Principle of sound absorption measuring device
To determine the sound absorption properties in situ, two measurements have to be carried out, one measurement with specimen and one without specimen (free field).
The first measurement with specimen results in an impulse response containing a direct component, a reflected component from the specimen and further unwanted reflected components caused by possible obstacles nearby. Latter ones can be excluded by properly applying a specific time window. The second measurement without specimen (free field) results in an impulse response containing only a direct component.
Thus, the sound reflection factor and the sound absorption coefficient of the specimen to be characterized are determined by substracting the calculated impulse responses.
Fields of application
Originally, the measurement method was developed to evaluate the acoustic properties of road surfaces in situ without destruction. Over the years, the measurement method was applied to determine the acoustic properties of noise barriers as well.
Thus the measuring system AFD 800 can be applied to:
- measure the sound absorption coefficient and the sound reflection factor of road surfaces according to DIN ISO 13472-1 (sound absorption coefficient > 0,15)
- measure the sound absorption coefficient and the sound reflection factor of noise barriers according to DIN CEN/TS 1793-5 and DIN EN 1793-5
- determine the Miller index of noise barriers and the Miller index difference of crowns of noise barriers (influence of crowns of noise barriers on the sound diffraction) according to DIN CEN/TS 1793-4 and DIN EN 1793-4
- determine the sound insulation of noise barriers according to DIN CEN/TS 1793-5 and DIN EN 1793-6
On the basis of the fundamental measurement method, also
- the sound absorption coefficient and the sound reflection factor of plates of open porous materials
can be evaluated. In contrast to the operation of the impedance tube AFD 1000/1200 – AcoustiTube®, the sound absorption coefficient cannot only be determined for perpendicular sound incidence, but for statistical sound incidence as well.
By applying a specific model to consider the effect of sound diffration at edges, these results can be directly compared to measurement results gained in the reverberation chamber according to DIN EN ISO 354.
The measurement system AFD 800 is supported by the analysis software AFD 801.