Researchers from Chalmers University of Technology in Sweden have demonstrated that one of the most common artificial warning sounds used by electric vehicles (EVs) at low speeds is very difficult for humans to locate, especially when multiple similar vehicles are in motion at the same time.
Unlike internal combustion engine (ICE) vehicles which, as the name suggests, are powered by a noisy internal combustion engine, electrified vehicles must instead rely on one of three common types of artificial warning signals generated when at low speed, also known as AVAS, an Acoustic Vehicle Alerting System.
In Europe, China, and Japan, for example, electric and hybrid vehicles travelling at a speed below 20km/h must emit a warning tones or noises so as to allow pedestrians, cyclists, and other non-car users to be able to detect them. In the United States, these warning sounds are required for vehicles travelling at speeds of up to 30km/h.
However, according to experiments involving 52 test subjects conducted in a soundproofed and anechoic acoustics laboratory by researchers at Chalmers University of Technology, at least one of these common signal noises is very difficult for humans to locate.
Specifically, the Chalmers’ researchers emulated real-world conditions such as might be found in a large carpark by placing test participants in the centre of the acoustics laboratory, surrounded by 24 loudspeakers placed in a ring at chest height.
Three types of simulated vehicle sounds were played on the loudspeakers to represent 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 pre-recorded background noise from a quiet city carpark.
Test participants were required to point a toy gun that had been converted into a laser pointer in the direction they believed the noise was coming from.
According to the Chalmers’ researchers, the signal comprising two tones coming from three vehicles simultaneously was the hardest to detect, and none of the test subjects managed to locate all the two-tone signals within a ten-second time frame.
“The requirements placed on car manufacturers relate to detection, or detectability, not about locating sound direction or the number of vehicles involved,” said Leon Müller, a doctoral student at the Department of Architecture and Civil Engineering at Chalmers.
“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.”
Conversely, test subjects were easily able to detect and locate the sound corresponding to an internal combustion engine, a sound which Müller explained consists of short pulses across all frequencies which is easier for the human ear to perceive than a fixed tone at a single frequency.
“Naturally, as acousticians, we welcome the fact that electric cars are significantly quieter than internal combustion engines but it’s important to find a balance,” says Müller.
According to Wolfgang Kropp, a professor of acoustics at the Department of Architecture and Civil Engineering at Chalmers and one of the authors the research, “The way the requirements are worded allows car manufacturers to 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.”
Previous research conducted into AVAS has focused primarily on detectability and detection distance, but there had been no research into what happens when two or three cars emit the same type of signal.
“From a traffic safety point of view, it would be desirable to find a signal that’s as effective as possible in terms of detection and localisation, but which doesn’t affect people negatively; something our previous research has shown to be true of traffic noise,” says Kropp.
The research, Auditory Localization of Multiple Stationary Electric Vehicles, was published in The Journal of the Acoustical Society of America.
Joshua S. Hill is a Melbourne-based journalist who has been writing about climate change, clean technology, and electric vehicles for over 15 years. He has been reporting on electric vehicles and clean technologies for Renew Economy and The Driven since 2012. His preferred mode of transport is his feet.