Abstract:
Embodiments of infrasound sensors comprising multiple matched-responsivity pressure sensors are presented. Infrasound sensors in accordance with the present invention have limited volume, which enables them to observe wind velocity at the same point that infrasound is monitored. The small size and matched-responsivities enable infrasound sensors in accordance with the present invention to obviate the need for complex and costly spatial filters that degrade the signal-to-noise ratio of prior-art infrasound sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/255,663, filed Oct. 28, 2009, entitled “Infrasound Sensor,” (Attorney Docket: 123-140us), which is incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to displacement sensors in general, and, more particularly, to infrasound sensors. 
       BACKGROUND OF THE INVENTION 
       [0003]    Infrasound is sonic energy characterized by a frequency lower than the range of human hearing (i.e., less than approximately 20 Hz). Infrasound manifests as pressure waves having wavelengths that are kilometers to hundreds of kilometers. As a result, infrasound can propagate over extremely long distances—even around the globe. Detection of infrasound, therefore, can provide useful information about geophysical phenomena such as avalanches, earthquakes, tsunami, and volcanic eruptions. Infrasound detection also forms the basis of much of the international network that monitors nuclear explosions for the purpose of test ban treaty enforcement. 
         [0004]    Conventional infrasound sensing systems typically comprise an array of inlets arranged about a relatively bulky microbarometer contained in a housing. The inlets are connected to an opening at the top of the housing to fluidically couple the inlets to the diaphragm of the microbarometer. The diaphragm divides the housing into a front (i.e., top) chamber and a back chamber. The diaphragm vibrates with pressure disturbances in the air, much like a conventional audio microphone, thereby providing an output signal based on the pressure at the inlets. 
         [0005]    The response time of the infrasound sensor is based on how fast pressure equalizes between the front and back chambers. A small capillary tube is typically used to fluidically couple the front and back chambers so as to provide a “leak” between them. The time constant for the equalization of the pressures in the front and back chambers is based on the size of this capillary tube. 
         [0006]    To date, it has been difficult, if not impossible, to differentiate pressure changes due to wind from infrasound energy. Wind interferes with infrasound sensing because it produces an additional pressure fluctuation that cannot be easily discerned from infrasound signals significantly degrading the signal-to-noise ratio of the detection system. To mitigate the effects of wind-induced noise, the inlets of the infrasound sensor are typically connected with the housing through a complex arrangement of inlet hoses. The hose configuration works as a spatial filter for wind noise. In operation, this mechanical spatial filter samples the atmosphere at a series of points spaced around the microbarometer. The pressure signals from these points are mechanically added at the union of the plurality of tubes at the single microbarometer. 
         [0007]    While these hose arrangements are somewhat effective in reducing wind noise, they present many other challenges. Phase delays along the lengths of the pipes arise due to finite sound velocity. This limits the size of the area of the mechanical filters over which response can be averaged. Further, because of the size of these spatial filters there is no practical way of measuring their response. Still further, the complexity of the installation is substantial and the deployment of such a sensor in remote locations can be extremely challenging. Hard-walled pipes, such as PVC or metal pipes, produce undesirable resonance affects. As a result, porous garden houses, such as those that can be bought at a local retail store, are normally used. The quality control for such hoses is virtually non-existent. In addition, their actual porosity to air in real conditions has not been evaluated in a scientific manner. Although the use of garden hoses mitigates the problem of resonance effects, it is not clear what additional problems are being introduced by their use in this manner. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an infrasound sensor that overcomes some of the disadvantages of the prior art. Embodiments of the present invention are particularly well suited for detecting explosions; avalanches; earthquakes; severe weather; shock waves due to meteor strikes and supersonic jets; variations in local magnetic field strength during auroral displays; and microbaroms related to kilometer size standing waves on the ocean. 
         [0009]    In some embodiments, an array of pressure ports is symmetrically arranged about a central point. The pressure ports are located within a small volume such that they are substantially uniformly affected equally by infrasound energy. Each of the pressure ports is fluidically coupled with a pressure sensor comprising a displacement sensor having very high sensitivity and very small footprint. Each displacement sensor is based on an optical beam splitter that distributes the optical energy of an input beam into a reflected beam and a transmitted beam. A processor receives output signals based on each of these reflected and transmitted beams. The compactness of the infrasound sensor enables it to observe wind velocity and infrasound at substantially the same point. 
         [0010]    It is an aspect of the present invention that the beam-splitter-based pressure sensors have substantially matched responsivities. The matched responsivities, coupled with the small volume of the sensor, enable differences in the output signals of the pressure sensors to be attributed to affects of wind noise. Wind noise, therefore, can be directly cancelled. This obviates the need for complicated hose-based spatial filters and/or isolation vaults typically required for prior-art infrasound sensing systems. As a result, embodiments of the present invention are significantly cheaper and less complex than prior-art infrasound sensing systems. Further, embodiments of the present invention can have improved sensitivity as compared to prior-art infrasound sensors. 
         [0011]    An embodiment of the present invention comprises an apparatus for sensing infrasound energy, wherein the apparatus comprises: a plurality of pressure sensors symmetrically arranged about an origin, each pressure sensor comprising: (a) a pressure port; and (b) a displacement sensor fluidically coupled with the pressure port, the displacement sensor comprising a beam splitter that distributes an input signal into a first signal and a second signal based on a pressure at the pressure port, wherein the displacement sensor is dimensioned and arranged to provide a pressure signal that is based on at least one of the first signal and the second signal; wherein the responsivity of each of the plurality of pressure sensors is within ±2.5% of the average responsivity of the plurality of pressure sensors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  depicts a schematic diagram of a cross-section of an infrasound sensor in accordance with the prior art. 
           [0013]      FIG. 2  depicts an infrasound measurement installation in accordance with an illustrative embodiment of the present invention. 
           [0014]      FIG. 3  depicts operations of a method for sensing infrasound in accordance with the illustrative embodiment of the present invention. 
           [0015]      FIG. 4  depicts a schematic drawing of a cross-sectional view of an infrasound sensor in accordance with the illustrative embodiment of the present invention. 
           [0016]      FIG. 5  depicts a schematic diagram of a pressure sensor in accordance with the illustrative embodiment of the present invention. 
           [0017]      FIGS. 6A and 6B  depict top and cross-sectional views, respectively, of a beam splitter in accordance with the illustrative embodiment of the present invention. 
           [0018]      FIG. 7  depicts a model of pressure versus position for a stationary object subjected to wind. 
           [0019]      FIG. 8  depicts the velocity distribution associated with wind incident on a stationary object. 
           [0020]      FIGS. 9A and 9B  depict top and cross-sectional views, respectively, of a beam splitter in accordance with an alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following terms are defined for use in this Specification, including the appended claims:
       Operatively Coupled is defined as a condition wherein a first object, which might be remote from a second object, affects the second object or has some effect on a third object through the second object, etc. For example, consider a rigid linkage having a first end and a second end. The first end attaches to a plate and the second end abuts a wall. The linkage is capable of transferring, to the wall, a force that is received at the plate. The linkage and the plate can therefore be considered to be operatively coupled for transmitting a force. Operatively-coupled objects need not be in direct contact with one another and, as appropriate, can be coupled through any medium (e.g., semiconductor, air, vacuum, water, copper, optical fiber, etc.). The coupling between operatively-coupled objects can transmit, as appropriate for the nature of the coupling and the objects, any type of force, signal, charge, electrical current, optical energy, etc. Consequently, operatively-coupled objects can be electrically-coupled, hydraulically-coupled, magnetically-coupled, mechanically-coupled, optically-coupled, pneumatically-coupled, thermally-coupled, fluidically-coupled, etc.   Fluidically Coupled is defined as that, with respect to two regions, fluid (i.e., liquid, vapor, gas) can move between the two regions or that a change in pressure in one region can affect the pressure in the other region, etc.       
 
         [0024]      FIG. 1  depicts a schematic diagram of a cross-section of an infrasound sensor in accordance with the prior art. Sensor  100  comprises barometer  102 , chamber  104 , manifold  106 , conduits  108 , and inlets  110 . Prior-art examples of infrasound sensors such as sensor  100  include two- and three-dimensional configurations. 
         [0025]    Barometer  102  is a pressure sensor that is contained within chamber  104 , which is fluidically coupled with manifold  106 . 
         [0026]    Manifold  106  is fluidically coupled with each of the plurality of inlets  110  through conduits  108 . 
         [0027]    Conduits  108  are conduits that fluidically couple inlets  110  and manifold  106  (and, therefore, barometer  102 ). 
         [0028]    Wind interferes with infrasound sensing by producing pressure fluctuations that cannot be easily discerned from infrasound energy. As a result, wind represents a noise source that can devastate the performance of a conventional infrasound sensor. In order to provide a spatial filter for wind, inlets  110  are arrayed, substantially symmetrically, about manifold  106 . In many common arrangements, inlets  110  are arranged in symmetric clusters  112 , which are symmetrically arranged about secondary manifolds  114 , which are further arranged symmetrically about manifold  106 . 
         [0029]    Since it is desirable to deploy infrasound sensors throughout the world, these systems are often deployed in remote areas such as tropical rainforests, arid deserts, mountainous regions, and the like. Further, wind noise rejection is dependent upon the relative separation between inlets  110 ; therefore, their positions relative to one another, as well as manifold  106 , are critical. Still further, wind noise rejection improves as the relatively separations of inlets  110  increases. Sensor  100 , for example, is characterized by inlet separations that range from a few meters to nearly 100 meters. As a result, the installation complexity of arrangements such as that of sensor  100  is quite substantial and the deployment of such a sensor in remote locations can be challenging. 
         [0030]    In addition to the challenges associated with deploying system  100 , the choice of material for conduits  108  present further issues. Using hard-walled conduits, such as PVC or steel pipes, for conduits  108  can result in undesirable resonances in the output signal of barometer  102 ; therefore, porous conduits are more desirable. Due to its porosity and ready availability world-wide, the most common choice for conduits  108  is the common garden hose. The use of garden hoses to fluidically couple inlets  110  and barometer  102  presents many other challenges, however. 
         [0031]    First, the quality control of the garden hose is not rigorous since, for most applications, it is simply not important. 
         [0032]    Second, the actual porosity to air in real conditions has not been evaluated in any scientific manner. 
         [0033]    Third, the reliability and life time of the common garden hose is dramatically affected by the environmental conditions in which it is deployed. Reliability and lifetime are severely degraded when the hoses are exposed to the harsh conditions inherent to tropical or sub-tropical rainforests or arid desert regions. Plasticizer in the hose walls outgases over time, making a garden hose more brittle. Hose material degrades due to exposure to sunlight, extreme temperatures, and the like. Animal attack (e.g., mouse bites, etc.) or animals nesting within the hoses are commonly reported. These practical considerations significantly reduce the desirability of the use of garden hoses in an infrasound installation. 
         [0034]    The present invention enables measurement of infrasound without the need for large-area arrays of sensors that are interconnected by conduits. Embodiments of the present invention utilize a small-area array of sensors having well-matched responsivities to observe wind velocity and monitor infrasound signals at the substantially same point. Because their responsivities are well matched, disparities between the outputs of different sensors can be directly attributed to wind-induced pressure fluctuations and, therefore, readily cancelled. 
         [0035]      FIG. 2  depicts an infrasound measurement installation in accordance with an illustrative embodiment of the present invention. Infrasound measurement installation  200  comprises sensor  202 , which is deployed at measurement site  204 . 
         [0036]    Sensor  202  is an infrasound detector that provides output signal  210 . Sensor  202  mitigates the effects due to wind at measurement site  204 , which results in a higher signal-to-noise ratio output for infrasound. Sensor  202  is described in more detail below and with respect to  FIG. 4 . 
         [0037]      FIG. 3  depicts operations of a method for sensing infrasound in accordance with the illustrative embodiment of the present invention. Method  300  begins with operation  301 , wherein sensor  202  is deployed at measurement site  204 . 
         [0038]      FIG. 4  depicts a schematic drawing of a cross-sectional view of an infrasound sensor in accordance with the illustrative embodiment of the present invention. Sensor  202  comprises pressure sensors  402 - 1  through  402 - 6 , housing  404 , and processor  406 . 
         [0039]    Each of pressure sensors  402 - 1  through  402 - 6  (collectively referred to as pressure sensors  402 ) comprises a displacement sensor that is fluidically coupled with a pressure port. Pressure sensors  402  are described in more detail below and with respect to  FIG. 5 . 
         [0040]    Housing  404  is a substantially rigid, three-dimensional, substantially spherical housing that locates pressure sensors  402 . Housing  404  positions each pressure sensor  402  such that its respective pressure port is located in the center of a different face of a substantially symmetric cube centered at origin  408 . Pressure sensors  402 - 1  and  402 - 2  are displaced from one another along the y-direction. Pressure sensors  402 - 3  and  402 - 4  are displaced from one another along the x-direction. Pressure sensors  402 - 5  and  402 - 6  (not shown for clarity) are displaced from one another along the z-direction. 
         [0041]    In some embodiments, housing  404  is non-spherical. Although the illustrative embodiment comprises six displacement sensor/pressure port assemblies, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments of the present invention that comprise any number of displacement sensor/pressure port assemblies. It is preferable, although not required, that the pressure ports be substantially equidistant about origin  408 . Preferable three-dimensional arrangements include, therefore, sensors that comprise platonic polyhedrons having 6, 8, 12, 20 . . . etc. sides. In some embodiments, the pressure ports are arranged in non-three-dimensionally symmetric arrangements such as toroid, or other shape. 
         [0042]    Processor  406  is a general purpose processor that is suitable for providing control signals  410  to pressure sensors  402 , receiving pressure signals  412 - 1  through  412 - 6  (collectively referred to as pressure signals  412 ) from pressure sensors  402 , generating a compensation factor for wind incident on sensor  202 , and providing output signal  210  based on infrasound received by sensor  202 . 
         [0043]      FIG. 5  depicts a schematic diagram of a pressure sensor in accordance with the illustrative embodiment of the present invention. Each of pressure sensors  402  comprises a pressure port  502  that is fluidically coupled with a displacement sensor  504 . 
         [0044]    Pressure port  502  is an inlet of suitable size and shape for enabling a pressure at the orifice of the pressure port to induce a pressure signal from its corresponding displacement sensor  504 . Pressure ports  502  are separated by a distance of less than a few tens of millimeters, and preferably by a distance of only a few millimeters. In some embodiments, one or more pressure ports  502  include a barrier that deter or prevent entry and/or damage by animals or insects. The size and configuration of pressure port  502  is application dependent and it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use pressure ports  502 . 
         [0045]    Displacement sensor  504  comprises beam splitter  506 , chamber  508 , source  512 , detector  518 , and detector  520 . Displacement sensor  504  provides pressure signal  412 , which is based on the pressure immediately outside pressure port  502 . Pressure signal  412  comprises output signals  522  and  524 . 
         [0046]    Each of displacement sensors  504  is an optical displacement sensor that has an extremely small footprint. Displacement sensors  504  are analogous to displacement sensors described in U.S. Pat. No. 7,355,723, issued Apr. 8, 2008 (Attorney Docket No.: 123-010US), which is incorporated herein by reference. The small size of displacement sensors  504  enables formation of sensor  202  having a total volume of less than 10 cm 3 . For example, the size of sensor  202  is approximately 6 mm×6 mm×5 mm. 
         [0047]    It is an aspect of the present invention that displacement sensors  504  are characterized by responsivities that are matched within 5% (preferably within a few percent) and are characterized by self-noise that is less than or equal to 1 mPa RMS. 
         [0048]    The high-performance characteristics of displacement sensors  504  arise from the performance of beam splitter  506  and the manner in which it is used. Beam splitter  506  is an optically resonant cavity whose cavity length is based on the position of a movable layer. The position of the movable layer is based on the presence and magnitude of a pressure differential across it. The ratio of light reflected and transmitted by the optically resonant cavity is, therefore, based on this pressure differential as well. 
         [0049]    By detecting both the light transmitted by beam splitter and the light reflected by the beam splitter, displacement sensor  504  provides a pressure signal that has very high sensitivity, very high dynamic range, and low self-noise. In some embodiments, detection of only one of the transmitted and reflected light can be used to generate a pressure signal having suitably high sensitivity and dynamic range as well as low self-noise. In some embodiments, beam splitters  506  are fabricated using MEMS-based technology, which facilitates achieving matched responsivities across a number of beam splitters. Beam splitter  506  is described in more detail below and with respect to  FIG. 6 . 
         [0050]    In some embodiments, each of displacement sensors  504  is co-located with its respective pressure port  502 . In some embodiments, one or more of displacement sensors  504  is physically displaced from its respective pressure port  502  but fluidically coupled to the pressure port through a conduit. 
         [0051]    Source  512  is a laser diode capable of emitting monochromatic light having a wavelength of approximately 1380 nanometers (nm) with a spectral-width of less than ten (10) nanometers, and preferably less than three (3) nanometers. In some embodiments of the present invention, source  512  comprises a light-emitting diode. In still some other embodiments, source  512  comprises a super-luminescent light-emitting diode. In still some other embodiments of the present invention, source  512  comprises a narrow-wavelength-band filter that reduces the spectral bandwidth of source  512 . 
         [0052]    Detectors  518  and  520  are photodetectors sensitive to the wavelength of the output light from source  512 . Detectors  518  and  520  provide output signals  522  and  524 , respectively, to processor  406 . Output signals  522  and  524  are based on the intensity of reflected beam  514  and transmitted beam  516 , respectively. It will be clear to those skilled in the art, after reading this specification, how to make and use detectors  518  and  520 . 
         [0053]    Although the size of pressure sensor  402  is design dependent and limited only by practical fabrication considerations, one skilled in the art will recognize that pressure sensor  402  will typically have a volume that is within the range of approximately 1 mm 3  to approximately 4000 mm 3 . More particularly, in the illustrative embodiment, pressure sensor  402  is approximately 6 mm×6 mm×5 mm. 
         [0054]      FIGS. 6A and 6B  depict top and cross-sectional views, respectively, of a beam splitter in accordance with the illustrative embodiment of the present invention. Beam splitter  506  comprises substrate  602 , layer  604 , layer  606 , and spacers  608 . Beam splitter  506  receives optical beam  510  from source  512  and splits the optical energy of optical beam  510  into reflected beam  514  and transmitted beam  516 . The ratio of optical energy in reflected beam  514  and transmitted beam  516  is dependent upon the characteristics of optically resonant cavity  614 , as described below. Optically resonant cavity  614  is formed by surface  610  of layer  604  and surface  612  of layer  606 , which are separated by cavity length L. 
         [0055]    Substrate  602  is a 500 micron-thick silicon wafer. Substrate  602  provides a mechanical platform for layer  604 . Substrate  602  is substantially transparent for the wavelengths of light included in optical beam  510 . In some embodiments, substrate  602  comprises a through-hole through which optical beam  510  is directed so as to mitigate absorption of optical energy of optical beam  510  by substrate  602 . In some embodiments of the present invention, substrate  602  is a material other than silicon. Suitable materials for substrate  602  include, without limitation, glass, III-V compound semiconductors, II-VI compound semiconductors, ceramics, and germanium. In some embodiments, the thickness of substrate  602  is other than 500 microns. 
         [0056]    Each of layers  604  and  606  is a layer of silicon-rich silicon nitride preferably having a thickness substantially equal to n*λ/4, where λ, is the wavelength (within layer  604 ) of the light in optical beam  510  and n is an odd-integer. Layers  604  and  606  are translucent for optical beam  510 . Suitable materials for use in layers  604  and  606  include, without limitation, silicon, glass, silicon dioxide, silicon oxide (SiOx, where x is in the range of 0.1 to 4), titanium nitride, polysilicon, non-stoichiometric silicon nitride (Si 3 N 4 ), stoichiometric silicon nitride (Si 3 N 4 ), III-V compound semiconductors, and II-VI compound semiconductors. 
         [0057]    It will be appreciated by those skilled in the art that the distribution of optical energy into the reflected beam and transmitted beam is dependent upon the thickness and index of refraction of each of layers  604  and  606 . In addition, it will be appreciated by those skilled in the art that thicknesses of layer  604  other than λ/4 can provide suitable performance, such as any odd-order of λ/4 (e.g., 3λ/4, 5λ/4, etc.). In some embodiments of the present invention, (e.g., wherein a different ratio of transmitted light to reflected light or different mechanical characteristics for one or both of layers  604  and  606  are desired) the thickness of layers  604  and  606  is approximately an even-order of n*λ/4 (e.g., λ/2, λ, 3λ/2, etc.), and n is an even-integer. In still some other embodiments of the present invention, the thickness of layer  604  is made different than any order of n*λ/4. 
         [0058]    In some embodiments, at least one of layers  604  and  606  is a glass substrate having a thickness within the range of approximately 1 micron to approximately 200 microns. In such embodiments, surfaces  610  and  612  would comprise a coating having a reflectivity for optical beams  510 ,  514 , and  516  that is within the range of approximately 50% to approximately 85%. Further, the surfaces of layers  604  and  606  distal to surfaces  610  and  612 , respectively, would typically comprise an anti-reflection layer. 
         [0059]    It should be noted that a thick substrate, such as a 200 micron-thick glass substrate, is suitable for use as layer  606  because such a substrate is typically characterized by little or no tensile stress and thus has higher compliance than many thinner membranes having large tensile stress. Further, infrasound-sensing applications do not require extremely high sensitivity; therefore, a thicker substrate can be used as a deformable membrane than would normally be suitable for a more conventional microphone application. 
         [0060]    In some embodiments of the present invention, layer  604  is not present and the optically resonant cavity is formed by a surface of layer  606  and a surface of substrate  602 . 
         [0061]    Spacers  608  have a thickness of approximately 110 microns. Spacers  608  are formed by etching cavity  616  in a sacrificial layer that interposes layers  604  and  606 . The formation of cavity  616  results in suspended membrane  606 . The thickness of spacers  608  is determined by the desired performance characteristics of beam splitter  506  for the light in optical beam  510 . In some embodiments, spacers  608  are precision spacers interposed between layers  604  and  606  to create cavity  616 . Materials suitable for spacers  608  include, without limitation, ceramics, silicon, metals, epoxies, solder, silicon dioxide, glass, alumina, III-V compound semiconductors, and II-VI compound semiconductors. Although the illustrative embodiment comprises spacers that have a thickness of approximately 110 microns, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that comprises spacers that have a thickness of other than 110 microns. 
         [0062]    Surface  610  of layer  604  and surface  612  of layer  606  collectively define optically resonant cavity  614 . Optically resonant cavity  614  has cavity-length L. In the absence of a pressure differential across membrane  606 , cavity-length L is equal to the thickness of spacers  608 . When infrasound energy or a pressure wave due to wind is received at pressure port  502 , however, a pressure differential develops across membrane  606  that moves the membrane thereby changing cavity length L. As cavity length L changes, the ratio of optical energy in reflected beam  514  and transmitted beam  516  changes. 
         [0063]    Prior-art infrasound sensors are typically based on microbarometers and, as a result, are bulky. The size of the microbarometers has historically been considered an advantage, since the perception has been that large sensors are necessary to reliably measure sound at very low frequencies. In reality, however, the limiting factor for low frequency microphone is the amount of time that it takes to equalize the pressure from the front volume to the back volume. 
         [0064]    Holes  618  are included in layer  618  for two primary purposes: (1) they provide access for the etchant used to form cavity  616 ; and (2) they provide a means of controlling the rate at which air flows into and out of cavity  616  in response to motion of membrane  606 . Design considerations for membrane holes are discussed in detail in “Phenomenological model for gas-damping of micromechanical structures,” Greywall, Busch, and Walker,  Sensors and Actuators , Vol. 72, pp. 49-70, 1999, which is incorporated by reference herein. Holes  618  are micron-sized holes that are formed in layer  606  by conventional reactive ion etching. In some embodiments, holes  618  are formed using a different suitable technology, such as laser drilling and the like. The presence of holes  618  enables membrane  606  and cavity  616  to be scaled in size to less than a few millimeters, while still providing a time constant for pressure equalization that exceeds that of prior-art microbarometers. One skilled in the art will recognize that the size, number, and positions of holes  618  are matters of design and it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use holes  618 . 
         [0065]    It should be noted that in some embodiments, as few as one hole  618  is required to achieve a desired performance for sensor  202 . 
         [0066]    In embodiments wherein substrate  602  comprises a through-hole, the area of layer  604  disposed over the access hole forms a second membrane that enables fluidic coupling between optically active membrane  614  and a larger volume cavity fluidically coupled with beam splitter  506 . Such an arrangement enables displacement sensor  504  to be designed with a greater range of time constants. 
         [0067]    In some embodiments, an anti-reflection layer for the light in optical beam  510  and reflected beam  514  is disposed on surface  620  of substrate  602 . 
         [0068]    At operation  302 , processor  406  generates output signal  210  based on pressure signals  412 - 1  through  412 - 6 . Since sensor  202  is much smaller than the wavelength of infrasound  208 , incident infrasound energy imparts substantially the same pressure on each pressure sensor  402  included in sensor  202 . In the absence of wind, therefore, pressure signals  412 - 1  through  412 - 6  are substantially equal. Further, as discussed below and with respect to  FIGS. 7 and 8 , wind flowing across sensor  202  induces a pressure gradient that affects pressure sensors  402  unequally. As a result, in some embodiments, output signal  210  is generated based upon the common-mode characteristics of pressure signals  412 - 1  through  412 - 6 . 
         [0069]    At operation  303 , sensor  202  generates a wind-compensation factor based on wind  206 . 
         [0070]      FIG. 7  depicts a model of pressure versus position for a stationary object subjected to wind. Plot  700  depicts a two-dimensional plot of the relative pressure changes produced around cylinder  702  by a wind blowing across the cylinder. Cylinder  702  is a stationary 2 cm-diameter cylinder positioned within a 20 m/s wind field that is blowing along direction  704  as shown. A wind speed of 20 m/s represents the typical maximum value experienced in most areas of the world. Cylinder  702  approximates sensor  202  as subjected to wind  206 . At 20 m/s, the maximum pressure change across sensor  202  due to the wind is approximately 50 mPa. 
         [0071]      FIG. 8  depicts the velocity distribution associated with wind incident on a stationary object. Plot  800  depicts the velocity distribution about cylinder  702  assuming Stokes flow and using no-slip boundary conditions. Since cylinder  702  is small relative to the coherence length of the wind vector fields, the simulation disregards the effect of turbulence. 
         [0072]    Plots  700  and  800  demonstrate that the effects of wind-induced pressure fluctuations are extremely predictable. In addition, since sensor  202  is much smaller than the wavelength of infrasound, incident infrasound energy imparts substantially identical pressure on each pressure sensor  402 . Differences between pressure signals  412 , therefore, can be attributed entirely to airflow rather than infrasound energy enabling the effects of wind-induced pressure fluctuations to be directly canceled. Further, using displacement sensors whose responsivities (1) are matched to within a few percent, (2) have high dynamic range as compared to displacement sensors of prior art infrasound sensors, and (3) are characterized by self-noise of 1 mPa or less, embodiments of the present invention can observe wind velocity at the same point that infrasound energy is monitored, obviating the need for widely spaced hose arrays. 
         [0073]    In order to generate a wind-compensation factor, processor  406  determines a pressure differential based on at least two of the plurality of pressure signals  412 . As evinced by plot  700 , wind across an object induces a well-characterized pressure distribution about the object. Specifically, in response to wind  206 , a high-pressure region develops on the windward side of sensor  202  while a low-pressure region develops on the leeward side of the sensor. As a result, pressure sensor  402 - 3  registers an increase in pressure while pressure sensor  402 - 4  registers a decrease in pressure. 
         [0074]    Using the difference between pressure signals  412 - 3  and  412 - 4 , processor  406  computes the wind-compensation factor for wind  206 . 
         [0075]    At operation  304 , processor  406  generates compensated output signal  210  based on pressure signals  412 - 1  through  412 - 6  and the computed wind-compensation factor. 
         [0076]    One skilled in the art will recognize that wind across sensor  202  that is not aligned with one of the x-, y-, or z-directions will induce pressure signal differentials across more than one of the pressure sensor pairs aligned with these directions. It will be clear to one skilled in the art, after reading this specification, how to calculate a wind-compensation factor for such cases. 
         [0077]      FIGS. 9A and 9B  depict top and cross-sectional views, respectively, of a beam splitter in accordance with an alternative embodiment of the present invention. In operation, acceleration sensor  600  is analogous to beam splitter  506 . 
         [0078]    Beam splitter  900  comprises substrates  602 - 1  and  602 - 2 , layers  604  and  902 , mirror  904 , and spacers  916 . Beam splitter  900  is analogous to beam splitter  506  described above and with respect  FIGS. 6A and 6B . 
         [0079]    Layer  902  is a layer of silicon-rich silicon nitride that has been etched to form through-hole  906 . Layer  902  is analogous to layer  604 . 
         [0080]    In some embodiments, layer  902  is a layer of metal that is stamped, cast, etched, or photo-etched to form through-hole  906  and holes  618 . In some embodiments, layer  902  comprises a central plate supported from a perimeter region by one or more tethers. In such embodiments, open regions that define the tethers are analogous to holes  618  and contribute to the control of airflow through layer  902 . 
         [0081]    Layer  902  is disposed on substrate  602 - 2 . Substrate  602 - 2  comprises through-hole  908 . As a result, a portion of layer  902  forms membrane  910 , which moves in response a pressure differential across its thickness. 
         [0082]    Mirror  904  is a block of material suitable for transmission of light contained in input light signal  510 . Although its dimensions are a matter of design choice, an exemplary mirror is a 1 mm by 1 mm square that has a thickness of 0.5 mm. Materials suitable for use in mirror  904  include, without limitation, soda-lime glass, borosilicate glass, fused silica, high-dielectric constant glasses, and the like. In some embodiments, surface  912  is coated to enhance its functionality in optically resonant cavity  914 . 
         [0083]    Mirror  904  is attached to membrane  910  such that the mirror is aligned with through-hole  906 . As a result, light transmitted by optically resonant cavity  914  passes through layer  902  without incurring further reflection or absorption. 
         [0084]    Spacers  916  precision spacers that interpose layers  604  and  902  when beam splitter  900  is assembled. Spacers  916 , in conjunction with the thickness of mirror  904 , define the value of cavity length, L, in the absence of wind and infrasound energy. In some embodiments, substrate  602 - 2  is not included and layer  610  is disposed on spacers  916 . 
         [0085]    In similar fashion to the operation of beam splitter  506 , beam splitter  900  receives optical beam  510  from source  512  and splits the optical energy of optical beam  510  into reflected beam  514  and transmitted beam  516 . The ratio of optical energy in reflected beam  514  and transmitted beam  516  is dependent upon the characteristics of optically resonant cavity  914 . Optically resonant cavity  914  is formed by surface  610  of layer  604  and surface  912  of mirror  904 , which are separated by cavity length L. 
         [0086]    It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.