Automobile and airplane engines run more and more quietly from year to year.
This phenomenon is useful for the folks, but not always pleasant for car owners. A man buys an expensive sports car and wants to enjoy not only the speed and curves but also an exhilarating roar of the engine. In addition, it is very useful to hear the sound produced by the engine, for instance for preventing its overload.
A contradiction occurs.

On the one part, the car engine must run loudly so as to give pleasure to its owner and signal about overload.
On the other part, the engine roar must not be heard at all because it annoys people.

 

One of the methods for resolving this contraction is using a temporal approach. Since the noise problem is topical for the city, let the engine run quietly within the limits of the city and let the driver enjoy the “songs” of his engine in the country. This can be realized by installing two silencers on a car – one for the city and the other one for the highway.
This contradiction can also be solved in space. If a driver wants to hear well the sound produced by his engine, let the sound be loud in the zone of his head. If the folks about him do not want to hear this sound, let the sound be quiet for them. To do this, it is enough to place loudspeakers in the driver's headrest and connect them through an amplifier with the microphones installed in the engine compartment.