A sound wave consists of pressures above and below the normal undisturbed pressure in the gas. P total instantaneous - P static . The instantaneous sound pressure at a point is the total instantaneous pressure at that point minus the static pressure.
Static pressure is the normal atmospheric pressure.
Particle Displacement A sound wave passing through a gas medium displaces the molecules of gas from their normal positions. Conversational speech at a distance of 10 ft. displaces the molecules of air about 1 millionth of an inch. Our ears are not sensitive to the total acoustical energy but to the rate at which the energy arrives. This rate determines loudness. The rate at which an instrument radiates energy is its acoustic power output.