April 17, 2009 Although the environmental benefits of hybrid vehicles are well known, their near silent operation when running off batteries does pose a danger to pedestrians who are used to the rumble of an engine to alert them to oncoming vehicles. Active Noise Control technologies from Lotus Engineering address this problem by projecting engine sounds externally to improve pedestrian safety, while also employing noise-canceling technology internally to reduce unwanted cabin noise.
Making use of the same technology employed in noise canceling headphones, Lotus’ Road Noise Cancellation (RNC) and Engine Order Cancellation (EOC) systems reduce noise levels in the cabin, particularly at frequencies that are audibly unpleasant. In the case of RNC, the system reduces broadband noise levels at frequencies below 250Hz whereas EOC tackles harmonic frequencies generated by the engine. Input signals from the engine (for EOC) or sensors mounted to the suspension system (for RNC) are fed into the electronic controller, as are sound signals, measured by microphones located in the cabin. The software algorithms of the controller then calculate what sound is needed to provide cancellation and the speakers of the in-car entertainment system are used to put this into the cabin. All this takes just a few thousandths of a second and repeats and adapts constantly to changes in speed or road condition. Anyone worried that there rendition of Barry Manilow’s Coca Cabana will be rendered inaudible by the system will be relieved to learn the cancellation system operates solely on the input signals so other noise in the vehicle such as the audio system and speech are not interfered with or cancelled.
While Lotus’ Electronic Sound Synthesis (ESS) system serves to alert pedestrians to an oncoming vehicle, the system also has applications inside the vehicle to provide audio feedback to the driver in relation to engine speed and throttle position. The control system uses an engine speed signal, a throttle position sensor and the in-car entertainment system to add sound and, since the system provides specified electronic sound models, even the most sedate family car can be made to sound like a V12 Lamborghini. External sound is generated through a waterproof loudspeaker system positioned behind the grille allowing the sound to emanate from the front of the vehicle, while sound can also be synthesized from the rear of the vehicle to provide a warning when the vehicle is reversing.
When a car is operating on the electric motor only, throttle and speed dependent synthesized sound projects an engine sound in front of the vehicle. The sound is designed to mimic the pitch and frequency behavior of a conventional engine to help identify vehicle distance and speed. If the hybrid’s engine starts operating, due to higher speeds, throttle demands or lower battery levels, the control system automatically stops the external synthesis in a way that is designed to be as seamless as possible. In order to generate a realistic engine sound, recordings of a suitable donor engine are made and analyzed to establish the characteristic frequencies at different engine speeds. These frequencies are then entered into the synthesis controller in the form of a ‘voice’ which outputs the sound through an amplifier and out through the loudspeakers. Alternatively, more futuristic sounds for electric vehicles can be created using sampled sounds and generated waveforms.
While we’re a little worried the ESS system could be abused by the same people that like to advertise their taste in music by cruising around with their audio system at full boar, we have to admit using sound sampled from the Millennium Falcon on the family wagon would be hard to resist. And the technology could have applications beyond the automotive market. One day not only our cars, but also our buildings could be sound-optimized using active noise control technologies – a vision of the future that everyone who has had to endure noisy neighbors is sure to embrace.
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