# Concert Halls as Nearly Adaptive Spaces
## 2 Main Adaptive Factors for Room Acoustic Quality in Concert Halls
### Curtains' Systems  
In concert halls, to adjust the reverberant field when satisfying the need of chamber orchestra music or contemporary electroacoustic music, textile curtains are often hung on the lateral walls of the hall, on the stalls side, as good sound absorbers.  
Their acoustic properties depend both on their textile, the drapery fullness, and the backing condition, that is, the spacing between the fabric and a rigid backing wall, or the absence of a backing itself in the case of a freely hanging curtain that reduces the active volume of the hall.  
Several prediction models for calculating sound absorption quantities have been developed.  
A curtains' acoustic characteristics are usually described by the sound absorption coefficient, the areic mass, the air flow resistance [23], and the thickness of the air cavity between the curtain and the rigid wall, and the classical prediction model considers the thickness of the air cavity between the fabric and a rigid wall as well as the specific airflow resistance of the fabric as input parameters [16].  
This model assumes that no sound-induced fabric vibrations occur, which is fulfilled if the fabric is much heavier than the surrounding air layer. For a long time, in fact, only heavy curtains of velvet material were installed to provide the requested absorption.  
Recently, new lightweight and translucent textile curtains have been developed that exhibit good absorption properties, in which sound-induced vibrations affect the sound absorption characteristics. New models have been investigated, and several authors have proposed methods that consider these effects [24; 25; 26] for evaluating their acoustic properties.  
In the prediction phase, testing in a reverberation room can be executed [27], where the edge diffraction at the specimen [28] and the non-uniform intensity distribution of the incident sound are considered even if the absorption coefficient for normal incidence must be validated first in an impedance tube [29]. On site, various measures can be available to quantify the resulting quality of the hall [30].  
Considering the reverberation time RT derivative formula (Equation (1)), the walls covered by the curtains cannot be part of the reflective surfaces with Si included in the equivalent absorption area A (Equation (3)) (for which a residual absorption coefficient is referred) because of their higher absorption coefficient, and so they must be specified separately. The walls' surface ratio \(\text{CLW}_{c}\), which is covered by the curtains, is introduced. Equation (1) is then as follows:\[\text{RT}=\frac{55.3\;\text{V}}{C*\left(\sum_{1}^{n}\alpha_{i}S_{i}+\text{OC}_{A} \alpha_{p}S_{s}+(1-\text{OC}_{A})\alpha_{s}S_{s}+\text{CLW}_{c}\alpha_{c}S_{hw}+( 1-\text{CLW}_{c})\alpha_{i}S_{hw}+\text{mV}\right)} \tag{4}\]  
where \(\alpha_{i}\) is the residual absorption coefficient in octave bands of the surface i, from 125 Hz to 4 kHz, and refers to the reflecting surfaces \(S_{i}\) of the hall, with walls coverable by the curtains \(S_{hw}\) excluded.  
\(\text{OC}_{A}\) is the occupancy ratio (number of spectators/audience capacity).  
\(S_{s}\) is the audience surface [m\({}^{2}\)].  
\(\alpha_{p}\) is the absorption coefficient of the people [21].  
\(\alpha_{s}\) is the absorption coefficient of the empty seats at a given frequency.  
\(\text{CLW}_{c}\) is the curtain ratio referred to the coverable walls (walls' surface covered by curtains/total surface of coverable walls by curtains).  
\(\alpha_{c}\) is the curtains absorption coefficient.