Bass absorbers – measuring the equivalent absorption surface

Bass absorber – measuring the equivalent absorption surface

The measurement of the equivalent absorption surface of bass absorber was implemented in a reverberation chamber (125m²) of the RWTH Aachen University. All in all 16 different combinations of speaker- and microphone installations per study were analyzed. The researched frequency range spreads out over the spectrum from 40Hz to 8Hz.
The results were divided in thirds. As a measurement signal, a sine sweep with duration of 20 seconds was used. The reverberation time was determined after the third-filtration of the impulse response with help of the backward-integration (after Schroeder). The differences of the reverberation times between the rooms with and without bass absorbers and absorption materials show the absorption results.

Concerning the positioning of the edge absorbers (4 large, 8 small) to different placements of the edge absorbers were chosen:

A: edge absorbers in an edge area, built up by 2 walls

B: edge absorbers in an edge area, built up by 2 walls and the floor

The edge absorbers were always positioned in a way that the long arm of the triangle was pointing towards the edge of the room as it is displayed on our website. This results in the fact that the bass absorber reaches further into the room and a larger surface is subject to the sound (bass), which makes it more effective.

In version b the wedges were placed with one of the front surfaces on the ground. Because of that positioning, the surface reaching into the room/being subject to the sound declines. This leads to lower absorption coefficients at high frequencies.

For the 8 small edge absorbers at first two of each were placed on each other, resulting in the fact that only one front surface is facing into the room.

During the positioning, there were single absorbers placed in all room edges and the front surfaces were facing directly into the room.

The results can be found in the following diagrams and tables.

The other displayed abscissa values are the so called equivalent absorption area for each element. This is an area with an absorption value of 100%.
For example: A material with an absorption coefficient of α=0.35 and an area of 10m² has an equivalent absorption surface of 3.5m².
The results are striking: an absorber positioned in the edge of the room reaches a value of more than 3m² at a frequency of 100Hz.

Measurement results for the large edge absorber, positioned in the edge of a room. Four edge absorbers were measured, being placed in the four corners of the reverberation room.

        

 

 

Measurement results for a small edge absorber, positioned in the edge of the room.

 

Measurement results for a small edge absorber, positioned in the edge of the room. Eight edge absorbers were measured being distributed asymmetrically in the four bottom edges of the reverberation room.

 

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