The "active noise control" system of Toyota's Crown Hybrid reduces the muffled noise coming from the engine to the car interior. The Crown Hybrid was released in May 2008.
The rotational frequency of its engine is as low as 1,000 to 3,000rpm when the vehicle is running at 20 to 40km/h (12 to 25 mph). However, if no measure is taken, the muffled noise of about 50-150Hz is transmitted to the car interior, impairing the comfort of the car.
The first active noise control system of Toyota detects the muffled noise inside the car with three microphones and generates antiphase signals from the speakers to counter the noise, enhancing the quietness by about 5 to 8dB, the company said.
Though the Crown Hybrid is equipped with the same hybrid system as the Lexus GS, the engine of the Crown Hybrid has a lower rotational frequency at low speed for better mileage. In the developmental stage, the muffled noise occurred inside a car when it was running in urban areas. But the active noise control system enabled to improve fuel economy and ensure quietness at the same time.
The system is always in operation when the car is running. Neither a driver nor a passenger can manually switch on or off the system.
"There is no point in having a switch that cannot turn on the system," Toyota said. "So, we made the system that is always operating."
The Crown Hybrid is equipped with three noise canceling speakers: two on the right and left front doors and one rear woofer. The active noise control system works even when music is being played by the in-car audio system.
The acoustic transmission in a vehicle is complex because the sound echoing off walls and the sound directly reaching the ears are mixed.
"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."