Turning the Vehicle into a Giant Microphone

Harman’s sound-and-vibration sensor functions as a vehicle’s external listening ‘ear.’

When an emergency vehicle is approaching but its blaring siren isn’t heard by nearby motorists, all are put at risk. Engineers at Harman International have developed novel sensor technology that detects both the sound and its direction, in effect piping that screaming siren into vehicles so-equipped, to alert the driver.

Close-up image of Harman’s external vehicle microphone. (Harman)

“What we’re in essence doing is turning the vehicle into a giant microphone,” Mitul Jhala, senior director of automotive embedded audio for Harman, explained in an SAE Media interview.

Harman’s sound and vibration sensor functions as a vehicle’s external “listening ear” to detect sound frequencies from 20 Hz up to 6 kHz. The sensor can “distinguish siren tones via post-processing algorithms, so it can tell whether the sound is a police, fire, or ambulance siren,” Jhala said. He noted specific tones vary for the U.S., European and Asian markets. This sensor can be used alone or alongside Harman’s new external microphones.

600-m range

Harman’s sound and vibration sensor provides acoustic sensing of the vehicle’s exterior. (Harman)

Using multiple external microphones to create a multi-element array, post-signal processing algorithms, such as beam forming, can be used to determine the siren’s direction.

“Localization is very useful when it’s not possible to see the emergency vehicle because it’s behind a building or there’s another visibility obstruction,” Jhala said. The distance range for siren detection is approximately 1968 ft (600 m), depending on factors such as whether the emergency vehicle is stationary or moving.

A sound-and-vibration sensor as well as an external microphone are two products expected to reach the U.S. market in late 2023, according to Harman’s Mitul Jhala, shown. (Harman)

To prevent wind interference, a single sound and vibration sensor would be mounted on the B-side of the exterior panel between the interior panel trim and the exterior panel. The optimal placement location for external microphone(s) depends on the vehicle type, as different vehicle profiles mean different aerodynamics, which equates to different noise profiles. “We’ve done testing on small cars, hatchbacks, minivans and SUVs with microphones integrated on different locations, such as the rooftop’s shark-fin antenna or the front bumper, with single- and multi-arrays,” Jhala said.

Exposure to harsh conditions, especially wind turbulence and water, required a microphone design unlike anything the automotive industry’s microphone technology leader has done in the past. “It took lots of engineering hours to design a unique, conical-shaped grille pattern for the microphone,” said Jhala. The resulting grille pattern includes drainage holes to protect against a direct water hit to the electronics. “There is also a protective membrane in front of the microphone itself that serves as a secondary layer of water and environmental protection,” he explained.

Acoustic vehicle monitoring

When technology development started in 2019, Harman’s sound-and-vibration sensor and the external microphone products were intended as an alert system for autonomous vehicles. But it soon became apparent that human drivers could also benefit from acoustic sensing of the vehicle exterior. One of the use-case examples is emergency-vehicle detection.

Another example is speech commands, such as ‘open liftgate’, or ‘close liftgate’,” Jhala said. When used separately or together, the sound and vibration sensor and external microphone solutions also enable acoustic vehicle monitoring. For instance, an alert can be sent to a cell phone if vehicle glass breakage is detected by the sound and vibration sensor.

Acoustic emergency-vehicle alerts and other notifications can be communicated to drivers via a vehicle’s infotainment screen, gauge cluster, or head-up display. “It’s also possible to alert drivers with an audible transmitted through the vehicle’s audio system,” he said. The first passenger-vehicle application of the design-validated sound and vibration sensor or the external microphone product is expected in the U.S. market in late 2023. Asian and European market applications are expected in 2024 or later.