Static pressure

Professional Definition

In physics and acoustics, we must precisely distinguish between the state of rest and the state of excitation (vibrations). Static pressure describes this equilibrium state.

Static pressure – the pressure that exists at a given point in a medium when there are no acoustic vibrations present; this is atmospheric pressure.

This means that it is a value that varies freely, independent of the temporary disturbances caused by sound sources. Under Earth conditions, it is simply the atmospheric pressure resulting from the weight of the column of air above us.

Acoustics in Simple Words

To understand this concept, it is best to return to the analogy of water. Imagine a perfectly smooth surface of a lake on a windless day. The water level is constant and motionless. This is the equivalent of static pressure.

Only when you throw a pebble into the water do waves appear on the surface – “hills” and “valleys” that rise above and fall below this established level. These waves are sound (acoustic pressure). But for a wave to occur, it must have water to propagate in.

In the air, it works the same way:

• Static pressure is the “density” and air pressure that surrounds us all the time (the same one you hear in the weather forecast, e.g., 1013 hPa).

• Sound is just tiny, momentary fluctuations of that pressure.

Interestingly, static pressure is gigantic compared to acoustic pressure. Atmospheric pressure (static) is about 100,000 Pascals. Meanwhile, normal conversation generates pressure fluctuations (acoustic) of just 0.02 Pascal. Sound is therefore just a gentle tremor of the powerful force that is atmospheric pressure.

Summary

Static pressure is the equilibrium state of a medium (e.g., air) in which sound does not propagate. It is the base, the background, and the carrier on which acoustic waves are superimposed. Without the existence of static pressure (i.e., in a vacuum), sound could not propagate.