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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