Acoustic metamaterial

Abstract

A metamaterial comprising, a plurality of acoustic vector field sensors, each configured to sense an acoustic vector field of a fluid within a fluid-filled space in response to fluid waves, and producing an electrical signal corresponding to the sensed acoustic vector field; a processor configured to perform a time and space transform on the electrical signal; and at least one phased array transducer, configured to emit fluid waves according to a produced acoustic vector field pattern dependent on a result of the time and space transform, a within a portion of the fluid.

Claims

1. A metamaterial, comprising: a plurality of vector flow sensors, each of the vector flow sensors comprising at least one displaceable conductive sensing fiber and being configured to sense a vector flow of a gas due to primary acoustic waves within a first portion of the gas past the at least one displaceable conductive sensing fiber by quantifying a displacement of the at least one displaceable conductive sensing fiber due to the vector flow of the gas, wherein the at least one displaceable conductive sensing fiber is suspended in a magnetic field, and transduces the displacement to an electromagnetically induced voltage corresponding to the primary acoustic waves; a transducer array comprising a plurality of acoustic wave emitters, configured to emit secondary acoustic waves within a second portion of the gas, the first portion of the gas and the second portion of the gas being separated by an intervening physical structure therebetween that interferes with propagation of the primary acoustic waves surrounding the intervening physical structure; and an automated control comprising an automated transform processor, configured to: receive an electrical signal corresponding to the electromagnetically induced voltage responsive to the sensed primary acoustic waves from the plurality of vector flow sensors; perform a time and space transform having a metamaterial approximation transfer function on the electrical signal; determine a vector flow pattern of the primary acoustic waves; and control the transducer array in accordance with a result of the time and space transform having the metamaterial approximation transfer function, to emit the secondary acoustic waves which at least partially cancel the primary acoustic waves.

2. The metamaterial according to claim 1, wherein the transducer array has an emission pattern which does not directly emit the secondary acoustic waves toward the plurality of vector flow sensors.

3. The metamaterial according to claim 1, wherein the transducer array is configured to emit the secondary acoustic waves in an emission pattern which emits the secondary acoustic waves which are at least one of directly and indirectly sensed by at least one of plurality of vector flow sensors.

4. The metamaterial according to claim 1, wherein each of the plurality of vector flow sensors comprises the at least one displaceable conductive sensing fiber extending under tension between attachments, having a portion which is displaced due to the primary acoustic waves, the displacement being detected based on the electromagnetically induced voltage.

5. The metamaterial according to claim 1, wherein the at least one displaceable conductive sensing fiber comprises at least three displaceable conductive sensing fibers disposed along different axes, each of the at least three displaceable conductive sensing fibers being displaceable due to the primary acoustic waves, to thereby detect a three- dimensional acoustic wave vector of the primary acoustic waves.

6. The metamaterial according to claim 1, wherein the intervening physical structure comprises a core, and the transducer array is controlled to emulate a core which is transparent with respect to the primary acoustic waves.

7. The metamaterial according to claim 1, wherein: the plurality of vector flow sensors surround the intervening physical structure which interferes with propagation of the primary acoustic waves across the intervening physical structure; and the transducer array is controlled to emulate a negative index of refraction with respect to propagation of the primary acoustic waves across the intervening physical structure.

8. The metamaterial according to claim 1, wherein the metamaterial approximation transfer function emulates a negative index of refraction.

9. The metamaterial according to claim 1, wherein the at least one displaceable conductive sensing fiber comprises a conductive coating formed on a polymer fiber.

10. The metamaterial according to claim 1, wherein absent emission of the secondary acoustic waves, the metamaterial has externally observed inhomogeneous properties due to at least the physical intervening structure, and the time and space transform having the metamaterial approximation transfer function causes the metamaterial to have externally observed homogeneous acoustic properties.

11. A metamaterial method, comprising: providing a metamaterial comprising: a plurality of vector flow sensors, each of the vector flow sensors comprising at least one displaceable conductive sensing fiber suspended in a magnetic field, and being configured to sense a flow of a gas in response to primary acoustic waves past the at least one displaceable conductive sensing fiber by quantifying a displacement of the at least one displaceable conductive sensing fiber due to the flow of the gas, producing an electromagnetically induced voltage corresponding to the primary acoustic waves; a transducer array; and an intervening physical structure, disposed between the plurality of vector flow sensors and the transducer array, that interferes with propagation of the primary acoustic waves surrounding the intervening physical structure; the metamaterial method further comprising: receiving, by an automated transform processor, an electrical signal corresponding to the electromagnetically induced voltage responsive to the movement of the gas from the plurality of vector flow sensors; performing a time and space transform having an approximated metamaterial transfer function on the electrical signal with the automated transform processor, to determine a pattern of gas movement; and emitting secondary acoustic waves within a portion of the gas with the transducer array comprising a plurality of acoustic wave emitters, responsive to a result of the time and space transform having the approximated metamaterial transfer function from the automated transform processor, to at least partially cancel the primary acoustic waves proximate to the metamaterial with the emitted secondary waves.

12. The metamaterial method according to claim 11, wherein the transducer array comprises a phased array transducer having a commonly controlled emission pattern.

13. The metamaterial method according to claim 11, wherein the transducer array is controlled as a phase array to produce a controlled emission pattern, and the plurality of vector flow sensors receive feedback from the transducer array, wherein the time and space transform is responsive to the feedback.

14. The metamaterial method according to claim 11, wherein the intervening physical structure comprises a core which interferes with acoustic wave propagation in the gas surrounding the core; the plurality of vector flow sensors being arranged in an array proximate to the core to sense at least an axis of propagation of the primary acoustic waves; and the transducer array being disposed on at least an opposite side of the core with respect to the plurality of vector flow sensors; the metamaterial method further comprising driving the transducer array according to the time and space transform to emulate a core which is transparent with respect to the primary acoustic waves.

15. The metamaterial method according to claim 11, wherein the at least one displaceable conductive sensing fiber of each vector flow sensor comprises a conductive portion extending between attachments, further comprising displacing the conductive portion due to viscous drag from the primary acoustic waves, and a detecting the displacement of the conductive portion based on the electromagnetically induced voltage.

16. The metamaterial method according to claim 11, wherein the at least one displaceable conductive sensing fiber comprises at least three displaceable conductive sensing fibers disposed along different axes, further comprising displacing the at least three displaceable conductive sensing fibers due to the primary acoustic waves, and detecting a three-dimensional acoustic propagation vector based on the electromagnetically induced voltage produced by the displacement.

17. The metamaterial method according to claim 11, wherein the transducer array is provided around the physical intervening structure comprising a core that interacts with the primary acoustic waves, further comprising controlling the transducer array in dependence on the electrical signal, to emulate a core which is transparent with respect to the primary acoustic waves.

18. The metamaterial method according to claim 11, wherein the time and space transform comprises at least one of a Fast Fourier Transform, an Inverse Fourier Transform, and a wavelet transform.

19. The metamaterial method according to claim 11, wherein the at least one displaceable conductive sensing fiber comprises a conductive coating formed on a polymer fiber.

20. A non-transitory computer readable medium containing instructions for controlling an automated processor to implement an active metamaterial control system for controlling a metamaterial, the metamaterial comprising: a plurality of acoustic wave propagation vector flow sensors, each of the acoustic wave propagation vector flow sensors comprising a displaceable conductive sensing fiber suspended in a magnetic field, which are responsive to a viscous drag of a gas flow from a primary acoustic wave propagating in a gas on the respective displaceable conductive sensing fiber, configured to electromagnetically induce a voltage upon a displacement; a phased array transducer; and an intervening physical structure separating the plurality of acoustic wave propagation vector flow sensors and the phased array transducer, the intervening physical structure interfering with the propagation of the primary acoustic wave between the plurality of acoustic wave propagation vector flow sensors and the phased array transducer, the instructions comprising: instructions to receive electrical signals corresponding to the electromagnetically-induced voltage from the plurality of acoustic wave propagation vector flow sensors responsive to the primary acoustic wave; instructions to perform a time and space transform having an approximated metamaterial transfer function on the electrical signals; and instructions to control the phased array transducer to emit secondary acoustic waves within a portion of the gas, responsive to a result of the time and space transform having the approximated metamaterial transfer function, to at least partially cancel the primary acoustic wave.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows predicted and measured velocity of 6 μm diameter fibers driven by sound.

(2) FIG. 2 shows predicted and measured velocity of thin fibers driven by sound show that the fibers motion is very similar to that of the air over a very wide range of frequencies.

(3) FIG. 3 shows a simplified schematic of a packaging for the nanofiber microphone

(4) FIG. 4 shows that a nanofiber microphone achieves nearly ideal frequency response.

(5) FIG. 5 shows a prototype circuit board for a microphone design.

(6) FIG. 6 shows an analysis of the magnetic field surrounding the fibers due to magnets positioned adjacent to the circuit board of FIG. 5.

(7) FIG. 7 shows the predicted effect of the diameter of a thin fiber or wire on the response due to sound at its mid-point.

(8) FIG. 8 shows that, when the diameter of the fiber is reduced sufficiently, the response becomes nearly independent of frequency.

(9) FIG. 9 shows predicted and measured electrical sensitivity of a prototype microphone, for a 3.8 cm length 500 nm conductive spider silk fiber.

(10) FIG. 10 shows the measured velocity of thin fibers driven by sound show that the fibers motion is very similar to that of the air in the low frequency range 0.8 Hz to 100 Hz.

(11) FIG. 11 shows the measured open circuit voltage E over the air motion in the low frequency range 1-100 Hz.

(12) FIG. 12 shows the real and imaginary portions of the viscous force over a range of radii.

(13) FIG. 13 shows Predicted and measured silk velocity relative to the air particle velocity for silks (L=3.8 cm) of various diameters: 500 nm, 1.6 μm, 3 μm.

(14) FIG. 14 shows a relative direction of flow of the fluid medium with respect to the fiber.

(15) FIG. 15 shows a predicted directional response of the fiber to waves in the fluid medium, independent of frequency.

(16) FIGS. 16A and 16B show test configuration, and a directional response of a fiber to a 3 Hz infrasound wave in air.

(17) FIG. 17 shows a measured and predicted directivity of a single fiber as a sensor to 500 Hz vibrations.

(18) FIG. 18 shows a metamaterial system having a plurality of acoustic vector field sensors and a phased array transducer.

(19) FIG. 19 shows a flowchart of a method according to the present invention.

(20) FIG. 20 shows a sensor array and phased transducer array formed around a core.

DETAILED DESCRIPTION OF THE INVENTION

EXAMPLE 1

(21) In order to verify the results of the analytical model for an acoustic sensor, measurements were obtained of the response of a thin wire due to a plane wave sound field. Stainless steel fiber having a diameter of 6 μm was obtained from Blue Barn Fiber (Hayden, Id.) [72]. This is intended to be spun into yarn for clothing. The fiber is in the form of continuous strands having a length of several centimeters.

(22) A single strand of stainless steel fiber was soldered to two wires spanning a distance of 3 cm. The fiber was not straight, in this experiment, which may influence the ability to accurately predict its sound-induced motion. The fiber was placed in an anechoic chamber and subjected to broad-band sound covering the audible range of frequencies. The sound pressure was measured in the vicinity of the wire using a B&K 4138 ⅛th inch reference microphone. The sound source was 3 meters from the wire which resulted in a plane sound wave at frequencies above approximately 100 Hz. Knowing the sound pressure in pascals, one can easily estimate the fluctuating acoustic particle velocity through equation (2).

(23) FIG. 1 shows comparisons of measured results with those predicted using equation (14). The response is found to vary with frequency but the general behavior of the curves show qualitative agreement. Predicted results based on an infinitely long, unsupported fiber, obtained using equation (12),

(24) $\frac{V_I}{U}=\frac{C\left(\omega \right)+\widehat{\mathrm{.Math.}}\omega M\left(\omega \right)}{C\left(\omega \right)+\widehat{\mathrm{.Math.}}\omega \left(M\left(\omega \right)+{\rho }_m\pi r^2\right)}.$

(25) In this case, the general slope of the curve versus frequency is consistent with the measured results but the absence of wave reflections from the supports causes the response to not account for resonances in the fiber. It should be emphasized that it was not attempted to accurately account for the boundary conditions of this thin fiber, and effects due to its curvature were neglected. Nonuniform behavior of the response over frequencies is most likely due to wave reflections (i.e., resonances) in the wire.

(26) The general qualitative agreement between the measured and predicted results shown in FIG. 1 indicates that the analytical model described above provides a reasonable way to account for the dominant forces on and within the wire. Based on this, equation (14) is used to predict the effect of significantly reducing the fiber diameter. As discussed above, the viscous fluid forces are expected to dominate over all mechanical forces associated with the material properties of the wire when the diameter is reduced to a sufficient degree.

(27) The results of reducing the wire diameter on the predicted response to sound are shown in FIG. 7. The figure shows the amplitude (in decibels) of the wire velocity relative to that of the air in a plane sound wave field. As expected, when the wire diameter is reduced to less than 1 μm, (i.e., on the nanoscale), the nature of the response changes significantly and resonant behavior appears to be damped out by the viscous fluid. The frequency response of the wire is nearly flat up to 20 kHz when the diameter is reduced to 100 nm.

(28) FIG. 1 shows predicted and measured velocity of a 6 μm diameter fiber driven by sound.

(29) FIG. 2 shows predicted and measured velocity of thin fibers driven by sound show that the fibers motion is very similar to that of the air over a very wide range of frequencies. Results are shown for man-made (PMMA) fiber along with those obtained using spider silk. This previously unexplored method of sensing sound will lead to directional microphones with ideal, flat frequency response.

(30) FIG. 3 shows a simplified schematic of a packaging for the nanofiber microphone

(31) FIG. 4 shows that a prototype nanofiber microphone achieves nearly ideal frequency response. Measured electrical sensitivity is shown for two prototype fibers as the micro- phone output voltage relative to the velocity of the air in a plane-wave sound field. Measurements were performed in the anechoic chamber. One fiber consists of natural spider silk which has been coated with a conductive layer of gold. The other is a man-made fiber electrospun using PMMA and also coated with gold. A magnet was placed adjacent to each fiber and the open circuit output voltage across the fibers were detected using a low noise SRS SR560 preamplifier. Each has a diameter of approximately 0.5 μm. The length of spider silk and PMMA is about 3 cm, and B is about 0.35 T based on a finite element model of the magnetic field shown in FIG. 4. This gives BL≈0.01 volts/(m/s), in close agreement to that shown here.

(32) An experimental examination of the effect of reducing the fiber diameter was conducted using PMMA fiber that is approximately 600 nm in diameter and 3 mm long. It thus is about one tenth the size of the steel wire discussed supra. The Young's modulus has been estimated to be approximately 2.8×10 N/m.sup.2 and the density is approximately 1200 kg/m.sup.3. The results are shown in FIG. 8 along with those shown in FIG. 1 for comparison. FIG. 8 also shows predicted results for this PMMA fiber based on equation (14). FIG. 8 shows that equation (14) accurately predicts that this factor of 10 reduction in fiber diameter results in nearly ideal flat response as a function of frequency.

(33) The results indicate that a wire that is sufficiently thin can behave as a nearly ideal sound sensor since it moves with nearly the same velocity as the air over the entire audible range of frequencies. It should therefore be possible to employ this wire in a transducer to obtain an electronic voltage that is in proportion to the sound pressure or velocity.

(34) FIG. 7 shows the predicted effect of the diameter of a thin fiber or wire on the response due to sound at its mid-point (x=L/2). The wire is assumed to be 3 cm long and have a diameter of 6 μm. The material properties are chosen to represent stainless steel.

(35) FIG. 8 shows that, when the diameter of the fiber is reduced sufficiently, the response becomes nearly independent of frequency. Measured and predicted results are shown for a PMMA fiber having a diameter of approximately 800 nm and length 3 mm. The results of FIGS. 1A and 1B are also shown for comparison.

(36) FIG. 9 shows predicted and measured electrical sensitivity of a prototype microphone which employs a 3.8 cm length of conductive, 500 nm diameter spider silk fiber. The predicted results were obtained by computing the velocity of the fiber averaged over its length and multiplying this result by the estimated BL product of BL≈0.0063 volts-seconds/meter. For the fiber of length 3.8 cm, this corresponds to a magnetic flux density of B≈0.2 Teslas (estimate for the neodymium magnet used in this experiment). No attempt was made to optimize the placement of the wire to maximize the magnetic flux density. The wire is attached to two supporting wires, which are then taped to the neodymium magnet. The measured results show qualitative agreement with the predictions up to a frequency of about 2 kHz. Above this frequency the noise in the measured signal dominates.

(37) FIG. 10 shows the measured velocity of thin fibers driven by sound show that the fibers motion is very similar to that of the air in the low frequency range 0.8 Hz to 100 Hz.

(38) FIG. 11 shows results of an experiment seeking to determine low frequency transduction of fiber motion. FIG. 11 shows which shows the open circuit voltage E over the air motion U, is about B×L: E/U=BL=0.35 T×0.038 m.

(39) An extremely convenient method of converting the wire's velocity into a voltage is to employ Faraday's law, in which the open circuit voltage across a conductor is proportional to its velocity relative to a magnetic field. The conductor should, ideally, be oriented orthogonally to the magnetic field lines as should the conductor's velocity vector.

(40) To examine the feasibility of detecting sound, a fine wire was supported on a neodymium magnet, which creates a strong field in the vicinity of the wire. If the magnetic flux density B of the field orthogonal to the wire is assumed to be reasonably constant along the wire length L, Faraday's law may be expressed as V.sub.o=BLV (equation (15)).

(41) Each end of the wire was input into a low noise preamplifier while the wire was subjected to a plane sound wave within the anechoic chamber. A Bruel & Kjaer 4138 ⅛th inch microphone sampled the sound field in the vicinity of the wire. FIG. 9 shows the measured transfer function between the measured output voltage and the incident sound pressure as a function of frequency. The figure also shows the predicted voltage output assuming a BL product of BL≈0.0063 volts-seconds/meter. The predicted voltage output was computed using equation (15) where V is the average wire velocity as a function of position along its length.

(42) Because the overall sensitivity of the microphone (in volts/pascal) will be proportional to the BL product in equation (15), this product is an important parameter, along with selecting a suitably diminutive diameter of the fiber. This product is typically made as large as is feasible. Neodymium magnets are available that can create a flux density of B≈1 Tesla. This leaves the choice of L, the overall length of the fiber.

(43) To estimate the BL product that would be appropriate for the microphone design, it is helpful to cast equation (15) in the form of the predicted overall sensitivity in volts/pascal. To do this, assume that the goal is to detect a plane sound wave in which the relationship between the pressure and acoustic particle velocity is P/V=ρ.sub.0c≈415 pascal×sec/meter, where ρ.sub.0 is the nominal air density and c is the speed of sound wave propagation. The acoustic sensitivity is V.sub.o/P=BL/ρ.sub.0c volts/pascal. Assume that input-referred noise spectrum level of the amplifier is approximately 10 nV/√Hz (value for current low-noise operational amplifiers), and a goal for the sound input-referred noise floor is 30 dBA (typical value for current high-performance hearing aid microphones); this noise floor corresponds to a pressure spectrum level (actually the square root of the power spectral density) of approximately 10.sup.−5 pascals/√Hz. Knowing the noise floor of the electronic interface of 10 nV/√Hz, and the acoustic noise floor target of 10.sup.−5 pascals/√Hz enables us to estimate the required sensitivity so that sound at the minimum sound level can be detected, H.sub.PV is shown by equation (17). Assume that a magnetic flux density of B=1 Tesla can be achieved, then the effective length of conductor that is required can be estimated,

(44) 0$\begin{array}{cc}L\approx {10}^{-3}\frac{{\rho }_{0^C}}{B}\approx 0.415m.& \left(24\right)\end{array}$

(45) If this length of conductor can be incorporated into a design, the microphone could achieve a noise floor of 30 dBA, based on the assumed electronic noise. Of course, the conductor must be arranged in the form of a coil as in common electrodynamic microphones. A proposed design approach to realize is discussed below.

(46) FIG. 5 shows a prototype circuit board for a microphone design.

(47) FIG. 6 shows an analysis of the magnetic field surrounding the fibers due to magnets positioned adjacent to the circuit board of FIG. 5, indicated a value of B≈0.3 Teslas.

(48) According to the design shown in FIG. 5, a set of parallel fibers are suspended in a space which is subject to acoustic wave vibrations. The fibers, though physically in parallel, are wired in series to provide an increased output voltage, and a constrained area or volume of measurement. Each strand may be 1-5 cm long, e.g., 3 cm long, and the total length may be, e.g., >0.4 meters. The entire array is subject to an external magnetic field, which is typically uniform across all fibers, but this is a preference and not a critical constraint. As shown in FIG. 6, the magnetic field is, e.g., 0.3 Teslas. Because the outputs of the various fibers is averaged, various mechanical configurations may be provided to impose known constraints. For example, sets of fibers may be respectively out of phase with respect to a certain type of sound source, and therefore be cancelling (differential). Similarly, directional and phased arrays may be provided. Note that each fiber has a peak response with respect to waves in the surrounding fluid that have a component normal to the axis of the fiber. The fibers may assume any axis, and therefore three dimensional (x, y, z) sensing is supported. It is further noted that the fibers need not be supported under tension, and therefore may be non-linear. Of course, if they are not tensioned, they may not be self-supporting. However, various techniques are available to suspend a thin fiber between two electrodes that is not tensioned alone an axis between the electrodes, without uncontrolled drooping.

(49) For example, a spider web type structure provides an array of thin fibers, which may be planar or three dimensional. Indeed, a spider web or silkworm may be modified to provide sufficient conductivity to be useful as a sensor. A natural spider silk from a large spider is about 2.5-4 μm in diameter, and thus larger than the 600 nm PMMA fiber discussed above. However, small spiders produce a silk less than 1 μm in diameters, e.g., 700 nm, and a baby spider may produce a silk having a diameter of less than 500 nm. Silkworms produce a fiber that is 5-10 μm in diameter.

(50) As shown in FIG. 5, the desired coil configuration may be achieved through circuit-board wiring of electrodes, wherein the fibers are themselves all linear and parallel (at least in groups).

(51) As discussed herein, the conductor length L to be comprised of a number of short segments that are supported on rigid conducting boundaries. The segments will be connected together in series in order to achieve the total desired length L. It is likely infeasible to construct a single strand of nanoscale conductor that is of sufficient length for this application, so assembling the conductor in relatively short segments is much more practical than relying on a single strand in a coil.

(52) By fashioning the conductor length as the series connection of short segments, it is also possible to control the static stiffness of the fiber. Since the purpose is to detect air velocity at audible frequencies, it is beneficial to attenuate the response due to very low frequency air fluctuations. This can be achieved by selecting the length of individual fiber segments to be small enough to set the lowest natural frequency, which may be obtained from equation (9).

(53) It is reasonable to set the lowest natural frequency, f.sub.l to be between 10 Hz and 20 Hz.

(54) Having selected appropriate material properties (such as Young's modulus E and density ρ), one may solve equation (9) for the desired length of each segment L with ω.sub.i=2πf.sub.l.

EXAMPLE 2

(55) In some applications, an infrasonic sensor is desired, with a frequency response f.sub.l that extends to an arbitrarily low frequency, such as a tenth of hundredth of a Hertz. Such a sensor might be useful for detecting fluid flows associated with movement of objects, acoustic impulses, and the like. Such an application works according to the same principles as the sonic sensor applications, though the length of individual runs of fibers might have to be greater.

(56) In addition, the voltage response of the electrode output to movements is proportional to the velocity of the fiber, and therefore one would typically expect that the velocity of movement of fluid particles at infrasonic frequencies would low, leading to low output voltages. However, in some applications, the fluid movement is macroscopic, and therefore velocities may be appreciable. For example, in wake detection applications, the amplitude may be quite robust.

(57) Generally, low frequency sound is detected by sensors which are sensitive to pressure such as infrasound microphones and microbarometers. As pressure is a scaler, multiple sensors should be used to identify the source location. Meanwhile, due to the long wave length of low frequency sound, multiple sensors have to be aligned far away to distinguish the pressure difference so as to identify the source location. As velocity is a vector, sensing sound flow can be beneficial to source localization. There is no available flow sensor that can detect infrasound flow in a broad frequency range with a flat frequency response currently. However, as discussed above, thin fibers can follow the medium (air, water) movement with high velocity transfer ratio (approximate to 1 when the fiber diameter is in the range of nanoscale), from zero Hertz to tens of thousands Hertz. If a fiber is thin enough, it can follow the medium (air, water) movement nearly exactly. This provides an approach to detect low frequency sound flow directly and effectively, with flat frequency response in a broad frequency range. This provides an approach to detect low frequency sound flow directly. The fiber motion due to the medium flow can be transduced by various principles such as electrodynamic sensing of the movement of a conductive fiber within a magnetic field, capacitive sensing, optical sensing and so on. Application example based on electromagnetic transduction is given. It can detect sound flow with flat frequency response in a broad frequency range.

(58) For the infrasound detection, this can be used to detect manmade and natural events such as nuclear explosion, volcanic explosion, severe storm, chemical explosion. For the source localization and identification, the fiber flow sensor can be applied to form a ranging system and noise control to find and identify the low frequency source. For the low frequency flow sensing, this can also be used to detect air flow distribution in buildings and transportations such as airplanes, land vehicles, and seafaring vessels.

(59) The infrasound pressure sensors are sensitive to various environmental parameters such as pressure, temperature, moisture. Limited by the diaphragm of the pressure sensor, there is resonance. The fiber flow sensor avoids the key mentioned disadvantages above. The advantages include, for example: Sensing sound flow has inherent benefit to applications which require direction information, such as source localization. The fiber flow sensor is much cheaper to manufacture than the sound pressure sensor. Mechanically, the fiber can follow the medium movement exactly in a broad frequency range, from infrasound to ultrasound. If the fiber movement is transduced to the electric signal proportionally, for example using electromagnetic transduction, the flow sensor will have a flat frequency response in a broad frequency range. As the flow sensor is not sensitive to the pressure, it has a large dynamic range. As the fiber motion is not sensitive to temperature, the sensor is robust to temperature variation. The fiber flow sensor is not sensitive to moisture. The size of the flow sensor is small (though parallel arrays of fibers may consume volume). The fiber flow sensor can respond to the infrasound instantly.

(60) Note that a flow sensor is, or would be, sensitive to wind. The sensor may also respond to inertial perturbances. For example, the pressure in the space will be responsive to acceleration of the frame. This will cause bulk fluid flows of a compressible fluid (e.g., a gas), resulting in signal output due to motion of the sensor, even without external waves. This can be advantages and disadvantages depends on the detailed applications. For example, it can be used to detect flow distribution in the buildings. If used to detect infrasound, the wind influence be overcome by using an effective wind noise reduction approach.

EXAMPLE 3

(61) To intuitively illustrate the transverse motion of spider silk due to fluctuating airflow in the direction perpendicular to its long axis, sound is recorded from the silk motion. The complex airborne acoustic signal used here contains low frequency (100 Hz-700 Hz) wing beat of insects and high frequency (2 kHz-10 kHz) song of birds. Spider dragline silk with diameter d=500 nm was collected from a female spiderling Araneus diadematus (body length of the spider is about 3 mm). A strand of spider silk (length L=8 mm) is supported at its two ends slackly, and placed perpendicularly to the flow field. The airflow field is prepared by playing sound using loudspeakers. A plane sound wave is generated at the location of the spider silk by placing the loudspeakers far away (3 meters) from the silk in our anechoic chamber. The silk motion is measured using a laser vibrometer (Polytec OFV-534).

(62) While the geometric forms (cob-web, orb-web, and single strand), size and tension of the spider silk shape the ultimate time and frequency responses, this intrinsic aerodynamic property of silk to represent the motion of the medium suggests that it can provide the acoustic information propagated through air to spiders. This may allow them to detect and discriminate potential nearby prey and predators [89, 90], which is different from the well-known substrate-borne information transmission induced by animals making direct contact with the silk [91-94].

(63) Knowing that the spider silk can capture the broadband fluctuating airflow, its frequency and time response is characterized at the middle of a silk strand. Three loudspeakers of different bandwidths were used to generate broadband fluctuating airflow from 1Hz to 50000 Hz. Note that the amplitude of air particle deflections X at low frequencies are much larger than those at high frequencies for the same air particle velocity V (X=V/ω, where ω=2πf, f is the frequency of the fluctuating airflow, and V is the velocity amplitude). A long (L=3.8 cm) and loose spider silk strand was used to avoid possible nonlinear stretching when the deflection is relatively large at very low frequencies. The nanodimensional spider silk can follow the airflow with maximum physical efficiency (V.sub.hair/V.sub.air≈1) in the measured frequency range from 1 Hz to 50 kHz, with a corresponding velocity and displacement amplitude of the flow field of 0.83 mm/s and 13.2 nm, respectively. This shows that the silk motion accurately tracks the air velocity at the initial transient as well as when the motion becomes periodic in the steady-state. The 500 nm spider silk can thus follow the medium flow with high temporal and amplitude resolution.

(64) The motion of a 500 nm silk strand (L=8 mm) is characterized at various locations along its length. Although the fixed ends of the silk cannot move with air, over most of the length, the silk motion closely resembles that of the airflow over a broad frequency range.

(65) If the silk and the surrounding medium to behave as a continuum, a model for the silk motion can be expressed in the form of a simple partial differential equation. This simple approximate analytical model is presented in Equation (25) to examine the dominant forces and response of a thin fiber in the sound field.

(66) $\begin{array}{cc}\mathrm{EI}\frac{{\partial }^{4}w\left(x,t\right)}{\partial x^4}+\rho A\frac{{\partial }^{2}w\left(x,t\right)}{\partial t^2}=C{v}_r\left(t\right)+M\frac{d{v}_r\left(t\right)}{dt}& \left(25\right)\end{array}$

(67) The left term gives the mechanical force due to bending of the fiber per unit length, where E is Young's Modulus of elasticity, I=πd.sup.4/64 is the area moment of inertia, w(x, t) is the fiber transverse displacement, which depends on both position, x, and time, t. The second term on the left accounts for the inertia of the fiber where ρ is volume density, and A=πd.sup.2/4 is the cross section area. The right term estimates the viscous force due to the relative motion of the fiber and the surrounding fluid. C and M are damping and added mass per unit length which, for a continuum fluid, were determined by Stokes (50). v.sub.r(t)=v.sub.air(t)−v.sub.silk(t) is the relative velocity between the air movement and fiber motion.

(68) It should be noted that the first term on the left side of Equation (25) accounts for the fact that thin fibers will surely bend as they are acted on by a flowing medium. This differs from previous studies of the flow-induced motion of thin hairs which assume that the hair moves as a rigid rod supported by a torsional spring at the base [1, 2, 82, 84, 85]. The motion of a rigid hair can be described by a single coordinate such as the angle of rotation about the pivot. In our case, the deflection depends on a continuous variable, x, describing the location along the length. Equation (25) is then a partial differential equation unlike the ordinary differential equation used when the hair does not bend or flex.

(69) It is evident that the terms on the left side of Equation (25) are proportional to either d.sup.4 or d.sup.2. The dependence on the diameter d of the terms on the right side of this equation is more difficult to calculate owing to the complex mechanics of fluid forces. It can be shown, however, that these fluid forces tend to depend on the surface area of the fiber, which is proportional to its circumference πd. As d becomes sufficiently small, the terms proportional to C and M will clearly dominate over those on the left side of Equation (25). For sufficiently small values of the diameter d, the governing equation of motion of the fiber becomes approximately:

(70) $\begin{array}{cc}0\approx C{v}_r\left(t\right)+M\frac{d{V}_r\left(t\right)}{dt}& \left(26\right)\end{array}$

(71) For small values of d, Equation (25) is then dominated by terms that are proportional to v.sub.r(t), the relative motion between the solid fiber and the medium. Since v.sub.r(t)=v.sub.air(t)−v.sub.silk(t), the solution of Equation (26) may be approximated by v.sub.air(t)≈v.sub.silk(t). According to this highly simplified continuum view of the medium, the fiber will thus move with the medium fluid instantaneously and with the same amplitude if the fiber is sufficiently thin.

(72) To examine the validity of the approximate analysis above, the velocity response of dragline silks (L=3.8 cm) from female orb-weaver spiders Araneus diadematus having various diameters: 0.5 μm, 1.6 μm, 3 μM were measured at the middle position. Predictions are obtained by solving Equation (25).

(73) FIG. 13 shows predicted and measured velocity transfer functions of silks using the air particle velocity as the reference. Predictions are obtained by solving Equation (26). In the prediction model, Young's modulus E and volume density ρ are 10 Gpa [96] and 1,300 kg/m.sup.3 [97], respectively. The measured responses of the silks are in close agreement with the predicted results. While all three of the measured silks can follow the air motion in a broad frequency range, the thinnest silk can follow air motion closely (V.sub.silk/V.sub.air˜1) at extremely high frequencies up to 50 kHz. These results suggest that when a fiber is sufficiently thin (diameter in nanodimensional scale), the fiber motion can be dominated by forces associated with the surrounding medium, causing the fiber to represent the air particle motion accurately. Over a wide range of frequencies, the fiber motion becomes independent of its material and geometric properties when it is sufficiently thin.

(74) The fiber motion can be transduced to an electric signal using various methods depending on various application purposes. Because the fiber curvature is substantial near each fixed end, sensing bending strain can be a promising approach. If nanowires are stacked into a rigid 3-dimension nanolattice [97], capacitive transduction is also possible. When sensing steady or slowly changing flows for applications such as controlled microfluidics, the transduction of fiber displacement may be preferred over velocity. Having an electric output that is proportional to the velocity of the silk is advantageous when detecting broadband flow fluctuations such as sound. While the detailed transduction approach (for example piezoresistive, piezoelectric, capacitive, magnetic, and optical sensing) may be different depending on the applications, the fact that the fiber motion is nearly identical to that of the medium in its vicinity will always prove beneficial. Advances in nanotechnology make the flow sensor fabrication possible [97-99].

(75) In an electromagnetic induction embodiment, the motion of the fiber is transduced to an open circuit voltage output E directly based on Faraday's Law, E=BLV.sub.fiber, where B is the magnetic flux density, and L is the fiber length. To examine the feasibility of this approach, a 3.8 cm long loose spider silk with a 500-nm diameter is coated with an 80 nm thick gold layer using electron beam evaporation to obtain a free-standing conductive nanofiber. The conductive fiber is aligned in a magnetic field with flux density B=0.35 T. The orientation of the fiber axis, the motion of the fiber, and the magnetic flux density, are all approximately orthogonal. Because the fiber accurately follows the airflow (V.sub.fiber/V.sub.air≈1) over most of the length, and the fiber motion is transduced linearly to a voltage signal, E/V.sub.air is approximately equal to the product of B and L in the measured frequency range 1 Hz-10 kHz. The open circuit voltage across the silk is detected using a low-noise preamplifier SRS Model SR560.

(76) This provides a directional, passive and miniaturized approach to detect broadband fluctuating airflow with excellent fidelity and high resolution. This device and technology may be incorporated in a system for passive sound source localization, even for infrasound monitoring and localization despite its small size. The sensor is sensitive to the flow direction with relationship e(t)=e.sub.0(t)cos(θ), where e.sub.0(t) is the voltage output when the flow is perpendicular to the fiber direction (θ=0°). As infrasound waves have large wavelength λ (λ=c/f, c is speed of sound), at least two pressure sensors should normally be used and placed at large separation distances (on the order of m to km) in order to determine the wave direction. Since velocity is a vector, in contrast to the scalar pressure, flow sensing inherently contains the directional information. This is very beneficial if one wishes to localize a source. The device can also work as a nanogenerator to harvest broadband flow energy with high power density [100]. For a conductive fiber (of length L, cross section area A, volume V=LA, resistivity ρ.sub.e, velocity amplitude V), the maximum generated voltage E.sub.0=BLV, the fiber resistance R=ρ.sub.eL/A, the maximum short circuit power per unit volume can be expressed as P/V=B.sup.2V.sup.2/ρ.sub.e. If B=1 T, V=1 cm/s, p.sub.e=2.44×10.sup.−8 Ω.Math.m, then P/V is 4.1 mW/cm.sup.3.

(77) The results presented here offer a simple, low-cost alternative to methods for measuring fluctuating flows that require seeding the fluid with flow tracer particles such as laser Doppler velocimetry (LDV) or particle image velocimetry (PIV). While good fidelity can be obtained by careful choice of tracer particles [101], these methods employ rather complicated optical systems to track the tracer particle motions. However, according to the present technology, a velocity-dependent voltage is obtained using simple electrodynamic transduction by measuring the open-circuit voltage between the two ends of the fiber when it is in the presence of a magnetic field.

(78) The motion of a fiber having a diameter at the nanodimensional scale can closely resemble that of the flow of the surrounding air, providing an accurate and simple approach to detect complicated airflow. This is a result of the dominance of applied forces from the surrounding medium over internal forces of the fiber such as those associated with bending and inertia at these small diameters. This study was inspired by numerous examples of acoustic flow sensing by animals [1, 2, 82, 83]. The results indicate that this biomimetic device responds to subtle air motion over a broader range of frequencies than has been observed in natural flow sensors. The miniature fiber-based approach of flow sensing has potential applications in various disciplines which have been pursuing precise flow measurement and control in various mediums (air, gas, liquid) and situations (from steady flow to highly fluctuating flow).

(79) All measurements were performed in the anechoic chamber at Binghamton University. The fluctuating airflow was created using loudspeakers. In order to obtain measurements over the broad frequency range examined, three different experimental setups were employed, each designed to cover a portion of the frequency range. The fluctuating airflow from 100 Hz to 50 kHz near the silk is determined using a measure of the spatial gradient of the pressure, ∂p(x, t)/∂x [102]. Knowing the sound pressure gradient, the acoustic particle velocity, v.sub.a(x, t), is calculated using Euler's equation: −∂p(x, t)/∂x=ρ.sub.0∂v.sub.a(x, t)/∂t, where ρ.sub.0 is the air density. The pressure is measured using a calibrated reference microphone.

(80) In the prototype typical transducer configuration, the orientation of the fiber axis, and the magnetic flux density, are orthogonal. Suppose θ is the angle between the flow direction and the fiber direction, as shown FIG. 14, the sensor has the maximum response e.sub.0(t), when the flow direction is perpendicular to the fiber direction, e.sub.0(t)=BLv(t).

(81) The sensor is sensitive to the flow direction with relationship, e.sub.0(t)=e.sub.0(t) cos(θ). A single sensor is expected to have a bi-directional (figure-of-eight) directivity. The directional response is independent of frequency. The predicted directional response is shown in FIG. 15.

(82) This suggests it could be incorporated in a system for passive flow source localization, even for infrasound monitoring and localization despite its small size. FIG. 16A shows a schematic test setup, and FIG. 16B shows the directional sensor response to a 3 Hz infrasound flow with wavelength about 114 m. As infrasound waves have large wavelength λ, λ=c/f, at least two pressure sensors should normally be used and placed at large separation distances (on the order of m to km) in order to determine the wave direction. Since velocity is a vector, in contrast to the scalar pressure, flow sensing inherently contains the directional information. This is very beneficial if one wishes to localize a source.

(83) The measured directivity of a single sensor at 500 Hz audible sound is shown in FIG. 17. The measured directivity matches well with the predicted directivity.

(84) The sound pressure near the silk is measured using the calibrated probe microphone (B&K type 4182). The measured microphone signal is amplified by a B&K type 5935L amplifier and then filtered using a high-pass filter at 30 Hz. To measure the frequency response of the spider silk in the frequency range of 1-100 Hz, a maximum length sequence signal having frequency components over the range of 0-50,000 Hz was employed. The signal sent to the subwoofer (Tang Band W6-1139SIF) was filtered using a low-pass filter (Frequency Devices 9002) at 100 Hz, and amplified using a Techron 5530 power supply amplifier. To measure the silk frequency response in the range of 100 Hz-3 kHz, the signal sent to the subwoofer (Coustic HT612) was filtered using a low-pass filter (Frequency Devices 9002) at 3 kHz, and amplify it using a Techron 5530 power supply amplifier. To measure the silk frequency response at 3-50 kHz, the signal sent to the supertweeter was filtered using a high-pass filter (KrohnHite model 3550) at 3 kHz, and amplified it using a Crown D-75 amplifier. The standard reference sound pressure for the calculation of the sound pressure level is 20 μPa. For the measurement of the open-circuit voltage E of the conductive fiber, the signal is amplified by a low-noise preamplifier, SRS model SR560. All of the data are acquired by an NI PXI-1033 data acquisition system.

(85) FIG. 18 shows a metamaterial system having a plurality of acoustic vector field sensors, each configured to sense an acoustic vector field of a fluid within a fluid-filled space in response to fluid waves, and producing an electrical signal corresponding to the sensed acoustic vector field. A processor is provided, configured to perform a time and space transform on the electrical signal and to produce a corresponding output. The output communicates a control signal for at least one phased array transducer, which is defined to cause the at least one phased array transducer to emit fluid waves according to a defined acoustic vector field pattern, dependent on a result of the time and space transform, within a portion of the fluid.

(86) FIG. 19 shows a flowchart of a metamaterial method, comprising: providing a plurality of acoustic vector field sensors, each configured to sense an acoustic vector field of a fluid within a fluid-filled space in response to fluid waves, and producing an electrical signal corresponding to the sensed acoustic vector field; receiving the electrical signal responsive to the acoustic vector field of the fluid within the fluid-filled space by the plurality of acoustic vector field sensors; performing a time and space transform on the electrical signal with an automated transform processor, e.g., a metamaterial transfer function having a negative index; and generating an output, which communicates a control signal for at least one phased array transducer, the control signal being defined to cause the at least one phased array transducer to emit fluid waves having an acoustic vector field pattern dependent on at least a result of the time and space transform within a portion of the fluid.

(87) FIG. 20 shows system having a plurality of acoustic vector field sensors surrounding a core which interferes with fluid wave propagation in the fluid surrounding the core. A plurality of acoustic vector field sensors are arranged in an array around the core, to sense at least an axis of propagation of the fluid wave. At least a portion of the at least one phased array transducer is disposed on at least an opposite side of the core from at least a portion of the plurality of acoustic vector field sensors. The at least one phased array transducer is driven to at least compensate for an interaction of the core with respect to the fluid waves.

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(198) It is understood that this broad invention is not limited to the embodiments discussed herein, but rather is composed of the various combinations, subcombinations and permutations thereof of the elements disclosed herein, including aspects disclosed within the incorporated references. The invention is limited only by the following claims. Each claim is combinable with each other claim unless expressly inconsistent.