July 15, 2008 Toyota has taken the principle behind noise-canceling headphones and applied it to its Toyota Crown Hybrid. The company’s ‘Active Noise Control’ system uses three microphones located on the roof of the interior of the car to detect the muffled noise of the engine and then generates antiphase signals from three speakers – two located on the right and left front doors and one rear woofer on the rear shelf – to counter the noise. The system, which was implemented after the car’s engine was found to be noisier than expected, reduces the interior engine noise by about 5 to 8dB, the company said.
Although equipped with the same hybrid system as the Lexus GS, the engine of the Crown Hybrid has a lower rotational frequency at low speed which results in better mileage, but also more noise. The acoustics inside a car also made getting the system right difficult. Unlike headphones, the in-car system not only has to deal with both direct noise, but also its echo bouncing off the car’s interior walls. Toyota ended up implementing the system so that it cancels out the engine sounds around head height - or rather ear height.
"We collected the data such as the distance between the microphone and the head of a passenger, the distance between each speaker and the head of a passenger, and the shape of a car interior," Toyota said. "We designed the vehicle so that the wavelengths of the system's sound and the muffled noise cancel each other out over the heads of passengers and that the car interior becomes as quiet as possible." The system, which is always in operation when the car is running and cannot be manually switched on or off, does not interfere with the car’s music system.