An image model for predicting the field performance of acoustic louvres from impulse measurements

Acoustic louvres are commonly used to provide natural ventilation while offering some reduction of noise break-in from external sources or control of noise break-out from machinery in the building. The sound insulation of acoustic louvres is not high, particularly at low frequencies, and therefore can be the dominant contribution to the overall performance of a façade or acoustic enclosure. Developments in acoustic louvre design have been hindered by the lack of an agreed simple measurement method that allows the effectiveness of prototypes to be assessed. In a companion paper, a method has been proposed for the direct measurement of the sound transmission loss of louvres by an impulse method, which circumvents the limitations imposed by standard methods. However, the field performance of louvres is more properly characterised by the sound insertion loss, rather than transmission loss, and a question remains on how impulse data can be transformed to predict field performance. In the present paper a description is given of the derivation of a transformation based on an image model of a test chamber with aperture. The resultant output data is in a form expected from measurements of insertion loss and includes directivity. The impulse measurement technique, combined with the transformation, promises an inexpensive method of assessing field performance of louvres without recourse to specialist acoustic test facilities.