Sound pressure  Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa). A sound wave in a transmission medium causes a deviation (sound pressure, a "dynamic" pressure) in the local ambient pressure, a "static" pressure. Sound pressure, denoted "p", is defined by where  In a sound wave, the complementary variable to sound pressure is the particle velocity. Together, they determine the sound intensity of the wave. "Sound intensity", denoted I and measured in W·m−2 in SI units, is defined by where  "Acoustic impedance", denoted "Z" and measured in Pa·m−3·s in SI units, is defined by where  "Specific acoustic impedance", denoted "z" and measured in Pa·m−1·s in SI units, is defined by where  The "particle displacement" of a "progressive sine wave" is given by where  It follows that the particle velocity and the sound pressure along the direction of propagation of the sound wave "x" are given by where  Taking the Laplace transforms of "v" and "p" with respect to time yields  Since formula_18, the amplitude of the specific acoustic impedance is given by  Consequently, the amplitude of the particle displacement is related to that of the acoustic velocity and the sound pressure by  When measuring the sound pressure created by a sound source, it is important to measure the distance from the object as well, since the sound pressure of a "spherical" sound wave decreases as 1/"r" from the centre of the sphere (and not as 1/"r"2, like the sound intensity):  This relationship is an "inverse-proportional law". If the sound pressure "p"1 is measured at a distance "r"1 from the centre of the sphere, the sound pressure "p"2 at another position "r"2 can be calculated:  The inverse-proportional law for sound pressure comes from the inverse-square law for sound intensity: Indeed, where hence the inverse-proportional law:  The sound pressure may vary in direction from the centre of the sphere as well, so measurements at different angles may be necessary, depending on the situation. An obvious example of a sound source whose spherical sound wave varies in level in different directions is a bullhorn.