Infrasonic windscreen

Infrasonic windscreens, designed for service at frequencies below 20 Hz, were fabricated from a variety of materials having a low acoustic impedance, and tested in a small wind tunnel against four specifications: (1) attenuation of wind‐generated sound (at a free‐stream wind speed of 9.4 m/s), (2) the transmission of low‐frequency sound from a known source (Janis subwoofer), (3) spectrum of sound generated from trailing vortices, and (4) water absorption (to determine the suitability for all‐weather service). Windscreen materials included three woods (pine, cedar, and balsa), polyurethane foam, and Space Shuttle tile material. The windscreen outside diameter ranged from 0.0254 to 0.1016 m (1 to 4 inches), and wall thickness from 0.003175 to 0.01905 m (1/8 to 3/4 inch). A windscreen made of polyurethane foam revealed a wind noise attenuation of 10–20 dB from 0.7–25 Hz, transmission coefficient near unity from 10–25 Hz, and a spectral peak of 23 Hz due to vortex‐generated sound. Data will be presented for a variety of windscreens.