A windscreen has been developed that features two advantages favorable for the measurement of outdoor infrasound. First, the sub-surface location, with the top of the windscreen flush with the ground surface, minimizes the mean velocity of the impinging wind. Secondly, the windscreen material (closed cell polyurethane foam) has a sufficiently low acoustic impedance (222 times that of air) and wall thickness (0.0127 m) to provide a transmission coefficient of nearly unity over the infrasonic frequency range (0-20 Hz). The windscreen, a tightly-sealed box having internal dimensions of 0.3048 x 0.3048 x 0.3556 m, contains a microphone, preamplifier, and a cable feed thru to an external power supply. Provisions are made for rain drainage and seismic isolation. A three-element array, configured as an equilateral triangle with 30.48 m spacing and operating continuously in the field, periodically receives highly coherent signals attributed to emissions from atmospheric turbulence. The time delays between infrasonic signals received at the microphones permit determination of the bearing and elevation of the sources, which correlate well with locations of pilot reports (PIREPS) within a 320 km radius about the array. The test results are interpreted to yield spectral information on infrasonic emissions from clear air turbulence

Burkett, Cecil G., Jr.

Comeaux, Toby

Shams, Qamar A.

Weistroffer, George R.

Zuckerwar, Allan J.

NASA Technical Reports Server

                  Sub-surface windscreen for the measurement of outdoor infrasound 
 
       Qamar A. Shams, Cecil G. Burkett, and Toby Comeaux 
                        NASA Langley Research Center 
                                                        Hampton, VA 23665 
 
                                                          Allan J. Zuckerwar  
                                         Analytical Services and Material 
                                                          Hampton, VA 23666 
 
                                                          George R. Weistroffer 
                                            Virginia Commonwealth University 
                                                          Richmond, VA 23284 
 
A windscreen has been developed that features two advantages favorable for the measurement of 
outdoor infrasound.  First, the sub-surface location, with the top of the windscreen flush with the 
ground surface, minimizes the mean velocity of the impinging wind.  Secondly, the windscreen 
material (closed cell polyurethane foam) has a sufficiently low acoustic impedance (222 times 
that of air) and wall thickness (0.0127 m) to provide a transmission coefficient of nearly unity 
over the infrasonic frequency range (0-20 Hz).  The windscreen, a tightly-sealed box having 
internal dimensions of 0.3048 x 0.3048 x 0.3556 m, contains a microphone, preamplifier, and a 
cable feed thru to an external power supply.  Provisions are made for rain drainage and seismic 
isolation.  A three-element array, configured as an equilateral triangle with 30.48 m spacing and 
operating continuously in the field, periodically receives highly coherent signals attributed to 
emissions from atmospheric turbulence.  The time delays between infrasonic signals received at 
the microphones permit determination of the bearing and elevation of the sources, which 
correlate well with locations of pilot reports (PIREPS) within a 320 km radius about the array.  
The test results are interpreted to yield spectral information on infrasonic emissions from clear 
air turbulence. 
 
INTRODUCTION 
 
A fundamental difficulty in the detection of outdoor infrasound is the so-called “wind noise” 
problem. Because of ever-present wind-generated turbulence, the atmosphere is inherently noisy 
at frequencies of interest (typically between 0.1 and 20 Hz). Accordingly, effective wind 
screening is vital to the success of outdoor infrasonic measurements. Past methods of screening a 
microphone from the wind include a piped array, a barrier, or an open-mesh (e.g., cloth or foam 
enclosure).    
 
Over the years, researchers at NASA Langley have developed and field tested different types of 
compact and structurally sound windscreens [1-4]. The windscreen principle is based on the high 
penetrating capability of infrasound through barriers, which is dependent upon the acoustic 
impedance ratio (wall-to-air) and wall thickness. Initially a family of windscreens as shown in 
Figure 1 were developed having an internal diameter of 0.0762 m (3 in.), internal depth of 0.23 
m (9 in.) and wall thicknesses of 0.01905, 0.0127, and 0.00635 m (3/4, ½, and ¼ in.) 
respectively. Such windscreens were used in the field as shown in Figure 2. Though this method  
https://ntrs.nasa.gov/search.jsp?R=20090009968 2019-08-30T06:25:55+00:00Z
was moderately successful, to eliminate wind flow, an advanced version of a nonporous 
windscreen called “sub-surface windscreen” was designed and developed. Following a 
description of the sub-surface windscreen, the principle of operation, and the experimental 
method, experimental data are presented.  
 
                                   
 
                                           FIG. 1. The family of tested windscreens. 
 
                                             
 
                                         FIG. 2.  Infrasonic windscreen in field service 
 
 
The principle of the windscreen is based on the great penetrating capability of infrasound 
through matter. Consider the model shown in the lower left corner of Figure 3. The sound power 
transmission coefficient from medium 1 to medium 3 is  
 
                    
x
Z
Z
Z
Zx
T
2
2
2
1
1
22 sin
4
1cos
1
⎟⎟⎠
⎞
⎜⎜⎝
⎛ ++
=   where  
2
2
c
fWx π=  
 
and f is the frequency, W the wall thickness, Z1 the acoustic impedance of air, Z2 the acoustic 
impedance of the wall material, and c2 the speed of sound in the wall. The transmission 
coefficient for a ½” –wall, made of a material with very low specific acoustic impedance (222 
times that of air, as in closed-cell polyurethane foam) transmits low-frequency sound with 
practically unity transmission coefficient. For reference, a ½” –wall made of steel (specific 
impedance ratio = 113 253), passes only the far infrasound.  
 
The second feature makes use of the fact that the horizontal wind speed approaches zero near the 
Earth’s surface. Figure 4 shows the vertical profile of the horizontal wind, having a magnitude of 
10 m/s at a height of 10m. The logarithmic profile is based on a scale length of 0.01 m (short 
grass), at which height the model specifies a zero wind speed.  
 
                            
Transmission coefficient of closed-cell polyurethane 
foam
0 
0.2 
0.4 
0.6 
0.8 
1 
Tr
an
sm
is
si
on
 c
oe
ffi
ci
en
t
0 0 1 10 100 1000 
Frequency, Hz
fc
   Steel
(113 253)
Polyurethane
foam (222)
Increasing Z  / Z2 1
.1.01
W = ½”
Tr
an
sm
is
si
on
 c
oe
ffi
ci
en
t
       1                2              3 
      Air          Wall         Air                    
xZ
Z
Z
Zx
T
2
2
2
1
1
22 sin
4
1cos
1
⎟⎟
⎟
⎠
⎞
⎜⎜
⎜
⎝
⎛ ++
=       
2
2
c
Wfx π=  
W 
 
   
                     FIG. 3.  Transmission coefficient of closed-cell polyurethane foam 
 
 
                            
Vertical profile of horizontal wind
 
0 
20 
40 
60 
80 
100 
120 
H
ei
gh
t, 
m
0 2 4 6 8 10 12 14 16 18 20 
Wind Speed, m/s
 10 m/sU   =10
Scale length = 0.01 m
 
     
                                          FIG. 4.  Vertical  profile of horizontal wind  
 
Figure 5 is a sketch of the windscreen installed in a hole below the ground surface. The 
windscreen is in the shape of a box of dimensions shown and of wall thickness ½”. The material 
is closed-cell polyurethane foam (“eight-pounder”), which passes propagating infrasound, as 
described above, but prevents convected pressure fluctuations from reaching the interior 
microphone. The lid of the box fits tightly snug. The box is surrounded by drainage rock, from 
which rain water passes through into a drainage pipe. The wall material is impervious to water. 
The microphone is a B&K type 4193 with a low-frequency adapter.  
 
                 
Sub-surface windscreen
Drainage
 
   
                                   FIG. 5. Hole for Microphone Box 
 
 
 
 
Figure 6 is a photograph of the windscreen installed in to the ground. The microphone is fitted 
with a rain cover, although we never had a single case of rain leaking into the box, and is fixed to 
a plate, which rests upon packing material for seismic isolation. The microphone cable passes 
through a tightly sealed feedthru.  
 
                                 
 
                                       FIG. 6. Sub-surface windscreen 
 
Figure 7 is a satellite image of a three-element microphone array installed in the field at Langley 
Research Center. The array consists of three stations, arranged in an equilateral triangle spaced 
100 ft. (30.48 m) apart. The red lines show drainage conduits to a drainage ditch surrounding the 
array site. The microphone power supplies are housed in a central control box, which also 
contains a weather station. The microphone output cables pass through a conduit to a control 
room about 200 ft (60.96 m) away. The control room contains the data acquisition system.  
 
                                 
Drainage Ditch
Array Layout
Satellite Image
100’
 
 
        FIG. 7. Satellite image of a three-element microphone array                                           
 
The microphone array operated continuously in the field during the latter part of 2006. Data were 
collected from a variety of sources – a situation that prompted the establishment of the following 
criteria for the association of received infrasound with a designated source: 
 
(1) Concomitancy. When the source is present, signals must be received. When the source is 
absent, the signals must disappear. As a corollary, if the location of the source is known, 
then the time of arrival must correspond to the source-microphone separation. 
(2) Directionality. Signals must arrive from the direction of a known source.  
(3) Characteristic signature. Signals must conform to the signatures of a known source. This 
criterion will be difficult to fulfill for the detection of infrasonic emissions from clear air 
turbulence, for the spectrum of the latter is unknown.  
(4) Signals must appear identical on all microphones in the array. In other words, the 
coherence among microphone pairs must be high.  
 
The data sampling interval was 30 s at 200 samples per sec. A data file block comprised 6 h of 
data, thus a sample population of 360/0.5 min = 720 sample blocks. Figure 8 shows a typical 30-
s snapshot on a day when coherent signals were received on the microphone array. It is noted 
that the signals appear to be similar on all three channels, indicating that they represent the 
propagation of sound across the array. Since the maximum delay from one microphone to 
another is 0.1 s, the resolution on the figure is too coarse to observe a delay; but a 200 Hz sample 
rate affords good resolution in determining the time delays, from which the angle of incidence is 
determined.  
 
                             
 
                              FIG. 8. Received infrasonic signals on 08/24/06 (0.2-4 Hz) 
                 
The incoming signals were filtered over the passband 0.2-4 Hz. The coherence over a six-hour 
data block is shown in the figure 9. The coherence is the geometric mean of the three 
microphone pairs. The ordinate is frequency and the abscissa is the time, spanning 360 min (i.e.  
 
six-hour block). The coherence is color coded: blue representing low coherence and red 
representing high coherence. The high coherence from zero to 70 min has not been explained. 
The high coherence from 150-360 min is interpreted to represent signals emitted from 
atmospheric CAT. Note that the coherence cuts off at frequencies beyond the passband of the 
filter and that the coherence appears to be intermittent, as to be expected from a CAT source.  
 
                           
Coherence of received infrasonic signals (0.2-4 Hz)
Geometric mean of three channels
Fr
eq
ue
nc
y,
 H
z
0
2
4
6
8
10
0
0.2
0.4
0.6
0.8
1.0
C
oh
er
en
ce
120 180 240 300 360600
Time, min 
 
 
       FIG. 9.  Coherence of received infrasonic signals (0.2-4 Hz) 
 
Figure 10 shows the bearing angles of the received infrasound, as determined from the time 
delays among the microphone pairs. Over the six-hour period of the data block signals are found 
coming from two direction: the first set at an angle near 00 (North) and the second just shy of 
1800 (slightly Southwest).  
 
 
 
 
 
 
 
 
 
 
                                 
Bearing angles of received infrasound
North
South
South
East
West
 
 
        FIG. 10. Bearing angles of received infrasound  
 
The figure 11 is based on a map of the map-Atlantic region. Pilot reports (PIREPS) are indicated 
by circles, which are color coded: blue, green, and red indicating light, moderate, and severe 
turbulence respectively. The directions of the received infrasound are indicated by the pink 
sectors, the spatial resolution of the directionality being +/-7.50. Note the PIREP indicating 
severe turbulence within the northern sector. The correspondence between the PIREP location 
and the infrasound bearing angle offers support for the hypothesized emissions of infrasound 
from CAT.  
 
                               
Bearing angles (±7.5º) of received infrasound vs. pilot 
report locations
NC
VA
PA
WV
MD
 
 
         FIG. 11.  Bearing angles of received infrasound vis. Pilot report locations 
 
 
CONCLUSIONS 
 
The measurements reported in this study lead to the following conclusions: 
(1) The sub-surface windscreen concept proved successful in suppressing wind noise.  
(2) Hypothesized infrasonic emissions from CAT were identified, located, and corroborated 
with PIREPS. 
(3) Three of the four acceptance criteria were fulfilled 
• Concomitancy: The reception of coherent infrasonic signals occurred  
    simultaneously with PIREPS, indicating the presence of CAT. 
• The direction of the received infrasound corresponds to the PIREP locations. 
• The coherence among the three microphone pairs was high. 
• The characteristic signatures of emissions from CAT remain unidentified but  
    will be the subject of future investigation.  
 
ACKNOWLEDGMENTS 
 
We acknowledge the work of Benjamin F. Guenther and George C. Hilton, Langley Research 
Center, for fabricating the windscreens. This research was supported by the Creativity and 
Innovative Program at NASA Langley Research Center.  
 
REFERENCES 
 
1Qamar. A. Shams, Bradley S. Sealey, Toby Comeaux, Allan J. Zuckerwar, & Laura M. Bott, 
“Infrasonic Windscreens,” 146th Meeting of the Acoustical Society of America.  
2Allan J. Zuckerwar, Qamar A. Shams, Krish K. Ahuja, & Robert Funk, “Soaker Hose vs. 
Compact non-porous windscreen: A comparison of performance at infrasonic frequencies,” 150th 
Meeting of the Acoustical Society of America. 
3Q. A. Shams, A. J. Zuckerwar, B. S. Sealey, “Performance Comparison of Compact Cylindrical 
and Spherical Windscreens,” 150th Meeting of the Acoustical Society of America. 
4Q. A. Shams, A. J. Zuckerwar, Bradley S. Sealey, “Compact Nonporous Windscreen for 
Infrasonic Measurements,” J. Acoust. Soc. Am. 118 (3), 1335-1340 (2005).  
 


Sub-Surface Windscreen for the Measurement of Outdoor Infrasound

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