was slightly higher but depended on control of the relative phase between the propellers; when phase was not controlled the average variability was about 10 dB. Noise reductions provided by the fuselage structure were in the range of 20 to 40 dB, when an exterior microphone in the plane of rotation of the propeller was used as reference.

Wilby, J. F.↗
NASA NTRS1990

Acoustic measurements of gas density

Sound transmission through gases in an enclosure is considered. Analytical results are given in terms of geometrical parameters, wave numbers, and source type for simple model problems, and are compared with data obtained by Haran (1983). It is concluded that density measurements can be made in a gas contained in an enclosure by measuring the sound pressure level at a receiver located near a dipole source driven at a constant velocity amplitude at low frequencies.

Shakkottai, P.↗
NASA NTRS1990

The ROTONET prediction system and initial comparisons with far-field acoustics measurements for the XV-15 tilt-rotor aircraft

The NASA Langley Research Center is developing the ROTONET prediction system, a comprehensive computer program for the prediction of full system rotorcraft noise, to provide an analytical tool for estimating the total noise signature of rotorcraft. It contains models of many rotorcraft noise generating mechanisms. NASA is also performing a series of flight tests to provide comprehensive validation data bases for the ROTONET System. A joint NASA/Bell Helicopter Textron Incorporated test of an XV-15 tilt-rotor aircraft is one of several flight test which has been performed. A data base consisting of spectra, noise level time histories, and effective perceived noise levels, incorporating actual meteorological conditions, and tilt-order aircraft flight dynamics, is being produced from this test. The ROTONET prediction system is described, the flight test methodology is explained, and initial comparisons are shown of ROTONET predictions with data from the flight test. EPNL, noise level time history, and narrowband noise spectra comparisons demonstrate the overall capabilities of the prediction system.

Golub, Robert A.↗
NASA NTRS1991

Optimum data analysis procedures for Titan 4 and Space Shuttle payload acoustic measurements during lift-off

Analytical expressions have been derived to describe the mean square error in the estimation of the maximum rms value computed from a step-wise (or running) time average of a nonstationary random signal. These analytical expressions have been applied to the problem of selecting the optimum averaging times that will minimize the total mean square errors in estimates of the maximum sound pressure levels measured inside the Titan IV payload fairing (PLF) and the Space Shuttle payload bay (PLB) during lift-off. Based on evaluations of typical Titan IV and Space Shuttle launch data, it has been determined that the optimum averaging times for computing the maximum levels are (1) T (sub o) = 1.14 sec for the maximum overall level, and T(sub oi) = 4.88 f (sub i) (exp -0.2) sec for the maximum 1/3 octave band levels inside the Titan IV PLF, and (2) T (sub o) = 1.65 sec for the maximum overall level, and T (sub oi) = 7.10 f (sub i) (exp -0.2) sec for the maximum 1/3 octave band levels inside the Space Shuttle PLB, where f (sub i) is the 1/3 octave band center frequency. However, the results for both vehicles indicate that the total rms error in the maximum level estimates will be within 25 percent the minimum error for all averaging times within plus or minus 50 percent of the optimum averaging time, so a precise selection of the exact optimum averaging time is not critical. Based on these results, linear averaging times (T) are recommended for computing the maximum sound pressure level during lift-off.

Piersol, Allan G.↗
NASA NTRS1993

X-Ray And Acoustic Measurements Yield Stiffnesses

Analytical technique combines ultrasonic scanning measurements of local velocity of sound in specimen of material with x-ray computed tomographic measurements of local mass density to compute local stiffness of material. Stiffnesses at various locations in specimen then used in finite-element mathematical model of elastic behavior of specimen to compute local stresses, local strains, and overall deformations. Technique enhances value of quantitative nondestructive x-ray and ultrasonic measurements. Especially useful in characterization of carbon/carbon composites and other advanced materials not homogeneous and for which customary simplifying assumption of constant density and/or constant stiffness not valid.

Madaras, Eric Irvine↗
NASA NTRSDate not supplied

Spacecraft Loads and Acoustic Measurement (SLAM)

The lack of measured flight vibration data for payloads launching on ESPA rings, is preventing Goddard engineers from developing lighter, less expensive, small satellites. The SLAM ESPA Payload will capture and transmit in- flight structural loads and vibroacoustic levels encountered by ESPA spacecraft. The SLAM Team will work with NASA's Launch Service Program (LSP) to validate the ESPA flight environment data and potentially update ESPA payload interface requirements. Goddard engineers will use the updated ESPA payload flight environment data to develop mass efficient ESPA payloads. Scientists can use the resources saved by these efficient small satellites, to develop more and/or larger science instruments.

Keon Harmon↗