Also there are sounds that the human ear cannot perceive, but can be detected by other living things or on special equipment. When, for example, you say a word the molecules of air around your mouth are pushed together forward in a straight line. When it moves back again past its original position and on to the other side, it leaves behind it an almost empty space with fewer molecules in it. The energy or push from the sound of the word keeps going down a whole row of molecules, pushing them further and further down the row. Meanwhile, the first molecules go back to their original positions.
Sound waves travel through the air by vibrating or moving the particles of matter in the air.
If there were no particles in the air, there would be no sound made. We are able to hear sound waves between 20 and 20,000 waves per second. Frequency is a term for the number of vibrations per second. If it is more than 20,000 waves per second, it is ultrasonic. The sound of music differs from noise in that the sound waves are regular and ordered.