The specific airflow resistance of an acoustical material is one of the properties that determine its sound-absorptive and sound-transmitting properties. Measurement of specific airflow resistance is useful during product development, for quality control during manufacture, and for specification purposes.

Valid measurements are made only in the region of laminar airflow where, aside from random measurement errors, the airflow resistance (R = P/U) is constant. When the airflow is turbulent, the apparent airflow resistance increases with an increase of volume velocity and the term “airflow resistance” does not apply.

The specific airflow resistance measured by this test method may differ from the specific resistance measured by the impedance tube method in Test Method C 384 for two reasons. In the presence of sound, the particle velocity inside a porous material is alternating while in this test method, the velocity is constant and in one direction only. Also, the particle velocity inside a porous material is not the same as the linear velocity measured outside the specimen.

1.1 This test method covers the measurement of airflow resistance and the related measurements of specific airflow resistance and airflow resistivity of porous materials that can be used for the absorption and attenuation of sound. Materials cover a range from thick boards or blankets to thin mats, fabrics, papers, and screens. When the material is anisotropic, provision is made for measurements along different axes of the specimen.

1.2 This test method is designed for the measurement of values of specific airflow resistance ranging from 100 to 10 000 mks rayls (Pas/m) with linear airflow velocities ranging from 0.5 to 50 mm/s and pressure differences across the specimen ranging from 0.1 to 250 Pa. The upper limit of this range of linear airflow velocities is a point at which the airflow through most porous materials is in partial or complete transition from laminar to turbulent flow.

1.3 A procedure for accrediting a laboratory for the purposes of this test method is given in Annex A1.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.