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  • 1
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    Verification of a one-dimensional analysis of sound propagation in a variable area duct without flow (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: (Previously announced in STAR as N82-12891)
    Keywords: ACOUSTICS
    Format: text
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  • 2
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    Pressure transfer function of a JT15D nozzle due to acoustic and convected entropy fluctuations (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: (Previously announced in STAR as N82-22951)
    Keywords: ACOUSTICS
    Type: Acoustical Society of America; vol. 72
    Format: text
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  • 3
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    Verification of elastic-wave static displacement in solids (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The solution of the nonlinear differential equation which describes an initially sinusoidal finite-amplitude elastic wave propagating in a solid contains a static-displacement term in addition to the harmonic terms. The static-displacement amplitude is theoretically predicted to be proportional to the product of the squares of the driving-wave amplitude and the driving-wave frequency. The first experimental verification of the elastic-wave static displacement in a solid (the 111 direction of single-crystal germanium) is reported, and agreement is found with the theoretical predictions.
    Keywords: ACOUSTICS
    Type: Applied Physics Letters; 37; Nov. 1
    Format: text
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  • 4
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    Propeller tip vortex - A possible contributor to aircraft cabin noise (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Wind tunnel model tests support the hypothesis that a propeller tip vortex may subject a downstream wing surface to greater excitation than would be experienced by the aircraft fuselage side wall exposed to propeller-generated noise, ultimately transmitting this structural response to incident dynamic pressure to the cabin interior. Even if structure-borne excitations are less efficient than airborne excitations in the creation of cabin noise, the higher level of the former could still govern cabin noise levels.
    Keywords: ACOUSTICS
    Type: Journal of Aircraft; 19; Jan. 198
    Format: text
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  • 5
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    Why credible propeller noise measurements are possible in the acoustically untreated NASA Lewis 8 ft by 6 ft wind tunnel (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: An explanation is presented for the lack of acoustic reflections in noise studies of propfan models in the NASA-Lewis 8 x 6 ft wind tunnel, where trials were run at Mach numbers 0.5-0.85. The highly directional propeller noise, i.e., mainly in the plane of rotation, experiences a convective effect due to the high subsonic axial Mach number. Reflected sounds are carried downstream, out of range of the acoustic sensors in the tunnel. Furthermore, reflected noise is less audible, and therefore does not affect measurements near peak values. It is suggested that some data contamination may occur below Mach 0.6, and that measurements be performed on higher harmonics generated by low level reflected noise.
    Keywords: ACOUSTICS
    Type: Acoustical Society of America, Journal (ISSN 0001-4966); 75; 1913
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  • 6
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    Input-output characterization of an ultrasonic testing system by digital signal analysis (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: Ultrasonic test system input-output characteristics were investigated by directly coupling the transmitting and receiving transducers face to face without a test specimen. Some of the fundamentals of digital signal processing were summarized. Input and output signals were digitized by using a digital oscilloscope, and the digitized data were processed in a microcomputer by using digital signal-processing techniques. The continuous-time test system was modeled as a discrete-time, linear, shift-invariant system. In estimating the unit-sample response and frequency response of the discrete-time system, it was necessary to use digital filtering to remove low-amplitude noise, which interfered with deconvolution calculations. A digital bandpass filter constructed with the assistance of a Blackman window and a rectangular time window were used. Approximations of the impulse response and the frequency response of the continuous-time test system were obtained by linearly interpolating the defining points of the unit-sample response and the frequency response of the discrete-time system. The test system behaved as a linear-phase bandpass filter in the frequency range 0.6 to 2.3 MHz. These frequencies were selected in accordance with the criterion that they were 6 dB below the maximum peak of the amplitude of the frequency response. The output of the system to various inputs was predicted and the results were compared with the corresponding measurements on the system.
    Keywords: ACOUSTICS
    Type: NASA. Lewis Research Center Analytical Ultrasonics in Materials Research and Testing; p 311-339
    Format: application/pdf
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  • 7
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    Pattern recognition characterizations of micromechanical and morphological materials states via analytical quantitative ultrasonics (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: One potential approach to the quantitative acquisition of discriminatory information that can isolate a single structural state is pattern recognition. The pattern recognition characterizations of micromechanical and morphological materials states via analytical quantiative ultrasonics are outlined. The concepts, terminology, and techniques of statistical pattern recognition are reviewed. Feature extraction and classification and states of the structure can be determined via a program of ultrasonic data generation.
    Keywords: ACOUSTICS
    Type: NASA. Lewis Research Center Analytical Ultrasonics in Materials Research and Testing; p 193-205
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  • 8
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    Laboratory experiments on active suppression of advanced turboprop noise (1985)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The noise generated by supersonic tip speed propellers may be a cabin environment problem for future propeller-driven airplanes. Active suppression from speakers inside the airplane cabin has been proposed for canceling out this noise. The potential of active suppression of advanced turboprop noise was tested by using speakers in a rectangular duct. Experiments were first performed with sine wave signals. The results compared well with the ideal cancellation curve of noise as a function of phase angle. Recorded noise signals from subsonic and supersonic tip speed propellers were than used in the duct to deterthe potential for canceling their noise. The subsonic propeller data showed significant cancellations but less than those obtained with the sine wave. The blade-passing-tone cancellation curve for the supersonic propeller was very similar to the subsonic curve, indicating that it is potentially just as easy to cancel supersonic as subsonic propeller blade-passing-tone noise. Propeller duct data from a recorded propeller source and spatial data taken on a propeller-drive airplane showed generally good agreement when compared versus phase angle. This agreement, combined with the similarity of the subsonic and supersonic duct propeller data, indicates that the area of cancellation for advanced supersonic propellers will be similar to that measured on the airplane. Since the area of cancellation on the airplane was small, a method for improving the active noise suppression by using outside speakers is discussed.
    Keywords: ACOUSTICS
    Type: NASA-TM-87129 , E-2740 , NAS 1.15:87129
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  • 9
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    Cruise noise of counterrotation propeller at angle of attack in wind tunnel (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The noise of a counterrotation propeller at angle of attack was measured in the NASA Lewis 8- by 6-Foot Supersonic Wind Tunnel at cruise conditions. Noise increases of as much as 4 dB were measured at positive angles of attack on the tunnel side wall, which represented an airplane fuselage. These noise increases could be minimized or eliminated by operating the counterrotation propeller with the front propeller turning up-inboard. This would require oppositely rotating propellers on opposite sides of the airplane. Noise analyses at different bandwidths enabled the separate front- and rear-propeller tones, as well as the total noise, at each harmonic to be determined. A simplified noise model was explored to show how the observed circumferential noise patterns of the separate propeller tones might have occurred. The total noise pattern, which represented the sum of the front- and rear-propeller tones at a particular harmonic, showed trends that would be hard to interpret without the separate-tone results. Therefore it is important that counterrotation angle-of-attack noise data be taken in such a manner that the front- and rear-propeller tones can be separated.
    Keywords: ACOUSTICS
    Type: NASA-TM-88869 , E-3275 , NAS 1.15:88869
    Format: application/pdf
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  • 10
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    An estimate of the enroute noise of an advanced turboprop airplane NASA-TM-87302 E-3020 NAS 1.15:87302 HC A02/MF A01 (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The enroute noise of an Advanced Turboprop powered aircraft was estimated. The measured noise levels were roughly equivalent in annoyance to the noise 15.24 m from an automobile traveling at 80 km/h. It is felt that these levels would not illicit noise complaints from urban areas during the day but might be a slight annoyance in rural areas or in urban areas at night. Although it is not felt that the enroute noise is a major problem, it is indicated that a reduction in the enroute noise could improve the acceptability of advance turboprop airplanes.
    Keywords: ACOUSTICS
    Type: NASA-TM-87302 , E-3020 , NAS 1.15:87302
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