Detection of infrasound disturbances from the Earth's stratosphere

Infrasound is usually defined as sound waves below 20 Hz, the nominal limit of human hearing. Infrasound waves propagate over vast distances the Earth's atmosphere: the CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organization) has 48 installed infrasound-sensing stations around the world to detect nuclear detonations and other disturbances. In February 2013, several CTBTO infrasound stations detected infrasound signals from a large bolide that exploded over Chelyabinsk, Russia. Some stations recorded signals that had circumnavigated the Earth, over a day after the original event. A balloon-borne infrasound sensor is expected to have two advantages over ground-based stations: a lack of wind noise and a concentration of infrasound energy in the “stratospheric duct” between roughly 5-50 km altitude. To test these advantages, we have built a small balloon payload with five calibrated microphones. This paper discusses the design, testing and expected sensitivity of the payload. Our current plans call for a launch in August 2016, coordinated with a ground-based explosion and a network of ground-based infrasound sensors.