Sound waves, sometimes referred to as pressure waves, are mechanical waves that result from the back and forth vibrations of a particle through a medium. The ear uses these longitudinal vibrations to determine sound. There are two aspects of sound waves: compressions and rarefactions. Compressions are where there is a high concentration of air particles. Rarefactions are where there is a low concentration of air particles. As sound waves are pushed into the outer ear, the canal gets compressed. As the wave moves away, the pressure is immediately relieved from the ear to bring it back to a normal state. Next, the air particles continue to move out of the ear and be sucked into the air -this is known as a rarefaction. The back and forth motion causes us to hear. How we differentiate sounds is by the wavelength of the period of the sound wave.