Sound Pressure Waves

Musical Theory and Technique

This Side of Sanity

    A vibrating body (such as vocal chords or a musical instrument) will compress the surrounding air when it moves outward from its resting state, increasing the air pressure on the surrounding air because more air molecules are squeezed into a smaller volume. This causes a temporary increase in air pressure known as compression.

    When the vibrating body moves back to its normal resting state the air pressure of the surrounding air will decrease because the air molecules will be spread out over a larger volume. This causes a temporary drop in air pressure known as rarefaction.

    As the vibrations continue, the air immediately surrounding the vibrating body will alternate between high and low pressure.

    These cyclic variations in air pressure will spread out from the vibrating body as the air pressure changes affect the surrounding air molecules.

    The process of alternating compression and rarefaction will radiate outward from the sound source at the speed of sound, approximately 1087 feet per second ot 331 meters per second in normal air at sea level.

    The air molecules do not move at the speed of sound. The sound-pressure waves move through the air molecules at the speed of sound.

    The number of cycles per second of vibration are perceived as pitch by the human ear, with lower number of vibrations per second being perceived as lower notes and with higher number of vibrations per second being perceived as higher notes.

    The number of cycles per second do not affect the speed of sound.