# Acoustic certificates

To determine the exact absorption behaviour of products it is necessary to have them measured in a reverberation room. For this reason, the majority of our acoustic elements have already been tested in a certified reverberation room of the Technical University Graz. All current certificates are available for download on our homepage. Below, you will find an overview and explanations of the most important terms of certification.

Go to certificates## Absorption coefficient

The absorption coefficient of a material or object indicates how much sound it absorbs. The value ranges between 0 (complete reflection) and 1 (complete absorption), the value usually differing depending on the frequency. The harder and smoother a material is, the worse its absorption coefficient. For example, a smooth stone floor has absorption coefficient of virtually 0.00 across all frequencies. In contrast, an open window represents an ideal absorber with an absorption coefficient of 1 across all frequencies. The sound exits through the open window and none of it is reflected.

In room acoustics, the absorption coefficient is specified across the frequencies 125 Hz to 4,000 Hz. Thin materials, such as carpeted floors, usually absorb better in higher frequency ranges and worse in low ones.

The absorption coefficient is necessary in order to determine the reverberation time in a room by means of the Sabine equation. In doing so, the surface area of the material used is multiplied by the respective absorption coefficient, in order to obtain the absorption surface area in m². Example: 1 m² carpeted floor with an average absorption coefficient of 0.5 has an average absorption surface area of 0.5m².

## Absorption class

Due to their absorption coefficient, materials or objects are categorised in absorption classes from A to E. A is the highest absorption class and therefore has the highest absorption coefficient.

The absorption class is used only for the rough classification of absorbing materials or products. However, as the distribution of the absorption across all frequency ranges is essential in room acoustic planning, the absorption class is not a sufficient parameter.

## Absorption surface area

While the absorption coefficient indicates the absorption per square metre, the absorption surface area indicates the absorption per object.

The absorption surface area is calculated from the absorption coefficient multiplied by the surface area of the object. Therefore, 1 m² carpet with an average absorption coefficient of 0.5 has an average absorption surface area of 0.5 m².

For some objects it is more appropriate to specify the absorption surface area per object instead of the absorption coefficient. Since calculating the surface area of complex three-dimensional objects proves difficult, the calculation can be performed here directly using the specified absorption surface area which is multiplied by the number of products used. In our example, an ordinary armchair would therefore have an absorption surface area of 1.5 m².