Analysis methods for acoustical systems based on the FFT and intensity techniques: Dissertation

A two-part study on the measurement and analysis of acoustical systems is presented, consisting of the development of various new methods based on the FFT and intensity, and a collection of experiments for testing their applicability.The common factor in the studies is the FFT: the measurements were made with a dual-channel FFT analyser, and the 2-D space-wavenumber FFT is used in some subsequent analyses.Most of the methods and experiments are further related to the two-transducer sound intensity techniques either directly or via some extension.In the method development part, the main theme is the direct measurement of acoustic impedance with the intensity technique. This is followed by additional developments for the determination of the incident power of an arbitrary sound field, the radiation efficiency, absorption coefficient and transmission loss.Two versions of the incident power method were formulated, one employs direct calculation of the Rayleigh integral the space domain, and the other uses holographic approach in the wavenumber domain.The impedance technique is also used in the 3-D acoustic modal analysis of cavities.Measurements of the flow intensity are treated as well.In addition, some current advanced techniques, including multiple input coherence analysis, spatial Fourier transform and nearfield acoustic holography are reviewed, for use in the supporting analyses of the applicability experiments with the new methods.In the experimental part, a number of simple acoustical systems are analysed using the new developed methods and, whenever possible, other wellknown methods for comparison purposes.The investigated systems include a wool absorbent, grass-covered ground surface, a loudspeaker, a two-pane window, a wind tunnel, and a simple model silencer.The main results of the experiments indicate that: (a) the direct impedance technique seems to work satisfactorily in a wide range of applications both for boundaries with incoming sound and for radiation from sources; (b) the direct determination of incident power was successful in both the tested cases, the well absorbing wool and the highly reflecting window; (c) acoustic modal analysis of simple cavities is possible using standard structural analysis software and intensity instrumentation; and (d) intensity-based methods offer a number of new applications for analysing sound propagation and attenuation in ducts.Consequently, the presented methods considerably extend the possibilities available for investigating more complicated acoustical systems such as the window.