Experimental Acoustic Field Characterization of Enclosures by Impedance Estimates and Modal Analysis

An experimental method to characterize the acoustic field inside an enclosure in terms of the active and reactive components is presented. The method uses a finite difference approximation of the sound pressure from two closely spaced microphones to estimate the specific acoustic impedance and particle acceleration throughout a cavity. The impedance is then used to decompose the sound pressure at a point into components caused by the progressive and standing waves. The reactive or standing wave components within the cavity are further characterized in a normal modes fashion by extracting the modal parameters from the acoustic particle acceleration frequency response functions. The underlying theory of the technique is discussed. An experimental evaluation consisting of an analysis of the cavity characteristics of a tube with several end terminations is presented. For a reflective termination the experimental natural frequencies differed by less than 7 Hz between the analytical solution and a finite element model. For a semiabsorptive termination the experimental method shows the degradation of the high frequency reactive field due to the increased absorption and also the minimal effect on the low order acoustic mode characteristics.