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Target Strength reflected wave incident wave a At r = 1 yd.

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Factors Determining Target Strength the shape of the target the size of the target the construction of the walls of the target the wavelength of the incident sound the angle of incidence of the sound

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Target Strength of a Convex Surface R1R1 R2R2 Incident Power Large objects compared to the wavelength

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Reflected Intensity R1R1 R2R2 (At r = 1 m)

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Special Case – Large Sphere a Note: Large means circumference >> wavelength TS positive only if a > 2 yds

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Large Spheres (continued) a 0o0o 180 o

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Example An old Iraqi mine with a radius of 1.5 m is floating partially submerged in the Red Sea. Your minehunting sonar is a piston array and has a frequency of 15 kHz and a diameter of 5 m. 20 kW of electrical power are supplied to the transducer which has an efficiency of 40%. If the mine is 1000 yds in front of you, what is the signal level of the echo. Assume spherical spreading.

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Scattering from Small Spheres (Rayleigh Scattering)

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Scattering from Cylinders L 2a Dimensions (L,a) large compared to wavelength

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Gas Bubbles Damping effect is due to the combined effects of radiation, shear viscosity and thermal conductivity. A good approximation is where f k is the frequency in kHz.

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Fish Main contribution for fish target strength comes from the swim bladder. This gas-filled bladder shows a very strong impedance contrast with the water and fish tissues. It behaves either as a resonator (frequencies of 500 Hz-2 kHz depending on fish size and depth) or as a geometric reflector (> 2 kHz). This swim bladder behaves very similar to gas bubbles. The difference in target strength between fish with and without swim bladder can be 10-15 dB. A semi-empirical model most often used is: Love (1978) This formula is valid for dorsal echoes at wavelengths smaller than fish length L.

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Form t TS=10log(t) SymbolsDirection of incidenceConditions Any convex surface a 1 a 2 = principal radii of curvature r = range k = 2 /wavelength Normal to surface ka 1, ka 2 >>1 r>a Large Spherea = radius of sphereAny ka>>1 r>a Small Sphere V = vol. of sphere = wavelength Any ka<<1 kr>>1 Infinitely long thick cylinder a = radius of cylinder Normal to axis of cylinder ka>>1 r > a Infinitely long thin cylinder a = radius of cylinder Normal to axis of cylinder ka<<1 Finite cylinder L = length of cylinder a = radius of cylinder Normal to axis of cylinder ka>>1 r > L 2 / a = radius of cylinder = kLsin At angle with normal Infinite Plane surfaceNormal to plane Rectangular Plate a,b = sides of ractangle = ka sin At angle to normal in plane containing side a r > a 2 / kb >> 1 a > b Ellipsoid a, b, c = semimajor axis of ellipsoid parallel to axis of a ka, kb, kc >>1 r >> a, b, c Circular Plate a = radius of plate = 2kasin At angle to normal r > a 2 / ka>>1

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Example What is the target strength of a cylindrical submarine 10 m in diameter and 100 m in length when pinged on by a 1500 Hz sonar? TS 10 o 5o5o

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Example What is the target strength of a single fish 1m in length if the fish finder sonar has a frequency of 5000 Hz?

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