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Flanking transmission of acousto-vibrational energy between adjacent acoustic cavities Wakefield, Clair William

Abstract

The effectiveness of partitions erected to, among other things, provide acoustic insulation between adjacent cavities in buildings, ships and aero-space structures is often limited by the presence of acoustical flanking paths. These paths, which can be provided by air gaps, ventilation ducts, etc., or by continuous walls or floors, allow sound and vibrational energy to pass between adjacent cavities without suffering the attenuation of the primary acoustic barrier (the partition). Techniques for the measurement of flanking transmission in existing structures and for its theoretical prediction have to date been awkward and imprecise, (the measurement technique requiring the erection of a second barrier to cover the barrier under test) and limited in range of application, (the theory only applying at frequencies for which the panel responses are mass controlled). The work described here was directed at removing these limitations on the present ability to measure and predict the effects of flanking transmission as they act to reduce the noise insulation attainable between two adjacent acoustic cavities. A cross correlation - Fourier transform technique was employed to measure the contributions of individual airborne flanking paths to the total sound field in a receiving cavity. It was discovered that cross correlation between two microphones, one in each of the source and receiving rooms, could not yield useful information about individual flanking paths because of the strong correlation of natural modes common to both cavities. This problem was overcome by replacing the source room microphone signal with the input signal to the noise source. The common room modes could not then correlate since only the receiving cavity microphone signal contained the mode components. This alteration, however, meant that only the relative magnitudes of the various flanking path transmission spectra could be obtained since the source room microphone signal was no longer available to provide a reference spectrum. However, for purposes of determining which flanking path contributes most to the receiving room sound field, relative magnitudes of their transmission spectra are all that is required. The altered technique allowed measurement of the transmission spectrum of an induced airborne path. Agreement with the measured difference in sound pressure levels with and without the flanking path, was very good. A relatively new structural dynamics technique known as Statistical Energy Analysis (SEA) was used to predict the noise reduction between two adjacent cavities, the boundaries of which were structurally coupled. SEA is based on an analogy to conductive heat transfer. Therefore, it becomes more accurate at higher frequencies as the wave fields in the acoustic cavities become more diffuse (have more spatially uniform energy densities). The SEA model developed here, then applied over all but the lower end of the experimental panel response range. This lower limit corresponds to the breakdown of diffuse wave field conditions in the smaller of the two experimental cavities. The model allowed the effects of the variation of panel internal damping and bending stiffness upon noise reduction to be investigated and was successful in predicting the noise reduction between two aluminum walled model rooms (for two different partition thicknesses) to within 2 dB over most of the frequency range 400 to 20,000 Hz. The results of the above two experiments and the corresponding SEA values of noise reduction, showed that, except at low frequencies where panel response is predominantly mass controlled, the increasing of primary barrier (partition) surface density does little to increase the noise reduction between cavities bounded and coupled by relatively lightweight, resilient walls. Increased panel internal or joint damping, however, increased the noise reduction at all frequencies but especially those near panel coincidences (when flexural wave speed in a panel equals the acoustic wave speed).

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