A fish finder uses a sonar device that sends 20,000-Hz sound pulses downward from the bottom of the boat, and then detects echoes. Part A If the maximum depth for which it is designed to work is 230 m , what is the minimum time between pulses (in fresh water)

 The minimum time between pulses (in fresh water)

= 0.3106 s

Explanation:

To calculate the speed of echoes sounds, we will use

Speed = 2x/t

Where x = distance and t = total time.

Please note that:

sound travels at 343 m/s in air. But it travels at 1,481 m/s in water (almost 4.3 times as fast as in air); 

Total distance = 2 × 230 = 460m

Using speed of sound in water

1481 = 460/t

t = 460/1481

t = 0.3106

this is poiselle's equation from fluid mechanic in physics. mech eng