Electrical audio signal processing systems and devices – Directive circuits for microphones
Reexamination Certificate
1996-05-17
2001-03-20
Chang, Vivian (Department: 2644)
Electrical audio signal processing systems and devices
Directive circuits for microphones
C343S893000, C367S905000
Reexamination Certificate
active
06205224
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to planar arrays having broad frequency range applications for source location, source imaging or target illumination with projected beams. Prior attempts to address planar array design where the number of array elements is restricted focus on single frequency application, don't address the issue of circular symmetry, and/or are for far-field application and thus do not comprehensively address near-field, circularly symmetric, and broad band application for source mapping or target illumination with projected beams.
Regular arrays are known in the state of the art whereby array elements are placed in a periodic arrangement such as a square, triangle, or hexagonal grid. In these arrangements, adjacent elements are required to be spaced within one-half wavelength of each other to prevent the array pattern from having multiple mainlobes in other than the steered direction, a phenomenon commonly referred to as spatial aliasing or grating lobes. This half-wavelength requirement can be cost prohibitive from the standpoint of the number of array elements required in broad frequency range applications because the lowest frequency for intended use drives the array aperture size larger (to achieve adequate array resolution), while the highest frequency drives the element spacing smaller (to avoid spatial aliasing).
Irregular arrays are known in the state of the art for providing a way to address grating lobe problems inherent in regular arrays because irregular arrays eliminate periodicities in the element locations. Random arrays are known in the state of the art as one form of irregular array. Random arrays are limited in ability to predictably control worst case sidelobes. When array element location can be controlled, an algorithm may be used to determine element placement that will guarantee irregular spacing and allow for more predictable control of worst case sidelobes. Prior art contains many examples of irregularly spaced linear arrays many of which are non-redundant, that is, no spacing between any given pair of elements is repeated. Non-redundancy provides a degree of optimality in array design with respect to controlling grating lobes.
Prior art for designing irregular planar arrays is largely ad-hoc. Only a few simple examples of non-redundant planar arrays—where there is either a relatively small number of elements or a simplistic element distribution such as around the perimeter of a circle—appear to exist in prior art. Prior art appears void of non-redundant planar array design techniques for locating an arbitrary number of elements distributed throughout the array aperture (as opposed to just around the perimeter) in a controlled manner to ensure non-redundancy and circular symmetry.
It is one object of the present invention to provide a planar array design substantially absent of grating lobes across a broad range of frequencies where the available number of elements is substantially less than that required to construct a regular (i.e., equally spaced element) array with inter-element spacing meeting the half-wavelength criteria typically required to avoid grating lobe contamination in source maps or projected beams.
Another objective of the present invention is to provide a planar array design that provides circular symmetry so that the source map resolution or projected beamwidth is not substantially array-dimension (i.e., azimuthal angle) dependent.
A further object of the invention is to provide a planar array design that makes optimal use of a fixed number of array elements in the sense that the array is non-redundant.
Still another object of the invention is to provide space density tapering flexibility in the array design to allow for trade-offs in the array design between array beamwidth and sidelobe levels.
Yet another object of the present invention is to provide a general method for distributing an arbitrary number of elements on an arbitrary diameter circular planar aperture in a manner that guarantees circular symmetry and non-redundancy in the spatial sampling space.
SUMMARY OF THE INVENTION
A planar array of sensing or transmitting elements (e.g., microphones or antennas) spaced on a variety of arc lengths and radii along a set of identical logarithmic spirals, where members of the set of spirals are uniformly spaced in angle about an origin point, having lower worst-case sidelobes and better grating lobe reduction across a broad range of frequencies than arrays with uniformly distributed elements (e.g., square or rectangular grid) or random arrays. The array is circularly symmetric and when there are an odd number of spirals, the array is non-redundant. A preferred spiral specification embodiment combines the location of array elements on concentric circles forming the geometric radial center of equal-area annuli with locations on an innermost concentric circle whose radius is independently selected to enhance the performance of the array for the highest frequencies at which it will be used. This result applies over a broad wavelength band, e.g. 10:1 ratio, making it useful for phased acoustic microphone or speaker arrays, or for phased electromagnetic antenna arrays. For small numbers of array elements, it is superior to a random array. Alternate spiral specification embodiments provide array space density tapering alternatives allowing for flexibility in array design and for array performance trade-offs between array beamwidth and sidelobe levels.
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IEE Proceedings—Microwaves, Antennas and Propagation, Aug. 1994, UK, vol. 141, No. 4, ISSN 1350-2417, pp. 321-325, XP002038893 Hall P.S. et al.: “Sequentially rotated arrays with reduced sidelobe levels” (p. 323-p. 325).
Chang Vivian
Gardner Conrad O.
The Boeing Company
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