Acoustic behavior of ceramic pots used in middle age worship spaces - a laboratory analysis

Abstract

This paper presents the theoretical analysis and the results of laboratory measurements of ceramic acoustic pots similar to those used in the vaults and walls of some Middle Age churches in Switzerland and other countries. The resonance frequency of the pots was found to be from 237 Hz to 444 Hz with a high quality factor (average 5.5). A good estimation of the resonance frequency is given by the formula of Alster (with a cylindrical modeling) or by the traditional formula for Helmholtz resonators, slightly adapted. Measurements of the pressure variations inside and outside the pots showed their response in the frequency domain, their natural modes, and also their weak reverberation time (0.13 to 0.48 s), all in accordance with the theory. The intensimetric and sound level measurements highlighted a weak-operating range for the pots (difference open/closed pot on a radius of 0.3 m) of the omnidirectional re-emitted wave by the pots at the natural frequency and at certain harmonics. The sound absorption measurements in reverberation chamber highlighted a small (less than 0.05) but very selective sound absorption. In conclusion, the acoustic pots present, around their resonance frequency and certain harmonics, light phenomena of amplification, sound absorption and diffusion.