AVAS Sound Design: Why Every Vehicle Needs Its Own Acoustic Identity

1.From Safety Requirement to Brand Statement
2.Why a Standard Sound Does Not Work Across Different Vehicles
3.The Principles That Guide Good AVAS Design
4.Frequency Selection and Tonal Design
5.How BESTAR Supports AVAS Development
6.Conclusion

From Safety Requirement to Brand Statement
When the use of EVs and hybrid vehicles began to proliferate on the public roads, policy-makers raised a serious safety concern. Depending on the volume of the vehicle's sound, EVs generate substantially less sound than traditional internal combustion vehicles at low speeds, making it hard to see before proximity to the vehicle for pedestrians, particularly those who are blind or visually impaired. Many markets have adopted a mandatory AVAS rule to deal with this problem.
The basic idea is very simple. At low speed (usually less than 20km/h), an EV, hybrid vehicle or electric motorcycle shall be equipped with an Acoustic Vehicle Alerting System-it shall warn near by pedestrians to be aware of its presence and whether it is moving forwards or backwards. Presently, AVAS regulations are required in the biggest markets, such as the European Union, the U.S., China, Japan and South Korea. Retailers in these areas must comply with the regulations.
There have been significant changes in the attitudes of the industry toward the implementation of AVAS in the last few years. At first, AVAS was considered just a compliance exercise. The engineers picked a sound that was below the minimum regulatory sound pressure level and determined whether or not it fulfills the related regulations. It's not working anymore. 
Today, auto makers are aware that a vehicle's noise is becoming a crucial element of their brand image. The iconic noise of a combustion engine has been a clear signal as to performance, heritage and character for decades. This acoustic signature is however no longer present in electric vehicles. AVAS is a chance to change it for a well-thought-out sound, which expresses the character of the vehicle. Accordingly, the strategic importance of acoustic branding as a core element of brand identity has never been greater. Automakers increasingly recognize that a vehicle's sound is one of the very first brand experiences customers encounter when they enter and begin driving the vehicle.
This mindset change has led to some significant expectations placed on the development of AVAS. Creating a "safe" sound that meets the requirements is easy. The task of developing a sound that is consistent, comfortable for driver and occupants, consistent in the real world and providing reliable pedestrian recognition is a much more challenging engineering exercise.

Why a Standard Sound Does Not Work Across Different Vehicles
AVAS became one of the earliest misunderstandings that there was a one-in-all solution, in that a sound for a given product lineup had to be tailored to just a few minor vehicles, and then for use in a single final assembly line. This is in fact impractical for several very compelling reasons.
A large electric SUV is different from a compact urban runabout designed for efficiency and convenience. A premium executive sedan operates in a different context. The sound each of these vehicles produces needs to match its physical character and the expectations of the people who drive it. A deep, authoritative tone that suits the SUV. A light, neutral tone that works for a compact commuter car would undermine the positioning of a luxury sedan. Sound must align with what the vehicle is, not just what the regulation requires.
The next is vehicle's physical design. The front-end designs of the various vehicle platforms are different, with each contributing to sound transmission from the vehicle, including size and shape of the grille opening as well as depth of the front cavity, position and angle of the AVAS speaker. These structural differences have an impact on sound radiation, frequency response, overall acoustics and others. The characteristic of the vehicle developed with one platform won't be the same if the characteristic is developed on a different platform. The reflections, resonances and diffraction patterns depend on the geometry of the vehicle, therefore, if no optimisation of the vehicle acoustics is made, the intended sound of the vehicle can be quite different from the sound heard by the pedestrians.
Thirdly, the Noise, Vibration and Harshness (NVH) environment of the vehicle. NVH refers to the ambient noise signals produced by the operation of these tire, wind, electric motor and structural vibrations. These sounds can partially cover or distort the AVAS signal in actual vehicles operation. The warning sound that is clearly heard on a control road can disappear significantly when this occurs in rough road conditions or in traffic conditions. However, AVAS development needs to be done in actual acoustic settings, as opposed to optimized laboratory conditions for this reason. To ensure reliable pedestrian detection and consistent acoustic performance, a realistic assessment of the system's performance under realistic operating conditions is crucial.

The Principles That Guide Good AVAS Design
Pedestrian detectability is the non-negotiable starting point. AVAS should create an audible sound that stands out from background noise in typical urban settings, from the sounds of a noiseless residential street at night to the noisy intersections during peak hours as traffic busy sounds overrun it. It should also be audible by people that have normal and less than normal hearing. Furthermore, overall direction is also a significant factor, pedestrians need to not only hear or see a vehicle approaching, but be able to hear or see its direction of approach.
Driver and passenger comfort is equally important and is sometimes treated as secondary when it should not be. The AVAS speaker faces outward, but sound does not travel in only one direction. Some portion of the output will find its way into the cabin. Over a typical urban commute with frequent low-speed sections, an intrusive or tonally unpleasant AVAS sound causes real listener fatigue. Good acoustic design uses directional speaker placement, cavity shaping, and signal processing to maximize outward radiation while minimizing cabin intrusion.
Dynamic response is what separates a static compliance sound from a genuinely well-designed AVAS system. It is dynamic and does not stay the same. The sound must change smoothly when the vehicle changes speed, accelerates and decelerates. There should be sufficient pitch, volume and tonal differences which will be intuitive for users to identify avenues of vehicle behaviour. Sudden and erratic movements could detract from the pedestrian's safety. The aim is to produce an acoustic effect that will naturally and reliably convey the action of a vehicle without the need for large, perceptible actions.

Frequency Selection and Tonal Design
AVAS frequencies are not just any frequencies. The minimum sound pressure level are usually specified in a regulatory framework within certain frequency ranges, which are frequently broken down into one-third octave measurement intervals. Besides regulations, psychoacoustic principles have to be taken into account in order to make sure that the sound is both audible and palatable for human ears.
Lower frequencies(from 160 Hz to 400 Hz) contribute to the sense of physical presence and weight. They communicate that something substantial is nearby. Middle and upper frequencies(from around 500 Hz to 2500 Hz) are where human hearing is most sensitive and directionally accurate. These frequencies carry the clarity and locatability that make a sound easy to detect and orient toward. Pure tones within this range can be effective for detectability but may become irritating with repeated exposure. A well-designed AVAS typically combines a structured harmonic profile with some broadband content to create a sound that is both detectable and tolerable over time.
After all, tonal design is crucial to acoustic branding. Fundamental frequency and harmonic structure are key ingredients to a sound's character. The AVAS signal changes into more than a warning at this point and instead becomes an integral part of the vehicle's sound.

How BESTAR Supports AVAS Development
BESTAR has a long history of involvement in the electro-acoustic field, having a wide range of expertise to design and manufacture acoustic components, speaker systems and car audio solutions. This experience helps the company address the more complex today's AVAS development programs.
AVAS systems for exterior vehicle use are built to handle the rigors of an interior vehicle environment, including high-durability, temperature resistance and protection from moisture, dust and mechanical shock with performance approaching that of an automotive system. BESTAR's engineering team not only provide hardware supplies but also offer acoustic simulation and acoustic placement test for speakers, as well as acoustic signal processing tuning (DSP tuning) after considering the specific architecture of the vehicles.

Conclusion
If designed correctly, AVAS incorporates three essential factors: regulation, acoustics and brand. These elements should be balanced in the vehicle system using the vehicle architecture, NVH aspects, target user expects and brand positioning.
When dealing with an acoustic solution, the same type of solution results in the same type of user experience. A properly designed AVAS system elevates a must-implement rule into a valuable driving and pedestrian experience. With careful design, AVAS can transform into a more than just a warning tool, it can become part of the vehicle's identity.
BESTAR's engineering team can support project-specific needs and requirements, including AVAS optimization for new vehicle platforms currently being developed or existing architectures. Provide us with your vehicle architecture, structural limits and regulatory targets and we’ll co-create an optimized acoustical solution for you.