Ultrasonic waves are produced by making use of

The waves produced by motor boat sailing on water are

Ultrasonics are sound waves of frequency:

Sonic boom is produced when a source of sound travels at a speed :

A sound wave travelling at a speed of 330 m/s produces 20 crests and 20 troughs in 0.1 second. The wavelength (λ) of the sound wave is:

Which of the following is not a musical instrument that produces sound by vibrating a string?