Acoustic background noise from sailplane-mounted microphones

One of many options for monitoring acoustic events at altitude is a sailplane-mounted sensor. In an attempt to determine the limiting background noise for acoustic measurements from a sailplane, in-flight measurements were made with microphones installed in a Schempp-Hirth Standard Cirrus and a Blanik L23. Recordings were made in steady gliding flight in a stable atmosphere at speeds ranging from 45 to 80 knots. The background noise from a sailplane-mounted microphone shows three distinct regions. The first region, above 100 Hz, is sensor-influenced flow noise and the pressure spectral density is well characterized by a non-dimensional relation. The second region, from several hertz to 100 Hz, is probably the result of sampling pre-existing pressure fluctuations in the atmosphere. For these measurements made in a convectively stable atmosphere, the pressure spectral density was between 0.005 and 0.01 Pa per root hertz (48 to 54 dB with respect to 20 μPa per root hertz). The third region, below 1 Hz, is dominated by pressure fluctuations that correlate strongly with altitude variations - either natural oscillations of the glider or pilot control inputs.