Patent Application
20170365248
This invention relates to a diffractive acoustic device based on extraordinary acoustic transmission.
In various embodiments, the present invention comprises an acoustic lens or diffractive acoustic device, including but not limited to, a sub-wavelength thickness lens or diffuser, comprising an array of Helmholtz resonators (HRs). Perfect sound transmission through a rigid barrier occurs with an array of one or more HRs confined within a waveguide and oriented so that one neck protrudes onto each side of the barrier. Extraordinary acoustic transmission (EAT) occurs when radiation (such as EM or acoustic radiation) incident on an opaque barrier perforated with sub-wavelength holes is transmitted at a rate higher than expected based on the areal coverage fraction of the holes. In the present invention, the transmission is independent of the direction of sound on the barrier and the relative placement of the necks.
Acoustic lensing and diffractive acoustic devices can be created using the phase characteristics associated with the phenomenon of EAT. In EAT, sound incident on a perforated barrier can be nearly perfectly transmitted (i.e., greater than 97%) in a narrow frequency range even though the area of the perforations is less than 7% of the total barrier area. In one embodiment, the perforations on each side of the barrier comprise the neck openings of a two-neck HR whose volume is within the barrier. The high transmission occurs in a band of frequencies about the resonant frequency of the HR.
Coincident with the high transmission, the phase of the sound undergoes a smooth continuous change as a function of frequency of about p radians. The phase characteristics of EAT are used to create an acoustic lens that focuses sound or a diffractive acoustic element that steers the incident acoustic wave in any desired pattern. In several embodiments, such devices use a two-dimensional array of HRs in a barrier.
A lens or diffractive acoustic device of the present invention is designed to work at a specific target wavelength. The phase profile of an acoustic wavefront at this frequency can be modulated as a function of position across the barrier. This spatial modification of the phase of the transmitted wavefront is accomplished by adjusting the HR frequency at each position of the array either above or below the target wavelength frequency. As an example, for a lens the phase delay would be greatest at the center of the array and become progressively smaller away from the center. This arrangement is analogous to a converging optical lens where there is a larger phase delay for the light that goes through the center of the lens, where the glass is thicker, compared to the phase delay at the edges, where the glass is thinner.
Lensing can be achieved with a single HR element, or an array of multiple HRs. Resonators are tuned such that the phase delay is greatest at the center, and gets progressively smaller with distance from the center. Arrays of HRs can be polygonal, linear, or other configurations. In one embodiment, a simple lens is created from a linear array of 7 HRs each spaced by 0.1 m. The lens operates at about 900 Hz in air and the sound comes to a distinct focus at about 0.47 m from the linear array.
In various embodiments, diffractive acoustic elements may be created in a manner similar to the design of diffractive optics. For example, with the definition of a desired target sound distribution in the far field, the phase of each HR element in an array can be determined by an optimization technique, such as, but not limited to, simulated annealing. Once the desired phase values are set, the resonant frequency of each Helmholtz resonator can be adjusted accordingly. The process functions well because the technique permits a continuous variation in the phase in contrast to diffractive optics in which the phase variations are generally quantized (2-level, 4-level, etc.) by the limitations of nanofabrication.
While the above examples have been presented in the context of a single frequency, the invention can operate at a two or more well-separated discrete frequencies. Two or more HRs in parallel do not interfere with each other's operation as long as the frequency separation is sufficiently large. The transmission at each frequency is unaffected by the presence of the second resonator.
The acoustic lens and other devices presented herein may be used in areas such as, but not limited to, sonar and ultrasonics. Advantages of the present invention compared to other techniques are the high throughput and the sub-wavelength thickness of the lens. In contrast, lensing technology based on zone plates loses more than 50% of the incident sound due to reflection and lenses based on modifying the effective velocity using arrays of rods or spheres are all require a thickness greater than a wavelength. Further, the ability to modify the phase profile of the present invention has applications including diffusers for architectural acoustics or sonar applications or the creation of patterned acoustic beams for sonar and ultrasound.
In various exemplary embodiments, the present invention comprises an acoustic lens or diffractive acoustic device, including but not limited to, a sub-wavelength thickness lens or diffuser, comprising an array of Helmholtz resonators (HRs). Perfect or near-perfect sound transmission through a rigid barrier occurs with an array of one or more HRs confined within a waveguide and oriented so that one neck protrudes onto each side of the barrier. Extraordinary acoustic transmission (EAT) occurs when radiation (such as EM or acoustic radiation) incident on an opaque barrier perforated with sub-wavelength holes is transmitted at a rate higher than expected based on the areal coverage fraction of the holes. In the present invention, the transmission is independent of the direction of sound on the barrier and the relative placement of the necks.
Acoustic lensing and diffractive acoustic devices can be created using the phase characteristics associated with the phenomenon of EAT. In EAT, sound incident on a perforated barrier can be nearly perfectly transmitted (i.e., greater than 97%) in a narrow frequency range even though the area of the perforations is less than 7% of the total barrier area. In one embodiment, as seen in
Coincident with the high transmission, the phase of the sound undergoes a smooth continuous change as a function of frequency of about p radians, from a frequency just below the resonant frequency to one just above the resonant frequency. The transmission amplitude 18 and phase 20 as a function of sound frequency for an HR with a resonant frequency of 900 Hz is shown in
The phase characteristics of EAT are used to create an acoustic lens that focuses sound or a diffractive acoustic element that steers the incident acoustic wave in any desired pattern. In several embodiments, such devices use a one-dimensional line or two-dimensional array of HRs in a barrier, as seen in
In one embodiment, a lens or diffractive acoustic device of the present invention is designed to work at a specific target wavelength. The phase profile of an acoustic wavefront at this frequency can be modulated as a function of position across the barrier. This spatial modification of the phase of the transmitted wavefront is accomplished by adjusting the HR frequency at each position of the array either above or below the target wavelength frequency. As an example, for a lens the phase delay would be greatest at the center of the array and become progressively smaller away from the center. This arrangement is analogous to a converging optical lens where there is a larger phase delay for the light that goes through the center of the lens, where the glass is thicker, compared to the phase delay at the edges, where the glass is thinner.
An example of an array configuration (i.e., multiple HRs) to achieve lensing is shown in
Arrays of HRs can be polygonal, linear, or other configurations. A computer image of a simple lens created from a linear array 70 of seven HRs each spaced by 0.1 m is shown in
In various embodiments, diffractive acoustic elements may be created in a manner similar to the design of diffractive optics. For example, with the definition of a desired target sound distribution in the far field, the phase of each HR element in an array can be determined by an optimization technique, such as, but not limited to, simulated annealing. Once the desired phase values are set, the resonant frequency of each Helmholtz resonator can be adjusted accordingly. The process functions well because the technique permits a continuous variation in the phase in contrast to diffractive optics in which the phase variations are generally quantized (2-level, 4-level, etc.) by the limitations of nanofabrication.
While the above examples have been presented in the context of a single frequency, the invention can operate at a two or more well-separated discrete frequencies. Two or more HRs in parallel do not interfere with each other's operation as long as the frequency separation is sufficiently large.
The acoustic lens and other devices presented herein may be used in areas such as, but not limited to, sonar and ultrasonics. Advantages of the present invention compared to other techniques are the high throughput and the planar nature and sub-wavelength thickness of the lens or acoustic element. This is important for the creation of lenses for sonar signals, for example, which can have wavelengths of many meters. In contrast, lensing technology based on zone plates loses more than 50% of the incident sound due to reflection and lenses based on modifying the effective velocity using arrays of rods or spheres are all require a thickness greater than a wavelength.
Further, the ability to modify the phase profile of the present invention has applications including diffusers for architectural acoustics or sonar applications or the creation of patterned acoustic beams for sonar and ultrasound. Because the phase on an acoustic wave front can be continuously modified between 0 and p radians by a one- or two-dimensional array of HRs, it is possible to build diffractive acoustics devices that can form the acoustic wave into any desired far field pattern. A simple example would be a diffraction grating of alternating regions of 0 and p radian phase shift that sends sound in specific symmetrical diffracted directions. A more complicated example would be to funnel transmitted sound in a single particular direction. For example, a barrier beside a roadway could be designed to send sound up into the air to reduce noise in a neighborhood. A similar design might be used in an architectural setting to channel sound away from certain areas. This application is analogous to diffractive optic devices that can create light in any desired output pattern. A key difference here is that the acoustic HR device can create a continuous phase variation between 0 and p, whereas most diffractive optic devices are binary in nature being composed of only the two levels 0 and p.
Thus, it should be understood that the embodiments and examples described herein have been chosen and described in order to best illustrate the principles of the invention and its practical applications to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited for particular uses contemplated. Even though specific embodiments of this invention have been described, they are not to be taken as exhaustive. There are several variations that will be apparent to those skilled in the art.
This application is a continuation of U.S. patent application Ser. No. 14/739,993, filed Jun. 15, 2015, which claims benefit of and priority to U.S. Provisional Application No. 62/012,376, filed Jun. 15, 2014, by William M. Robertson, and is entitled to those filing dates for priority. The specifications, figures, appendices, and complete disclosures of U.S. Provisional Application No. 62/012,376 and U.S. patent application Ser. No. 14/739,993 are incorporated herein by specific reference for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 62012376 | Jun 2014 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14739993 | Jun 2015 | US |
| Child | 15584346 | US |
