Sound is a phenomenon in which pressure disturbances propagate through an elastic material medium. In the context of physics, it is characterised as a mechanical wave of pressure or related quantities, whereas in physiological-psychological contexts it implies a hearing process. Though sensitivity to sound varies among all organisms, the human ear is sensitive to audio frequencies ranging from 20 Hz to 20 kHz. More broadly, acoustic waves encompass mechanical longitudinal waves in general. Examples of the significance and application of sound include music, medical imaging techniques, oral language and parts of science.