Prediction of flow-generated noise produced by an in-duct spoiler in a ventilation system using CIBSE Guide B5 methods

Abstract

Measurements of flow-generated noise produced by two different types of strip spoilers in the air duct and radiated from an open exhaust termination unit into a reverberation chamber have been made in 1/3 octave bands using sound power spectra. The measured values are compared with the predicted values using CIBSE Guide B5 method based on selected values of the constant C. It is found that the measured values agree well with the predicted values. The measured values are also compared with the predicted sound power levels using the generic equations given in the Appendix 5.A2 of the CIBSE Guide. It is found that the predicted values using the generic formulae and the single normalised spectrum given in the Appendix 5.A2 of CIBSE Guide do not agree well with the measured values, particularly at low frequencies. For very different in-duct elements, a single normalised spectrum for flow-generated noise may not be sufficient for all configurations of the in-duct elements. It is suggested that several representative normalised spectra for different types of in-duct elements should be given. Practical application: This study provides an evaluation of the accuracy of the CIBSE Guide B5 method and the CIBSE Appendix 5.A2 generic formulae for the prediction of flow-generated noise produced by in-duct spoilers in a ventilation system. It also gives comments on the method for improvements.