Determination of the depth of sea :
The ultrasonic waves are directed to the bottom of the sea. The time (t) that elapses between the emission of ultrasonic waves and the reception of the echo is recorded. Then, the depth of the sea = h = (vt) / 2, where v = the velocity of ultrasonic waves through sea water.
Detection of air craft, submarines etc :
Because of their short wavelength, ultrasonic waves are highly directional. A piezo-electric quartz crystal oscillator is used for sending out a beam of ultrasonic waves. This is reflected back from an aircraft, if one comes in its way. The reflected beam is detected by a quartz receiver. The entire system is called SONAR from Sound Navigation and Ranging.
Sound signalling :
The highly directional ultrasonic sound beam is used for purposes of signalling to a distant ship.
Chemical applications :
(i) Ultrasonics waves are used to form stable emulsions of even immiscible liquids like water and oil (or) water and mercury. This finds an application in the preparation of photographic films, face creams etc.
(ii) They are used to liquefy gels like aluminum hydroxide in the same way as they are liquefied by shaking.
(iii) They are used to coagulate fine solid or liquid particles in a gas; for example, dust, smoke, mist etc. Ultrasonics thus find use in collecting factory dust and purifying the air.
(iv) Ultrasonics act like a catalytic agent and accelerate chemical reactions. Ultrasonic waves accelerate crystalisation.
Biological applications :
Small animals like frog, fish, rat etc, are maimed or killed when exposed to ultrasonic waves. Ultrasonic waves can kill bacteria and therefore they are used for sterilising milk.
Medical applications :
They are used for
(i) treatement of neuralgic and rheumatic pains
(ii) relieving pain in Arthritis
(iii) restoring contracted fingers
(iv) extraction of broken teeth etc.
When ultrasonic waves are focussed on a sharp instrument, it becomes capable of destroying tissues without loss of blood. Such instruments are used in performing bloodless brain surgery.
Industrial applications :
(i) A glass rod oscillating with ultrasonic frequency can be used to bore holes in steel and other hard metals.
(ii) Ultrasonic waves are used to detect cracks or flaws in metal structures.
(iii) They are used for producing alloys of uniform composition.
(iv) They are used for cleaning clothes and parts of watches etc.