Noise Source Identification for Arbitrary Acoustic Radiators, 18-9050

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Principal Investigators
James F. Unruh
Paul Till

Inclusive Dates:  10/01/97 - 10/01/99

Background - The level of noise radiated from transportation vehicles, industrial plant machinery, and consumer products is important to consumer acceptance, even beyond meeting environmental regulations. To implement adequate noise control measures in a cost-effective manner, the location, strength, and radiation efficiency of various acoustic sources in multicomponent equipment must be identified. The technique of scanning the sound field in a planar surface close to the acoustic source and mapping the source over the three-dimensional region extending from the surface of the source to infinity has undergone development and evaluation during the past decade. Near-field acoustic holography (NAH) techniques are used to reconstruct the sound source in terms of surface velocities. These velocities are, in turn, projected into far-field radiated pressures. Present NAH techniques are limited to sources of planar, cylindrical, or spherical geometry, thereby greatly restricting the ability to identify major acoustic sources of geometrically complicated acoustic sources.

Approach: The approach uses advanced numerical solutions to the generalized Helmholtz Integral Equation, which governs noise propagation. The advanced boundary element solution capabilities allow modeling of arbitrary shaped vibroacoustic sources, via a distribution of surface velocities, with no restriction on near- or far-field radiation. Measured hologram pressures establish a distribution of surface source velocities to identify source locations and strengths with subsequent radiation to the far-field via an inverse solution employing boundary elements to develop the surface velocity to pressure influence matrix. The generalized acoustic holography (GAH) technique should not exhibit any restrictions on source or receiver geometries.

Accomplishments: Several simulated vibroacoustic source fields were modeled using the generalized boundary element modeling technique for arbitrary geometry sources. Excellent results were obtained in identifying source locations and predicting far-field noise radiation. In the laboratory, experimental verification also showed the system to be quite robust. The GAH technique was applied to the cabin of a general aviation aircraft to identify panel groups responsible for noise transmission into the aircraft.

(a)

(b)

Using the generalized acoustic holography technique, engineers conducted an in-flight noise source evaluation of a general aviation cabin employing (a) measured hologram pressures and (b) predicted cabin panel velocities.

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