Warning signals from electric vehicles at low speeds are intended to alert unprotected road users. However, a new study by researchers at Chalmers University of Technology shows that one of the most common signal types is very difficult for people to locate, especially when several similar vehicles are in motion at the same time.

In a recently published open-access study, researchers from Chalmers 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. The researchers’ tests 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.

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.

—Leon Müller, a doctoral student at the Department of Architecture and Civil Engineering at Chalmers

Today’s electric and hybrid vehicles meet the requirements set for acoustic warning systems according to international standards. In Europe, plus China and Japan, for example, vehicles traveling at a speed below 20 km/h must emit a warning signal consisting of tones or noise, to allow pedestrians, cyclists and other non-car users to detect them. In the United States, warning signals are required from vehicles traveling at speeds of up to 30 km/h.

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.

—Wolfgang Kropp, professor of acoustics at the Department of Architecture and Civil Engineering at Chalmers

The experiments involved some 52 test subjects and were conducted in Chalmers’ acoustics laboratory in soundproofed, anechoic chambers. The aim of the tests was to emulate real conditions in, say, larger carparks. The subject was placed at the center 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 pre-recorded 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 two-tone signals within the ten-second time limit.

The test subjects were easily able to locate the sound corresponding to an internal combustion engine. Leon Müller says 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.

Existing research has focused 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.

In a follow-up study, the researchers have begun investigating how AVAS signals are perceived and what effect they may have on non-road users.

Resources

• Leon Müller, Jens Forssén, Wolfgang Kropp; Auditory localization of multiple stationary electric vehicles. J. Acoust. Soc. Am. 1 March 2025; 157 (3): 2029–2041. doi: 10.1121/10.0036248