reverberations: sound news from around the world

Hybrid cars are too quiet

Less air pollution is great, but tests are showing that hybrid cars are too quiet for pedestrian safety: people can’t hear them coming.

Following the passing of a congressional bill to study this problem, which especially affects blind pedestrians, Lawrence D. Rosenblum and Ryan L. Robart from The University of California, Riverside recently presented their findings in the 157th (Acoustical Society of America Meeting in Portland Oregon.

To compare the relative audibility of combustion and hybrid engine cars, they surveyed (sighted) subjects’ reaction time in hearing recordings of a hybrid Toyota Prius and a combustion engine Honda Accord moving at 5 mph slow speeds in a parking lot, using a variable level of background sound. The test results with the background sound made the task very difficult for the Prius especially. Test subjects were unable to determine the approach direction of the hybrid until it had actually passed by the location of the listener by 0.2 (or 1.4 feet) seconds, compared to the approach direction of the combustion car being determined when it was 3 seconds (or 22 feet) away.

In the real world this would mean in a situation with normal background sound, a blind person (or someone relying on sound to give them their ‘all clear’ signal) would find it especially difficult to hear a hybrid Prius.

What sound and when?
The reaction to this has been suggestions for a required “minimum sound” for all late model cars. As we move to fully electric vehicles, this problem will become amplified and even more urgent.

But what sound should quiet cars make to announce their vector? Patrick Nyeste and Michael S. Wogalter from North Carolina State University tested 24 subjects’ response to six categories of sounds that could be used to make quiet vehicles more noisy. These were: engine, horn, hum, siren, whistle, and white noise. Three variations of each type of sound were tested and their findings were published in the Proceedings of the Human Factors and Ergonomics Society, 52nd Annual Meeting, Not surprisingly engine noise was the favourite due to the people’s association to the current sound of combustion cars.

On communicating the ‘need for noise’ to hybrid car manufacturers, there have been further suggestions for a “smart” system that ups the noise level when a car is not making enough noise (i.e. when traveling slowly and only using the very quiet electric motor) but then turning off when the car is producing an adequate level (i.e. traveling faster using its combustion engine). One manufacturer in particular - Lotus Engineering -has already introduced "Safe and Sound," which links to the accelerator pedal and uses the sound of a donor engine when the vehicle is in electric-only mode.

Surely this is a great opportunity for creativity. Why not give drivers a range of sounds to choose from: F1 Grand Prix car, battle tank, Harley Davidson bike, classic Ferrari… the fun we could have! More seriously, we do believe there is scope for manufacturers to design branded engine sounds so that everyone gets to know the characteristic tone of a Honda from a Toyota.

Links:
HFES research into ‘what sound?’
ASA report on ‘Are Hybrid Cars Too Quiet?’
Lotus Active Noise Technologies

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