Patent Publication Number: US-7916887-B2

Title: Wind-shielded acoustic sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Provisional Patent Application Ser. No. 60/540,058, filed Jan. 30, 2004, entitled WIND-SHIELDED ACOUSTIC SENSOR, the teachings of which are expressly incorporated herein by reference. 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to an acoustic sensor, and more particularly, to an acoustic sensor used in windy environments found on aircraft, moving ground vehicles, wind tunnels and in naturally windy conditions. 
     Signal detection afforded by acoustic sensors or microphones are limited in windy conditions by at least two distinct forms of wind-induced noise. The first form is due to disturbances in the wind created by the acoustic sensor, which is solely caused by interaction of the wind and the aerodynamics of the sensor and/or sensor windscreen. The second form of wind-induced noise involves complex velocity and pressure fluctuations that are an inherent component of most wind. 
     The solution of the first form of wind-induced noise is proper aerodynamics. Proper aerodynamics design seeks to minimally disturb the wind, avoid separation of flow from the surface of the windscreen and thereby prevent unsteady, noisy flow from developing. Various distinct design techniques have been proposed for achieving proper aerodynamics. However, the low-noise achievement of these design techniques has been limited to the condition that winds are approaching in a given direction. That is, when winds alter their course, the low-noise performance of these aerodynamic design techniques is negated. 
     The second form of wind-induced noise caused by complex velocity and pressure fluctuations inherent to most winds is more difficult to address. Currently, foams, fabrics and other porous materials have been used to lessen the effects of these natural fluctuations on the acoustic sensors. The most common of these techniques is the use of an open cell, reticulated foam ball. However, all such approaches only offer limited immunity to inherent wind fluctuations, and are not sufficiently rugged for many applications. 
     BRIEF SUMMARY OF THE INVENTION 
     A wind-shielded acoustic sensor is provided to effectively reduce both categories of wind induced noises. The acoustic sensor has a microphone housing that employs an aerodynamic cross-section operative to redirect the bulk fluid flow around the sensing elements while causing minimal disturbance to the fluid flow. The housing may be formed in many different shapes, but the preferred embodiment is symmetrical (like a disc) or nearly-symmetrical about an axis, allowing the windscreen to present a similar aspect to the fluid flow for many given flow directions. In addition, separate sound ports and/or structural components that are semi-transparent to sound are formed to capture both the sound signal to be measured and the random-like pressure fluctuations that may be inherent in winds. The sound ports bring the detected signal and pressure fluctuations into a central mixing cavity, which serves to remove the random-like pressure fluctuations through a process of uncorrelated averaging and intensify the detected signal, which, by contrast, is well correlated. 
     The housing includes sufficiently thin and pliable regions that will deform subject to wind. In addition, sound ports are formed to extend through the housing. The deformation of the housing regions and the sound ports allow sounds, including the acoustic signals and wind-related, random-like pressure fluctuations, to transmit through the housing and enter the cavity enclosed by the housing. Addition of the acoustic signals and the wind-related, random-like pressure fluctuations occurs within the cavity. As the negative components and the positive components of the wind-related, random-like pressure are substantially equal, the addition thus substantially removes wind-related, random-like pressure. The microphone can thus detect the acoustic signals with greatly reduced wind noise. The acoustic sensor may be secured to the ground or a flat surface by gripping the housing from two adjacent edges or by inserting a rigid shaft into the center of the housing along the longitudinal axis. 
     In one embodiment, the housing may further comprise a streamlined surface extending from the circular disc to a mounting unit. The mounting unit as well as the housing can be supported by a mating shaft connected to a perpendicular shaft. To avoid vibration, a cross shaft may also be affixed to the mating shaft and the perpendicular shaft diagonally. It is also contemplated that alternative mounting arrangements may be utilized as may be appropriate for a given application of the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These as well as other features of the present invention will become more apparent upon reference to the drawings therein: 
         FIG. 1  is a top view of a wind-shielded acoustic sensor comprising a microphone housing; 
         FIG. 2  is a side view of the wind-shielded acoustic sensor as shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the wind-shielded acoustic sensor as shown in  FIG. 1 ; 
         FIG. 4  is a variant of the microphone housing for forming the wind-shielded acoustic sensor; and 
         FIG. 5  shows the mounting scheme of the wind-shielded acoustic sensor as illustrated in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for purpose of illustrating preferred embodiments of the present invention only, and not for purposes of limiting the same,  FIG. 1  shows a top view of an embodiment of a wind-shield acoustic sensor  10 ,  FIG. 2  shows a side view of the acoustic sensor  10 , and  FIG. 3  shows a cross-sectional view of the acoustic sensor  10  along line  3 - 3 ′. As shown, the acoustic sensor  10  includes a housing  16  having a streamline, aerodynamic profile that is everywhere continuous and symmetric about a latitudinal axis  12 . In this embodiment, the profile is in the form of a circular disc that is also symmetric about a longitudinal axis  14  thereof. With the streamlined and symmetric profile, the housing  16  can be oriented such that wind may flow along any direction roughly perpendicular to the longitudinal axis  14 , which itself is made to be perpendicular to the ground or the mounting surface. The profile thus prevents the creation of unnecessary disturbances in wind-flow and therefore prevents the creation of wind induced self-noise. 
     The acoustic sensor further includes a plurality of sound ports  18  extending through the housing  16 . Preferably, the sound ports  18  are uniformly spaced along several circumferences centered about the longitudinal axis  14 . The diameters of these sound ports  18  are in excess of ten times smaller than the smallest sound wavelength to be detected. In addition to the sound ports  18 , the sound may also enter the housing  16  through deformation within the regions  16 A that are sufficiently thin and pliant. Both acoustic signals and wind related, random-like pressure fluctuations are transmitted through the sound ports  18  and the housing  16 A into an internal cavity  19  enclosed by the housing  16 . The internal cavity  19  and the sound ports  18  together form a lumped element acoustic resonator. The housing  16  serves as a structural resonator that is coupled to the lumped element acoustic resonator. The housing  16  is designed such that combined resonance frequencies of the resonators are greater than the largest sound frequency to be detected. The sound ports  18  also aid in flattening the frequency response of the internal cavity  19 . Addition of the acoustic signals and random-like pressure fluctuations from the sound ports  18  and the housing regions  16 A occurs in the internal cavity  19  and the housing region  16 A. Random-like pressures are removed upon addition owing to the statistical fact that there are as many negative random-like pressure fluctuations as there are positive random-like pressure fluctuations. Signal pressure contributions from the sound ports  18  and the housing regions  16 A are roughly equal and therefore add together to form a larger pressure. 
     The acoustic sensor  10  further comprises a microphone  24  supported in place by a pair of microphone seats  22  to measure a signal in which the wind induced, random-like pressure fluctuations have been greatly reduced. The housing  16  may be secured to the ground or a flat surface by gripping the housing  16  from two adjacent edges or by inserting a rigid shaft into the center of the housing along the longitudinal axis  14 . 
     An alternative configuration of the housing  16  is illustrated in  FIG. 4 . As shown, a streamlined surface  26  is extended from the circular disc of the housing and culminates in a mounting unit  28 .  FIG. 6  depicts a mounting scheme, which includes a shaft  30  to mate the mounting unit  28  and to extend towards a perpendicular shaft  32 . To reduce vibrations, a cross-shaft  34  is fixed to the mated shaft  30  and the perpendicular shaft  32  in a diagonal manner. 
     While an illustrative and presently preferred embodiment of the invention has been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art. For example, it should be understood that the acoustic sensor  10  can take any of a variety of aerodynamic shapes that are suited for a particular application and/or orientation relative an oncoming stream of wind. It should also be understood that the acoustic sensor  10  may be mounted in a number of different configurations, depending on the specific application for which the sensor is utilized. It is likewise contemplated that the acoustic sensors of the present invention may be utilized in any and all relevant applications known in the art, as well as utilized with all known and later developed signal processing technologies, such as frequency filters, noise cancellation and the like. Accordingly the invention and its application should be construed as broadly as possible.