Sound Pressure (p)

Professional Definition

Before we move on to simpler explanations, let’s look at the formal definition that we will encounter in standards and technical literature. It is two-part, as one concept directly results from the other.

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

Acoustic pressure (p) – the difference between the pressure existing at a given point in the medium at the moment of the passage of an acoustic wave and the static pressure.

In short, acoustic pressure is not an absolute value but a temporary change, a fluctuation around the "normal" pressure that surrounds us.

Acoustics in Simple Words

Let’s imagine a perfectly calm lake surface. That is our resting state, which is static pressure (atmospheric). It is present around us all the time, although we do not feel it. Now let’s throw a stone into the water. A wave is created that spreads across the surface. This wave has its "peaks" and "troughs".

Acoustic pressure is exactly the same, only in the air. A sound source (e.g., a guitar string, our vocal cords) acts like that stone – it sets air particles into vibration. These vibrations travel, creating microscopic, alternating zones:

• Compressons (temporary increase in pressure, a "peak" on the wave).

• Rarefactions (temporary decrease in pressure, a "trough" on the wave).

These rapid fluctuations in pressure around a constant level of atmospheric pressure are what constitute a sound wave. When it reaches our ear, it moves the eardrum, and our brain interprets these vibrations as sound.

The greater the amplitude of these fluctuations – meaning the higher the "peaks" and deeper the "troughs" – the greater the acoustic pressure, and the louder the sound we perceive.

It is important to realize how incredibly sensitive the human ear is.

• Atmospheric (static) pressure is approximately 100,000 Pascals (Pa).

• The threshold of hearing, or the quietest sound we can hear, corresponds to an acoustic pressure of just 0.00002 Pa (i.e., 20 micropascals, µPa).

It is this value (20 µPa) that has been adopted as a universal reference point in acoustic measurements.

Key Conclusions

Operating at such small and simultaneously varied values (from the threshold of hearing to the pain threshold at around 20 Pa) is very inconvenient. Therefore, in practice, we rarely use the absolute value of acoustic pressure in Pascals. Instead, we relate it to the threshold of hearing (20 µPa) and convert it to a much more convenient, logarithmic scale – decibels (dB). We will explore this topic, namely the level of acoustic pressure, in one of the next articles.

Remember:

• Sound is a physical, measurable phenomenon – rapid fluctuations in pressure in a medium.

• Acoustic pressure is the amplitude of these fluctuations.

• Greater acoustic pressure means (in simplified terms) a louder sound.