Traditional methods for forming and steering beams produced by an array of acoustic transducers involve phased or time-delayed acoustic pulses and require that each stave of the array be sampled as a separate hardware channel. Although this approach may produce effective, high-resolution imaging systems, it also requires substantial support electronics for each hardware channel, which increases the expense, size, weight, and power requirements of the system.
The radar community has used frequency to position beams using a frequency scanning radar technique. This technique employs delay lines in an antenna array that provide appropriate phase shifts so that the frequency determines the steering angle of the array's main beam. Frequency-steered beamforming systems have also be used in sonar systems with phase shifting electronics and multi-channel acoustic arrays. These systems use specific array designs and broadband pulses to map angular imaging information into the frequency domain. The beamformer for such a system may be designed around time-frequency (e.g. spectrogram, wigner) or time-scale (e.g. wavelets) decomposition data processing techniques. This approach allows multiple independent beams to be simultaneously formed using a single hardware channel.
Frequency-steered acoustic systems use angular spectral dispersion analogous to the dispersion of light incident on a prism or a diffraction grating to form spatially distinct beams. In the field of optics, diffraction gratings may be designed to take advantage of a unique set of discrete angles along which, for a given spacing d between facets, the waves diffracted from each facet are in phase with the waves diffracted from any other facet and the waves therefore combine coherently. The classical transmission grating equation is as follows:
                                          θ            ⁡                          (              λ              )                                =                      arcsin            ⁡                          [                                                m                  ⁢                                                                          ⁢                  λ                                d                            ]                                      ,                            (        1        )            Where m is the “order” or number of wavelengths, λ, between the facets.
For a given grating design defined by the variables m and d, Equation (1) provide the mapping between angle and frequency. In a blazed diffraction grating, the individual facets are rotated away from the general plane of the array by some groove angle χ. Several important aspects of a diffraction grating with respect to a frequency-steered system are noted when θ, the angle between the beam and a plane normal to the plane of the grating is plotted versus wavelength for m=−2, −1, 0, 1, and 2. First, the zero order is frequency-independent and is real for all frequencies. Because the zero order beam is not steered as a function of frequency, this beam has been used in conventional systems, where the beams are steered with phase shifts or time-delays. However, this frequency-independent zero order beam is typically not useful in a frequency-steered system and therefore must be suppressed so that it will not produce ambiguous responses.
The first negative and first positive order beams enter the visible region (−90° to 90°) from what is commonly called the ‘end-fire’ orientation (perpendicular to the array normal) at λ/d=1. As frequency is increased, the first order beams are joined by the second order beams an octave higher in frequency, at λ/d=0.5. At all angles in between −90° and 90°, the first and second order beams are separated by one octave of spectral bandwidth. The second order beams may create ambiguities if more than one octave of spectral bandwidth is used.
The classical transmission grating equation is the fundamental frequency-steered acoustic beamforming equation. A simple frequency-steered beamforming and processing system is illustrated schematically in FIG. 1. From left to right, the diagram shows the flow of a broadband acoustic pulse 12 produced by a pulse generator and composed of acoustic beams having a range of frequencies f0 . . . fn. The electrical signal output from broadband pulse 12 is input to an acoustic beamformer composed of projector electronics 14 and a frequency-steered array 16. The acoustic array is designed to produce a frequency-dispersed sound field 18 having a known, nonlinear relationship between angular space (θ) and frequency f given by Eq. (1). In this way, a broadband signal containing many acoustic frequencies is sent into a frequency-steered array and emerges as a set of acoustic beams having different angular directions depending on frequency.
The frequency-dispersed sound field 18 from blazed array 16 interacts with the ambient environment and/or a target 20 and a backscattered, frequency dispersed sound field 22 is incident upon a receiver array 24, formed as a frequency-steered array, and receiver electronics 26 and is recombined into a broadband signal 28. Thus, reflected signals are received from the same angle they were transmitted and are recombined by the frequency-steered array to form a single broadband receive signal. Analog and digital processing techniques may then be applied to the broadband signal to separate out the frequencies and create and display an image similar to that of medical ultrasound systems.
One system for frequency-steering an acoustic sound field employs a “blazed array” having active faces of acoustic elements arranged at an angle from the general plane of the array. U.S. Pat. No. 5,923,617 describes a sonar system employing a blazed acoustic array including a plurality of stepped acoustic elements formed in an echelon array, with adjacent acoustic elements being displaced from one another. The blazed arrays described in the '617 patent are first order (m=1) arrays, having a single wavelength spacing between facets. The disclosure recognizes that higher-order and multi-order modes could be designed.
The simplest implementation of the blazed array and time-frequency beamforming is in a single channel 2D imaging sonar system. Data collected using a single channel blazed array and a spectrogram-based beamformer is presented in R. L. Thompson et al., “Two Dimensional and Three Dimensional Imaging Results Using Blazed Arrays,” IEEE Oceans 2001 proceedings, pp. 985-988, vol. 2. This publication also describes a blazed array implementation in combination with conventional array design and beamforming techniques to produce 3D volumetric imaging. One 3D configuration employed a blazed array oriented vertically and flown horizontally to create a horizontal synthetic aperture. Several views rendered from 3D blazed synthetic aperture sonar data set are presented. Both the 2D and 3D systems were implemented with a single hardware channel.