This research deals with the characterisation of low frequency sound transmission between two rooms via a flexible panel, a problem of considerable importance in the automotive, aerospace and building acoustics industries. Low frequency sound insulation has become more important recently, due to the increasing presence of noise sources in this frequency range. In practice, panels are often tested in finite sized reverberant chambers, where the acoustic field is dominated in the low frequency range by a few normal modes and does not satisfy diffuse field conditions. The use of different facilities can lead to a wide spread of results. An analytical formulation for the calculation of the sound reduction index of flexible partitions has been presented and the results compared when the panel is mounted in an infinite baffle with free field conditions on both sides and when the effects of the transmission suite have been taken into account. A fully coupled model has been maintained in the numerical simulations. The results obtained in the transmission suite (15 m3 ) and in the free field show discrepancies of greater than 20 dB in the low frequency range.
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| Geometrical arrangement of the source and receiving room connected through the partition (left) and the sound reduction index (right) estimated under diffuse field excitation conditions (blue) and in the transmission suite with four near field optimised loudspeakers (red) and four far field uncorrelated loudspeakers (green) | |
To make the measurement of the incident power more reliable at low frequencies, the use of an array of loudspeakers in the near field of the object to be tested has been proposed, where the sources have been optimised to reproduce a diffuse pressure field on the surface of the panel. This new approach has been tested numerically. It has been shown that, for the particular configuration analysed here, an array of four near field loudspeakers is able to remove the modal influence of the source room in the low frequency range. The new approach proposed is able to provide a measure of the sound reduction index that only depends on the properties of the panel itself, avoiding the modal problems presented in the classical method. The dimensions of the source room could be significantly reduced, even for low frequency testing, if such a near field array was used.
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| New methodology proposed for the synthesis of a diffuse pressure field using four loudspeakers well driven (left) and results for the panel sound reduction index (right) when excited by an ideal diffuse field (blue), using the classic approach (green) and experimentally determined with the electro-acoustic device (red) | |
