Sound energy distribution in Italian opera houses

Transcription

1 Sound energy distribution in Italian opera houses Massimo Garai, Simona De Cesaris, Federica Morandi, and Dario D Orazio Citation: Proc. Mtgs. Acoust. 28, (2016); doi: / View online: View Table of Contents: Published by the Acoustical Society of America Articles you may be interested in Acoustics of Italian Historical Opera Houses The Journal of the Acoustical Society of America 138, 769 (2015); / Human listening experiments provide insight into cetacean auditory perception Proceedings of Meetings on Acoustics 29, (2016); / Proposal of suspended floor structure for reducing interior noise of railway vehicle Proceedings of Meetings on Acoustics 29, (2016); / Review of aspects that shape the aural experience in worship spaces Proceedings of Meetings on Acoustics 28, (2016); / Acoustical analysis of Aligarh Muslim University s central auditorium Proceedings of Meetings on Acoustics 28, (2016); / Acoustical evaluation of the Teatro Colón of Buenos Aires Proceedings of Meetings on Acoustics 30, (2017); /

2 Volume nd International Congress on Acoustics Acoustics for the 21 st Century Buenos Aires, Argentina September 2016 Architectural Acoustics: Paper ICA Sound energy distribution in Italian opera houses Massimo Garai, Simona De Cesaris, Federica Morandi and Dario D Orazio DIN, University of Bologna, viale Risorgimento 2, Bologna, Italy; massimo.garai@unibo.it, simona.decesaris2@unibo.it, federica.morandi6@unibo.it, dario.dorazio@unibo.it A typical Italian opera house is a complex system of coupled volumes: fly tower, orchestra pit, cavea (the volume where the stalls are), boxes, loggione (gallery). The way of propagation of the sound energy between one volume and the others is still a subject of research. The present work gives a contribution to the discussion by applying the Barron's revised theory to the analysis of recent measurements done in several Italian theatres. The averaged values of sound strength versus the distance from the sound source is plotted inserting in the Barron's equation either the classical reverberation time or the early decay time and different volume values. The significance of the different choices and their agreement with the experimental values are discussed. It is concluded that the spatial distribution of sound strength depends on the sound source position. Published by the Acoustical Society of America 2017 Acoustical Society of America [DOI: / ] Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 1

3 1 Introduction In Italian opera houses, the acoustics perceived by listeners sitting in the stalls is mainly conditioned by the following factors: the presence of the seats, usually sound absorbing; the reflections from a polished strip on the walls surrounding the stalls at the height of the listeners ears (a plaster made of marble powder and slaked lime, called marmorino); the presence of the boxes; the volume of the hall. As in a sabinian space the reverberation time is related to the volume, some authors [1, 2, 3] proposed that the volume to be taken into account be the one of the main hall, excluding the volumes of the fly tower, the boxes and the gallery. A proposal by Hidaka and Beranek [1], accepted also by Prodi et al. [2], splits the opera houses in two groups, respectively group A and group B. Group A includes theatres returning substantial energy from the backside of the sound source to the hall (partial draperies or empty stages, large stage house or hard reflecting surfaces, orchestra enclosure). Group B includes theatres returning no substantial energy from the backside of the sound source to the hall (high absorption in the stage house, sparse stage set, no orchestra enclosure). Group A and Group B have been defined on the basis of two regression curves of the measured values of reverberation time versus the volume of the audience space. It is worth noting that Hidaka and Beranek [1] and Prodi et al. [2] found different fitting curves for each group of theatres. The same authors studied the theatres also using the trend of the ratio EDT/V versus the spatial averaged value of the sound strength G, according the hypotheses of the diffuse field theory ([2], Fig. 6). In this work, relying on the large amount of measurements collected in eleven historical opera houses [4, 5, 6], the Barron s revised theory, originally conceived for concert halls only, is extended to study the spatial behaviour of the sound energy in Italian theatres. 2 Measurements The eleven theatres were investigated in an unoccupied state according to ISO [7]. The measurement sessions were carried out between May 2014 and March For the purpose of this study, the positions of the omnidirectional sound source on the stage were chosen similarly in all theatres: the first position (SS1) on the longitudinal axis of the stage at 1 m from the edge (fore stage or proscenium), the second one (SS2) at the centre of the area closed by the curtains (centre stage) see Fig. 1. An high powered omnidirectional sound source was used [8]. Table 1: Architectural features of the eleven investigated Italian historical opera houses. Theater, City Seats V hall (m 3 ) V stage (m 3 ) Scenic arch (m x m) T 30,M (s) BON Bonci, Cesena x ALI Alighieri, Ravenna x MAS Masini, Faenza x ROS Rossini, Lugo x STI Stignani, Imola x GOL Goldoni, Bagnacavallo x DRA Dragoni, Meldola x RUS Comunale, Russi x CES Comunale, Cesenatico x CER Comunale, Cervia x PET Petrella, Longiano x Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 2

4 (a) BON (b) ALI (c) MAS (d) ROS (e) STI (f) GOL (g) DRA (h) RUS (i) CES (j) CER (k) PET Figure 1: Plans of the eleven investigated theatres with the source positions on the stage (metric scale). Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 3

5 2 T30,M (s) 1.5 GOL BON A 1 B 0 1,000 2,000 3,000 4,000 V hall (m 3 ) Figure 2: Plot of the reverberation time T 30,M averaged in the octave bands of 500 and 1000 Hz versus the volume of the halls V. The dashed gray lines are the regression curves, respectively for theatres of group A and B, found by Prodi et al. [2]. In the stalls, impulse response measurements were performed at all seats for the two source positions. In each box, measurements were performed placing the microphone in the front position. In the gallery, measurements were taken in correspondence to the seats in the boxes, with some slight differences depending on the setting of the gallery. Over 50,000 IRs were processed [9]. According to ISO the height of the microphone was kept at 1.2 m [7]. The curtains were set for a standard performance of a medium-sized orchestra in the larger theatres and in the standard configuration in the smaller theatres. For the latter ones, the minimum amount of 500 m 2 of absorptive material on the stage suggested in the Charter of Ferrara [10] could not be satisfied. It is important to note that the volume of the fly tower is measured below the trellis. Room criteria have been extracted from impulse responses using optimized algorithms [11, 12, 13]. In line with the previous literature [1, 2], Tab. 1 shows the main characteristics of the theatres under study; the subscript M denotes the average over the octave bands centred at 500 Hz and 1,000 Hz. 2.1 Reverberation time and early decay time The volumes of the main halls of the two biggest theatres, BON and ALI, are comparable (see Tab. 1), but the respective reverberation times are quite different. This is due to the fact that the number of seats in the audience is different: 370 seats in ALI, 222 seats in BON. Moreover, while in BON the seats are poorly absorbing (wooden seats with a velvet lining), in ALI the seats are heavily upholstered. This causes a reduction of reverberation time of about 27%. BON is also characterised by a huge stage house, unusually large compared to the main hall. In Fig. 2 GOL is highlighted too, as its behaviour does not follow the general trend, due to finishings and the geometry of the hall. While the reverberation time T 30 is a constant of the hall, due to gaussian properties of the energy decay curve between -5 and -35 db, the EDT is more variable, being related to the deterministic part of the impulse response, and may help in identifying the transition between the early and the late reverberation. In fact, the Italian theatre is a complex system of reverberating volumes, partially coupled the one with the other. Moreover, in the Italian historical opera houses, the typical impulse responses are characterised by Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 4

6 10 6 EDTM/V (10 6 s/m 3 ) Prodi et al. Hidaka & Beranek 10 6 EDTM/V (10 6 s/m 3 ) Prodi et al. Hidaka & Beranek G M (db) (a) SS1 (proscenium) V = V hall G M (db) (b) SS2 (centre stage) V = V hall 10 6 EDTM/V (10 6 s/m 3 ) Prodi et al. Hidaka & Beranek G M (db) (c) SS2 (centre stage) V = V hall + V stage Figure 3: Plot of EDT M /V M versus G M. The theatres in group A are marked with empty squares, the ones in group B are marked with black dots. the presence of strong early reflections provided by the proscenium arch, the vault and, for the stalls area, from the smooth side walls. Thus, decays of this kind can be identified as cliff-type decays [15], being the EDT values smaller than the T 30 ones. Due to its variability EDT is also useful to qualify the difference between the various parts of the audience: stalls, boxes, gallery. 2.2 Sound Strength The natural amplification of the hall (Sound Strength) is an important factor when the sound sources are mainly singers and the orchestra is often reduced compared to the symphonic ones. In the small- or mid-sized theatres under study the measured sound strength values are high enough to assure to the listeners a good intelligibility of the singers voice [6]. It has been proved that if the Sabine s theory is valid, then G is proportional to T 30 and inversely proportional to V [1]. Thus, Fig. 3 shows the plot of the EDT/V ratio vs the G values. Hidaka and Beranek found a high correlation between EDT/V and G for the theatres with highly absorbing stage houses (group B in Ref. [1]); the importance of the draperies is confirmed also by the set of data provided in Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 5

7 the present work. The linear regressions calculated over the eleven halls with the sound source on the fore stage (proscenium) have an angular coefficient different from the one predicted by the diffuse field theory, with a trend that recalls closely the one found in Ref. [2]; this highlights the greater relative weight of the energy of the direct field to that of the reverberant field when the sound source is placed on the proscenium, i.e. between the coupled volumes of the stage house and the main hall. It should be noted that in the present study the values measured in the stalls of the theatres of the two groups (A and B) are well fitted by the same curve. This result agrees in some manner with the analogue analysis in Prodi et al. [2] and differs from the results of Hidaka and Beranek [1] for a set of larger halls in which the two curves are clearly distinguishable. 3 Method The aim of this work is to prove on an experimental basis that the sound energy inside an Italian opera house is related to the acoustic volume of the hall seen by the sound source during the measurements. In other words, the spatial distribution of G depends on the sound source position. For this purpose the Barron s revised theory and the classical diffuse field theory are compared on the same set of measurements. The revised theory of the semi-reverberant sound field [14] gives a relationship between the expected sound strength value at distance r from the source, G(r) (in db), the volume of the hall, V (in cubic metres) and the mean reverberation time measured in the hall, T (in seconds): [ 100 G(r) = 10 log r T ] V e 0.04r/T (db) (1) While in a classical concert hall the volume of the hall is unequivocally defined, in an opera house constituted by several coupled volumes it is more problematic to assess which is the acoustic volume seen by the sound source. In other words, it is not clear which volume value must be taken into account in Eq. (1): the volume of the cavea only (as proposed by Hidaka and Beranek and Prodi et al.) or both the volumes of the hall and the fly tower? In order to answer to this question, two formulations of the Barron s revised theory have been tested: in the first one V = V hall, in the second one V = V hall + V stage. Moreover, studying the effect of the balconies, Barron suggested the use of the EDT instead of T 30. Following this suggestion, and taking into account the coupling of the volumes of the stage house and the cavea in Italian opera houses, the original relationship Eq. (1) may be formulated in terms of T 30 as well as of EDT. In addition, in order to verify the effectiveness of the coupling effect the Late Decay Time (LDT ) may also be used, being LDT the reverberation time calculated from a line fitted over the range db of the energy decay curve. This gives rise to six possible combinations, summarized in Tab. 2, which should be repeated for each sound source position (SS1 on the fore stage, SS2 at the centre of the stage). Table 2: Possible combinations of volume and reverberation time values to be used in Eq. (1) for each sound source position and colour code used in Figg. 5 and 6. EDT M T 30,M LDT M V = V hall violet red orange V = V hall + V stage cyan green dark blue Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 6

8 (a) BON (b) ALI Figure 4: Averaged G values for the larger theatres under study. Left side: sound source in SS1. Right side: sound source in SS2. Colour code in Tab Results and discussion The sound strength values measured in the eleven theatres (see section 2) have been plotted versus the distance r from the sound source. The sound source positions SS1 (fore stage) and SS2 (centre stage) have been used (see section 2). For each theatre the measured sound strength values (black points) have been compared with the curves of the Barron s revised theory, calculated using in Eq. (1) as volume the volumes of the cavea only or the sum of the volumes of the cavea and the fly tower and as reverberation time the measured EDT M, T 30,M and LDT M values, averaged over the receivers in the stalls. Fig. 5 shows the results for the mid-sized theatres, Fig. 6 shows the results for the small-sized theatres. The behaviour of the Barron s revised theory curves is quite different for the two sets of theatres. For the mid-sized theatres the experimental results match the revised theory with V = V hall. For the small-sized theatres some measured results seem uncorrelated with the theoretical curves (e.g. see CER in Fig. 6); this may be due to the reduced size of the halls where the source-receiver distance and the critical distance of reverberation have comparable values. Nevertheless, some examples of small-sized theatres show a general agreement with the choice V = V hall when the sound source is in SS1. (e.g. see RUS and PET in fig. 6). Thus, the measurements suggest that, if the sound source is placed on the fore stage (SS1), the Barron s revised theory holds considering the volume of the cavea, i.e. the volume of the stalls audience only, according also with the fitting curve of Fig. 3(a). It may be argued that the different trend in this figure of the measured values with respect to the fitting curve calculated from the classical theory supports the application of the revised theory. Increasing the dimension of the theatre the averaged values of the sound strength decrease with respect to the diffuse field theory (Fig. 4). When the sound source is placed at centre stage (SS2) the Barron s theory seems no more applicable, as shown by the comparison with measurements. In fact the experimental sound strength values do not Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 7

9 (a) MAS (b) ROS (c) STI (d) GOL Figure 5: Averaged G values for the mid-sized theatres under study. Left side: sound source in SS1. Right side: sound source in SS2. Colour code in Tab. 2. Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 8

10 (a) RUS (b) DRA (c) CER (d) CES (e) PET Figure 6: Averaged G values for the small-sized theatres under study. Left side: sound source in SS1. Right side: sound source in SS2. Colour code in Tab. 2. Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 9

11 show any decreasing trend increasing the distance from the sound source. In this conditions the sound field found in the stalls seems to match the diffuse field theory, if both the volumes of the hall and the fly tower are taken into account (see Fig. 3(c)), as suggested also by the offsets of the curves in Fig.5, right side. Remarkably, the general trend of the experimental sound strength vs. distance shows a different behaviour for the two theatres of the A group (BON, GOL). The effective volume seen by SS2, according to the measurements, seems to be an intermediate value between V hall and V hall + V stage. Further comparisons between the equivalent absorption area of the fly tower and of the stalls may allow to better understand the behaviour of group A theatres. It is worth noting that, on the contrary of the concert halls studied by Barron, in the Italian opera houses the sound strength increases for the receivers near the backside of the cavea (and, more in general, at the edges of the stalls). This is due to the peculiar shape of these theatres, being more evident when they are elliptical in plan (e.g. see the measured sound strength of ROS and STI in Fig.5 and their plans in Fig. 1). 5 Conclusions The energy distribution of the sound field in eleven Italian historical theatres has been investigated, comparing the measured values of sound strength with the predictions of the Barron s revised theory. When the sound source is on the fore stage (proscenium) the revised theory is confirmed. When the sound source is placed at centre stage the revised theory seems not applicable: indeed the sound strength does not show any significant dependence on the distance from the source. This may be due to the fact that some of the hypotheses (on the behaviour of the reflections) underlying the Barron s revised theory are not applicable when the sound source is not inside the same volume of the audience, but inside a second volume (the stage house) coupled to the first [17]. Moreover, an interesting relationship has been found between the G values and the volume of the opera house: when the sound source is on the fore stage, only the volume of the hall contributes to the reverberation; when the sound source is at centre stage both the volume of the stage house and the volume of the hall must be taken into account. This fact has been proved in two ways: first by using some results of the Barron s revised theory, and secondly by comparing the measured values of EDT/V versus G with the diffuse field theory. This findings extend some proposals of the previous literature in which only the volume of the hall has been considered. The categorization of opera houses in group A and group B, proposed in the previous literature, seems relevant also for the sound energy distribution. While group B includes normal opera houses, in group A theatres the acoustic volume seen by the sound source at centre stage is about V hall + V stage /2. Further studies may be able to correlate this behaviour with the ratio between the equivalent absorption areas of the stage and of the hall. Finally, the method proposed in this study allows to identify as small theatres the ones smaller than a threshold, below which the proposed method seems to be meaningless. This happens when the theatre dimensions are comparable to the critical distance of reverberation: in this case the sound field is dominated by the direct sound field and the first reflections, thus not fulfilling the hypoteses underlying the Barron s revised theory. Acknowledgements This research project was supported by CIRI - Edilizia e Costruzioni (POR-FESR ). The authors gratefully acknowledge the directions of the studied theatres and the students who contributed to the measurement sessions. Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 10

12 References [1] T. Hidaka and L. L. Beranek (2000). Objective and subjective evaluations of twenty-three opera houses in Europe, Japan, and the Americas. J. Acoust. Soc. Am., 107(1), [2] N. Prodi, R. Pompoli, F. Martellotta, and S. Sato (2015). Acoustics of Italian Historical Opera Houses. J. Acoust. Soc. Am. 138(2), [3] D. Stanzial, D. Bonsi and N. Prodi (2000). Measurement of new energetic parameters for the objective characterisation of an opera house. J. Sound Vib., 232(1), [4] F. Morandi, S. De Cesaris, D. D Orazio, M. Garai (2015). Energy criteria in Italian historical opera houses: a survey over eleven theatres. Proc. of Inst. of Acoustics, 37. [5] S. De Cesaris, F. Morandi, D. D Orazio, L. Loreti, M. Garai (2015). Notes about the early to late transition in Italian Theatres. 22th International Congress of Sound and Vibrations, Florence, July [6] M. Garai, F. Morandi, D. D Orazio, S. De Cesaris, and L. Loreti (2015). Acoustic measurements in eleven Italian opera houses: Correlations between room criteria and considerations on the local evolution of a typology. Build. Environ. 94(2), [7] ISO Acoustics - Measurement of room acoustic parameters. Part 1: Performance spaces, Geneva, [8] D. D Orazio, S. De Cesaris, P. Guidorzi, L. Barbaresi, M. Garai, R. Magalotti (2016). Room acoustic measurements using a high SPL dodecahedron, 140th AES Convention, Paris, May [9] P. Guidorzi, L. Barbaresi, D. D Orazio, M. Garai (2015). Impulse responses measured with MLS or Swept-Sine signals applied to architectural acoustics: an in-depth analysis of the two methods and some case studies of measurements inside theaters. Energy Procedia, 78, [10] R. Pompoli and N. Prodi (2000). Guidelines for acoustical measurements inside historical opera houses: procedures and validation. J. Sound Vib., 232, [11] E. Dietrich, M. Guski, M. Pollow, B. Masiero, M. Müller-Trapet, R. Scharrer and M. Vorlænder (2012). ITA-Toolbox - An Open Source MATLAB Toolbox for Acousticians. Proc. of DAGA, , Darmstadt, March [12] D. D Orazio, S. De Cesaris, M. Garai (2012). Measuring reverberation time using preprocessed energy detection, Internoise 2012, New York City, [13] S. De Cesaris, D. D Orazio, F. Morandi, M. Garai (2015). Extraction of the envelope from impulse responses using pre-processed energy detection. J. Acoust. Soc. Am., 138, [14] M. Barron and L. J. Lee (1988). Energy relations in concert auditoriums. J. Acoust. Soc. Am., 84(2), [15] M. Barron (1995). Interpretation of Early Decay Times in Concert Auditoria. Acustica, 81, [16] M. Barron (2015). Theory and measurement of early, late and total sound levels in rooms. J. Acoust. Soc. Am., 137(6), [17] E. Cirillo and F. Martellotta (2005). Sound propagation and energy relations in churches, J. Acoust. Soc. Am, 118(1), Proceedings of Meetings on Acoustics, Vol. 28, (2017) Page 11