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We are have heard sounds. But how exactly do scientist define sound and what exactly is sound?
Sound is any disturbance that travels through air, ground, or water to be heard by the ears. When something vibrates, or moves back and forth, a disturbance of the surrounding air, water, etc. moves outward in straight lines in the form of waves. The effect these waves produce upon the ear is perceived as sound. Sound is thought to be the waves caused by the vibration of the object, whether or not they are heard by the ears. Thus, people who are deaf may sometimes feel the vibration of sound although they cannot hear the sound. Also there are sounds that the human ear cannot perceive, but can be detected by other living things or on special equipment.
How does this happen? When, for example, you say a word the molecules of air around your mouth are pushed together forward in a straight line. 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. The crowding together of the molecules of air is called compression and a lessening in the air of the molecules is called rarefaction.
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 (Hz). Frequency is a term for the number of vibrations per second.
If the sound waves are below 20 waves per second, it is subsonic.
If it is more than 20,000 waves per second, it is ultrasonic.