Ford Europe has developed a tailored engine noise for the new Ford Focus powered by a 1.6-L EcoBoost engine. The system is called the Sound Symposer, which is a box with four chambers, two in the engine bay and two outside. The chambers are separated by a movable flap that opens when the engine is under high load, so that sound is transmitted to the outlet chambers. The system boosts engine volumes into passenger compartment in 200-to-450-Hz frequency range.
Watch the youtube videos to hear it work.