Acoustic characterization and numerical simulations ...

AIA-DAGA 2013 Merano

Acoustic characterization and numerical simulations of Rossini Theatre in Lugo and Masini Theatre in Faenza Davide Spada, Dario D’Orazio1 , Simona De Cesaris, Massimo Garai 1

University of Bologna, 40136 Bologna, Italy, Email: [email protected]

Introduction The Rossini Theatre of Lugo (RA) was built in 175760, following the design of Ambrogio Petrocchi and completed in the stage, seating and balconies by Antonio Galli Bibiena. The Masini Theatre of Faenza (RA), was built in 1780-87, following the design of Giuseppe Pistocchi. The two theatres have never been measured and they represent a case study to evaluate the reliability of the numerical simulations of 18th century Italian Theatres. Impulse responses with different source-receiver combinations have been acquired, and monaural descriptors have been calculated. An hybrid ray-tracing software c (Odeon ) has been used in order to test the same monaural parameters, following similar studies from literature. Considering that the two theatres have comparable general aspects (volume, number of seats, materials...) but they have different geometrical elements and surface treatments that influence the acoustic field, different hypotheses on the scattered field and different resolution of numerical simulation have been compared. Thanks to the multitude of measured positions, a point-topoint comparison between the simulated and measured parameters permitted the validation of both numerical models.

Characterisation of theatres On one hand both theatres have common features: horseshoe shape, four tiers of boxes and gallery, volume , number of seats, and wooden structure. On the other hand the two theatres present differences concerning surface treatments and material, mainly as a result of restoration: vault shape and treatment, proscenium arch and cavea decoration and flooring material. The Rossini theatre was restructured first by Marconi in 1919-21, who gave to the building a new connotation with the stucco decorations , then by Cervellati in 198486, who further consolidated the theatre restructuring and reinstating the most recent modifications, which give the auditorium an ¨eaura´ı of the 17th-18th century. Structural work proved to be indispensable, particularly in order to address humidity problems; it was thus necessary to build foundations and to reconstruct and restore parts deteriorated or missing - flooring, stage structures, decorations and furnishings - replacing deteriorated elements deemed to be unsalvageable or integrating missing ones. The auditorium, with the typical Italian style, has four tiers of boxes and a gallery that, with the main floor, has 448 seats. The plan, not perfectly elliptical, is the one designed by Leandro

Figure 1: Photo of the cavea in the Rossini theatre.

Marconi in 1819-21 and floors of the main seating area and the stage have been rebuilt in wood , according to the original flooring. Several careful restoration were done on all the walls of the cavea and the vault: the stucco decorations create a coffered design on the vault, and on the balustrades of the boxes, theatre masks are alternated with composite motifs. The orchestra pit too was restored, redesigned and enlarged, and the stage was rebuilt according to tradition. At last also curtain are new. In the Masini theatre the Architect had adopted the circle as the floor plan’s driving shape: the boxes, hallways, and main floor are inscribed within three concentric circles. The horseshoe shape of the audience was accentuated only after the first restoration works to improve perspective vision. The Masini theatre was then restaured by the architect Crispino Tabenelli in 1984 in order to ensure compliance with safety regulations, to reduce fire risks and to solve the building’s static problems. In particular the seriously degraded flooring was replaced with a wooded one in the boxes, with a Venetian battuto in the perimetral hallways and with cotto in the gallery.

Figure 2: Photo of the vault in the Masini theatre.

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A comparison between the two theatres features is shown in Table 1. Theatre

Description Volume Schroeder frequency Cavea Volume Stage Volume Cavea shape Structure Places Tiers of boxes Stage Vault surface Proscenium Arch Cavea surfaces Boxes flooring

ROS 4800 m3 33 Hz

MAS 5000 m3 36 Hz

1500 m3 1000 m3 Horseshoe Wood, brick and concrete 450 Four and gallery Wood Flat Bass-relief

2300 m3 1200 m3 Horseshoe Wood

Bass-relief Cotto

Bass-relief Wood

500 Four and gallery Wood Curved Painted

Table 1: Comparison between theatre’s features: Rossini (ROS) and Masini (MAS)

Measurement Impulse response measurements were performed at 48kHz with MLS signals, dodecahedron and sub-woofer. A statistical analysis was conduced for the parameters EDT, T15 , T20 , T30 , C80 in the frequency range 63 to 4000 Hz. Data obtained were filtered by Pearson’s correlation coefficient r=0.98. IR were taken with a forward source on stage and a centre stage source in the audience, in all tiers of boxes and in the gallery. All measurements were made when the theatre was unoccupied and the microphone height was set at 1.2 m from the floor throughout the measurements; stage curtains were open, in the same position used during representations.

Numerical simulation c The 3D model of both theatres was created using Rhino 4.0 according to the assumption of infinite surface compared to wavelength. The flat surfaces were kept as large as possible while curve surfaces were divided in divided into flat surfaces, paying particular attention to concavity and convexity which can create unwanted focus. The c Odeon Software (ver. 12.0) was used for simulation. The imported models have approximately 3500 mesh, number higher than that recommended by the Guide, but necessary to describe the geometry of theatres, and it is divided into structural categories that are catalogued in layers to better manage the properties of materials. Due to Schroeder frequencies and to geometrical aspects of the theatre the impulse response was set on 2500 ms with a resolution of 5 ms, the number of rays awarded for room is 35000, and the TO was set on 0. Absorption coefficients were obtained from reference materials already present

c in Odeon and were suitably modified. The scattering calculation method chosen was Oblique Lambert, so c Odeon computes scattering considering the coefficient indicated in the material list. The model setting are calibrated on parameters EDT , T30 and C80 . Nine representative measuring points are chosen in audience according to the forward source on stage , according to guidelines for acoustical measurements inside historical opera houses [1]. Then the values have been re-checked with the rearmost source on stage and at last with the values of tiers and gallery.

Rossini theatre The Rossini theatre presents several decorations on the proscenium arch therefore numerous attempts have been made in order to set the suitable scattering coefficient. Matter of importance was to group appropriately surface in the model in order to assign the absorption and scattering coefficients. Two features of the theatre have a huge influence on the sound field: the almost elliptical cavea shape and the flat shape of the vault. At last the data obtained from simulation are compared with those measured in the theatre.

Masini theatre The theatre presents numerous decorations on balconies, on the proscenium arch, statues on the gallery external wall and bass-relief on the vault, therefore numerous attempts have been made in order to set the suitable scattering coefficient. All these not flat surfaces made trouble in setting scattering coefficients. Matter of importance was to group appropriately surface in the model in order to assign the absorption and scattering coefficients. Three features of the theatre have a huge influence on the sound field: the marked bend in the cavea plan, the curved shape of the vault, and curved ceiling in boxes. At last the data obtained from simulation are compared with those measured in the theatre.

Method of investigation The different surface treatment leads to different model setting: the definition of scattering coefficient is essential to tune the 3D model. Comb. Comb-1 Comb-2 Comb-3 Comb-4 Comb-5 Comb-6 Comb-7 Comb-8 Comb-0

GR 0.9 0.05 0.9 0.05 0.05 0.05 0.9 0.9 meas.

Scattering values VA P.A. 0.9 0.9 0.05 0.05 0.05 0.05 0.9 0.05 0.05 0.9 0.9 0.9 0.05 0.9 0.9 0.05 meas. meas.

Table 2: Scattering coefficients configurations: the graticcio, the vault and the proscenium arch are supposed to affect the scattered field.

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Some elements were chosen that are supposed to affect the scattered field - i.e. the vault, the graticcio and the proscenium arch - and several simulations were c performed using Odeon 12.0 for both theatres [2]. In each simulation, the scattering coefficient of these elements was changed and the response of the hall was analyzed. The attempt was to analyse the behaviour of EDT and C80 with the barycentric source position on the stage. Using Oblique Lambert calculation method [3], scattering coefficients of 0.05 and 0.9 were assigned to the three selected surfaces: graticcio, vault and proscenium arch. Eight configurations of scattering coefficients were tested, as shown in the Table 2. The criteria evaluated in each simulation were compared. The first comparison deals with the trend of the criteria for each combination (COMB-1 to COMB-8) related to the values measured (COMB-0)in the theatre, as shown Figure 3 for the Rossini Theatre, and Figure 5 for the Masini Theatre. In

(a) EDT

Figure 4: Comparison between EDT and C80 derived from simulation with graticcio’s, vault’s and proscenium arch’s scattering coefficient variation 0.05 and 0.9; ROS.

Elem.

125 GR 3.89 VA 4.28 PA 27.05

EDT Ratio*1000/m2 250 500 1000 2000 3.89 3.89 3.93 3.89 4.27 4.31 4.36 4.31 26.80 27.05 27.05 27.05

4000 3.93 4.31 26.76

Table 3: Scattering coefficients configurations: EDT variation per unit area of graticcio, vault and proscenium arch; 0.85 gap. ROS. (b) T30

Elem. GR VA PA

250 -0.4 -0.4 0.0

C80 Delta*1000/m2 500 1000 -0.4 -0.4 -0.9 -0.9 -2.7 -2.7

2000 -0.4 -0.9 -2.7

Table 4: Scattering coefficients configurations: C80 variation per unit area of graticcio, vault and proscenium arch; 0.85 gap. ROS. (c) C80

Figure 3: Comparison between EDT , T30 and C80 derived from scattering simulation and from measurements; ROS.

the second step, the scattering coefficient of one surface at time was changed (e.g. COMB-1 vs COMB-6), as shown Figure 4 for the Rossini Theatre, and Figure 6 for the Masini Theatre. The same analysis was developed in the third step by evaluating the variation of each criteria per unit area of the specific surface, as shown in the Table 3 and 4 for the Rossini Theatre, and in the Table 5 and 6 for the Masini Theatre.

Conclusions Acoustic characterization and numerical simulations of Rossini and Masini Theatre show the evolution of the so called Italian Theatre. In the 17th century the typical features of Italian Theatre are not still defined. The typical horseshoe shape is better identifiable in the Masini theatre were the marked bend of the cavea plan deeply influences the sound fields and reflections so that the EDT and T30 trend is markedly decreasing. In the Rossini theatre instead the rather elliptical plan together with flat surfaces and structural rigidity, makes the EDT and T30 trend flat. Furthermore the ample presence of

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(a) EDT

(b) T30

Figure 6: Comparison between EDT and C80 derived from simulation with graticcio’s, vault’s and proscenium arch’s scattering coefficient variation 0.05 and 0.9; MAS. (c) C80

Elem. Figure 5: Comparison between EDT , T30 and C80 derived from scattering simulation and from measurements; MAS.

Elem.

125 GR 6.01 VA 4.20 PA 28.94

EDT Ratio*1000/m2 250 500 1000 2000 5.88 5.87 5.92 5.92 4.14 4.09 4.16 4.16 29.00 28.70 29.46 29.16

4000 5.98 4.29 29.77

Table 5: Scattering coefficients configurations: EDT variation per unit area of graticcio, vault and proscenium arch; 0.85 gap. MAS.

velvet surfaces produces low values of T30 .

GR VA PA

250 0.0 -0.8 0.0

C80 Delta*1000/m2 500 1000 0.0 0.0 -0.8 -0.4 0.0 -2.9

2000 0.0 -0.8 -5.9

Table 6: Scattering coefficients configurations: C80 variation per unit area of graticcio, vault and proscenium arch; 0.85 gap. MAS.

[3] Shtrepi L. et Al.: Influence of scattering coefficient on the prediction of room acoustic parameters in a virtual concert hall through three different algorithms. Proc. of EURONOISE 2012, Prague (CZ), 2012. pp. 1116-1120.

The results of scattering investigations show that the variation of scattering coefficient of a single surface does not affect the acoustic criteria. Though the same analysis reported per unit area shows that the contribution of the proscenium arch is overwhelming.

References [1] Prodi, N.; Pompoli, R.: Guidelines for acoustical measurements inside historical opera houses: procedures and validation. J. Sound Vib. 232 (2000), 281-301 [2] Odeon Room Acoustics Program (ver. 12.0) Industrial, Auditorium and Combined Editions, Odeon A/S, Denmark, 2012.

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