How AVAS Works: A Complete Technical Breakdown of the Acoustic Vehicle Alerting System

1. Introduction
2. System Architecture: The 4 key parts
3. Step One: Data Collection and ECU Input
4. Step Two: DSP and Sound Synthesis
5. Step Three: Pitch Modulation and Speed Mapping
6. Step Four: Power Amplification
7. Step Five: Acoustic Speaker via the External Speaker
8. Sound Advice: A New Direction
9. Conclusion

Introduction
EV are silent. It is one of their most hymned attributes. But at low speed, there is a literal safety issue of that silence. Invasive petrol engine is noisy at all times. People perceive a car moving prior to its sight. They change their route, have a look at a curb, or look at the phone. This sound alarm takes place automatically, without one even considering it.
EV automobiles eliminate such an audio signal completely. A speed lower than 30 km/h means that the tyre and wind noises are insignificant to draw the attention of most pedestrians. Particularly under threat are visually impaired individuals. But it is a problem that touches all: the children, bikers, and people, who walk down the streets and are not concentrated enough to pay enough attention to avoid accidents.
UN R138 standard has a market usability in Europe, South Korea and Australia among other markets. The United States is governed by FMVSS 141 standard. China follows GB/T 37153. Their application is the same with all of these regulations: the electric and hybrid cars have to produce some fake warning sound when they drive low.
This sound-generating system is known as the Acoustic Vehicle Alerting System(AVAS). It has the task to provide the lost engine sound with a fake sound that people walking around can hear and intuitively comprehend. It must resemble a car. It should react to instantaneous variations in the speed. And it must work when needed, on a daily basis and at any time of the year.
In this blog, we are going to describe in details how AVAS to do.

System Architecture: The 4 key parts
AVAS is not a device. It is a sequence of elements, which combine. All the parts perform only a single task, and the quality of the final product is based on the proper functionality of all the links in the chain.
It consists of four main parts: the control unit, the digital signal processor, the power amplifier and the external speaker. These components have the following data flow: the vehicle is fed into the processor via communication, the amplifier processes the signal and finally the sound is sent back out of the speaker.
The system is linked to the car via the onboard network of communications. The vast majority of vehicles today have a CAN bus or CAN FD bus. This is the highway of data which transmits information among various systems in a vehicle. This bus is read by the AVAS control unit continuously and the information it obtains is used to determine when to turn on, what sound to make and how loudly to make the sound.

Step One: Data Collection and ECU Input
Following the initial step, Data Collection and ECU entrance.
The initial task of the AVAS control unit is to be aware of what the vehicle is doing at each point in time. The electric vehicle has the brain, which is known as the Vehicle Control Unit (VCU). It also measures speed sensors, gear condition and dozens of others. The AVAS system measures this in response to real-time data fed in the CAN bus. Speed and direction of travel of the vehicle are the most significant inputs.
The system is dependant on speed to prevent or allow the system to be in operation. The laws stipulate that AVAS has to be used within a specified range of speed. Using UN R138, the system should make a sound during the start up until 20 km/h. In FMVSS 141 this maximum speed is 30 km/h. Control unit obtains the data on the speed and forms the simpled binary decision: is the vehicle in the activation range or not?
Direction of travel is also of concern. Different sounds can be required in forward movement and reverse movement. Most of the regulations state that AVAS has to produce sound as well whilst the car is in reverse. The system captures the gear state of the bus and changes the sound profile of the system as needed.
This step of data collection occurs on a continuous basis. Speed and direction are checked a number of times a second by the system. Whenever there is any state alteration in the vehicle, a sound output is updated in real-time.

Step Two: DSP and Sound Synthesis
When the control unit has received information that it is in the vehicle and the desired range of speed, it informs the digital signal processor (DSP) to produce sound.
It is DSP in which the real sound is generated. It does not play recorded file. It rather produces sound on-the-fly in mathematical models. Wavetable synthesis and frequency modulation synthesis are the two most prevalent techniques of synthesis.
The principle of wavetable synthesis is that the waveform data is cycled quickly by loading it into memory. Consider it playing a very short high speed loop of sound. The DSP is capable of achieving a flowing engine-like sound by regulating the velocity, pitch and overlaying these loops.
Sound Frequency modulation synthesis is a technique to create sound by modulating one frequency signal by another one. The technique is also able to create more complex tones which have multiple harmonics that is what gives a real engine its typical timbre or tonal colour.
The DSP involves clustering a basic base frequency with other harmonic layers forming layers above it. The pitch of the sound is the base frequency in which it is based. The harmonic strata add texture and realism to it. The aim is a sound that will be instantly identified by a pedestrian as a car and it is not the hum of a machine or an alarm.
Speed of processing is important in this case. Once a driver presses a pedal (acceleration and deceleration), the sound should shift instantly without being audible any longer. This DSP aims to update the output on the milliseconds scale. This maintains the sound closely in line with the real car action.

Step Three: Pitch Modulation and Speed Mapping
A tone played at constant volume would be enough to meet the minimum regulatory criteria. But it wouldn't do the job well. Pedestrians want to know more than that a car is there. They want to know what it's doing.
The faster the vehicle is traveling, the higher the pitch of the sound will be. The slower it's moving, the lower the pitch. It's exactly how a petrol engine sounds when it accelerates and decelerates. Most people intuitively know that if engine pitch is rising, it's speeding up. If it's falling, it's slowing down. AVAS takes advantage of this intuition that already exists.
The relationship between speed and pitch isn't arbitrary. Pitch isn't the only thing that changes with speed. Volume does too. Sound pressure level increases with vehicle speed. This allows the warning to cut through the masking effect of higher speed road noise, making sure it can be heard at all speeds within the activation zone.
Gradation is smooth for both pitch and volume changes. There are no abrupt jumps in either. The end result is an output that smoothly varies in volume and pitch, resulting in a natural sounding signal that mimics the behaviour of a real engine.

Step Four: Power Amplification
The synthesised audio signal from the DSP is a low power electrical signal. It is clean and accurate, but it does not have enough energy to drive a speaker at useful volume levels outdoors. The power amplifier changes that.
Most AVAS systems use a Class D amplifier. Class D amplifiers are highly efficient. They convert a large percentage of input power into audio output, wasting very little as heat. This matters in a vehicle context because every watt consumed by the AVAS system is a watt drawn from the battery. An inefficient amplifier in a product used continuously in urban driving would add up to a measurable impact on driving range.
The amplifier also plays a filtering role. Digital synthesis can introduce small amounts of quantisation noise into the signal. The amplifier circuit includes output filtering that removes this noise before the signal reaches the speaker. The result is a clean, stable current signal that accurately represents the intended sound waveform.
The output power level is matched to the speaker load and the required sound pressure level. The system is tuned to meet regulatory minimums comfortably while avoiding overdriving the speaker, which would cause distortion and reduce component lifespan.

Step Five: Acoustic Speaker via the External Speaker
The last component of AVAS is the speaker. It changes the electrical signal that becomes stronger to sound waves that are real in the air.
The AVAS speaker is attached to the outside of the vehicle usually to the front. And this is an unfriendly place. Rain, road spray, mud, extreme temperatures, direct sunshine and vibration (driving force and vehicle chassis) all expose the speaker to elements and factors potentially damaging the speaker. And all this it must continue to do over the life span of the vehicle service, which is usually a decade or more.
All of these conditions are considered by Bestar in the designing of its AVAS speakers. The acoustic seal and housing are ruggedised. The speaker elements are chosen and confirmed to be used in long term outdoor usage. The protection ratings like IP67 will make sure that the unit is water resistant.
The acoustic design of a speaker is also optimised to the propagation in the outdoors. The purpose of indoor speakers is to fill a room. AVAS speakers must broadcast sound that is directed both in front of the vehicle and to its sides, and a pedestrian height and range over a varying distance. The directional nature of the speaker, together with the geometry of the housing is designed to provide a uniform coverage of the pedestrian danger area surrounding the vehicle.

Sound Advice: A New Direction
AVAS regulatory level has been already determined. The industry is not just adapting these new regulations but is becoming more able.
Adaptive volume control on the basis of ambient noise is one of the fields of active development. The concept is simple. An automobile travelling in a desert area in a residential street in the evening does not require making the same sound as an automobile in a construction area. Adaptive system takes the measure of the noise in the vicinity and increases or decreases the AVAS output volume accordingly. This does so by minimizing unnecessary noise pollution where unneeded noise is not required and enough warning is maintained where background noise is high enough to provide adequate warning.
Another trend that is gaining momentum is brand signature sound. Standards define minimum performance criteria, but allow manufacturers to select their desired sounding within these standards. The AVAS sound is becoming more of a brand identity element to automakers. Heard coming of a car is a matter of design choice as artistic as the appearance of the car. 

Conclusion
The vehicle makes a sound. Pedestrians hear it. Safety is improved.
The technical reality behind that outcome is more complex. The system must read vehicle data accurately, synthesise realistic audio in real time, modulate pitch and volume continuously, amplify the signal efficiently, and project it through a speaker that must survive years of outdoor exposure without degradation.
A weakness at any one of these steps degrades the overall performance. A slow DSP introduces lag between speed changes and sound changes. An inefficient amplifier drains battery. A poorly sealed speaker fails in wet conditions. None of these failures are acceptable in a safety critical system.
Bestar approaches AVAS as a complete acoustic solution. Bestar expertise in piezoelectric technology, acoustic engineering and automotive grade manufacturing which provides the speaker components and system knowledge that help vehicle manufacturers meet global regulatory requirements reliably. For automotive customers who need a trusted supply chain partner for AVAS hardware, Bestar offers the combination of technical depth, certification credentials, and production scale that the industry demands.