An Experimental Study on Factors That Influence Encapsulation Efficiency

Absorptive and isolating encapsulations or enclosures are commonly encountered around different noise-emitting components within the car industry. Not least for electric drive units, whose air borne noise shares often are dominant in the 2-6 kHz region, encapsulations can provide a cost and weight efficient noise abatement solution. The main constrains related to the acoustic performance when designing an encapsulation for electric drive units are surface coverage due to geometrical complexities, allowable package space (setting limits for maximum thickness of the encapsulation), weight and finally cost. The numerical simulation techniques for quantifying the acoustic performance in terms of insertion loss are challenging, since the encapsulations are partly compressed and far from homogeneous for example. Impedance tube measurements of absorption and transmission loss are standard for material samples, but the link between those metrics and the insertion loss of the complete encapsulation has been unknown. This work aimed to diminish this knowledge gap by performing an experimental study of the impact of the three parameters coverage, transmission loss and absorption on the insertion loss of a complete encapsulation. For enabling many tests and still low measurement uncertainty, a speaker mounted inside an empty housing of an electric drive unit was acting artificial source while insertion loss measurements of numerous encapsulations with known sample characteristics were performed sequentially. Multiple linear regression analyses revealed a strong correlation between the three parameters and insertion loss and the impact of each parameter could be quantified.