The considerable effort invested by automobile manufacturers to attenuate various noise sources within the passenger compartment has resulted in other sources such as induction noise having become more noticeable. This study was undertaken to investigate the feasability of using a non conventional noise cancellation technique to improve the acoustic performance of the induction system by introducing exhaust noise into the intake system through a manifold bridge.

The effectiveness of this technique was first investigated using Ricardo Wave, a computational, engine simulation, software program. Using a one-dimensional, finitedifference approach to analyse the dynamics of the pressure waves, mass flows, and energy losses within the ducts, an optimized bridge configuration was determined. A physical model incorporating the design of the optimized bridge was installed and tested on a motored engine for comparison to the numerical results.

The realized attenuation of induction noise due to the manifold bridge was evaluated using 1/12th octave frequency spectra and three-dimensional colour maps of both the unmodified and bridged engine for steady state and transient engine cases. A sound quality analysis was also performed using various psychoacoustic metrics including loudness, sharpness, roughness and fluctuation strength.

Both the numerical and experimental models demonstrated reductions in the overall sound level measured at the intake opening with the experimental results being more favourable. While the results of the sound quality analysis correlated well between the numerical and experimental models, the success of the bridging technique was somewhat ambiguous, depending on the sound quality metric used. As with the traditional analysis techniques, the reported loudness was lower for the numerical and experimental bridged engines. Sharpness was found not to be a relevant metric in this study due to a lack of high frequency content to the noise. Depending on the engine speed, values for roughness and fluctuation strength were either improved or diminished with the implementation of the manifold bridge.

For the conditions tested, implementation of the manifold bridge has demonstrated promise. Before it can be declared commercially viable, however, further considerations such as the effects of exhaust gas recirculation and fired engine tests, are warranted.