Chapter 3 - Introduction to Multibeam
Sonar: Projector and
Hydrophone Systems
The previous chapter examined how multibeam sonar can be ud to make up for many of the short comings of single-beam sonar. It introduced the concept of directivity and narrow projector beams. This chapter describes how
•groups of projectors, called projector arrays, and groups of hydrophones, called hydrophone arrays, can be ud to produce narrow transmit and receive beams, a process called beam forming
•the narrow beams can be targeted at specific angles using beam steering process
• a hydrophone array can be ud to simultaneously record sound from many steered beams •projector and hydrophone arrays are combined in a Mills Cross arrangement
•all of the techniques are employed in the SEA BEAM 2100 system
Projector Arrays and Beam Forming
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Recall from the ction, “A Single-Beam Depth Sounder,” in Chapter 2, that a ping from a simple single-beam echo sounder expands spherically with uniform amplitude as it propagates through water, spreading its acoustic energy equally in all directions. This symmetric spreading is called an isotropic expansion, and the projector that produces it is called an isotropic source. A good example of a wave with isotropic expansion is the circular pattern produced when a small stone is dropped in a quiet pond (e Figure Chapter 3 - -1).
Projector and Hydrophone Systems Multibeam Sonar Theory of Operation
Figure Chapter 3 - -1: Isotropic Expansion
平假名表An isotropic source is not ideal for a depth-sounding sonar for two reasons:
•The spherically expanding pul strikes the ocean floor in all directions. There is no way to determine the direction of the return echoes, so no detailed information about the bottom can be discerned.
finish的过去式•The power of the transmitted pul is nt equally in all directions, so much of it is squandered, ensonifying areas that may not be interesting.
Fortunately, groups of isotropic sources, called projector arrays, can be ud to transmit non-isotropic waves or sound waves who amplitude varies as a function of angular location (still spreading spherically), allowing projected puls to have a degree of directivity. Directed puls can be ud to ensonify specific areas on the ocean floor, causing stronger echoes from the locations. Ranges can then be found to tho locations, generating more detailed information about the bottom.
Recall from the ction, “The Physics of Sound in Water,” in Chapter 2, that a sound wave is compod of a ries of pressure oscillations. The circular solid lines in Figure Chapter 3 - -1 reprent high pressure peaks. Spaced half-way between the lines are low pressure troughs reprented by dashed lines. Alone, an ideal single-point projector always produces an isotropically expanding wave. Operating at a constant frequency, it creates a continuous ries of equally spaced, expanding peaks and troughs, which look similar to what is pictured in Figure Chapter 3 - -1.which是什么意思
If two neighboring projectors are emitting identical isotropically expanding signals, their wave patterns will overlap and interfere with each other. This situation is depicted in Figure Chapter 3 --2. At some points in the surrounding water, the peaks of the pattern from one projector will coincide with peaks from the other, and will add to create a new, stronger peak. Troughs that coincide with tro
ughs will create new, deeper troughs. This is called constructive interference. At other points, peaks from one projector will coincide with troughs of the other and will effectively cancel each other. This is called destructive interference.
In general, constructive interference occurs at points where the distances to each projector are equal, or where the difference between the two distances is equal to an integer number of wavelengths. Destructive interference occurs at positions where the difference between the distances to the projectors is half a wavelength, or half a wavelength plus an integer number of wavelengths (1.5, 2.5, 3.5, and so forth). If a hydrophone is placed at the positions of constructive interference, a combined wave would be measured with an amplitude twice that of the signals emitted by each projector individually. A hydrophone placed at a position of destructive interference would measure nothing at all. Where are the places?
Figure Chapter 3 - -2: Constructive and Destructive Interference
otc什么意思啊Projector and Hydrophone Systems Multibeam Sonar Theory of Operation In Figure Chapter 3 - -3, two projectors P 1 and P 2 are parated by a distance d (referred to as
the element spacing ). Consider a point located distance R 1 from P 1 and R 2 from P 2. If this point i
s
located anywhere on the perpendicular bictor of line P 1P 2, then R 1 and R 2 are equal. Any point along this line will witness constructive interference.
Figure Chapter 3 - -3: Positions of Constructive Interference (Example 1)
The locations of other constructive interference are less obvious, but they can be found with some simple geometry. In Figure Chapter 3 - -4, two projectors P 1 and P 2 again have a spacing d .
Consider a point at a location R 1 from P 1 and R 2 from P 2. The direction to this location (labeled R 0in the figure) intercts a line perpendicular to the spacing d with an angle θ0. Next, assume that
the point you are considering is very far away compared to the spacing of the projectors—meaning that R 1 and R 2 are much larger than d. For a typical operating environment for a sonar,
this is a good approximation—projectors are spaced centimeters apart (d = cm) and the ocean floor they are ensonifying is hundreds or thousands of meters away (R 0 = 100 m to 1000 m,
meaning R 0/d = 1000 to 10000). This is called the far field approximation, and it is necessary to
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keep computations simple. In this situation, the lines R 0, R 1, and R 2 are treated as parallel, and all
subtleintercting angles θ0, θ1, and θ2 as equal.
Figure Chapter 3 - -4: Positions of Constructive Interference (Example 2)
The difference between R
1 and R
2
is the line gment labeled A in Figure Chapter 3 - -4. If all
angles are equal, this distance is:
A = d× cos (90 - θ
), (3.1) or more simply:
incidentA = d× sin θ
.
(3.2) Recall that constructive interference occurs when A is an integer number of wavelengths:
A/λ = 0, 1, 2, 3, 4, . . . . . . . . . .etc., (3.3) where λ reprents wavelength.
Substituting Equation 3.2 for A:
(d/λ) × sin θ
= 0, 1, 2, 3, 4, . . . . . . . .etc.(3.4) Similarly, destructive interference will occur where:
(d/λ) × sin θ
mtc= .5, 1.5, 2.5, 3.5, . . . . . . .etc.(3.5) From the equations you can e that locations of constructive and destructive interference are
dependent on the projector spacing d, the wavelength of the sound emitted λ, and the angle θ
to the location. Both d and λ remain constant for a typical sonar installation—the only remaining variable is θ
设计师培训. This indicates that two projectors in the configuration described will transmit constructively interferin
g (that is, high amplitude) waves in certain directions, while in others it will transmit nothing due to destructive interference. Knowing d in terms of λ, you can determine which directions will have constructive and destructive interference.
