Bodén H., Knutsson M., Varma Nadampalli R.

This paper presents the results of an experimental study of the acoustic properties of an automotive air intake system. Modern air intake systems for personal cars are largely made of plastic materials. When trying to make an acoustic model for the sound transmission through the intake system it is questionable if the walls of different components can be modeled as acoustically rigid. The acoustic losses which determines the transmitted sound level at break-through frequencies may be especially difficult to model. To find out if acoustic losses associated with wall vibrations or with flow interaction are dominating for a typical intake system an experimental investigation has been performed. The acoustic plane-wave transmission matrices have been measured, using the two-source replacement technique, for the complete intake system and for separate parts. Measurements were made in a flow test rig without flow and for two different flow speeds. The measurements were then repeated with sand loading on the walls to reduce wall vibrations. The results indicate that flow related losses dominates at low frequencies wile losses associated with acoustically non-rigid walls dominate at higher frequencies, giving an additional transmission loss in the order of 2 dB for the complete system. Both effects will therefore have to e taken into account in an accurate model. Comparisons will also be made between results from a FEM model and experimental results.