February 5, 2024
blog
Thanks to recent efforts to improve passenger safety and comfort, the cars on our roads today have more computing power and functionality than the supercomputers of decades ago. Self-driving cars seem to be getting closer and closer to reality.
This automotive revolution is largely due to the integration of a wealth of sensors that have been introduced to build situational awareness and allow on-board computers to perceive the world around them.
Cameras, LiDAR, radar, and ultrasonic sensors are just a few examples. In this situation, the adoption of traditional audio sensing is limited. However, several use cases are emerging that require the ability to pick up sounds from outside the vehicle to supplement data from other sensors, such as external voice commands and emergency vehicle detection.
Microphones are currently used for this purpose, but they tend to fail when exposed to harsh elements such as snow, dust, water, and other contaminants. Recently, an alternative sound pickup mechanism with vibration sensing has provided a new solution to address this need while being largely unaffected by the elements. Additionally, the sensor package is completely sealed, reducing integration complexity.
The adoption of microphones to pick up sounds inside cars is already widespread, enabling several use cases, from hands-free calling and voice commands to road noise cancellation (RNC). More recently, the proliferation of advanced driver assistance systems (ADAS) and advances in in-vehicle infotainment have opened the door to new use cases aimed at improving safety and comfort while relying on the collection of sounds from outside the vehicle. was opened (Figure 1). These applications can be divided into two broad categories: context awareness and external voice pickup.
Figure 1: Automotive use case using an external microphone to pick up sound.
- context awareness: Full and advanced driving automation (SAE Level 4 and Level 5, also known as eyes-off and hands-off) requires systems to detect and respond to dynamic driving situations, such as the approach of emergency vehicles. Even SAE Level 3, the true first step in autonomous driving, requires the driver to take back control to place the vehicle in a safe position in certain situations. Detecting an approaching emergency vehicle allows drivers and automated driving systems to react early, long before visual sensors detect potential hazards, giving them more time to drive safely. You will be able to do it.
- audio pickup: External voice commands allow users to effortlessly open the car trunk or door when approaching the car or when their hands are occupied. This application can replace unnatural gestures such as kicking under the trunk with a natural voice-based user interface that is already used to work with smart speakers, smartphones and other electronic devices. .
The use case above requires an external microphone to capture sound. However, traditional microphones require sound waves to enter the sensor package through an acoustic port to detect sound (Figure 2.a). Portholes inherently make them vulnerable to contaminants such as water, snow, and dust, which can obstruct the acoustic path and prevent the microphone from working properly. Something as innocuous as driving to a car wash can result in a trip to the dealership for repairs. The traditional approach is to shield the acoustic path with a membrane, effectively sacrificing sensitivity for protection. This increases the cost and complexity of the solution. However, microphones are at risk of failure in the field.
Figure 2: Integration of a traditional bottom-port MEMS microphone (a) and vibration sensor (b).
High-bandwidth, low-noise vibration sensors offer a practical solution to this problem. These are single-axis accelerometers that measure sound-induced vibrations caused by sound waves hitting the vehicle’s surface. Because sensing vibrations does not require portholes or acoustic channels, vibration sensors are essentially independent of external elements and are much easier to integrate (Figure 2.b). Vehicles have several metal, plastic, and glass panels that are perfect for taking advantage of such technology. Vibration sensors can be installed in locations such as the vehicle’s windshield, rear glass, door panels, side mirrors, and bumper (Figure 3). Additionally, the sensor can be mounted on the inside surface of a car panel (for example, behind the side mirrors). This completely hides the exterior of your car, providing obvious benefits to your car’s aesthetics while also allowing outside sounds to come in.
Figure 3: Possible vibration sensor locations for optimal sound pickup.
The Knowles V2S200D vibration sensor provides performance and signal capture similar to a regular microphone in the frequency band of interest. Emergency vehicle sirens rely on a tonic tone that sweeps from 500Hz to 1.5kHz, with most harmonic power concentrated below 8kHz. In Figure 4, his V2S200D siren audio pickup mounted on a car door is compared to a reference MEMS microphone, showing excellent agreement between the two sensor outputs up to the required bandwidth of 8kHz. . Emergency vehicle detection tests conducted with sensors mounted at different locations on vehicles traveling at different speeds yielded promising results.
Sample audio recordings are available here.
Figure 4: Siren pickup (500Hz to 1.5kHz swept signal): Comparison of a vibration sensor and microphone mounted on a car door.
Audio vibration sensors provide a viable and preferred alternative to traditional MEMS microphones for picking up external sound in automotive applications due to their superior environmental immunity and low system integration costs.