Spinoff is NASA’s annual publication featuring successfully commercialized NASA technology. This commercialization has contributed to the development of products and services in the fields of health and medicine, consumer goods, transportation, public safety, computer technology, and environmental resources.

NASA’s special infrasound microphone can pick up the ultra-low frequencies generated by turbulence in the skies. This technology is now being tested on the Stratodynamics HiDRON glider for both turbulence detection and aeronautical research. (Credit: NASA)

Everything in the atmosphere can make a sound — volcanoes rumble and waterfalls crash — but there’s more to that sound than what our ears perceive. Much like how infrared light consists of frequencies that aren’t visible to the naked eye, there’s an audio analogue called infrasound that consists of pitches too low (0.001 to 20 hertz) to be heard by the human ear.

The sudden turbulence sometimes experienced when flying is called clear-air turbulence, so named because there are no visible clouds or atmospheric features to warn of the disruption. Turbulent invisible air can seemingly come out of nowhere and wreak havoc on aircraft.

Though it isn’t easily detected visually, clear-air turbulence has a definite infrasound signature. Researchers Qamar Shams and Allan Zuckerwar at NASA’s Langley Research Center in Hampton, VA realized that if air traffic controllers or pilots could listen to these whirling vortices before airplanes encounter them, an alternate route could be plotted.

Their experiments began in 2007 but initial tests showed them that they couldn’t grab just any off-the-shelf microphone and expect it to work with infrasound. The long-wave frequencies get overpowered by higher-frequency sounds, which results in interference.

The pair began developing something that could listen to these low frequencies in high fidelity. Microphones use a moving diaphragm to pick up audio where sound waves cause the surface to vibrate. The researchers used a low-tension diaphragm with a wide radius paired with a large, sealed air chamber behind it to allow the microphone to hear these ultra-low sound waves that travel great distances. The infrasonic microphones are manufactured by PCB Piezotronics of Depew, NY under contract with Langley.

Typically launched by a weather balloon, the HiDRON has soared to more than 100,000 feet in the air. (Credit: Stratodynamics)

With the sensor completed, testing began. When the microphones were placed in an equidistant triangular pattern around Langley’s runway, they were able to pick up and locate atmospheric turbulence more than 300 miles away.

By 2017, the technology had won the NASA Commercial Invention of the Year award, been tested on the ground for the Department of Defense, and been researched at Sandia National Laboratories to validate its performance but it hadn’t flown aboard any aircraft. Interest in turbulence detection from Stratodynamics Lewes, DE, would soon change that.

Stratodynamics realized the microphone system had significant potential as an in-flight turbulence detection sensor and looked for opportunities to test the technology. After licensing the patents from NASA, the company began to implement the sensor on an uncrewed stratospheric glider known as the HiDRON.

Stratodynamics has lifted their balloon-launched HiDRON glider to heights of more than 100,000 feet, from which it slowly makes its way back down to Earth. With the assistance of the infrasound microphone and wind probe, the UAV measures the intensity of turbulence in its path at a distance and may possibly detect thermal columns to keep the plane sailing longer. Currently, more work is underway to design the algorithms required to understand the turbulent signature’s intensity and range.

The researchers hope the data provided by the infrasonic microphone will become ubiquitous in detecting and forecasting turbulence, air traffic control decision-making, and aviation route planning.

Read this story and other NASA Spin-Offs on NASA.gov .


Tech Briefs Magazine

This article first appeared in the September, 2021 issue of Tech Briefs Magazine.

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