Under-Balcony Acoustics Improvement with Simple Electro-Acoustic Means: A Theoretical Development and Numerical Simulations

Abstract

In concert halls, the acoustic quality below a large balcony is often reduced compared with the acoustic quality of the main volume. This is caused by significant differences in the reverberated energy behavior between these two volumes. The main difference is a global lack of reverberated energy under the balcony, which can be theoretically estimated by the Sabine's theory applied to coupled spaces. A solution for enhancing the reverberated energy below the balcony is to inject amplified sound picked up in the main volume. This can be performed with simple electro-acoustic channels such as those used in a regenerative reverberation enhancement system. Thus, each channel is composed of a microphone in the main volume, an amplification processing unit and a loudspeaker under the balcony. In this paper, a theoretical development based on the Sabine's approach is presented to explain and predict how the electro-acoustic channels increase the coupling coefficient between the main volume and the volume below the balcony. A new coupling criterion is also proposed and the theoretical effect of the electro-acoustic system on this criterion is exposed. Additionally, the limits of this coupling enhancement technique are presented and discussed, especially with respect to feedback problems of electro-acoustic installation in a room. In order to prove the validity of these theoretical developments, numerical simulations based on ray-tracing methods have been implemented. These numerical simulations also show that a properly calibrated electro-acoustic coupling enhancement system (EACES) can sensitively decrease the acoustic shadow effect of the balcony with respect to room acoustic criteria. Hence, electro-acoustic systems appear to be an efficient solution for coupling volumes in concert halls.