(Principle of ultrasonic depth measurement)
(i) Define ultrasound and state its audible limit for the human ear.
(ii) A ship determines the depth of the sea by sending a brief pulse of ultrasound towards the seabed and listening for the reflected pulse. Explain clearly why the depth is obtained by multiplying the speed of sound in sea water with the total travel time and dividing the product by 2.
(iii) List any one practical advantage of using ultrasound instead of audible sound for such depth measurements.