Micropascal (µPa)

The micropascal is a unit of pressure equal to one millionth of a pascal. Its symbol is µPa. It is one of the smallest pressure units in practical use, and it owes its place to a single number: 20 micropascals, the reference pressure against which every sound level in air is measured.

That figure is the threshold of hearing — roughly the quietest sound a healthy young ear can detect at a frequency of about 1,000 hertz. When acousticians defined the decibel scale for airborne sound they needed a fixed pressure to compare against, and they chose that threshold. Zero decibels means a sound pressure of 20 micropascals; every decibel figure quoted for a road, a machine or a concert is a ratio to that number.

The scale that follows is steep. Twenty decibels is ten times the reference pressure, 200 micropascals. Sixty decibels, ordinary conversation, is a thousand times it, or 20 millipascals. A hundred and twenty decibels, the threshold of pain, is a million times it: 20 pascals. The ear covers a range of a million to one in pressure, and the micropascal sits at the bottom of it.

Underwater acoustics uses the same unit but a different reference. Sound in water is referred to 1 micropascal rather than 20, because the threshold of human hearing is meaningless in the sea. That difference matters enormously: a level quoted in decibels underwater is not comparable to one in air, and the gap between the two conventions is about 26 decibels before any other correction. Sonar figures, whale-song measurements and shipping-noise studies all carry the re 1 µPa qualifier for that reason.

Outside acoustics the micropascal appears in vacuum science. A good high vacuum is around 100 micropascals, and ultra-high vacuum, the regime used for surface physics and particle accelerators, runs from a few micropascals down to nanopascals. Pumping a chamber to that level takes hours of baking to drive adsorbed gas off the walls.

The unit also shows up in radiation pressure and in the very small pressure differences that laboratory instruments resolve. Anything measured in micropascals is a measurement rather than a force anyone would feel, which is exactly what makes the reference-level convention so useful.

One micropascal equals 0.000001 pascals, 0.001 millipascals, 1,000 nanopascals, or about 0.000000000145 pounds per square inch.

Micropascal conversion table