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The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm. In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.
The pascal is used to measure sound pressure. Loudness is the subjective experience of sound pressure and is measured as a sound pressure level (SPL) on a logarithmic scale of the sound pressure relative to some reference pressure.
Definition. Pressure is the amount of force applied perpendicular to the surface of an object per unit area. The symbol for it is "p" or P. [2] The IUPAC recommendation for pressure is a lower-case p. [3] . However, upper-case P is widely used.
