Theory of compact nonporous windscreens for infrasonic measurements.

The principle of the compact nonporous windscreen is based on the great penetrability of infrasound through matter. The windscreen performance is characterized by the ratio of the sound pressure at an interior microphone, located in the center of a windscreen, to the incident sound pressure in the free field. The frequency dependence of this pressure ratio is derived as a function of the windscreen material and geometric properties. Four different windscreen geometries are considered: a subsurface, box-shaped windscreen, a cylindrical windscreen of infinite length, a cylindrical windscreen of finite length, and a spherical windscreen. Results are presented for windscreens made of closed-cell polyurethane foam and for typical dimensions of each of the above geometries. The cylindrical windscreen of finite length, featuring evanescent radial modes, behaves as a unity-gain, low-pass filter, cutting off sharply at the end of the infrasonic range. The remaining geometries reveal a pass band that extends well into the audio range, terminated by a pronounced peak beyond which the response plummets rapidly.