Sound, Infrasound and Ultrasound

Sound is a mechanical wave that propagates longitudinally in material media. The speed of sound varies according to the density of the medium in which it propagates, so the denser the medium, the greater the speed of sound.

Sound waves are classified into:

Sound

Mechanical waves produced by a source that have frequencies audible to humans – ranging between 20 Hz and 20 000 Hz.

infrasound

Mechanical waves where the frequency is less than 20 Hz. This frequency cannot be heard by humans, however, there are some animals that emit and hear sounds at these frequencies, such as the elephant, the dog and the bat.

ultrasound

These are mechanical waves that have a frequency greater than 20,000 Hz. In this case, the sound also cannot be heard by the human being.
Ultrasound is widely used for clinical examinations, on ships, etc.

Note : The frequency of any periodic wave is thenumber of oscillationsthat occur per unit of time, at a point through which it passes. The frequency of the wave is equal to the frequency of what generated it, that is, its source. Thus, if a source vibrates at 50 Hz, the sound emitted by it will also be 50 Hz and, consequently, our eardrum will vibrate at 50 Hz.