01054355

Component

This paper describes how noise that radiates from interior surfaces of aircraft passenger cabins are difficult to localize because of the variation in radiation efficiency of the interior surfaces and the reflections that occur inside the cabin. Measurement of the spatial distribution of the vibration of surfaces is not sufficient enough to determine the contributions to the overall noise inside the cabin made by the vibrations of localized area patches. This is where mapping of the acoustic field by nearfield acoustical holography is useful. In addition, measurement of surface vibration is difficult with contact sensors, such as accelerometers that make scanning of surfaces difficult. Here again, nearfield acoustical holography is useful in determining surface vibration from measurements of pressures by conformal arrays of microphones located close to the surface of interest. This paper explores the feasibility of using nearfield acoustical holography methods in mapping sound pressure levels inside an aircraft passenger cabin from measurements of unsteady pressures made near an interior surface suspected of making a significant contribution to the overall acoustic field inside the cabin. The proposed process involves the reconstruction of the velocities on the surface under the array by back propagation from the array to the surface, followed by the reconstruction of the interior acoustic field from the reconstructed surface velocities. This should result in the acoustic field produced by the vibration of the surface under the array without interference by radiation from other surfaces. The array could then be moved and the process repeated to estimate contributions to the acoustic field inside the cabin by a series of surface areas. Comparison of sound pressure levels generated via this process to the overall measured sound pressure levels of noise inside the cabin would localize surface areas that are major contributors to the interior noise. The process proposed here involves two propagations: back propagation from the array to the surface under the array and forward propagation from the surface to locations inside the cabin of interest, such as locations of passenger ears. For the back propagation, a matrix of nearfield Green's functions from array to surface locations is proposed. This Green's function involves the numerical integration over surface elements of the classical point-to-point baffled Green's function. The integration renders well-behaved Green's function which results in a back propagation matrix that is well-behaved. For the forward propagation, a point-to-point Green's function from each surface location to each location inside the cabin will include the effects of the interior surfaces. The complexity of aircraft passenger cabins preclude the use of analytic functions and renders numerical models too computationally intensive to be usable. Thus, measurements of the back propagation Green's functions are proposed. Once these Green's functions are determined, they are applicable for all source distributions inside the aircraft of interest. In this paper, simulations in a capped cylinder are used to explore the applicability of the above two-step nearfield acoustical holography process to noise generated in enclosed spaces by vibrating surfaces. The simulation modeling is described in Section 2 and the results from exercising the model are presented and discussed in Section 3 followed by a summary of conclusions in Section 4.

01054353

English

pp 911-921

2004

Noise-Con 04. The 2004 National Conference on Noise Control Engineering

CD-ROM

Figures (15) ; References (1)

Aircraft; Aircraft cabins; Aircraft noise; Noise control; Noise sources; Passenger comfort; Sound transmission; Vibration; Vibration isolation

Nearfield acoustical holography; Quantification

Aviation; Environment; Vehicles and Equipment

TRIS, TRB

Jul 26 2007 2:08PM