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-
sipate wind energy in the form of heat.
5 Such a windscreen is effective at audio
frequencies (>20 Hz) but not at infra-
10 sonic frequencies (<20 Hz).
An array of pipes used as a windscreen
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
of an athletic field — and its perform-
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.
Transmission Coefficient, dB
10 20 40 60 100
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
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