Building Acoustics

196 Building acoustics

5.6 Measurements of material parameters

In general, there exist several methods for determining the material parameters needed
for the models described here. Most of them are laboratory methods and it is certainly an
advantage having a method which allows direct determination of each individual
parameter. There is an international standard for determination of the important
parameter airflow resistance.

5.6.1 Airflow resistance and resistivity

The airflow resistivity is one of the most important parameters to characterize porous
materials. Porous materials, as applicable in building acoustics, could be mineral wool
products, plastic foam materials as well as porous fabric, curtains etc. The terms and
symbols used for characterising the resistance of materials may sometimes be confusing.
We shall here use the international standard, ISO 9053, as a directive, represented in
Table 5.2.

Table 5.2 Terms and symbols for airflow resistance.

Quantity Symbol Unit

ISO 9053

1) Airflow resistance

2) Specific airflow

resistance

3) Airflow resistivity

R

RS

r

Pa⋅s/m^3 (N⋅s/m^5 )

Pa⋅s/m (N⋅s/m^3 )^

Pa⋅s/m^2 (N⋅s/m^4 )

The ISO standard specifies two different methods for determining these quantities.
Both methods are based on measuring the pressure difference across a disk, cut out from
the test material, when a known volume of air is passing through. The difference between
the methods is that one method (method a) uses a constant airflow (DC flow) while the
other uses alternating low frequency airflow (AC flow) of frequency 2–4 Hz. In the
former case one measures the pressure difference by a manometer while the latter uses a
microphone. Sketches showing these two principal methods are shown in Figure 5.32.

d

V P

P 1

2

d

p

a) b)

Figure 5.32 Principal set-up for measurement of airflow resistance according to ISO 9053. a) Direct airflow
method; b) Alternating airflow method.