Underwater locator beacons (pingers) are used to find the flight data and cockpi
ID: 1573313 • Letter: U
Question
Underwater locator beacons (pingers) are used to find the flight data and cockpit voice recorders (black boxes) of commercial aircraft that crash into the ocean. These beacons are triggered when they come into contact with water. When activated, they emit ultrasonic sound waves at a frequency of 37.5 kHz. The approximate intensity level of these waves is one million W/m2 at a distance of 1 m from the beacon. The pings can be detected as far as 3 km from the source. a) Determine the wavelength of the generated waves. Remember that these waves are underwater! b) How long would it take these waves to travel 3 km? c) Assuming no attenuation of the signal, find the approximate intensity level of these pings in W/m2 3 km from the source.
Explanation / Answer
given
f = 37.5 kHz
we know, velocity of sound in ocean water, v = 1500 m/s
a) using, v = lamda*f
==> lamda = v/f
= 1500/(37.5*10^3)
= 0.04 m or 4 cm
b) time taen to travel 3 km, t = d/v
= 3*10^3/1500
= 2 s
c) Intesity = power/area
Power = Intensity*area
so,
I1*4*pi*r1^2 = I2*4*pi*r2^2
==> I2 = I1*(r1/r2)^2
= 1*10^6*(1/(3*10^3))^2
= 0.111 W/m^2
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