Reviving the Low-Frequency Response of a Rupestrian Church by Means of FDTD Simulation

Abstract

Rupestrian churches are spaces obtained from excavation of soft rocks that are frequently found in many Mediterranean countries. In the present paper the church dedicated to Saints Andrew and Procopius, located close to the city of Monopoli in Apulia (Italy) is studied. On-site acoustical measures were made, obtaining a detailed description of the acoustics in the current state pointing out, thanks to a combination of analysis techniques, the presence of significant modal behavior in the low frequencies, causing reverberation time to be about 2 s, four times longer than in the other bands, as well as being strongly dependent on source and receiver position (with variations of about 1 s when source is moved outside the chancel). However, as the church is characterized by significant degradation of surfaces and large amounts of debris cover the floor, the original acoustic conditions can be expected to somewhat differ. Acoustical modelling can be very helpful in grasping the original conditions, but given the small dimensions of the space, conventional geometrical acoustic prediction methods cannot be applied to simulate the low-frequency behavior. Thus, the present paper proposes an application of finite-difference-time-domain (FDTD) computation to simulate the low-frequency behavior and analyze a possible reconstruction of the original state. Results showed that a very good agreement was obtained between predictions and measurements, both in terms of resonance frequencies and reverberation times that differed by less than 5%. Modal response strongly affected the acoustical conditions also in the hypothetical reconstruction of the original state, although the sound field proved to be more uniform than in the current state.

Acoustics, Soundscapes and Sounds as Intangible Heritage

“…Since acoustic measurements cannot be performed in the church in its current condition, the simulation models are tuned taken as a reference a set of acoustic measurements previously conducted in two structures in the nearby region built with similar materials. Material tests are again necessary during the modelling for determining the sound absorption characteristics of tuff rocks [15]. This acoustic study assesses the evolution of the acoustical performance of the church over time based on three different acoustic models of the space: Phase I (without frescoes), Phase I (with frescoes), and Phase II (with frescoes and narthex), in an attempt to understand the context of the use of a Middle-Byzantine church for different cultural practices.…”