Constant Beamwidth Research Articles

Spiral membrane speaker enclosure

Acoustical transmission lines formed into a spiral configuration possess the latent property of having different lengths along adjacent lines when traversing along a radius: for an acoustical spiral, sound waves in adjacent transmission lines will progress from an antiphase relation at the center of the spiral to an in-phase relation at some radius r (λ) from the center. This is a transformation dependent only upon wavelength. When those transmission - line walls that are facing the environment are covered with a low-mass, tightly stretched membrane, radiation from this secondary surface will occur at the appropriate time. Radiation of a transverse wave rotating about the spiral axis occurs when sound waves in adjacent transmission lines are in phase. In this paper, several radiation modes and phasing properties are discussed. Owing to automatic adjustment of aperture size with change in frequency, the spiral membrane enclosure has a broad and constant beamwidth. An acoustical source, such as a loudspeaker cone each side of which is contiguous to the end of an acoustical transmission line, generates within those transmission lines longitudinal waves that are antiphase relative to each other. Constant loading of the source occurs over a wide range of frequencies, providing uniform electrical/acoustical performance. An experrimental model is described. Subject Classification: 85.60; 20.40.

An electronically scanned beacon antenna

An electronically scanned cylindrical array having a minimum number of active devices has been designed. A saving of over 3 to 1 in diode elements as compared to conventional approaches has been achieved. In addition to providing instantaneous positioning to any azimuth direction, a constant azimuth beamwidth for an elevation fan beam has been realized. Extremely low azimuth sidelobes are obtained, and a straightforward synthesis procedure can provide elevation aperture designs to meet typical elevation pattern requirements. A modified Myer geodesic lens combined with a diode sequencing switch for coarse steering and a phased matrix feed for fine steering form the basis of the system. Sum and difference patterns show virtually no variation with scan or elevation angle and an omni-pattern is also available. Detailed pattern calculations as well as measured data on prototypes of key elements of the system are presented to verify the feasibility of the approach.

Parametric Acoustic Arrays

Parametric acoustic array performance has been demonstrated with several different transducers. Three-inch-diameter transducers with primary frequencies between 250 and 300 kHz have produced good parametric acoustic beams at 10, 20, and 40 kHz difference frequencies with nearly constant beamwidth across this frequency band. A parametric acoustic array operating at 20-kHz difference frequency was compared to a 100-kHz conventional transducer for bottom profiling capability in Lake Washington. Echograms were obtained demonstrating the capability of the lower frequency beam to produce subbottom, as well as bottom returns.

Octave Bandwidth Constant Beamwidth Underwater Receiving Array

By a method described at the 79th meeting of the Society, a seven-element linear broadside array was designed to give a constant beamwidth of 26° from 10 to 20 kHz. Five elements spaced 12λ at 10 kHz comprised one array. Using three of these and two more, a half-size array with spacing of 12λ at 20 kHz was obtained. Each was filtered then combined to give the array output. The filter functions as specified by the directivity function were approximated since they were not realizable by real elements. The predicted and measured characteristics of the array are presented.

Wide-Bandwidth Constant-Beamwidth Acoustic Array

A linear broadside array can be made to have constant beamwidth over wide frequency ranges by using logarithmic element spacing and the proper filtering and combination of signals from the elements. Such an array is achieved by choosing the array size and linear element spacing to give the desired beamwidth and side-lobe suppression at the lowest frequency of interest. An identical array at twice the frequency will be half as long with half the spacing. When the signals from these two arrays are properly filtered and combined constant beamwidth over an octave band results. When the arrays are superimposed, redundancy allows a reduction in the number of elements. This process can be continued to cover the desired bandwidth and a log spacing results. Results obtained from a five-element array covering 0.3 to 3 kHz are presented. [Partially supported by LTV-Electrosystems.]

Constant beamwidth transducers for use in sonars with very wide frequency bandwidths

The use of sonars with very wide frequency bandwidths in order to obtain more information about the detected target than is possible with their narrow-band counter-parts necessitates the use of special techniques for transmitting and receiving the acoustic signals if the amplitude of the target echoes is not to vary with their position in the transmitting and receiving “beams”. A transmitting device using nonlinear interaction in the medium and a specially shaped receiving array, both having constant beamwidths over a wide range of frequencies, are described and data of their performance is presented. The transmitting and receiving devices were subsequently used in a sonar with a wide band capability (20–60 kHz) to investigate the frequency response of two types of target, a solid iron sphere of 12 cm diameter and the bed of a sea-loch, and the results of this investigation are included.

Directional Loudspeaker System for a Big Hall

ABSTRACTA wide band directional loudspeaker system with a nearly constant beam width of 30° in the vertical plane and 120° in the horizontal plane has been described here. Systems using straight line columns of speakers satisfy the directivity requirements over a limited frequency range. A uniform directivity over a wide range of frequency is achieved if radiators are arranged in a curve (arc of a circle). Vertical directivity was achieved by placing the seven speakers along a 60° arc of a circle of radius 5½ ft. and for horizontal directivity three such columns were placed in a 120° arc of a circle of radius 1¼ ft. The directivity remained substantially constant between 200 c/s. and 8 Kc/s. The system was tested in SWP hanger of Roorkee University having floor section of 166 × 142 ft. and metallic parabolic roof 34 ft. high. A considerable improvement in articulation index was achieved.

Effects of Shading on Minor-Lobe Level and Beamwidth in Symmetrical Uniformly Spaced 6- and 4-Element Arrays

The effects of shading on number of minor lobes, level of minor lobes with respect to the principal lobe, and beamwidth are analyzed for symmetrically shaded, uniformly spaced 6- and 4-element arrays. For a 6-element array shaded 1:a:b:b:a:1, there are no minor lobes above the parabola 125−30 a+a2−20b=0. Between this parabola and 5−2 a+a2−4b=0, there is one minor lobe, while between the second parabola and the straight line b−3a+5=0 there are two minor lobes, which may or may not be equal in amplitude. Below the straight line, there is again only one minor lobe. Curves in the a-b plane for constant first and second minor-lobe levels are presented, as well as curves of constant beamwidth for the half-wavelength spacing. For the 4-element array shaded 1:c:c:1, there are no minor lobes when c is greater than 3. Curves for minor-lobe level and beamwidths for the half-wavelength spacing as a function of c are presented.

Broad-band constant beam-width transducers

Arrays having a constant beam-width (in one or more planes) over a wide frequency band have an important application in several branches of acoustical engineering, and particularly in sonar systems. Methods of obtaining this result using linear or planar arrays with electrical networks have been described earlier; the present paper is concerned primarily with methods involving the shaping of the surface of the transducer or array. Particular attention is given to transducers with cylindrical or spherical curvature and to twisted arrays. The results indicate that there is no restriction on the band-width ratio over which a reasonably constant beam-width can be maintained.

Log periodic dipole arrays

A new class of coplanar dipole arrays is introduced. The antennas described provide unidirectional radiation patterns of constant beamwidth and nearly constant input impedances over any desired bandwidth. The broad-band properties are achieved by making use of the principles of log periodic antenna design. Models are discussed which are capable of providing 8- to 9-db directive gain with an associated input standing wave ratio of 1:2:1 on a 75-ohm feeder, and this performance is independent of frequency. The free-space properties of several of these arrays have been measured and the results are presented. The antenna configuration is simple, permitting practical methods of fabrication, and the design should prove useful in many applications. It makes possible, for example, the construction of all-wave rotatable beams of very low cross section for use in the hf to uhf spectrum.

Arrays with Constant Beam-Width over a Wide Frequency-Range

IN many sound-ranging and listening systems (principally used under water) it is necessary to receive signals over a wide range of frequency, the ratio of upper to lower frequency being of the order of ten or more. Transducers (or hydrophones) with a sufficiently wide frequency response are available, but usually have to be used as omni-directional receivers because otherwise their beam-width would be inversely proportional to frequency. Clearly, the use of directional receivers would be very advantageous in many applications, but an essential condition is that the beam-width must be constant over the appropriate range of frequencies. An approach to the solution of this problem is to connect the outermost units of the transducer to the receiving amplifiers through low-pass filters, so that the transducer is virtually made smaller at the higher frequencies. There are, however, many difficulties and complications in this process, which is, at best, rather crude.