The doctoral thesis deals with the decay of sound energy in an enclosure in the low-frequency range below the Schroeder frequency. The Schroeder frequency is a frequency which marks the transition from individual, well-separated modes to many overlapping modes in an enclosure. Due to the modal behaviour of the room, the measured sound decay functions can have double or multiple slopes and the use of standardised measurement methods can become erroneous. In this work, the use of the window-width optimized time-frequency Stockwell transform to determine the decay time and modal frequencies from an experimentally measured room impulse response was investigated.
The developed method was used to characterize theoretical impulse responses with known properties and to analyze real impulse responses. We have shown that the presented method provides more accurate results compared to existing methods when analysing impulse responses where multiple modes with closely spaced frequencies and different amplitudes are present, as well as when analysing room impulse responses where noise is present. The method also overcomes the main weaknesses of standard procedures that use broadband frequency filters. We have demonstrated the usefulness of the time-frequency method using the example of measuring the absorption coefficient of materials in the low-frequency range in a small reverberation chamber (α). In this way we were able to increase the usable frequency range of small reverberation chambers.
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