Those looking forward to quieter city streets as a result of near-silent electric vehicles (EVs) and hybrids replacing internal combustion engine vehicles on our roads may have to think again. To ensure that cyclists and pedestrians, including the visually impaired, are aware of stealthy oncoming vehicles, researchers have been looking at different noises that can be applied to EVs. Toyota has also already announced plans to sell an onboard audio alert system for its Prius but it looks like such systems are set to become more widespread with the Infiniti M35h to be the world’s first hybrid to get an audible pedestrian warning system as standard.

Infiniti says, when it goes on sale in Europe towards the middle of 2011, the M35h will travel further and faster in silent electric-only mode than any other hybrid on the market. It combines a 306 PS (225 kW) 3.5-liter petrol V6 with a 68 PS (50 kW) electric motor and is capable of accelerating from 0-100 km/h (62 mph) in under 5.9 seconds. The help reduce the chance of cleaning up any oblivious pedestrians when operating in electric mode, the vehicle’s Approaching Vehicle Sound for Pedestrians (VSP) system will utilize a range of high-low sounds at different volume levels.

As the result of a research program, (involving cognitive and acoustic psychologists no less), to find the most effective, but least intrusive, noise, the system will emit a sine wave that extends from 2.5kHz at the high end to a low of 600Hz. The company says this is a range that is readily audible to all age groups and avoids a sound range (around 1,000Hz) that would add unnecessary noise to the environment.

The noise will be at its loudest when the vehicle is started to give a clear indication it is beginning to move, and will sweep from high to low frequency depending on the vehicle’s speed and whether it is accelerating or decelerating. An intermittent tone will be used to indicate reversing.

Although the Infiniti M35h can travel at up to 80 km/h (50 mph) on engine-noise free electric power alone, the VSP system will cut out at speeds over 30 km/h because sufficient road noise is generated above that speed.

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