Infrasonic horn and ribbon microphone

Abstract

Infrasound measurements normally utilize seismic sensors (geophones and accelerometers) with deployment on the ground surface, the seafloor, ice packs, etc. Here, the infrasound propagating in an overlying fluid medium such as air or water, interacts with the solid medium, thereby imparting its signature. These sensors primarily detect ground motion in terms of the particle velocity or acceleration (force) of the ground in contact with the sensor. Pressure microphone sensing of infrasound in the atmosphere has also been explored. We examine here the atmospheric sensing of infrasound, using the well-established technique of the ribbon microphone, which employs a thin metallic ribbon, suspended between two magnets to detect the particle velocity of the sound propagating in a fluid medium. Movement of the ribbon in the sound field generates a voltage according to Faraday’s Law. Although ribbon microphones are fixtures in audio band acoustics, their application to infrasound is not, and is explored here. Ribbon microphones made from suspension of conductive mylar tape between two neodymium magnets have been fabricated and tested in an infrasound band around a few Hertz. The results show a viable approach to vector sensing in this band. Work supported by ARL:UT Austin.