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ReSound Corp. in Redwood City, California, has
developed a miniature, ear-mounted, hands-free,
microphone/receiver for use by technicians, police,
firefighters, or others who need an unobtrusive
communication system. When used in the field,
however, wind-induced noise tended to mask voice
communication. A foam covering reduced the noise,
but was bulky and obtrusive. Through a Reimbursable
Space Act Agreement, ReSound collaborated
with the Experimental Physics Branch personnel to
mitigate the wind-induced noise without resorting to
bulky foam coverings.
To achieve the objective, an artificial head was
installed in a ducted fan flow in the AIP acoustics
laboratory (figure 1), and a series of experiments was
conducted on wind-induced noise of the baseline
ReSound microphone over a 3-month period. Using
technology developed at Ames for measurement of
aircraft model noise in wind tunnels, a screen-covered
forebody was designed that protected the
microphone sensor yet conformed to the shape and
size of the ear-mounted device (figure 2). The
forebody had an aerodynamic shape to minimize
flow perturbations; the screen inhibited unsteady
pressures at the sensors while passing sound waves.
New information was acquired on the baseline and
on improved microphone designs regarding effects of
turbulence, wind speed, head-tilt angle, and yaw
angle. The fluid-mechanic sources of wind noise
were isolated. Turbulence, airspeed, and noise were
measured in the fan flow, in the free-wind outdoors,
in a moving truck in calm air, and in the low turbulence
7- by 10-foot wind tunnel. Results showed that
the new wind screen gave improved noise reduction
relative to the ReSound baseline with foam covering
and that it has a more robust, streamlined shape that
lends itself to unobtrusive operation by the user.
Microphone response was carefully correlated with
flow parameters such as turbulence, airspeed, and
flow angle so that future changes can be evaluated
using the database.
Further improvements in the design may be
possible by optimizing the screen impedance and the
forebody shape.
Point of Contact: C. Horne/ P. Soderman
(650) 604-4571/6675
chorne@mail.arc.nasa.gov
psoderman@mail.arc.nasa.gov
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Fig. 1. Experimental set-up in acoustic lab with
artificial head model, wind simulator, and instrumen-tation
for measuring flow characteristics and micro-phone
noise.
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Fig. 2. Close-up detail of artificial head model with
ear-mounted microphone and alignment template.
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