SOME OBSERVATIONS ON THE THEORETICAL PREDICTION OF STRENGTH AND CLARITY INSIDE HIGHLY DIFFUSING ROOMS

The paper reports the results of an acoustic survey which took into account nine Romanesque churches built in Apulia (Italy), with reference to the measurements of strength and clarity. These values were compared with those predicted by means of theoretical formulas. The results showed that strength is generally underestimated near the source and overestimated at distant points, while clarity is generally overestimated. This behaviour was thought to be due to the highly diffusing architectural elements which were inside the analysed churches. A revised formula is finally discussed in order to obtain better predictions. INTRODUCTION In recent years many acoustic parameters have been gaining more and more importance and most of them have been standardized by means of ISO 3382 [1]. Strength (G) and clarity (Ct) are two of the most widespread parameters because they provide essential information about the acoustic quality of a room and can be determined quite easily from monaural impulse responses. These acoustic parameters are much more position-sensitive than reverberation time, therefore they provide a more detailed description of the acoustic characteristics of a room. As a consequence of the growing importance of both strength and clarity many attempts have been made in order to predict their values by means of formulas, or, better, of theoretical models. Many simple relations may be found between room average values of both parameters and mean values of reverberation time. As the strength factor depends on the total acoustic absorption, the room geometry (represented by its volume) plays an important role as well [2]. However, when a parameter is position sensitive, its room average value provides only a partial information, and, in order to predict its local values, a more comprehensive theory is needed. Barron and Lee [3] proposed a model to predict strength and clarity which proved to be significantly reliable in many auditoria. The model is based on simple assumptions and accounts for the reverberation time (RT), for the room volume (V), and for the source-receiver distance (r). However, Barron and Lee observed that the model did not perform well in rooms with highly diffusing ceiling, where a steeper decrease of the values with the distance was observed. A similar behaviour was found in many churches and other worship places [2,4-6], most of which had a wooden roof with trusses and many other scattering elements such as columns, arches, and niches. Sendra et al. [4] proposed a modification to Barron’s model in order to predict strength in Mudejar-Gothic churches, but the same model proved not to be reliable in predicting clarity [5]. In this paper, the results of an acoustic survey, carried out in some Romanesque catholic churches built in Apulia (Italy) were used to analyse the problem and to propose some corrections to Barron’s theory in order to achieve better performance in predicting strength and clarity inside this kind of places. THE CHURCHES SURVEYED The churches analysed in this survey were built in the Romanesque period and share many architectural features such as the basilican plan, with a main nave and side aisles, the wooden roof with trusses, the marble floor with wooden pews, and hard limestone walls and columns. Nevertheless, they have different dimensions, with volumes ranging from 32000 to 1500 m (see Table 1), and specific features briefly described below. St. Nicholas Basilica has a wooden ceiling with painted canvases and two columns which separate the nave from the transept. Bari Cathedral has false women’s galleries (i.e. the side aisles are higher and roofed), and on the crossing there is a dome. Bitonto Cathedral has a wooden roof with densely spaced trusses and a pulpit on the right pillar of the triumphal arch. Barletta Cathedral is partly Romanesque and partly Gothic; the first part has wooden roof, the latter has ribbed cross-vaults and a choir with radial chapels. Bisceglie Cathedral has only one big apse on the main nave, on the presbytery area there are carpets and a wooden choir. Ruvo Cathedral has no women’s galleries and the aisles are cross vaulted. Bovino Cathedral has no women’s galleries but both the nave, and the aisles are roofed; the walls are plastered. Ognissanti church has a domed nave, while the aisles are barrel-vaulted. Table 1. Geometric details of the 9 churches surveyed. Id Church Volume Floor area Total area Length RTmid (m) (m) (m) (m) (s) A St. Nicholas Basilica, Bari 3200