Validation of guidance for the prediction of the directivity index of ventilation louvres

The accurate prediction of external noise levels received at sensitive locations from building services plant is crucial at the design and planning stages of many construction projects. Guidance for the prediction of the directivity index of ventilation louvres found in the literature has been reviewed and partially validated. CIBSE Guide B4 is widely used by acousticians in the UK and other parts of the world. It presents guidance for predicting the directivity index of wall-mounted building ventilation louvres emitting noise to atmosphere at various angles. For louvres of large surface area it predicts very high values of frequency independent attenuation. This is not consistent with theoretical understanding of source directivity. The origin or basis for this correction has not been found in the literature. Experimental results strongly suggested that that the CIBSE B4 prediction method is not accurate for predicting the directivity index of large external ventilation louvres and that the guidance is questionable for small louvres. Of the other prediction methods found in the literature and tested in this study, one method showed stronger agreement with the measurements. Further investigation into this method is proposed to determine its suitability as a valid prediction alternative.