Auto Acoustic Revolution: Context Awareness Through Vibration Sensing

March 28, 2024

Cars on the road today have more computing power than ever, thanks largely to technology designed to improve passenger safety and comfort. This automotive revolution is due, in large part, to the integration 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.

Use cases such as external voice commands and emergency vehicle detection (Figure 1) require sound pickup outside of the vehicle. However, traditional MEMS microphones are prone to fail when faced with natural elements like snow, dust, water, and more.

Recently, vibration sensing technology has created an element-proof solution to these needs while enabling lower integration complexity compared to traditional MEMS microphones.

Automotive Use Cases

The adoption of microphones for in-vehicle sound pickup is already widely available for hands-free phone calls, voice commands, and road noise cancellation (RNC). More recently, Advanced Driver Assistance Systems (ADAS) and the advancement of in-vehicle infotainment systems have opened the doors to new use cases aimed at increasing safety and comfort while relying on sound pick up 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 condition. 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 full. This application can replace unnatural gestures like kicking under the car boot 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. 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 those 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 like the Knowles V2S200D offer a great solution to this challenge. 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 immune to external elements and much easier to integrate (Figure 2). 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 such as 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.

Results

The Knowles V2S200D vibration sensor achieves 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 the V2S200D mounted on a car door is compared with a reference MEMS microphone, showing an excellent match between the two sensors 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 show promising results. 

Sample audio recordings are available here.