Specific air flow resistance (Rs)

Professional Definition

Specific airflow resistance (denoted by the symbol Rs or σ) is a physical quantity that determines the resistance a porous material poses to the airflow passing through it. It is defined as the ratio of the difference in static pressure on both sides of the material layer to the airflow velocity through that material.

In acoustic engineering, this is a fundamental parameter describing sound-absorbing materials (such as mineral wools, nonwovens, or open-cell foams). The unit in the SI system is Pa·s/m² (pascal-seconds per square meter). The value of Rs is essential for calculating the acoustic impedance of the material and modeling its sound absorption coefficient. A value that is too low means that the material is acoustically transparent, while a value that is too high means that it behaves like a hard barrier reflecting sound.

Acoustics in Simple Words

To understand this parameter, imagine sound as wind and acoustic material as a dense forest. If the forest is very sparse (low airflow resistance), the wind passes through it almost without loss of speed – this is how overly airy materials behave, which do not dampen noise. Conversely, if we turn the forest into a solid wall (very high resistance), the wind will simply bounce off it.

An effective acoustic material must be something "in between" – like a dense sponge. Specific airflow resistance tells us how difficult it is for air (and thus the sound wave) to squeeze through microscopic channels inside the material. During this squeezing, due to the friction of air particles against the material fibers, acoustic energy is converted into heat. It is this resistance that makes noise disappear within acoustic panels rather than returning to the room as an echo.

Summary

Specific airflow resistance is the most important technical indicator when selecting materials for bass traps and broad-spectrum absorbers. It allows for precise determination of whether a given material will effectively absorb specific frequencies or will prove ineffective in that application.