Side Looking Sonar Transducer

Abstract

A side looking sonar transducer having an elongated active radiating, or receiving, surface which is wider at the ends of the transducer than at the middle. This arrangement provides an energy distribution which allows for a greater depth of focus.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

Ser. No. 889,415 filed Dec. 31, 1969.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention in general relates to transducers, and particularly to a side looking sonar transducer which transmits acoustic energy to, or receives reflected acoustic energy from, a narrow strip on a target area such as the sea bottom.

2. Description of the Prior Art

In side looking sonar systems an elongated transducer is generally mounted on a carrier vehicle which travels along a course line. Acoustic energy is propagated in a very narrow fan-shaped beam pattern toward a target area, the sea bottom, by the transducer and energy reflected from the bottom or objects on the bottom is picked up by a similar receiver transducer. As the carrier vehicle continues along its course line an indicating apparatus, such as a storage tube or paper recorder, portrays a picture of the bottom in accordance with each reflected transmitted signal. The resultant display is similar to a picture on a television screen in that the entire picture is made up of a plurality of parallel sweeps with each sweep being the portrayal of a reflected transmitted signal.

One type of side looking sonar transducer has been developed wherein the transducer is of a curved elongated configuration. The curved transducer is used for high-resolution applications and the carrier vehicle travels at a relatively short altitude above the bottom, for example 20 feet.

The curved transducer has a radiating surface arranged on the arc of a circle whose radius is the design altitude and acoustic energy is focused along a line of focus plane on the sea bottom. The curvature of the arc approximately matches the wave front curvature of the reflected acoustic energy and any variation in the design altitude results in defocusing and a consequent degradation of the display.

It is therefore an object of the present invention to provide a focused side looking sonar transducer that provides for a greater depth of focus than transducers of the prior art.

Another type of side looking sonar transducer is the straight line configuration. This type is not focused along a line of focus and can tolerate relatively large altitude excursions. The resolution of the desired target area relatively close to the transducer is however limited by the width of the transducer.

It is a further object to provide a straight line side looking sonar transducer that effectively increases the depth filed of the display, that is, increases the resolution for target areas relatively close to the transducer.

SUMMARY OF THE INVENTION

A side looking sonar transducer means is provided and includes an elongated active surface with the width of the surface being greater toward one, and preferably both ends of the transducer. The transducer may be used as a transmitter and/or a receiver of acoustic energy and the variable-width active surface results in a transducer having an effective length L for governing response at relatively large distances from the transducer and an effective length L', of a magnitude smaller than L, for governing response relatively close to the transducer. The apparent shortening of the transducer results in a greater depth of focus or depth of field at those positions relatively close to the transducer.

In the preferred embodiment the elongated transducer has an active face which assumes or lies on a straight line or the arc of a circle, and the transducer is made up of a plurality of short segments of transducer material positioned in end-to-end relationship to form an array and wherein the end elements of the array are wider than the elements near the center of the array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show an elevational and a plan view, respectively, of a typical side looking sonar beam pattern;

FIG. 2 is a block diagram of the transmit/receive electronics portion of a typical side looking sonar system;

FIG. 3 illustrates a side looking sonar transducer of the prior art;

FIG. 4 illustrates the positional orientation of the transducer of FIG. 3;

FIG. 5 illustrates another transducer of the prior art;

FIG. 6 is a beam pattern in rectangular coordinates;

FIG. 7 illustrates the development of a beam pattern on a target area for a transducer such as illustrated in FIG. 5;

FIG. 8 shows an insonified area, or a receive area for a typical side looking sonar transducer;

FIGS. 9a through 9c serve to illustrate the matching and mismatching of acoustic energy with the curved transducer to aid in understanding of depth of focus of the transducer;

FIG. 10 illustrates the effect of shortening the transducer of FIGS. 9a through 9c;

FIG. 11 are curves illustrating the variation in depth of focus with increase in bottom range;

FIG. 12 illustrates a preferred embodiment of the present invention;

FIGS. 13 through 15 serve to illustrate the operation of the preferred embodiment of FIG. 12;

FIG. 16 illustrates a straight line transducer orientation with respect to a target area; and

FIG. 17 illustrates another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1a, illustrating a typical side looking sonar application, a carrier 10 proceeds along a course line (toward the viewer) at a predetermined altitude H above the sea bottom 12. Although sea bottom is mentioned it is obvious that the apparatus may be used over different target areas and in various bodies of water. Side looking sonar apparatus aboard the carrier 10 has associated therewith a pancake-shaped beam pattern 14 being wide in a generally vertical plane and being relatively narrow in a generally horizontal plane, as illustrated in FIG. 1b which is a view looking down on the apparatus of FIG. 1a.

For increased coverage of the target area, port and starboard transducer means may be provided and may simultaneously operate at slightly different frequencies of operation or may operate sequentially at the same frequency.

The transducer which provides such a beam pattern has a radiating or active surface which is very long compared to its width. For example, in terms of wavelengths, .lambda., its length L may be over 150.lambda. and its width W, less than 1.lambda. where .lambda. is the wavelength of the operating frequency, in the fluid medium in which the transducer operates.

FIG. 2 illustrates in block diagram form an operative side looking sonar system for one side. Where very low speed can be tolerated, a single transducer may be used in conjunction with proper switching equipment, as both a transmitter and a receiver of acoustic energy. For a relatively faster speed however two transducers are utilized, one for projecting acoustic energy and one for receipt of reflected acoustic energy, with the transducers being oriented relative to one another that the transmitted acoustic energy is projected at an angle with respect to the receiver. In FIG. 2, upon the application of a suitable command pulse 2, the transmitter 3 will provide an electrical signal to the transmitter transducer TR. Reflected acoustic energy is received by the receiver transducer REC the output signal of which is provided through a receiver means 5. In order to provide for somewhat uniform intensity of the return signal (in the absence of a target) the receiver output, which decreases in amplitude with respect to time, is operated upon by time-varying gain means 6 which varies gain in accordance with the illustrated curve. Such technique is well known to those skilled in the art. The detector 8, which receives the output of the time-varying gain means 6, provides an output signal to recorder means 9 which then provides the desired display. Such recorder means 9 may be for example magnetic tape storage for future display, cathode ray storage tube apparatus, or a helical wire-electrosensitive paper unit, to name a few.

For high-resolution work, use is made of a curved transducer such as illustrated in FIG. 3 to which reference is now made.

The transducer 18, representative of the prior art, is made up of a plurality of active elements 20 arranged in end-to-end relationship such that the center of each element 20 lies substantially along the curve A, A being a section of a circle having the center 0 and a radius R. Each element 20 (more particularly the center of each rectangular surface illustrated) is at the same distance R from point 0. If the transducer 18 is designed to travel at an altitude H equal to the distance R, then point 0 0 will lie on the sea bottom 12 and the energy radiated at any instant of time from any of the active elements 20 will arrive at point 0 simultaneously with the arrival of the energy transmitted by the remaining active elements. Conversely, if transducer 18 is a receiver, energy transmitted, or reflected, from point 0 and radiating spherically, will simultaneously arrive at each of the active elements 20. If 0 is extended perpendicularly to the plane of the circle then each of the active elements 20 will be equally distant from any point on that line. This is illustrated in FIG. 4.

FIG. 4 illustrates an XYZ coordinate system with the transducer 18 of FIG. 3 being represented by T which is the curve A between points a and b. T lies in the XZ plane at an altitude or distance R from the origin 0 and each point on T is at the same distance R from 0 since T is the arc of the circle centered about 0. Line F, known as the line of focus is perpendicular to the plane of the circle at point 0 and any point on line F is equidistant to all points on the transducer T. A line from point 24 drawn to the intersection of the transducer T and the Z axis is designated 25 and has a length S, S being the slant range to point 24. The lines 25 joining the end points of the transducer with point 24 are also of a distance equal to S. Lines joining point 24 with each and every point on the transducer T would form a section of a cone.

Perpendiculars drawn to the faces of the active elements in FIG. 3 will all be parallel to one another and to the Y axis in FIG. 4. Maximum energy is directed or received along that perpendicular. In order to direct more energy onto the sea bottom the individual active elements are generally tilted about the arc A by a certain depression angle, as illustrated in FIG. 5, such that the perpendicular to each face passes through a point on the line of focus F, such point R.sub.m being at maximum range. For example, and with additional reference to FIG. 4, if point 24 is the maximum range point R.sub.m then the lines 25 will be identical with the lines 26.

The transducer 18 whether it is a transmitter or receiver of acoustic energy has a radiation pattern or receive pattern, respectively, associated therewith. FIG. 6 illustrates a typical beam pattern on the sea bottom. In FIG. 6 the horizontal axis represents units of distance from an origin 0, 0 for example being any representative point on the line of focus F. The vertical axis represents normalized acoustic sound pressure. That is, the maximum acoustic sound pressure (at the particular point in question) has been given an arbitrary value of 1 with all sound pressures being relative to the maximum and therefore falling between 0 and 1. The vertical scale of FIG. 6 has also been labeled in decibels db. This designation is another manner of stating relative pressures and since the pressures will be a maximum of 1 or less, the pressure designation will be -db. or "db. down." For example, the maximum point, 1.0 on the relative pressure scale would be equivalent to 0 db. a pressure of approximately 0.7 of maximum is said to be -db. or 3 db. down; and a pressure of approximately 0.1 is said to be -20 db. or 20 db. down. The db. values are approximate and may be calculated exactly from known formulas. The width of the beam is generally given at some reference level. By way of example, the -3 db. points may be chosen as the reference, as is commonly done, and the width of the beam represented by the pattern of FIG. 6 therefore will be equal to x units of distance.

Each point along the line of focus F has a beam pattern such as illustrated in FIG. 6. In FIG. 7 the beam pattern 30 is the pattern associated with point P.sub.2 at a slant range of S.sub.2 from the transducer T. The -3 db. points are designated 31. Another beam pattern 34 is illustrated for point P.sub.1 at a slant range of S.sub.1 from transducer T and the -3 db. points are designated 35. The distance between the -3 db. points (x in FIG. 6) may be approximated by the relationship

where .lambda. is the wavelength of the operating frequency, L the length of the transducer T, and S the slant range. From the relationship it may be seen that as the slant range S increases, the distance x between the -3 db. points also increases. It is obvious that the beam pattern could be drawn for every point along the line of focus F. If a line is drawn through all the -3 db. points on one side of the line of focus F and through all the -3 db. points on the other side of F then there is defined a wedge-shaped area 38 as illustrated in FIG. 8. The area 38, sometimes known as the insonified area, is the area which contains maximum acoustic energy. Conversely, if the transducer T is a receiver then it will receive substantially the energy reflected from the area, or any target in the area, 38 and will provide a corresponding output signal in response to receipt of such energy. In actual operation transducer T provides an acoustic pulse 39 which initially impinges upon the sea bottom (the Xy plane) for example at point 40 and proceeds outwardly along the line of focus F to a maximum point 42, to thereby define the area 38.

Consider now a situation wherein FIG. 8 represents a receive situation and transducer It is a receiver transducer. Reflected acoustic energy from the area 38 or from a target within the area 38 diverges substantially omnidirectionally. That is, a reflected wave front may be thought of as an expanding sphere emanating from the point of reflection. When reflected energy from area 38 reaches the transducer T the wave front of such energy will substantially match the curvature of transducer T which accordingly will provide a corresponding output signal.

The transducer is designed for a critical altitude and deviation from that altitude will place the line of focus F above or below the XY plane.

Even if the critical altitude is maintained it will be apparent that reflected energy may emanate from a point above or below the area 38 due to a reflection from a physical target or land contour. It may be shown that for relatively distant ranges, for example, near the vicinity of point 42, the wave front of energy reflected from a point above or below the line of focus F by a considerable distance will still match the curvature of transducer T to such an extent as to provide meaningful output signal. However, considering a relatively close in range in the vicinity of point 40, as soon as the reflecting point goes above or below the line of focus F by a relatively small amount, a proper signal indicative of that target point will not be provided. This defocusing effect is further illustrated in FIGS. 9a, b and c.

FIG. 9a, is a view looking down the line of focus F toward the XZ plane and illustrates acoustic energy reflected from point 40. FIG. 9a represents a particular wave front 50 as it emanates from point 40 toward the transducer T. It is seen that wave front 50 after having traveled for a distance R will exactly match the curvature of transducer T, every section of which will provide an output signal in accordance with the wave front and in phase with every other section of the transducer T.

In FIG. 9b the situation is depicted wherein acoustic energy is reflected from point 40' above the line of focus F. Wave front 52 proceeds from point 40' and at some later time just touches transducer T and is designated 52'. As the wave front proceeds it will touch the ends a and b of transducer T and a crescent 55 will be formed between the wave front designated 52" and the transducer T. If the height h of this crescent is more than a predetermined value then it may be stated that a proper output signal will not be provided. Depending upon system design and by way of example, if h is greater than 0.38.lambda. then the output signal will be degraded such that point 40' will not be properly displayed on the recorder apparatus. For radiations effectively emanating from point 40" the crescent area would be below the line T and if the height of such crescent was greater than the critical distance h, a meaningful output signal would not be provided.

The reason for signal degradation is graphically depicted in FIG. 9c which shows several active elements 20 of the transducer T in conjunction with the wave front 52' of FIG. 9b. A dot has been drawn at the center of the active face of each element 20 to serve as a reference point and it is seen that when the wave front 52' just touches the dot of the active element 20 on the Z axis it will be at a progressively increasing distance from the remaining active elements.

In the FIG. .delta..sub.1 =0 and .delta..sub.2 <.delta..sub.3 <.delta..sub.4 <.delta..sub.5. Each active element therefore contributes an individual signal which is out of phase with the signals provided by the remaining active elements. The signals are provided by suitable electrode means, not shown, but well known to those skilled in the art. A vector addition of these individual signals provides a resultant signal of a substantially reduced magnitude. In contrast, a wave front proceeding from point 40 would simultaneously impinge on each active element 20 and the individual output signals, all being in phase, would result in a significantly higher output signal.

If the height h of the crescent 55 formed by the wave front of energy emanating from point 40' could be reduced below the critical value 0.38.lambda.the information relative to point 40' could also be properly displayed.

As illustrated in FIG. 10, if the length of the transducer T is shortened from points a and b to points a' and b' respectively then the wave front 52 emanating from point 40' forms a crescent 55' of a height h which is significantly reduced in magnitude such that information relative to point 40' may now be displayed.

It may be shown that for bottom ranges from directly beneath the transducer out to maximum range, the depth of focus varies inversely as the square of the length of the transducer. For bottom ranges approximately equal to or less than the magnitude of altitude R, the depth of focus is approximately proportional to R.sup.2 /L.sup.2. For greater bottom ranges the depth of focus is approximately proportional to R/L.sup.2.

Variation in depth of focus is illustrated in FIG. 11 wherein the horizontal axis represents the bottom range, in feet, along the line of focus, and the vertical axis represents vertical distance, in inches. Curve 60 represents the depth of focus above, and curve 60' the depth of focus below the line of focus, for a transducer such as in FIG. 9b with the following design parameters;

operating altitude R ..... 20 feet

length L ..... 22 inches

.lambda. ..... 0.04 inches

width W of active surface ..... 0.03 inches (3/4.lambda.)

critical distance h ..... 0.38.lambda.

Curve 60 shows that directly below the transducer, that is at a bottom range of zero on the horizontal scale the depth of focus is +13.7 inches and curve 60' shows a depth of focus of -15.4 inches. A similar transducer reduced in length to 14 inches would have a depth of focus, at that same bottom range of zero, of +31.1 inches and -42.0 inches. The positive and negative depth of focus for such 14-inch transducer is depicted in curve 62 and 62' respectively.

Past approximately 20 feet, curve 60 and 60' diverge so that at a bottom range of for example 100 feet the depth of focus would be +73.1 inches and -74.8 inches. For the 14 -inch transducer these respective values are extended to +179.0 inches and -189.9 inches respectively. From the curves of FIG. 11 it is seen that for distant ranges a relatively large transducer altitude deviation may be tolerated. For close in ranges the maintenance of a specified relative altitude is critical although it is seen that it is less critical for the 14 -inch transducer than for the 22-inch transducer. Use of a 14-inch transducer therefore would allow for a greater depth of focus however, from equation (1) the width x of the insonified area at maximum range would also be increased, thus degrading system resolution. The situation is presented therefore where it is desired to have a relatively long transducer for maintaining system resolution while at the same time a relatively shorter transducer is desired for increasing the depth of focus for close in ranges. The present invention provides for an increased depth of focus at close in ranges without sacrificing system resolution. This is accomplished by an elongated transducer means having a radiating or receiving active surface and wherein the width of the active surface is greater at least one end portion of the transducer means and reduces in width toward the center portion of the transducer means. In the preferred embodiment the width of the active surface is greater at both end portions and the width decreased towards the center.

A preferred embodiment of the invention is illustrated in FIG. 12. The transducer 65 is similar in many respects to the transducer of FIG. 5 in that it is made up of a plurality of active elements forming an active surface, the elements being arranged substantially on an arc A of a circle of radius R and extending between points a and b and is symmetrical about a line (the Z axis in FIG. 12) perpendicular to a line joining points a and b. Preferably each active element is depressed by some predetermined angle.

The active elements may be of any well-known transducer material, barium titanate being one example, although it is understood that the transducer need not be piezoelectric or piezoceramic, but may also be magnetostrictive. Transducer operation is conventional in that when supplied with proper electrical energy the transducer 65 will transmit acoustic energy in a certain beam pattern or if a receiver, the transducer 65 will provide a corresponding electrical output signal in response to the receipt of proper acoustic energy and in accordance with its receive beam pattern. The active surface of the transducer 65 is effectively wider at the end portions thereof than in the middle. By way of example the two active elements 67 at each end portion are wider than the remaining active elements 68. By proper design the transducer 65 may be made to simulate the 14-inch transducer discussed in FIG. 11, to increase the depth of focus for close in ranges while also simulating the 22-inch transducer for maintaining system resolution.

It may be shown such operation will result if each active element 68 (there are seven such elements) has a length of 2 inches and a width of 0.03 inch and each active element 67 (there are four such elements) has a length of 2 inches and width of 0.045 inch. Other parameters are identical to that described with respect to FIG. 11, that is, an operating altitude of 20 feet with a frequency of operation such that the wave length .lambda. is 0.04 inches. This apparent duality of length of a single transducer is best explained with reference to FIG. 13, 14 and 15.

In the following discussion the transducer will be treated as a transmitter. It is to be understood that by reciprocity the principles apply to the transducer as if it were a receiver. In such case the individual elements would be responsive to point sources of radiation and to obtain equal response from a source of radiation at a distant point as in FIG. 15 the receiver response of the wider elements 67 may be varied with respect to the response of elements 68.

In FIG. 13 an active element's surface 74 lies in the XZ plane on the Z axis and has a length l and a width W. A planar surface is illustrated, however the active element may have a curved surface. Line 76 is perpendicular to the surface 74 and point 79 is a point in the ZY plane. A line drawn from the intersection of perpendicular 76 and surface 74 to the point 79 has a length S and lies at an angle .phi. with respect to perpendicular 76. The pressure at any point such as point 79 is given by the beam pattern formula:

If the angle .phi. (.phi. is in radians) is equal to 0 then point 79 falls on line 76 and equation (2) reduces to:

K is a proportionally constant dependent upon the length l and the input power supplied to the element. With a given depression angle .phi. the orientation of FIG. 14 results. The perpendicular 76 intersects the line of focus F at point m where the pressure is P.sub. m and the distance from the face 74 to point m is designated S.sub. m. The pressure at any other point n along the line of focus F between o and m is P.sub.n and the distance from the face 74 to such point n is designated S.sub. n. The ratio of the pressure at point n to the pressure point at point m is given by the relationship:

With the above beam pattern formulas, the transducer embodiment of FIG. 12 will be analyzed in the simplified arrangement of FIG. 15 which shows two active elements, element 68 having a width of 0.03 inch and an end element 67 of a greater width, 0.0425 inch. In the following analysis, parameters associated with the element 67 have been given primed reference characters. Both elements have been given a depression angle such that the line S.sub.m and S'.sub.m are equivalent to the perpendicular 76 illustrated in FIG. 14. Point n is directly below the transducer at the origin of the coordinate system and accordingly the distances S.sub. n and S'.sub. n are equal to a radius R as in FIG. 12. The line S.sub. m makes an angle .phi.' with line S.sub. n and line S'.sub. m makes an angle .phi.' with line S'.sub. n. In this illustration S.sub. m = S'.sub. m ; S.sub. n = S'.sub. n ; and .phi.=.phi.' . From equation (3) the pressure at point m due to element 68 is

P.sub.m =K/S.sub.m Eq. (5)

The pressure at point m due to element 67 is similarly

P'.sub.m =K'/S'.sub.m Eq. (6)

Although not a requirement, for purposes of illustration, the pressure at point m from element 67 will be made equal to the pressure at point m from element 68, that is P.sub.m =P'.sub.m. Since S.sub. m equals S'.sub. m the pressures are made equal by relative adjustment of the constants K and K'. The element 67 has a larger active surface area than element 68 and therefore K and K'are made equal by proportionately reducing the power supplied to element 67.

The pressure at point n due to the element 68 may be determined from equation (4) wherein .lambda.=0.04 inch; W=0.03 inch; .phi.=1.22 radians (70.degree.) and S.sub. n /S.sub. m = Cos. .phi.. Solution of equation (4) with these parameters result in P.sub. n =0.124.sup.. P.sub. m which demonstrates that element 68 contributes to the pressure at point n as will all elements 68 of FIG. 12.

The pressure at point m due to the wider element 67 may also be determined from equation (4) with the width W being 0.0425 inch. The solution of equation (4) with this increase in width results in P'.sub. n =0. The same will be true with respect to every other element 67 in FIG. 12. It is therefore seen that the total pressure at point m is the combination of all the individual pressures provided by elements 67 and 68 and since each element is 2 inches long the pressure at point m is the result of a 22-inch transducer since in the present example there are a total of 11 elements. The wider elements 67 contribute no pressure at point n the total pressure at which is provided solely by elements 68. In the present example there are seven such elements each 2 inches long so that in effect at point n the transducer appears to be a 14-inch transducer.

The foregoing principles may also be applied to a straight line transducer such as transducer T in FIG. 16. The transducer T may be similarly made up of a plurality of active elements each depressed by a certain angle to insonify a portion of the sea bottom. Since the transducer T is generally parallel to the X axis the acoustic energy radiating therefrom is not focused along a line of focus.

A view looking down on the transducer T and the sea bottom is illustrated in FIG. 17. If the transducer has a length L its -3 db. beam width in the near field will be that beam width depicted by lines 90, 90'. Starting from the transducer it is seen that the beam width decreases to a minimum value of 0.6L after which it diverges to form the far field -3 db. beam width. The distance of the narrow region from the transducer is approximately given by the value of L.sup.2 /2.lambda.. If it is desired to increase the resolution within the near field then the end elements 67 may be made wider than the central elements 68, exactly as described in FIG. 12, except for the fact that the elements now lie on a straight line. With such an arrangement it will be remembered that the end elements 67 contributed nothing to the region directly below the transducer, effectively simulating a smaller length transducer. Such simulated smaller length transducer is designated by the length L ' and its beam width is depicted by the lines 92, 92'. This latter beam width decreases to a value of 0.6L' at a region closer to the transducer, since L' is smaller than L. By applying the principles discussed herein therefore, the beam pattern for the transducer for close in ranges will be 92, 92' since elements 67 contribute no or little energy to the region, whereas for the far field consideration the beam pattern will be described by 90, 90'. The resolution up to distance of L.sup.2 /2.lambda. therefore is increased.

The variable width of transducer may be a curved or straight line receiver either one being utilized in conjunction with a curved or straight line transmitter of nonvariable width. Conversely the variable width transducer may be the transmitter used in conjunction with a curved or straight line transducer of either variable or nonvariable width. If the transducer is constructed of a plurality of active elements the depression angle of the elements may be varied, as disclosed in copending application Ser. No. 889,415 filed Dec. 31, 1969 and assigned to the assignee of the present invention.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made by way of example and that modifications in constructional details and variations of the present invention are made possible in the light of the above teachings.

Claims

We claim as our invention:

1. Side looking sonar apparatus comprising:

a. a transducer means having an elongated radiating surface extending between end points for radiating acoustic energy;

b. the width of said radiating surface being nonuniform;

c. said transducer means being for travel through a body of water;

d. electrical means;

e. said transducer means being responsive to said electrical means for radiating the said acoustic energy toward a relatively narrow area on the bottom of said body of water.

2. Apparatus according to claim 1 wherein:

a. the width of the radiating surface is greater at at least one of the end points than toward the center portion of the radiating surface.

3. Apparatus according to claim 2 wherein:

a. the radiating surface is symmetrical about a line perpendicular to a line joining the end points.

4. Side looking sonar apparatus comprising:

a. a transducer means having an elongated receiving surface extending between end points for receipt of acoustic energy;

b. the width of said receiving surface being nonuniform;

c. said transducer means being for travel through a body of water;

d. said transducer means providing an output signal upon receipt of the said acoustic energy propagating through said water toward said transducer means.

5. Apparatus according to claim 4 wherein:

a. the width of the receiving surface is greater at at least one of the end points than toward the center portion of the receiving surface.

6. Apparatus according to claim 5 wherein:

a. the receiving surface is symmetrical about a line perpendicular to a line joining the end points.

7. Side looking sonar apparatus comprising:

a. a plurality of transducer active elements each having an active surface of a certain length and width;

b. said elements being arranged in end-to-end relationship and lying substantially along a line extending between end points;

c. the width of said elements nearer said end points being greater than the elements near the center of said line;

d. electrical means;

e. said elements being connected to said electrical means for collective operation;

f. said collective operation providing a predetermined beam pattern associated with said transducer means.

8. Apparatus according to claim 7 wherein:

a. the line is a curved line.

9. Apparatus according to claim 8 wherein:

a. The line is an arc of a circle of radius R; and

b. the apparatus is for positioning over a target area at a distance R.

10. Apparatus according to claim 9 wherein:

a. the elements are oriented with a certain depression angle such that a line perpendicular to an active surface will intersect the target area.

11. Side looking sonar apparatus comprising:

a. elongated transducer means having an active surface for transmission and/or reception of acoustic energy

b. said active surface having a width that varies in the range of from greater than .lambda. to less than .lambda., where .lambda. is the wavelength of the operating frequency in the medium in which the transducer operates.

12. Apparatus according to claim 11 wherein:

a. the length of the active surface is greater than 150.lambda..

13. Side looking apparatus comprising:

a. elongated transducer means having an active surface for transmission and/or reception of acoustic energy;

b. said active surface having a width so varied that during operation over a target area said elongated transducer means has a length L with respect to points relatively distant on said target area, and has a simulated length L' with respect to points relatively close on said target area, where L>L'.