Turbochargers are a key component of internal combustion engines in the automotive industry. They contribute to the downsizing trend and allow constructors to respect more and more severe emissions norms. However, the turbocharger behaves as an acoustic source in the air intake system, and has an influence in the acoustic waves travelling in it. The passive and active acoustic characteristics of the turbocharger can be used as a starting point for the design and development of acoustic devices in the air intake system, or as an input of acoustic 1D simulations of engine intake lines. In this paper both the active and passive acoustic behaviours of a compressor stage of two different automotive turbochargers were studied.

The active acoustic effect of the compressor was studied by its characterization as an acoustic source. This is performed experimentally by the computation of the acoustic power at the inlet and outlet of the compressor. This power was obtained by performing plane wave decomposition with the beamforming method over all the working points of the compressor map. Using the pressure decomposition, the acoustic intensity is calculated and then the acoustic power. In this way the acoustic power delivered by the compressor on the air intake system is measured.

The passive acoustic effect is studied by the measurement of the acoustic Transmission Loss (TL) of the compressor through the calculation of its scattering matrix under two port considerations. Using also the beamforming method to decompose the incident and reflected waves at the inlet and outlet of the compressor, the transmission coefficients in both upwards and downwards directions are computed. Experimentally the TL of a static turbocharger was measured and results were extrapolated by a model to obtain the TL of the compressor over different working points of the compressor map. Numerically, 3D acoustic simulations were performed in order to compare to the model results. Numerical and experimental results were compared, and a predictive model for TL is proposed. This model will be compared in the future to real TL measurements on a working turbocharger.