Read the following passage and answer the questions from 21 to 25 :
Sound waves are formed due to vibrations of particles and propagate in a material medium in the form of longitudinal waves. The sound waves of frequency less than 20 Hz are called infrasonic waves. The human ear can hear the sound waves between frequency range 20 Hz to 20000 kHz. This is the audible range. The sound waves of frequency greater than the audible range are audible to certain animals such as cats, dogs, bats, etc. These are the ultrasonic waves. While flying, a bat produces ultrasonic waves. These waves strike obstacles in their path and are reflected. The bat can use echoes to detect the obstacles and hence avoid collisions with them. Thus a bat can find its way even is the dark.
Ultrasonic waves are produced by piezoelectric oscillators and magnetostriction oscillator method and detected by Kundt's tube, Quartz crystal and Thermal detector methods.
The waves having more frequency, possess more energy. Hence ultrasonic waves possess high energy. Like sound waves, ultrasonic waves also get reflected, refracted and absorbed. Their velocity increases with frequency.
Ultrasonic waves are used in many industrial, scientific and medical sciences. Sound navigation and ranging (SONAR) is the instrument which can produce, transmit and receive ultrasonic waves. This technique is used to determine the distance and location of underwater objects such as submarines, icebergs, etc. using ultrasonic waves. It can be used to determine the depth of the sea. For this a transmitter and a detector are installed in a ship. Ultrasonic waves are also used for ship to ship communication, welding plastic surfaces, to kill bacteria in liquids like milk to preserve them, in echocardiography, for imaging internal organs of the human body, in industry to clean parts of machines, to detect cracks and flaws in metal blocks.