<P> Applications of acoustics are found in almost all aspects of modern society, subdisciplines include aeroacoustics, audio signal processing, architectural acoustics, bioacoustics, electro - acoustics, environmental noise, musical acoustics, noise control, psychoacoustics, speech, ultrasound, underwater acoustics, and vibration . </P> <P> Sound is defined as "(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation . (b) Auditory sensation evoked by the oscillation described in (a)." Sound can be viewed as a wave motion in air or other elastic media . In this case, sound is a stimulus . Sound can also be viewed as an excitation of the hearing mechanism that results in the perception of sound . In this case, sound is a sensation . </P> <P> Sound can propagate through a medium such as air, water and solids as longitudinal waves and also as a transverse wave in solids (see Longitudinal and transverse waves, below). The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker . The sound source creates vibrations in the surrounding medium . As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave . At a fixed distance from the source, the pressure, velocity, and displacement of the medium vary in time . At an instant in time, the pressure, velocity, and displacement vary in space . Note that the particles of the medium do not travel with the sound wave . This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, the vibrations of particles in the gas or liquid transport the vibrations, while the average position of the particles over time does not change). During propagation, waves can be reflected, refracted, or attenuated by the medium . </P> <P> The behavior of sound propagation is generally affected by three things: </P>

What are the 3 properties of a medium that effect sound