We've seen that the frequency, or wavelength, of pressure waves defines the pitch of a sound. The amplitude of a pressure wave defines the loudness of the associated sound. In physics, pressure is defined as force per unit area. Force in turn is defined as mass times acceleration. The unit of mass is the kilogram, and the unit of acceleration is meters per second per second, commonly stated as meters per second squared. The unit of force described in these terms is the product of these units, or kilogram meters per second squared. This unit is cumbersome to use and in practice is simply called a Newton (abbreviated as "N") after the famous physicist. One Newton is the force necessary to accelerate a mass of 1 kilogram at a rate of 1 meter per second squared. The unit of pressure is Newton per square meter of area, or Pascal (abbreviated as "Pa").
Air pressure at sea level is about ten thousand Pascal. Sound pressures vary over a wide range. The weakest sound pressure audible to the human ear is twenty one-millionth of a Pascal, while an extremely loud sound may be two hundred Pascal. Most sound pressures we hear in our daily environment are below one Pascal. To avoid expressing these as decimal fractions, it is easier to use units of micro Pascal, or one-millionth of a Pascal. Using micro Pascal, the above range of sound pressures can be expressed as twenty micro Pascal for the weakest audible sound and two hundred million micro Pascal for an extremely loud sound. Most sound pressures are below one million micro Pascal.