Evaluation of rosette infrasonic noise-reducing spatial filters
Michael A.H. Hedlin
Institute of Geophysics and Planetary Physics, University of California, San Diego
Benoit Alcoverro
Commissariat a l'Energie Atomique, Departement Analyse et Surveillance de
l'Environnement
Gerald D'Spain
Marine Physical Laboratory, Scripps Institution of Oceanography, University of
California
The spatial noise filter currently preferred for use at new International Monitoring System
(IMS) infrasound array sites consists of an array of low-impedance inlets connected by
solid tubes to a microbarometer. We present results from recent tests of "rosette"
infrasonic noise-reducing spatial filters at the Pinon Flat Infrasound test-bed in southern
California. At wind speeds up to 5.5 m/s, the 96 inlet 18-meter rosette filter reduces wind
noise levels above 0.2 Hz by 15 to 20 dB. Under the same conditions, the 144 inlet 70-
meter rosette filter provides noise reduction of up to 15 to 20 dB between 0.02 and 0.7 Hz.
Standing wave resonance inside the 70-meter filter degrades the reception of acoustic
signals above 0.7 Hz. Synthetics accurately reproduce the noise reduction and resonance
observed in the 70-meter filter at all wind speeds above 1.25 m/s. Experiments with
impedance matching capillaries indicate that internal resonance in the rosette filters can
be removed. Rosette filters are tuned to vertically incident energy. Attenuation of signals
by the 70-meter rosette filter at frequencies above 3.5 Hz arriving at grazing angles of less
than 15