Airflow resistance

(definitions acc. ISO 9053)
The resistance against airflow passing through a porous material with thickness t can be measured with the pressure drop p for an airflow with velocity v.

Airflow resistance R=p/v [Pa/m]

The measured R/t is called the airflow resistivity r and is a product property independent of the actual thickness:

Airflow resistivity r=R/t [Pa/m2]

The airflow resistivity increases with increasing density and decreasing fibre thickness. Beside density and fibre thickness the wool structure (orientation, spot contents etc.) influences the airflow resistivity.

The absorption of energy from a sound wave mainly depends on the airflow resistivity of the wool. If the resistivity is too high, the sound wave is reflected and only a minor part is absorbed. If the resistivity is too low, the sound wave can pass through the wool without being absorbed. So depending on the application (thickness of the material, adjacent materials etc.) and the interesting frequency, different optima for the airflow resistivity may occur.