Is there a risk of not hearing the traffic? Image by Tim Sandle
Electric vehicles are growing more popular, yet their warning sounds may not be doing enough to protect pedestrians. A new Swedish study shows that these signals are hard to locate, especially when multiple vehicles are involved. Consequently, this leaves people unable to tell where danger is coming from or how many cars are nearby.
Researchers from Chalmers University of Technology investigated how well people can locate three common types of warning (or AVAS Acoustic Vehicle Alerting System) signals from hybrid and electric vehicles moving at low speeds.
To test this, the researchers’ showed that all the signal types were harder to locate than the sound of an internal combustion engine. For one of the signals, the majority of test subjects were unable to distinguish the direction of the sound or determine whether they were hearing one, two or more vehicles simultaneously.
According to lead researcher Leon Müller: “The requirements placed on car manufacturers relate to detection, or detectability, not about locating sound direction or the number of vehicles involved. But if you imagine, say, a supermarket carpark, it’s not inconceivable that several similar car models with the same AVAS signal will be moving at the same time and in different directions.”
Confusing for pedestrians
Car manufacturers often design their own signature sounds. These warning signals are often tested without the complication of background noise. But in a real traffic environment, there are usually many different types of sound.
As Müller explains today’s electric and hybrid vehicles meet the requirements set for acoustic warning systems according to international standards. In Europe, China and Japan, vehicles travelling at a speed below 20 kph must emit a warning signal consisting of tones or noise, to allow pedestrians, cyclists and other noncar users to detect them. However, in the U.S., warning signals are required from vehicles only travelling at speeds of up to 30 kph. This speed difference is often the difference between life and death.
Whilst accidents at 20 kph can be catastrophic, with the risk of serious injury, this exponentially increases with impact speed. For instance, a pedestrian hit at 30 kph has a significant chance of being killed.
Experimental data
The scientific experiments involved some 52 test subjects and were conducted in an acoustics laboratory contained within soundproofed, anechoic chambers. The aim of the tests was to emulate real conditions in, say, larger carparks.
The subject was placed at the centre of the room and surrounded by 24 loudspeakers placed in a ring at chest height. Three types of simulated vehicle sounds were played on the loudspeakers, corresponding to the signals from one, two or more electric and hybrid vehicles, plus an internal combustion engine. One of the signals consisted of two tones, one had multiple tones and one was just noise.
The test subjects heard a vehicle warning signal at about 7.5 meters away, mixed with prerecorded background noise from a quiet city carpark. When they heard the signal, the subjects had to mark the direction it was coming from as quickly as possible. The signal comprising two tones coming from three vehicles simultaneously was the most difficult and none of the test subjects managed to locate all the twotone signals within the tensecond time limit.
The test subjects were easily able to locate the sound corresponding to an internal combustion engine. Müller points out this sound consists of short pulses comprising all frequencies; something that is easier for the ear to perceive than a fixed tone at a single frequency. The fact that people can more easily perceive this type of sound may also be because of its familiarity.
Current research tends to focus mainly on detectability and what is usually referred to as “detection distance.” No previous studies have investigated what happens when two or three cars emit the same type of signal. The researchers see a major need for further knowledge of how people react in traffic situations involving electric vehicles.
From a traffic safety point of view, the researchers contend that it would be desirable to find a signal that’s as effective as possible in terms of detection and localisation, yet which does not affect people negatively; something our previous research has shown to be true of traffic noise.
The new study features in The Journal of the Acoustical Society of America, titled “Auditory localization of multiple stationary electric vehicles.”
