Volume 104, Issue 1, July 1998
Index of content:
- AEROACOUSTICS, ATMOSPHERIC SOUND 
Measurements of the two-frequency mutual coherence function for sound propagation through a turbulent atmosphere104(1998); http://dx.doi.org/10.1121/1.423242View Description Hide Description
An array of 32 microphones spanning 675 m of range was used to measure the frequency coherence for pulse signals propagating through atmospheric turbulence near the ground. Frequency coherence is the correlation between the fluctuations in received signals of different frequencies as expressed through the two frequency mutual coherence function evaluated at the same point in space and time. The experiments were conducted for frequencies between 200 and 1000 Hz under both downward and upward refracting propagation conditions. Measurements and theory are shown to be in good agreement for line-of-sight propagation. In the acoustic shadow region the frequency coherence bandwidth shows a strong dependence on mean frequency. The effects of refraction and diffraction, which cause wandering in the pulse arrival time and broadening of the pulse width, respectively, are clearly distinguished. The measurements provide experimental characterization of signal fluctuations for sound fields propagating outdoors. They are important for acoustic remote sensing and detection applications and for the validation of theoretical and numerical developments in sound propagation modeling.
104(1998); http://dx.doi.org/10.1121/1.423260View Description Hide Description
The numerical implementation of the Green’s function parabolic equation (GFPE) method for atmospheric sound propagation is discussed. Four types of numerical errors are distinguished: (i) errors in the forward Fourier transform; (ii) errors in the inverse Fourier transform; (iii) errors in the refraction factor; and (iv) errors caused by the split-step approximation. The sizes of the errors depend on the choice of the numerical parameters, in particular the range step and the vertical grid spacing. It is shown that this dependence is related to the stationary phase point of the inverse Fourier integral. The errors of type (i) can be reduced by increasing the range step and/or decreasing the vertical grid spacing, but can be reduced much more efficiently by using an improved approximation for the forward Fourier integral. The errors of type (ii) can be reduced by using a numerical filter in the inverse Fourier integral. The errors of type (iii) can be reduced slightly by using an improved refraction factor. The errors of type (iv) can be reduced only by reducing the range step. The reduction of the four types of errors is illustrated for realistic test cases, by comparison with analytic solutions and results of the Crank–Nicholson PE (CNPE) method. Further, optimized values are presented for the parameters that determine the computational speed of the GFPE method. The computational speed difference between GFPE and CNPE is discussed in terms of numbers of floating point operations required by both methods.
104(1998); http://dx.doi.org/10.1121/1.423261View Description Hide Description
In April 1994, the USAF Armstrong Laboratory, in cooperation with USAF Test Pilot School, conducted an experimental study of controlled focus boom generated by supersonic maneuvers. The objective of this study was to collect focus and postfocus booms and to assess the ability of aircrews to control the placement of the focal region during basic maneuvers. Forty-nine supersonic passes were flown and included level linear acceleration, level turn, accelerating dives, and climbout-pushover maneuvers. These flights were flown under calm and turbulent atmospheric conditions. Turbulent conditions had a defocusing effect which caused distortions in the focus region and resulted in smaller maximum overpressures. Sonic booms were collected by up to 25 boom event analyzer recorders (BEARs) placed in a 13 000-ft linear array. The BEAR units were spaced 500–2000 ft apart with the denser spacing at the expected focal region. This spacing was chosen to evaluate the thickness of both the focal and postfocal regions. The target location varied from 2000–5000 from the uptrack end of the array. Of the 49 flights, a focus boom was placed within the array 37 times and within feet of the target point 27 times, demonstrating the ability to place controlled focus booms.
104(1998); http://dx.doi.org/10.1121/1.423262View Description Hide Description
In this paper the acoustic absorption due to an orifice plate in a duct supporting a mean flow is studied theoretically. Absorption takes place as the acoustic field energizes a vortex field which is generated at the orifice rim. A linearized approximation is made to the absorption mechanism. This work presents an analytical extension of Howe [Proc. R. Soc. London, Ser. A 366, 205–233 (1979)]. The latter deals with unsteady high Reynolds numberflow through a circular aperture in a thin infinite screen. To the author’s knowledge, such an extension has not been previously made. The problem is formulated analytically insofar as it is possible and numerical results are presented. A Green’s function series expansion is used in the formulation. A difficulty arises with the convergence of this expansion. It is solved by a renormalization technique, which has been developed for this problem. The technique appears to be a novel method for dealing with convergence problems associated with term by term differentiation of Green’s function series expansions. To provide a check on the solution, it is shown that when the radius of the duct tends to infinity the present expression for the Rayleigh conductivity of the orifice plate limits to the expression obtained by Howe for an aperture in a thin infinite screen. With respect to the numerical results, it appears that for orifice mean flowMach numbers an orifice to duct open area ratio of 0.3 provides near optimal average absorption, for the band of frequencies limited by the first symmetric mode cutoff frequency.