Question

A submarine uses sonar (sound pulses) to locate objects. The sub radiates the sound like a point source in all directions at a power of 700 W and there are no losses. (give units in all answers)

a) What is the sound intensity impinging on an object at a 150 m distance? Answer: 2.476 mW/m2
b) What is the ‘sound intensity level’ measured at a distance of 150 m? Answer: 93.94 dB
c) The sub reflects power like a flat metal plate 25 m2 in area, what is it at 150 m? Answer: 6.19(10-2) W

Answer 1

(A)

Intensity = power / area

here, propagation of sound is in all directions,

Intensity = P / 4r²

Intensity = 700 / 4 * 150²

Intensity = 2.475e-3 W/m²

or

Intensity = 2.475 mW / m²

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(b)

sound level can be found as

L = 10 log ( I / 10^-12 )

L = 10 log ( 2.475e-3 / 1e-12)

L = 10 * 9.394

L = 93.94 dB

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(c)

P = Intensity * area

P = 2.476e-3 *  25

P = 6.19e-2 W