Automobile Acoustics: Context Awareness through Vibration Sensing
February 05, 2024
Thanks to the push in recent years to improve passenger safety and comfort, cars on the road today have more computing power and capabilities than supercomputers from a few decades ago. Self-driving cars seem ever closer to reality.
This automotive revolution is due, in large part, to the integration of a wealth of sensors deployed to build contextual awareness and help the on-board computers create a perception of the world around them.
Cameras, LiDAR, radar, and ultrasonic sensors are just a few examples. Traditional audio sensing has seen limited adoption in this context. However, several use cases are emerging, including external voice commands and emergency vehicle detection that require the ability to pick-up sound outside the vehicle to complement the data sourced from the other sensors.
Today, microphones are used for this purpose, but they are prone to failure when exposed to harsh elements such as snow, dust, water, and other contaminants. Recently, an alternative sound pickup mechanism via vibration sensing has presented a novel solution to address this need while being practically immune to the elements. Additionally, it enables a lower integration complexity due to the fully sealed package of the sensor.
Emerging Automotive Use Cases
The adoption of microphones for in-vehicle sound pickup is already ubiquitous and enables several use cases ranging from hands-free phone calls and voice commands to road noise cancellation (RNC). More recently, the proliferation of Advanced Driver Assistance Systems (ADAS) and the advancement of in-vehicle infotainment have opened the doors to new use cases aimed at increasing safety and comfort while relying on sound pickup from outside the vehicle (Figure 1). These applications can be grouped into two broad categories: context awareness and external voice pickup.
Figure 1:Automotive use cases relying on external microphones for sound pick-up.
- Context awareness: full and high driving automation (SAE Level 4 and Level 5, also referred to as Eyes-off Hands-off) require the system to detect and respond to dynamic driving situations, such as an approaching emergency vehicle. Even SAE Level 3, which represents the first true step in autonomous driving, demands the driver to regain control in specific situations to put the vehicle in a safe position. Detecting an approaching emergency vehicle allows the driver or the autonomous driving system to react early, well before the potential danger is spotted by vision sensors, allowing more time to perform safety maneuvers.
- Voice pickup: external voice commands allow users to effortlessly open the car trunk or the vehicle doors on approach or when their hands are occupied. This application can replace unnatural gestures like kicking under the trunk with a natural voice-based user interface, which we already use to engage with smart speakers, smartphones, and other electronic devices.
Challenges with Existing Technologies
The use cases described above require external microphones to capture the sounds. However, traditional microphones need the soundwaves to enter the sensor package through the acoustic port to detect the sound (Figure 2.a). The porthole makes them inherently vulnerable to contaminants, such as water, snow, and dust, that might obstruct the acoustic path, preventing the microphone from operating correctly. Something as innocuous as driving through a carwash could lead to a visit to the dealership for repairs. The traditional approach is to shield the acoustic path with a membrane, effectively trading sensitivity for protection. This adds to the cost and complexity of the solution. Yet the microphones remain at risk of failure in the field.
Figure 2: Traditional bottom-port MEMS microphone (a) vs. vibration sensor (b) integration.
Sensing Sound through Vibrations
High bandwidth and low noise vibration sensors offer a practical solution to this problem. They are single-axis accelerometers that measure sound-induced vibrations generated by acoustic waves hitting the vehicle’s surface. Sensing vibrations does not require any porthole or acoustic channel, making vibration sensors intrinsically immune to external elements and much easier to integrate (Figure 2.b). A vehicle has several metal, plastic, and glass panels that are ideal candidates to leverage such a technique. Vibration sensors can be mounted to the body of a vehicle in locations like on the front and rear windshield, door panels, side mirrors, and bumpers (Figure 3). Moreover, the sensor can be attached to the internal surface of a car panel (e.g., behind the side mirror). This makes it completely invisible, with a clear benefit for the vehicle’s aesthetics, while still able to capture external sounds.
Figure 3: Possible vibration sensor locations for optimal sound pickup.
Knowles V2S200D vibration sensor achieved similar performance and signal capture to a regular microphone in the frequency bands of interest. Emergency vehicle sirens rely on a main tone sweeping between 500Hz and 1.5kHz, with most harmonic power concentrated below 8kHz. In Figure 4, the siren sound pickup of V2S200D mounted on a car door is compared with a reference MEMS microphone, showing an excellent match between the two sensor outputs up to the desired bandwidth of 8kHz. Emergency vehicle detection tests conducted with the sensor mounted at different locations on a vehicle traveling at various speeds showed promising results.
Sample audio recordings are available here.
Audio vibration sensors represent a viable and preferred alternative to traditional MEMS microphones for picking-up external sounds in automotive applications thanks to their superior environmental robustness and lower system integration cost.