Active noise cancellation (ANC) has been part of automakers’ quest for the ultimate high-end vehicle experience for over 40 years. Eliminating or reducing engine and combustion-process-related noises, including exhaust noise, was part of the initial implementation, and road-tire, heating, ventilation, and air conditioning (HVAC) systems, vibration, and wind-related noises have always been targets for ANC systems. However, the recent transition to electric-motor-powered vehicles has changed the focus and need for improved noise-cancellation technologies. With the engine and exhaust noise eliminated, passengers can now more readily detect other noises.
The active noise cancellation process starts with the microphones that are strategically placed within the vehicle to monitor sound levels and frequencies that are transferred through the air and the vehicle’s structure. Then a digital signal processor (DSP) takes that information and generates a new signal with an inverted phase. When the vehicle’s speakers play the new distinct sound, it interacts with the existing (measured) sounds and partially or completely cancels them out so the resulting sound is either inaudible or less audible to passengers.
In vehicles, unlike the MEMS microphones widely used in headphones, the operating and storage temperatures as well as other more stringent operating environment characteristics have to be taken into consideration. For MEMS microphones used in the vehicle, the Automotive Electronics Council (AEC) established the AEC-Q103-003 standard, “Failure Mechanism Based Stress Test Qualification for MEMS Microphone Devices.”
Both digital and analog microphones that meet the automotive operating temperature range from -40°C to +105°C have been qualified to the new AEC-Q103-003 standard. Since vehicle infotainment systems already have built-in loudspeakers for playing audio from entertainment, cell phones and route guidance, the ANC system can leverage a significant portion of the available audio system and minimize additional costs.
Noise outside of the vehicle
The lack of engine noise generated by EVs has also created a problem for pedestrians’ ease of detecting an approaching EV. As a result, many governments have required minimum sound levels external to these vehicles. For example, under the Pedestrian Safety Enhancement Act of 2010 (PSEA), the U.S. National Highway Traffic Safety Administration (NHTSA) requires that an EV emits an alert sound that meets certain minimum requirements to aid visually impaired and other pedestrians to detect a vehicle’s presence, direction, location, and operation.
To meet the requirements of PSEA, FMVSS No. 141 requires EVs and HEVs to emit a pedestrian alert sound while operating in certain conditions and defines the test process. Testing to determine if a vehicle meets the requirements of FMVSS No. 141, test procedure TP-141-01, “Minimum Sound Requirements for Hybrid and Electric Vehicles” defines the acoustic sound measurement system for measuring the sound pressure level and requires three microphones for testing.
In an Acoustic Vehicle Alerting System (AVAS), artificial vehicle sounds proportional to vehicle parameters, such as velocity, gas pedal, and gear position, are generated using loudspeakers or actuators. As an alternative to speakers, actuators or exciters are directly attached to a structural element of the vehicle to generate the sound.
References
DOT HS 812 347 Minimum Sound Requirements for Hybrid And Electric Vehicles
AEC-Q103-003 standard Failure Mechanism Based Stress Test Qualification for MEMS Microphone Devices
FMVSS 141 proposed regulations, Federal Motor Vehicle Safety Standards; Minimum Sound Requirements for Hybrid and Electric Vehicles
Automotive Case Study 2
cancellation
