Volume 113, Issue 1, January 2003
Index of content:
- AEROACOUSTICS, ATMOSPHERIC SOUND 
113(2003); http://dx.doi.org/10.1121/1.1527927View Description Hide Description
There is a practical need to fully understand the mechanisms involved in the flow/pressure fluctuations around a screened microphone. A stream of uniform flow with low-frequency turbulence encountering a rigid, impermeable spherical windscreen is considered in this study. Pressure distributions on the surface of the sphere are determined by the flow structure. Pressure fluctuations at the center of the sphere are then calculated based on the integration of surfacepressure distributions. Because of the low-frequency assumption, results from steady-state laminar flows can be used to investigate the Reynolds numbereffects on wind noise reduction. Three types of flow have been studied in this paper: an inviscid case, a low-Reynolds-number Stokes flow, and intermediate- and high-Reynolds-number flows. A Reynolds-number/wind-noise-reduction correlation shows that the wind noise reduction increases with decreasing Reynolds number.
113(2003); http://dx.doi.org/10.1121/1.1520547View Description Hide Description
The occurrence of self-excited pressure oscillations routinely plagues the development of combustion systems. These oscillations are often driven by interactions between the flame and acoustic perturbations. This study was performed to characterize the structure of the acoustic field in the near field of the flame and the manner in which it is influenced by oscillation frequency, combustor geometry, flame length and temperature ratio. The results of these calculations indicate that the acoustic velocity has primarily one- and two-dimensional features near the flame tip and base, respectively. The magnitude of the radial velocity components increases with temperature ratio across the flame, while their axial extent increases with frequency. However, the acoustic pressure has primarily one-dimensional characteristics. They also show that the acoustic field structure exhibits only moderate dependencies upon area expansion and flame temperature ratio for values typical of practical systems. Finally, they show that the local characteristics of the acoustic field, as well as the overall plane-wave reflection coefficient, exhibit a decreasing dependence upon the flame length as the area expansion ratio increases.
Application of an acoustic noise removal method to aircraft-based atmospheric temperature measurements113(2003); http://dx.doi.org/10.1121/1.1528591View Description Hide Description
An acoustic noise removal method is used to reject engine acoustical disturbances from aircraft-based atmospheric temperaturemeasurements. Removal of engine noise from atmospheric temperaturemeasurements allows a larger wave number range to be fit while quantifying the magnitude of atmospheric temperatureturbulence. The larger wave number range was found to result in a more statistically certain spectral slope estimate, with up to a 50% reduction in the standard deviation of measured spectral slopes. The noise removal technique was found to break down under conditions of weak atmospheric temperatureturbulence where the engine acoustical disturbance can be several orders of magnitude larger than atmospheric temperatureturbulence.
113(2003); http://dx.doi.org/10.1121/1.1518469View Description Hide Description
Estimating the direction of arrival of sound in three-dimensional space is typically performed by generalized time-delay processing on a set of signals from a fixed array of omnidirectional microphones. This requires specialized multichannel A/D hardware, and careful arrangement of the microphones into an array. This work is motivated by the desire to instead only use standard two-channel audio A/D hardware and portable equipment. To estimate direction of arrival of persistent sound, the position of the microphones is made variable by mounting them on one or more computer-controlled pan-and-tilt units. In this paper, we describe the signal processing and control algorithm of a device with two omnidirectional microphones on a fixed baseline and two rotational degrees of freedom. Experimental results with real data are reported with both impulsive and speechsounds in an untreated, normally reverberant indoor environment.