Infrasonic windscreens, designed for service at frequencies below 20Hz⁠, were fabricated from a variety of materials having a low acoustic impedance, and tested against four specifications (the first three in a small wind tunnel): (1) wind-generated noise reduction (“insertion loss”) at a free-stream wind speed of 9.3m∕s⁠, (2) transmission of low-frequency sound from a known source (subwoofer), (3) spectrum of sound generated from trailing vortices (aeolian tones), and (4) water absorption (to determine suitability for all-weather service). The operating principle is based on the high penetrating capability of infrasound through solid barriers. Windscreen materials included three woods (pine, cedar, and balsa), closed-cell polyurethane foam, and Space Shuttle tile material. The windscreen inside diameter ranged from 0.0254to0.1016m (1to4in.)⁠, and wall thickness from 0.003175to0.01905m (18to34in.)⁠. A windscreen made of closed-cell polyurethane foam revealed a wind noise reduction of 10–20dB from 0.7to25Hz⁠, transmission coefficient near unity from 10to20Hz⁠, and spectral peaks beyond 20Hz due to vortex-generated sound. Following a description of past methods, the principle of operation, and the experimental method, experimental data are presented for a variety of windscreens.

Topics
Acoustical properties, Microphones, Infrasonic, Acoustic phenomena, Weather forecasting, Aeronautics, Aerodynamics, Signal processing, Space instruments, Polymers
1.
F. B.
 
Daniels
, “
Noise reducing line microphone for frequencies below 1Hz
,”
J. Acoust. Soc. Am.
 
31
,
529
–
531
(
1959
).
Google Scholar
Crossref
Search ADS
 
2.
R.
 
Burridge
, “
The Acoustics of Pipe Arrays
,”
Geophys. J. R. Astron. Soc.
 
26
,
53
–
69
(
1971
).
Google Scholar
Crossref
Search ADS
 
3.
B.
 
Alcoverro
, “
Proposition d’un systeme de filtrage acoustique pour une station infrason IMS
,” CEA-DASE Scientific Report, No. 241 (
1998
).
Google Scholar
 
4.
M. A. H.
 
Hedlin
 and
R.
 
Raspet
, “
Infrasonic wind-noise reduction by barriers and spatial filters
,”
J. Acoust. Soc. Am.
 
114
,
2379
–
1386
(
2003
).
Google Scholar
 
5.
P. V.
 
Bruel
, “
Aerodynamically Induced Noise of Microphones and Windscreens
,” B&K Technical Review No. 2-
1960
, Copenhagen, Denmark.
Google Scholar
 
6.
L. J.
 
Anderson
, “
Windscreen for Microphones
,” U.S. Patent 2,520,706 (
1950
).
Google Scholar
 
7.
H. N.
 
Ballard
, “
Multiple Wind Screen Noise Attenuation System
,” U.S. Patent 3,550,720 (
1970
).
Google Scholar
 
8.
S.
 
Morgan
 and
R.
 
Raspet
, “
Investigation of the Mechanisms of Low-Frequency Wind Noise Generation Outdoors
,”
J. Acoust. Soc. Am.
 
92
,
1180
–
1183
(
1992
).
Google Scholar
Crossref
Search ADS
 
9.
LAST-A-FOAM FR-3708, manufactured by General Plastics, Manufacturing Company, Tacoma, WA 98409. This formation is a closed cell rigid polyurethane foam, having a density of 8lb∕ft3 (128kg∕m3) per manufacturer’s specification.
10.
Low-temperature reusable surface insulation (LRSI). See
K. M.
 
Joels
,
Space Shuttle Operator’s Manual, Revised Edition
(
Ballantine Books
, New York,
1988
).
Google Scholar
 
11.
L. E.
 
Kinsler
 and
A. R.
 
Frey
,
Fundamentals of Acoustics
(
Wiley
, New York,
1962
), p.
136
.
Google Scholar
 
12.
ANSI S1.17–2004∕Part 1, “
Microphone Windscreens—Part 1: Measurements and Specification of Insertion Loss in Still or Slightly Moving Air
” (
Acoustical Society of America
, Melville, NY,
2004
).
13.
This is attributed to the high porosity of the Space Shuttle tile material. See
T. W.
 
Clyne
,
I. O.
 
Golosnoy
,
J. C.
 
Tan
, and
A. E.
 
Markaki
, “
Porous Materials for Thermal Management under Extreme Conditions
,”
Philos. Trans. R. Soc. London, Ser. A
(to be published).
2005
U.S. Government
You do not currently have access to this content.