While highway engineers looked for new ways to soften the noise of passing vehicles, automakers concentrated on making life quieter for the drivers. A quiet ride has traditionally been a symbol of luxury. But as vehicles more and more become rolling technology centers, with cellular telephones, televisions, and computer equipment, silence is becoming a necessity for everyone. Many electronic communication systems, such as hands-free cellular phones using voice-recognition technology, work better if background noise is kept low.
Much of the noise in a vehicle comes from the engine. To combat engine noise, engineers are working to develop more effective mufflers, including ones using active noise cancellation. This technology takes advantage of a property of sound waves known as destructive interference, in which the peak of one sound wave collides with the trough of another wave. When this happens, the two waves cancel each other out, thereby eliminating the sound. Active noise-cancellation systems use electronics to analyze the wave of an unwanted sound and generate an opposite “antinoise” wave to eliminate it.
One of the first mufflers using active noise cancellation was developed in the late 1980's by engineers at the NCT Group, Inc., of Westport, Connecticut. The NCT Group's device is quite different from a traditional muffler, which routes exhaust gases from the engine through a series of perforated tubes so they can expand and cool before exiting the tailpipe. In contrast, the electronic muffler uses a microphone to continuously monitor the vibrations of the exhaust pipe and feeds the information to an electronic signal processor. The processor then computes the appropriate vibration-canceling waves and causes a transducer (vibration-producing device) to generate the waves and send them toward the engine. There, the sound waves and antinoise waves collide and cancel each other out.
(Source: How stuff works?)