An acoustic vehicle alert system (AVAS), sometimes referred to as a pedestrian warning system (PWS), is designed to alert vulnerable road users (VRUs) like pedestrians, cyclists, and children to the proximity of nearby electric vehicles (EVs). The AVAS regulations detail the level of sound that must be emitted and the vehicle speed at which the sound is required. The AVAS regulations also apply to hybrids, plug-in hybrids, and fuel-cell cars.
There are several regulations related to AVAS, including:
- UN R138 is used in Europe, Australia, South Korea, South Africa, and other countries.
- Federal Motor Vehicle Safety Standard (FMVSS) 141 in the U.S.
- In Japan, guidelines have been issued for implementing AVAS.
- GB/T 37153 in China covers the use of AVAS in electric and hybrid vehicles. It’s mostly aligned with UN R138, except all minimum noise levels are 2dB higher.
Why low speeds?
There are several sources of sound from moving vehicles. Electric motors are quieter than internal combustion engines, but motors and engines are only two of several noise sources. Tire noise, road noise, and whistling noise are also significant.
Tire noise and road noise are related. Tire noise depends on the road’s condition and the tires’ design; the two factors interact to create noise. The tire and road noise are much lower at slower speeds compared with higher-speed driving conditions. The same applies to the whistling sound produced by the airflow around vehicles. Whistling is louder at faster speeds.
So, while EVs are quieter than IEC vehicles at slow speeds, at speeds above about 20 kph, the noise the two types of vehicles produce is similar (Figure 1). Hence, the need for AVAS only at low speeds.
Figure 1. At speeds up to about 20 kph, EVs are quieter than ICEs. (Image: International Congress & Exposition on Noise Control Engineering)
Louder at higher speeds
The sound emitted by AVAS is expected to change as the vehicle speeds up. UN R138 mandates a maximum sound level of 75 dB(A) measured at 2 m from the front of the vehicle or 66 dB(A) at 7.5 m. The regulation also requires that the frequency range varies at least 0.8% per kph.
Figure 2 shows the minimum sound level specified for each third-octave band between 10 and 20 kph. At least two of the minimum sound levels must be realized to satisfy the standard’s requirements.
Figure 2. UN R138 AVAS required sound levels in dB on vertical axis and frequencies on horizontal axis for speeds of 10 kph (left) and 20 kph (right), along with the overall (OA) minimum sound levels. (Image: UBA)
UN R138 and FMVSS 141 have similar but not identical technical requirements. FMVSS is based on the SAE J2889-1 specification but with several changes. UN R138 is based on the test specifications of ISO 16256, which mirrors the requirements of SAE J2889-1. UN R138 requires that AVAS operate until the vehicle reaches 20 kph while FMVSS 141 requires AVAS operation up to 30 kph. Some of the other differences in the FMVSS 141 requirements are:
- A stationary test for directivity testing using a third microphone.
- A third cruising speed test at a minimum of 30 kph
- No frequency or pitch changes, but a relative volume change of 3 dB is required for each cruising speed condition (10, 20, and 30 kph).
- The third-octave band testing defines two paths to compliance:
- Using four non-adjacent bands in the standard nine bands range
- Using two non-adjacent one-third octave bands between 315 and 3150 Hz, with one band below 1000 Hz and the other band at or above 1000 Hz.
Three purposes of AVAS
Before EVs, most vehicle noise regulations were focused on noise minimization. That changed with the inherently quiet EVs, particularly when driven at low speeds.
In EVs, AVAS is an important system for three reasons:
- Safety of VRUs. The risk of crosswalk and general road accidents involving VRUs is significantly reduced.
- Regulatory compliance. While increased safety is good, regulatory compliance is mandatory. That means that AVAS must be designed to meet the specific regulations of the country where the vehicle is sold.
- Sound branding. This is a new opportunity for vehicle makers to distinguish their products. Sound branding can also vary between regions and countries based on local sensibilities and preferences.
Summary
AVAS standards and regulations have been developed to improve the safety of VRUs near electric vehicles. The standards apply to low-speed driving since the sound pressure levels produced by EVs and IEC vehicles are similar at higher speeds. While the intent is the same for all AVAS regulations, there’s a wide range of significant and subtle differences between standards like UN R138 and FMVSS 141.
References
- Acoustic Vehicle Alerting Systems (AVAS) for EVs, Ansys
- AVAS: electric vehicle warning sounds, Siemens
- Car Noise: Are electric cars quieter than petrol cars?, Michelin
- Noise Assessment of the Acoustic Vehicle Alerting System (AVAS), UBA
- FMVSS 141, Minimum Sound Requirements for Hybrid and Electric Vehicles, US Department of Transportation
- Technical Comparison of FMVSS 141 and ECE R138, ISO
Images
- Figure 1, International Congress & Exposition on Noise Control Engineering, Page 3, Figure 1
- Figure 2, UBA, Page 65
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