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Compact Infrasonic Windscreen

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					             ⏐ ⏐
ther ado. If ⏐ rpi⏐ and ˆ B are not known
                        z                                           amount by which the phase at rpi leads           This invention is owned by NASA, and a
a priori, then it is necessary to deter-                            the phase at rB to (b) phase measure-         patent application has been filed. Inquiries
       ⏐ ⏐
mine ⏐ rpi⏐,the attitude, and the phase-                            ments for all of the GPS signals de-          concerning nonexclusive or exclusive license
                    ⏐ ⏐
correction term ⏐ rpi⏐ cos(βi) from a                               tected by the receiver.                       for its commercial development should be ad-
least-squares or other fit of (a) an ap-                              This work was done by Patrick W. Fink and   dressed to the Patent Counsel, Johnson Space
proximate geometric model of the                                    Justin Dobbins of Johnson Space Center.       Center, (281) 483-0837. Refer to MSC-23228.




Compact Infrasonic Windscreen
High values of infrasound-transmission and wind-noise-attenuation coefficients can be realized.
Langley Research Center, Hampton, Virginia
   A compact windscreen has been con-                               infrasonic pressures (which propagate at      ments: (1) it must attenuate noise gener-
ceived for a microphone of a type used                              the speed of sound) and convective pres-      ated by ambient wind, (2) it must trans-
outdoors to detect atmospheric infra-                               sure fluctuations generated by wind tur-      mit infrasound propagating across the
sound from a variety of natural and man-                            bulence. Hence, success in measure-           microphone, (3) it must be useable in
made sources. Wind at the microphone                                ment of outdoor infrasound depends on         all weather, and (4) it must not be sus-
site contaminates received infrasonic sig-                          effective screening of the microphone         ceptible to generation of infrasound
nals (defined here as sounds having fre-                            from the wind.                                through shedding of vortices.
quencies <20 Hz), because a micro-                                     To be effective, an infrasonic wind-          Past methods of wind screening in-
phone cannot distinguish between                                    screen must fulfill four basic require-       clude the use of cloth or open-cell foam,
                                                                                                                  and the use of an array of pipes. A wind-
                                                                                                                  screen made of cloth or open-cell foam
                                       –5                                                                         is thought to break up incident airflow
                                                                                                                  into very small turbulent eddies that dis-
                                           0
                                                                                                                  sipate wind energy in the form of heat.
                                           5                                                                      Such a windscreen is effective at audio
                                                                                                                  frequencies (>20 Hz) but not at infra-
      Attenuation, dB




                                       10                                                                         sonic frequencies (<20 Hz).
                                                                                                                     An array of pipes used as a windscreen
                                       15
                                                                                                                  consists, more specifically, of several per-
                                       20                                                                         forated pipes, called a “spider,” fanning
                                                                                                                  out radially from a microphone situated
                                       25                                                                         in an enclosed housing. The array is vast
                                                                                                                  — covering an area comparable to that
                                       30
                                                                                                                  of an athletic field — and its perform-
                                       35
                                                                                                                  ance as a windscreen is degraded by res-
                                               0.1   1              10              100             1000          onances that depend on the lengths of
                                                              Frequency, Hz                                       the pipes.


                                     10
      Transmission Coefficient, dB




                                      0




                                     –10




                                     –20




                                     –30
                                           10            20               40          60             100
                                                              Frequency, Hz


Figure 1. These Plots Are Results of Tests of the wind-noise-attenuation and infrasound-transmission              Figure 2. A Cylindrical Windscreen Covers a Mi-
properties of a polyurethane-foam windscreen.                                                                     crophone mounted on a pole outdoors.



NASA Tech Briefs, September 2005                                                                                                                              27
   The present compact windscreen is           the presence and absence of the wind-               ratio between detected sounds, with and
based on an entirely different principle:      screen was used as a measure of the atten-          without the windscreen, was taken as a
that infrasound at sufficiently large wave-    uation of wind noise by the windscreen.             measure of the transmission through the
length can penetrate any barrier of prac-         The windscreen that performed best               windscreen. The results for the portion
tical thickness. Thus, a windscreen having     in the wind-tunnel tests was a cylinder             of the spectrum from 10 to 100 Hz, plot-
solid, non-porous walls can block con-         made of polyurethane foam of a type                 ted in the lower part of Figure 1, show
vected pressure fluctuations from the          known in the industry as “eight-                    that this windscreen had a large trans-
wind while transmitting infrasonic             pounder,” having an inside diameter of              mission coefficient at frequencies below
acoustic waves. The transmission coeffi-       3 in. (7.62 cm), a wall thickness of 0.5 in.        25 Hz, even exhibiting a gain as high as
cient depends strongly upon the ratio be-      (1.27 cm), and a length of 12 in. (30.48            8 dB at 10 Hz, but then attenuated
tween the acoustic impedance of the            cm). The attenuation of wind-generated              sound at higher frequencies. Finally, a
windscreen and that of air. Several mate-      noise was quantified as the ratio between           soak test revealed that the water ab-
rials have been found to have impedance        the wind noises measured without and                sorbed by the polyurethane windscreen
ratios that render them suitable for use in    with this windscreen. The results, plot-            material amounted to only 2.1 percent
constructing walls that have practical         ted in the upper part of Figure 1, show             by weight.
thicknesses and are capable of high trans-     that this windscreen attenuated wind                   Figure 2 shows a windscreen installed
mission of infrasound. These materials         noise by amounts ranging from 12 to 20              over a microphone mounted on a pole
(with their impedance ratios in parenthe-      dB at frequencies ranging from 0.7 to 20            in the field. The windscreen has proved
ses) are polyurethane foam (222), space-       Hz. The large spikes in the spectrum                robust in weather conditions of all sea-
shuttle tile material (332), balsa (323),      represent aeolian tones generated by                sons and it survived Hurricane Isabel
cedar (3,151), and pine (4,713).               the wind passing over the windscreen,               with wind gusts up to 67 mph (30 m/s).
   A small wind tunnel was built to test the   but these lie above the infrasonic range.              This work was done by Allan J. Zuckerwar,
acoustical properties of a variety of wind-       For measurements of the infrasound-              Qamar A. Shams, Bradley S. Sealey, and
screen materials. A fan generated wind at      transmission coefficient of this wind-              Toby Comeaux of Langley Research Cen-
speeds up to 21 mph (9.4 m/s) across an        screen, a subwoofer was placed at an end            ter. For further information, contact the Lan-
infrasonic microphone. Tests were con-         of the wind tunnel and used to generate             gley Innovative Partnerships Office at (757)
ducted with and without the windscreen;        a tone that was swept over the frequency            864-3521.
the difference in the noises detected in       band from 10 to 200 Hz. In this case, the           LAR-16833-1




Broadband External-Cavity Diode Laser
This relatively simple, inexpensive device is suitable for use in survey spectroscopy.
John H. Glenn Research Center, Cleveland, Ohio
   A broadband external-cavity diode           light-emitting diodes, the ECDL offers              enhancement cells. A tunable filter —
laser (ECDL) has been invented for use         the greater brightness, simpler fiber               preferably, a monochromator or a spec-
in spectroscopic surveys preparatory to        coupling, and superior spatial propaga-             trometer — is used to select a portion of
optical detection of gases. Heretofore,        tion properties of a laser. For example,            the output spectrum.
commercially available ECDLs have              the broadband ECDL is easily coupled                   The optical configuration of the
been designed, in conjunction with so-         into multiple-pass optical-path-length-             broadband ECDL (see figure) is based
phisticated tuning assemblies, for nar-
row-band (and, typically, single-fre-
quency) operation, as needed for high                                                                                       Diffraction
sensitivity and high spectral resolution                                                                                     Grating
in some gas-detection applications. How-
ever, for preparatory spectroscopic sur-                 Diode Laser
veys, high sensitivity and narrow-band                                             Collection Optic
operation are not needed; in such cases,
the present broadband ECDL offers a
simpler, less-expensive, more-compact
alternative to a commercial narrowband
ECDL.
   To be precise, the output of the tun-
able, broadband ECDL consists of many                                                                                 Output
narrow spectral peaks spaced at narrow                                        Feedback
wavelength intervals that, taken to-                                           Mirror
gether, span a broad wavelength band.
The broadband ECDL can, therefore, be
likened to a light-emitting diode except
that the spectrum incorporates the ex-         The Feedback Mirror Is Made Curved (in contradistinction to flat) to make it select a range of wave-
ternal-cavity mode structure. Unlike           lengths (in contradistinction to a single wavelength).



28                                                                                                             NASA Tech Briefs, September 2005

				
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