Acoustic vehicle alert systems (AVAS) are a required feature for all types of electric vehicles (EVs), including hybrids and hydrogen-powered vehicles. In many instances, E-scooters and motorcycles must also integrate AVAS. These systems are designed to improve safety for vulnerable road users (VRUs) like pedestrians, cyclists, and children.
This FAQ begins with an overview of AVAS architectures, considers how the sounds are synthesized and the different regional regulations for AVAS sounds, and closes by looking at how AVAS sounds can be propagated.
AVAS systems have been designed using speakers and actuators that vibrate the vehicle’s structure. With the introduction of AVAS regulations like UN R138 and FMVSS 141, speakers have become the dominant solution since speakers provide more predictable results that can meet complex regulations. AVAS regulations call for the generation of sound when a vehicle travels at low speeds and when tire and wind turbulence noises are insufficient to alert VRUs of that vehicle’s presence.
In addition to increasing VRU safety, incorporating AVAS into a vehicle provides automakers another branding opportunity. In addition to visual aesthetics, vehicles can now be distinguished by their AVAS sound. Preferred types of sounds vary between countries. This is because the sounds suitable for use in Asia might not be ideal for European or North American cars.
Designing sounds and ensuring they meet the local regulations is only the beginning of the process. Two key system elements under the designer’s control are sound synthesis and propagation (Figure 1).
Figure 1. AVAS implementation showing the importance of sound synthesis (content) and propagation (directivity). (Image: Siemens)
There are several possible techniques for synthesizing AVAS sounds, including:
- Order-based synthesis was applied to early AVAS designs. It’s a technique to synthesize a combustion engine sound based on its physical properties.
- Wavetable synthesis is based on periodic reproduction of multiple arbitrary, single-cycle waveforms. Digital interpolation between adjacent waveforms allows for dynamic and smooth changes in the timbre of the tone produced.
- Granular synthesis uses an original sound sample broken down into time segments as short as 10 ms called grains. The grains can be reorganized as needed to deliver the desired effect.
Wavetable and granular synthesis provide the control needed when designing modern AVAS systems. Their use also provides the flexibility required to meet the diverse requirements of different AVAS regulations.
Location, location
Globally, the most common AVAS regulation is UN R138. It defines the minimum sound pressure level, the frequency content, and frequency characteristics in terms of a pitch shift of 0.8% per kph when a vehicle travels above five up to 20 kph. UN R138 applies across Europe, Australia, South Korea, South Africa, and other areas — but not in the US.
The US regulation is FMVSS 141, which differs from UN R138 in several key aspects. The American standard requires a sound when the vehicle is “idling” at a standstill, turned on, and ready to drive. And the maximum speed for AVAS implementation is 31 kph, compared with 20 kph in the UN regulation. The frequency requirements of FMVSS 141 are significantly different from the UN regulation (Figure 2).
As a result, an AVAS system designed to meet the requirements of most counties won’t necessarily be suitable for use in the US.
Figure 2. The AVAS frequency requirements differ between the US. (FMVSS 141) and other parts of the world (UN R138). (Image: 10th Convention of the European Acoustics Association)
Propagation and directivity
In most vehicles, a single speaker on the front bumper is used for AVAS. More advanced systems use multiple speakers to create directed sounds using beamforming techniques. Speaker placement is an important consideration in both cases. The vehicle structure can impact the speakers’ effectiveness. Also, the AVAS sound’s impingement on the vehicle’s interior must be considered.
Beamforming can be a powerful tool in AVAS implementations. If, for example, the vehicle “senses” the presence and location using LIDAR or cameras, beamforming can control a speaker array and direct the sound to the VRU location — enhancing the system’s effectiveness and minimizing unnecessary audio pollution.
Summary
All types of EVs are required to include AVAS for the protection of VRUs. The precise implementations vary based on regional regulations and local sensibilities.
Waveform and granular audio synthesis are common techniques used to generate AVAS sounds. While most systems use a single speaker in the front bumper, using a speaker array presents the opportunity to use beamforming, potentially increasing AVAS effectiveness and reducing sound pollution.
References
- Acoustic vehicle alerting system, Siemens
- Acoustic Vehicle Alerting Systems (AVAS) for EVs, Ansys
- AVAS Compliance: Meeting the Technical Requirements for Electric Vehicle Safety, Kepo
- AVAS Enhances Safety While Building Brand Reputation for EV OEMs, Microchip
- Design Guidelines for Improved AVAS Application Using Sound Radiation Analysis, 10th Convention of the European Acoustics Association
- Granular Synthesis 101: Breaking It Down, Unison Audio
- The Effect of a “Design-of-Awareness” Process on Recognition of AVAS Sound of Quiet Vehicles, MDPI applied sciences
Images
- Figure 1, Siemens, Page 6, Figure 2
- Figure 2, 10th Convention of the European Acoustics Association, Page 2, Figure 1
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