Greg, Lotus have done loads of work on this area, but I think the car cabin has too many different sources of sound, and resonances mean that a sound which is intense in one area is less noticeable in another, so the only pratical location for the sound removal devices is close to the ears of the users.
They use a system like this, built into the headphones/ear defenders of air force pilots.
Lotus did achieve exhaust silencing without the use of conventional silencers and, of course, this work lead to the idea of making a cheap front wheel drive shopping car sound like a Dodge Charger Hemi for no extra cost.
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