The standard “EN ISO 9053-1 Acoustics - Determination of airflow resistance - Part 1: Static airflow method”, which has its origins in a European standard and was subsequently published by the International Organization for Standardization (ISO), describes a procedure for measuring the determination of static airflow resistance in the laminar flow regime of porous materials for acoustic applications.
For the purposes of this standard, the following terms and definitions apply:
The principle of the described method is to measure the pressure drop between two free faces of a test specimen (a circular cylinder or rectangular parallelepiped) while it is subjected to a controlled unidirectional air flow.
The equipment used in these tests should generally consist of:
All instruments used for measurements must be calibrated at least every two years using conventional calibration methods.
The measuring cell shall be a circular cylinder or a rectangular parallelepiped (preferably with a square cross-section in the latter case). The diameter or smallest side of the measuring cell shall be selected depending on the test specimens. In any case, the minimum diameter or smallest side of the measuring cell shall be 29 mm. The different measuring cells may be used independently as long as they meet all the requirements of this standard.
The total height of the cell should be such that there is essentially a laminar unidirectional air flow into and out of the test sample. Its height should therefore be such that there is a free space at least equivalent to one diameter (or the larger transverse dimension) in front of the test sample. A length greater than the diameter (or the larger transverse dimension) of the sample holder is recommended behind the test sample, especially for thin test samples with low porosity (where flow disturbance has a non-negligible effect on the measurements).
The test specimen shall rest within the measuring cell (on a perforated support if necessary or preferably on a fine wire mesh) and shall be positioned sufficiently above the bottom of the cell to meet the above requirement. This support shall have at least 50 percent open area evenly distributed. The holes in the support shall be at least 3 mm in diameter. Tap points used for measuring pressure and air flow shall be leak-free.
In some cases, it may be necessary to increase the percentage of open area of the perforated support so as not to restrict airflow through the test specimen. The airflow resistance of the support should be less than 1 percent of the flow resistance measured when the specimen was tested. The specific airflow resistance of a perforated support with circular holes can be calculated according to the formula given in the standard.
In airflow generating devices, it is recommended to use water reservoir or vacuum pump type pressure reduction systems. Alternatively, pressurization systems (such as air compressors) can be used if they do not contaminate the air. Regardless of which airflow source is used, the setup should allow for precise control of the flow and ensure flow stability at the bottom of the test chamber. The airflow source should provide airflow rates that ensure the obtained velocities are sufficiently low to ensure that the measured airflow resistances are independent of velocity. It is recommended that the source allow for the acquisition of airflow velocities as low as 0,5 × 10−3 ms −1 .
The pressure tap of the volumetric air flow measuring device shall be placed between the source and the test specimen. The arrangement used shall permit the air flow to be measured with an accuracy of plus/minus 5 percent of the specified value.
The equipment used to measure differential pressure should allow measurement of pressures as low as 0,1 Pa. The arrangement used should allow measurement of differential pressure with an accuracy of plus/minus 5 percent of the specified value.
To ensure proper operation of the hardware and software, at least one calibrated test sample should be measured before a series of measurements. This measurement should be made at least once a day and after each significant change in pressure, temperature and hygrometer conditions (0,5 kPa or more, 5 degrees or more, 5 percent or more). This measurement should also be made after each operation or change to the hardware or software. The calibration test sample can be made of flat cylindrical perforations from which the measured value can be verified with a theoretical value. The value measured using the calibration test sample should correspond to the theoretical value within plus/minus 10 percent before measurements are made with test samples of unknown static air flow resistivity. As a reminder, the specific air flow resistance of a circular perforated support can be calculated using the formula given in the standard.
The test sample can be circular or rectangular, depending on the type of measuring cell available.
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