Frequency The number of complete sound waves (or oscillations) produced in one second is called frequency of sound wave.
Wavelength The minimum distance in which a sound wave repeats itself.
Echo The repetition of sound caused by reflection of sound waves is called an echo.
Full form of SONAR is Sound Navigation and Ranging.
Two applications of ultrasound are
(i) Ultrasound is used in industry for detecting flaws in metal blocks without damaging them.
(ii) Ultrasound is used to investigate internal organs of human body such as liver, gall bladder, pancreas, kidneys, etc.