Sound is a form of energy that propagates as longitudinal waves, requiring a medium. It is produced by vibrating objects and transmitted through compression and rarefaction variations in the medium. The human ear can detect sounds between 20 Hz to 20 kHz. Ultrasound with frequencies above this range has applications like medical imaging and industrial cleaning, while infrasound below 20 Hz is used for communication by some animals. Sonar uses ultrasound for underwater detection of objects.