As automakers attempt to make vehicles more energy-efficient, they're increasingly turning to lighter building materials. While doing so may indeed result in lightweight cars, these materials generally lack the sound-damping qualities of their heavier counterparts. That's why Harman and Lotus Engineering have developed the HALOsonic system, which cancels out road noise by making noise of its own.
Automakers such as GM and Toyota have already created active noise cancellation (ANC) systems that use sound to minimize engine noise. They do so by analyzing the engine noise within the cabin, and then generating a sound wave that has the same amplitude but an inverted phase. As a result, the sound waves from the engine and the ANC cancel each other out, so neither are heard by the passengers. It's the same principle used by noise-canceling headphones.
Things get a bit trickier when it comes to road noise, however. Because the baseline sound made by the engine is relatively consistent, ANC systems can pretty much just stay at one phase and get the job done. The vibrations made by the tires against the road surface are much more variable, though, which means that HALOsonic has to be able to constantly adapt.
It does so using accelerometers that measure the vibrations coming from the tires, in real time. An algorithm correlates these vibrations with noise detected in the cabin, to establish the vibration/noise relationship for that vehicle. The system then generates inverse sound waves that are played back through the speakers of the existing sound system, canceling out the road noise.
If music is being played back through those speakers at the same time, its sound quality will remain unaffected.
The basic system was created by Lotus a few years ago, mainly for use in its own cars. HALOsonic is intended more to be a feature that a variety of automakers can license for inclusion on their vehicles, although there's currently no word on which companies may be interested.