Quantification of the Acoustic Properties of the Alchija Gypsum Extraction Chamber

The Alchija gypsum chamber, located within the Knauf gypsum mine near Debar in North Macedonia, represents a distinctive underground space of significant geological and environmental value. The site is situated near thermal springs and Debar Lake. The Alchija chamber, with an approximate volume of 6500 m3, is characterized by stable microclimatic conditions and large crystalline gypsum formations. This study presents the first quantitative acoustic characterization of the Alchija Chamber, aiming to establish the frequency-dependent reverberation time and its spatial variability through in situ measurements, and to investigate additional acoustic parameters and their spatial variation through numerical modelling, with a view to assessing the chamber’s potential for future cultural use. The study combines in situ measurements of reverberation time, LiDAR-based three-dimensional geometric reconstruction, and ray-acoustic simulation in COMSOL Multiphysics 5.6. Reverberation time was measured in one-third-octave bands from 100 Hz to 5 kHz, together with background noise, air temperature, and relative humidity. An inverse characterization procedure was used to estimate frequency-dependent effective boundary absorption coefficients from the measured reverberation-time data. The calibrated model was then applied to examine additional objective parameters, including early decay time (EDT), the clarity indices C50 and C80, and the speech transmission index (STI), together with their spatial variation within the chamber. The mean measured reverberation time decreased from 1.79 s at 100 Hz to 0.93 s at 5 kHz and remained relatively uniform throughout the chamber. The inversely obtained effective absorption coefficients ranged from 0.17 to 0.34. Numerical results showed that, although reverberation time exhibited limited spatial variation, EDT, C50, C80, and STI varied more strongly with position, reflecting the influence of source–receiver geometry and the irregular chamber shape. EDT ranged from 0.30 to 1.59 s, while C50 and C80 ranged from approximately −0.62 to 9.89 dB and from 2.52 to 10.8 dB, respectively. STI ranged from 0.63 to 0.85. Overall, the results indicate acoustic characteristics potentially favourable for selected cultural and musical uses; however, suitability for specific applications requires further investigation, perceptual evaluation and measurements under representative occupancy and performance conditions.