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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Method and system for measuring sound velocity

A method and system for determining the speed of sound in a fluidic medium by determining the travel time of an acoustical signal a predetermined distance in a fluidic medium by generating a cyclical reference signal of a predetermined frequency and transmitting a portion of the reference signal through the medium. The transmitted portion of the reference signal is received after travelling a predetermined distance in the fluidic medium. The cycles of the cyclical reference signal are counted during the period of time between the transmitting and receiving of the portion of the reference signal wherein the travel time of the portion of the reference signal, is the number of cycle counts divided by the frequency. The speed of the acoustical signal through the fluidic medium is a function of the path length divided by the travel time.

Boegeman, Dwight E.↗
NASA NTRS1977

Tube method of sound-absorption measurement extended to frequencies far above cutoff

Equipment had been constructed for measuring sound absorption in the frequency range from 4 to 100 kHz in a large tube 25.4 cm in diameter and 4.8-m long. The technique employs a large moveable solid-dielectric capacitance transducer that completely fills the tube cross section and generates pulses of plane waves. An identical transducer terminates the other end of the tube and serves as a microphone to detect and reflect the sound pulses. Measurements in argon, nitrogen, and air indicate that the attenuation of the sound pulses differs by less than 1% from values calculated for the zero-order mode for frequencies up to 44 times the cutoff frequency for the first 'nonplane' mode. Above that frequency, the measured values are less than those predicted by theory by an amount that is approximately proportional to the wavelength to the -3.1 power. In a smaller tube of similar construction, the high-frequency deviation from theory is absent.

Shields, F. D.↗
NASA NTRS1992

Experimental validation of boundary element methods for noise prediction

Experimental validation of methods to predict radiated noise is presented. A combined finite element and boundary element model was used to predict the vibration and noise of a rectangular box excited by a mechanical shaker. The predicted noise was compared to sound power measured by the acoustic intensity method. Inaccuracies in the finite element model shifted the resonance frequencies by about 5 percent. The predicted and measured sound power levels agree within about 2.5 dB. In a second experiment, measured vibration data was used with a boundary element model to predict noise radiation from the top of an operating gearbox. The predicted and measured sound power for the gearbox agree within about 3 dB.

Seybert, A. F.↗
NASA NTRS1992

Sound power determination using the sound intensity scanning technique

A technique is presented for determining sound power from sound intensity measurements. Sound power from sound pressure measurements needs approximations, a specific measurement environment, and costly facilities. Sound intensity measurements are the primary quantity required to determine sound power levels, require no costly facilities, and can be made in the presence of extraneous noise. All results are presented in viewgraph format.

Krishnappa, G.↗
NASA NTRS1981

Propagation of sound through the Earth's atmosphere. 1: Measurement of sound absorption in the air: 2: Measurement of ground impedance

Parts were fabricated for the acoustic ground impedance meter and the instrument was tested. A rubber hose was used to connect the resonator neck to the chamber in order to suppress vibration from the volume velocity source which caused chatter. An analog to digital converter was successfully hardwired to the computer detection system. The cooling system for the resonant tube was modified to use liquid nitrogen cooling. This produced the required temperature for the tube, but the temperature gradients within each of the four tube sections reached unacceptable levels. Final measurements of the deexcitation of nitrogen by water vapor indicate that the responsible physical process is not the direct vibration-translation energy transfer, but is a vibration-vibration energy transfer.

Meredith, R. W.↗
NASA NTRS1984

Physical measures of sound and noise

The physical measurement of sound is examined through basic definitions and measuring techniques. The terminology of acoustics is presented with noise characterization, graphs, and mathematical formulas included.

Source record↗
NASA NTRS1989

Optical Measurement Of Sound Pressure

Noninvasive technique does not disturb field it measures. Sound field deflects laser beam proportionally to its amplitude. Knife edge intercepts undeflected beam, allowing only deflected beam to reach photodetector. Apparatus calibrated by comparing output of photodetector with that of microphone. Optical technique valuable where necessary to measure in remote, inaccessible, or hostile environment or to avoid perturbation of measured region.

Trinh, Eugene H.↗
NASA NTRS1980

Resonant tube for measurement of sound absorption in gases at low frequency/pressure ratios

The paper describes a resonant tube for measuring sound absorption in gases, with specific emphasis on the vibrational relaxation peak of N2, over a range of frequency/pressure ratios from 0.1 to 2500 Hz/atm. The experimental background losses measured in argon agree with the theoretical wall losses except at few isolated frequencies. Rigid cavity terminations, external excitation, and a differential technique of background evaluation were used to minimize spurious contributions to the background losses. Room temperature measurements of sound absorption in binary mixtures of N2-CO2 in which both components are excitable resulted in the maximum frequency/pressure ratio in Hz/atm of 0.063 + 123m for the N2 vibrational relaxation peak, where m is mole percent of added CO2; the maximum ratio for the CO2 peak was 34,500 268m where m is mole percent of added N2.

Zuckerwar, A. J.↗
NASA NTRS1984

Low-frequency sound absorption measurements in air

Thirty sets of sound absorption measurements in air at a pressure of 1 atmosphere are presented at temperatures from 10 C to 50 C, relative humidities from 0 to 100 percent, and frequencies from 10 to 2500 Hz. The measurements were conducted by the method of free decay in a resonant tube having a length of 18.261 m and bore diameter of 0.152 m. Background measurements in a gas consisting of 89.5 percent N2 and 10.5 percent Ar, a mixture which has the same sound velocity as air, permitted the wall and structural losses of the tube to be separated from the constituent absorption, consisting of classical rotational and vibrational absorption, in the air samples. The data were used to evaluate the vibrational relaxation frequencies of N2 and/or O2 for each of the 30 sets of meteorological parameters. Over the full range of humidity, the measured relaxation frequencies of N2 in air lie between those specified by ANSI Standard S1.26-1978 and those measured earlier in binary N2H2O mixtures. The measured relaxation frequencies could be determined only at very low values of humidity, reveal a significant trend away from the ANSI standard, in agreement with a prior investigation.

Zuckerwar, A. J.↗
NASA NTRS1977

Sound-tube measurements of the relaxation frequency of moist nitrogen

At audible frequencies, the vibrational relaxation of nitrogen contributes significantly to the absorption of sound in still air. The accurate measurement of the humidity dependence of the relaxation frequency of nitrogen, as yet, has eluded careful measurement due to the difficulty in measuring small absorption at low frequencies. Recently, equipment has been constructed for measuring sound absorption in air as a function of humidity over the frequency range from 4 to 100 kHz. In the experiment described here, the temperature and humidity range of this equipment has been extended so that it can be used to study the relaxation absorption in nitrogen at temperatures from 311 K to 418 K. The results indicate that, over this temperature range, the frequency of maximum absorption in moist nitrogen, f, can be given by f/P = 260 x h, Hz/atm, where h is the percent mole fraction of water, and P is the pressure in atm. To the accuracy of the measurements reported here, f is independent of temperature over the range of temperatures at which the measurements were made.

Chang, D.↗
NASA NTRS1985

Sound speed measurements in liquid oxygen-liquid nitrogen mixtures

The sound speed in liquid oxygen (LOX), liquid nitrogen (LN2), and five LOX-LN2 mixtures was measured by an ultrasonic pulse-echo technique at temperatures in the vicinity of -195.8C, the boiling point of N2 at a pressure of I atm. Under these conditions, the measurements yield the following relationship between sound speed in meters per second and LN2 content M in mole percent: c = 1009.05-1.8275M+0.0026507 M squared. The second speeds of 1009.05 m/sec plus or minus 0.25 percent for pure LOX and 852.8 m/sec plus or minus 0.32 percent for pure LN2 are compared with those reported by past investigators. Measurement of sound speed should prove an effective means for monitoring the contamination of LOX by Ln2.

Zuckerwar, A. J.↗