Charles D. Smith

Comparison of three methods for measuring acoustic properties of bulk materials

Three methods for measuring the acoustic properties of bulk materials have been evaluated. The methods differ from one another in their practical implementation and reliability. The two more convenient methods exploit changes in surface impedance of test samples resulting from a change in the test sample geometry or boundary conditions. The results from these surface methods are compared with that obtained using Scott’s method. In Scott’s method the propagation constant is obtained by a ‘‘direct’’ measurement inside the test material by means of a probe tube. The comparisons suggest that the surface methods will provide reliable and accurate results when the test sample behaves as a rigid porous structure.

Design and Evaluation of Modifications to the NASA Langley Flow Impedance Tube

The need to minimize fan noise radiation from commercial aircraft engine nacelles continues to provide an impetus for developing new acoustic liner concepts. If the full value of such concepts is to be attained, an understanding of grazing flow effects is crucial. Because of this need for improved understanding of grazing flow effects, the NASA Langley Research Center Liner Physics Group has invested a large effort over the past decade into the development of a 2-D finite element method that characterizes wave propagation through a lined duct. The original test section in the Langley Grazing IncidenceTube was used to acquire data needed for implementation of this finite element method. This test section employed a stepper motor-driven axial-traversing bar, embedded in the wall opposite the test liner, to position a flush-mounted microphone at pre-selected locations. Complex acoustic pressure data acquired with this traversing microphone were used to educe the acoustic impedance of test liners using this 2-D finite element method and a local optimization technique. Results acquired in this facility have been extensively reported, and were compared with corresponding results from various U.S. aeroacoustics laboratories in the late 1990 s. Impedance data comparisons acquired from this multi-laboratory study suggested that it would be valuable to incorporate more realistic 3-D aeroacoustic effects into the impedance eduction methodology. This paper provides a description of modifications that have been implemented to facilitate studies of 3-D effects. The two key features of the modified test section are (1) the replacement of the traversing bar and its flush-mounted microphone with an array of 95 fixed-location microphones that are flush-mounted in all four walls of the duct, and (2) the inclusion of a suction device to modify the boundary layer upstream of the lined portion of the duct. The initial results achieved with the modified test section are provided in this report, and a comparison of these results with those achieved using the original test section is used to demonstrate that the data acquisition and analysis with the new test section can be confidently used for impedance eduction.

Methodologies for duct liner impedance eduction

Methodologies and techniques are reviewed that are currently employed at Langley Research Center to educe (from primitive measurements) the impedance of acoustically absorbing liner structures. These structures are of interest for suppressing noise emission from aircraft engine nacelles. The accuracy and precision of the primitive measurements and their impact on the educed impedance of liners when exposed to high speed grazing flows is of special interest for aircraft engine nacelle applications. The test setups range from the classical standing wave tube for which the primitive measurement is a complex transfer function between two judiciously chosen locations, to an elaborate grazing flow duct arrangement (the Langley Grazing Incidence Tube) for which the primitive measurements (acoustic pressure and phase) are compromised by increased flow noise contamination due to high speed grazing flows up to a Mach number of 0.5. Results of different techniques/methodologies are compared on the basis of how the p...

Validation of Ray Tracing Code Refraction Effects

NASAs current predictive capabilities using the ray tracing program (RTP) are validated using helicopter noise data taken at Eglin Air Force Base in 2007. By including refractive propagation effects due to wind and temperature, the ray tracing code is able to explain large variations in the data observed during the flight test.

Helicopter aural detection as a function of reduced main rotor advancing blade tip Mach numbers

Acoustic tests were conducted to study far‐field noise levels and aural detection ranges associated with a Sikorsky S‐76A helicopter flying at various advancing blade tip Mach numbers spanning the range from 0.586 to 0.875. The responses of a sound jury of 12 persons, located along the flight path, were recorded at the moments in time when the aircraft was first heard. Simultaneously recorded with the subjective data were acoustic and aircraft flight operations data. Additionally, ground impedance characteristics were measured at numerous sites along the flight track. This paper presents a comparison between the normalized aural detection results obtained from the prediction code ICHIN (‘‘I Can Hear It Now’’) and the sound jury measurements. Predicted detection distances, which compare favorably to measured detection distances, are shown to improve by using measured ground impedances in the propagation module section of ICHIN.