Signal Processing and Speech Communication Laboratory
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Evaluating the decay of sound

Jamilla Balint
Gerhard Graber
Research Areas

The absorption coefficient is a central parameter in the room acoustic design process. It can be measured in an impedance tube for normal sound incidence or in the reverberation chamber for random incidence. To guarantee a standardized test environment for reverberation chambers, the international standard ISO 354 suggests a measurement procedure to increase the diffusivity of a sound field inside a laboratory environment by installing diffusers. The quality of the sound field diffusivity is evaluated by measuring the reverberation time and calculating the absorption coefficient of a sample. Unfortunately with this procedure it is not possible to achieve comparable results between different laboratories. As a consequence, the measured absorption coefficient shows unacceptable deviations in laboratories around the world. Therefore one of the most important input parameters for the room acoustic design process is not reliable.

In this thesis, the decay of sound in rooms of different shapes and sizes is investigated experimentally. A different framework to linear regression is proposed to calculate decay parameters, when the fundamental assumptions for the application of the reverberation formula in acoustics is not applicable. The decay parameters form the basis for calculating the absorption coefficient measured in the reverberation chamber. Results indicate that the beginning of the sound decay contains valuable information, which is mostly discarded for the evaluation of the reverberation time. The idea of absorption coefficient measurements leads to the topic of this work, namely the fundamental principle of measuring and calculating the reverberation time in various rooms.

The fulltext of this thesis can be downloaded here

Two listening examples (impulse responses) are prepared to illustrate the fundamental topic of this work:

Impulse Response 1:

Impulse Response 2:

What kind of space do you hear? ˆ What kind of space do you visualize? ˆ What is the difference between the two examples? ˆ Try to guess the reverberation time. ˆ Is the acoustics good or bad?