Acoustics of Margravial Opera House Bayreuth
Sebastian Krauss, Simeon Völkel, Christoph Dobner, Alexandra Völkel, Kai Huang
aa r X i v : . [ phy s i c s . pop - ph ] M a y Acoustics of Margravial Opera House Bayreuth
Sebastian Krauss, Simeon V¨olkel, Christoph Dobner, Alexandra V¨olkel, and Kai Huang Experimentalphysik V, Universit¨at Bayreuth, 95440 Bayreuth, Germany Email: [email protected]
Abstract
The Margravial Opera House Bayreuth, built between1745 and 1750, is a well preserved Baroque court theatredesigned by Giuseppe Galli Bibiena [1]. It provides anopportunity to experience not only the visual but alsothe acoustic design of opera theatres in the 18th cen-tury, as the bell-shaped auditorium along with the dec-oratively painted canvas remains intact. Using balloonsand hand-claps as sound sources, we characterize the im-pulse response of this opera house after its recent reno-vation. The reverberation time (RT), early decay time(EDT) and clarity factor are characterized and discussedin comparison to historical Italian theatres of a similarage.
Introduction
When the Margravial Opera House Bayreuth was beinginscribed in the UNESCO’s list of world cultural and nat-ural heritage in 2012, one justification for its outstandingvalue was that “The Margravial Opera House is a master-work of Baroque court theatre architecture by GiuseppeGalli Bibiena in terms of its tiered loge form and acous-tic, decorative and iconological properties.” [1] While theextraordinary interior decorations along with the 18thBaroque fa¸cade are readily perceivable through site toursand photographs, the acoustics of this opera house is farless discussed. Motivated by this fact, following a re-cent characterization of the Bayreuth Festspielhaus [2],we measure the impulse response of this opera house toprovide an objective evaluation of its acoustics. Thismeasurement is conducted roughly half a year after therecent renovation.Below is a short history of the Opera House adaptedfrom the book edited by Rainer [3], with an emphasison the main changes made to the opera house in thepast 270 years. This court opera house was originallybuilt to celebrate the marriage of the only daughter ofMargrave Friedrich and Margravine Wilhelmine in 1748.Since then, it had been regularly used by Wilhemine,who enjoyed writing librettos as well as composing mu-sic, despite of the high maintaining cost. After the deathof Wilhemine in 1758, the Margravial opera house hadrarely been used and hence kept largely intact until 1817,when the first renovation took place. The depth of thestage area for performance was reduced by roughly 45%.Warm air heating systems, lightening and a fire protec-tion system have been subsequently installed as the operahouse became more and more often used as a municipaltheatre. It was this opera house that attracted RichardWagner to Bayreuth for implementing his festival idea in 1871, although he later considered that this operahouse should better be kept exactly as it was. During thesecond renovation (1935-1936), the proscenium area hadbeen modified, including the replacement of the woodenrailing with a curving balustrade, new stairs in front ofthe stages, as well as a reduced size of stage opening.Those changes had been reversed to restore the originalsubstance in the most recent renovation (2013-2018), dur-ing which roughly 90% of the original painting layer hadbeen recovered, stabilized and strengthened. To summa-rize, beside the non-original wooden floor, the stage setand possible modifications to the proscenium area, theacoustics of the opera house nowadays should be veryclose to its original status in the 18th century.
Measuring Procedure
We characterize the acoustics of the Margravial operahouse by measuring the impulse response, based onwhich further characterizations on the strength, musicclarity, speech intelligibility, echoes, reverberation, andother features of a room can be subsequently quanti-fied [4, 5]. Nowadays, such characterizations are be-coming more convenient using hand claps and embeddedMEMS (micro-electro-mechanical systems) microphonesin smart phones [6, 7]. In a previous investigation, it wasdemonstrated that hand-claps and smart phone record-ing (HCSP) approach can be used to characterize thereverberation time of a room reasonably well, except forthe low frequency regime [2]. Here, in addition to HCSP,air balloons and condenser microphones are also used assound sources and recording devices, respectively. Therecorded signals are consequently analyzed with Matlabusing the ITA-Toolbox [8], which provides a standardroutine for room acoustic characterizations following ISO3382 [9].As shown in Fig. 1, the Margravial opera house inBayreuth has a bell shaped auditorium including stalls, agallery and three tiers of loges. This design resembles theplan of Lamberti’s “ideal” theatre [11]. The heavily dec-orative ornaments are designed on the one hand for thefestival, and on the other hand for improving the acous-tics inside, representing the belief of the architects [11].Except for the floor, the auditorium retains its originalbuilding material, i.e., wood [3]. More specifically, lin-den (tilia) was used for the sculptures, while fir (abies)and spruce (picea) were used for the other parts. Part ofthe wooden walls and the ceiling are covered with canvas.The auditorium has a total volume of about 5000m with520 seats distributed on the floor, three tiers of loges, thecourt loge and the gallery. S1SS3
Stage
XXX yx XXXX X ba ck d r op SS2
Figure 1:
Upper panel: The interior of the Margravial operahouse viewed from the court loge [10]. Lower panel: Groundplan of the Margravial Opera House Bayreuth adapted fromRef. [3] with the borders of the auditorium, stage, as well asthe locations of the sound sources (SS) and recording micro-phones (M) marked. M8 is located in the court loge oppositeto the stage. The gray shaded region marks orchestra area.See Table 1 for measured coordinates in the Cartesian systemdefined by dashed arrow lines. The thick red line correspondsto a iron curtain, which was closed for measurement SS4 andopen for SS1-3.
Table 1:
Positions (in meter) of sound sources and recordingdevices in the coordination system defined in Fig. 1(b).
SS1 SS2 SS3 SS4X -17.7 -2.7 -2.7 4.1Y 0 0 2.4 0M1 M2 M3 M4 M5 M6 M7 M8X 6.5 10.7 14.9 19.1 7.8 12.3 17.5 23.5Y 0 0 0 0 4.1 3.3 2.4 0The measurement was conducted on October 11, 2018.In the auditorium, the temperature and relative humidityin the auditorium was 18.9 degrees and 45%, respectively.Impulse signals are generated either by air balloons (AB)or hand-claps (HC). The balloons are inflated with apump to a diameter of 23 ± . ∼ . Results and Discussion -1-0.500.51 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -0.2-0.100.10.2 a) p a ud i . ( a r b . un it ) p a ud i . ( a r b . un it ) Time (s)Balloon Clap-30-20-100 b) -30-20-100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 c) E n e r gy d eca y c u r v e , E D C ( d B ) Time (s)EDC BalloonEDC Clap EDTT20
Figure 2: (a) Representative raw signals from one balloon-burst and one hand-clap at SS3 recorded by M3. (b, c) Corre-sponding energy decay curves of the impulse from the burstingballoon and from the hand-clap at the 1000 Hz octave band.The orange and green solid lines correspond to linear fits forobtaining the early decay time and T20, respectively. For theEDC curves, only every 600th data point is shown for a bettervisibility. igure 2 shows a comparison of two sample raw signalsgenerated by a balloon-burst as well as by a hand-clap.It shows that the sound pressure level (SPL) generatedby hand-claps is roughly one order of magnitude smallerthan that from balloon-bursts. Consequently, the EDCcurve in the former case decays rather quickly to thesurrounding noise level in comparison to the latter case.Therefore, the characterization of RT with T20 or abovebecomes unfeasible, as the fitted curve for T20 in (c)shows. In order to obtain reliable RTs, we set a thresh-old for the uncertainty obtained from a least square fit:Only RTs with a relative uncertainty smaller than 10%are used in the following analysis. With this criterion, un-realistic RTs [e.g. fit in (c) delivers T20=5 . ± . R e v e r b e r a ti on ti m e T ( s ) Frequency (Hz) SS4SS1
Figure 3:
Reverberation time (T20) of the auditorium atdifferent octave bands for two different conditions: With theiron curtain open (SS1) or closed (SS4). It is averaged overall microphones except for M2, which has technical issues.The error bars correspond to the maximum of the uncertaintyfrom fitting and that from data scattering among differentmicrophones.
The spectral variation of the reverberation time is shownin Fig. 3 and listed in Tab. 2. Similar to ‘regular’ Ital-ian historical opera houses, there exists a clear increaseof RT as frequency decreases to the 125 and 250 Hz oc-tave bands, arising from the extensive mid- and high-frequency sound absorptions due to the interior de-sign [16]. For instance, the loges act as resonators for lowfrequency sound, and meanwhile as effective absorbersfor mid- and high-frequency sound, owing to the deco-rative design and sound absorbing finishes. For a better visibility, only T20 values are shown here as representa-tives. The other RT values agree with T20 within exper-imental uncertainties, provided that the fitting criteriondescribed above is applied. The reverberation times ofthe auditorium for low (125 and 250 Hz octave bands)and middle (500 and 1000 Hz octave bands) frequencyranges, averaged over various sound sources and micro-phones are 1 . ± .
11 s and 1 . ± .
02 s, respectively.This results lead to a bass ratio (BR) of 1 .
29, matchingthe grand average (1.30 in unoccupied conditions) of 50traditional Italian opera houses characterized recently byProdi et al. [16].When the iron curtain is closed (condition for measure-ments taken at SS4), the overall reverberation time re-duces to 1 . ± .
05 s and 1 . ± .
03 s for low- and middle-frequencies, respectively. According to the Sabine for-mula [17, 4], RT ∝ V /S with total volume V and surfacearea S of the room. As V is dramatically reduced withclosed iron curtain, and in the meanwhile S does notchange with the same proportionality as V (due to thedecorative, sound absorbing surfaces), the overall RT isexpected to decrease substantially. The other obviousfeature is the vanishing spectral variation, which arisespresumably from the vanishing coupling between the au-ditorium and the stage [4]. Table 2:
Averaged reverberation time obtained without(SS1) and with (SS4) ironcurtain closed, unoccupied.
125 250 500 1000 2000 4000 HzSS1 1.79 1.61 1.37 1.24 1.18 1.11 sSS4 1.25 1.33 1.31 1.25 1.25 1.15 sTo estimate the transparency of music in this operahouse, we also characterize the clarity index C80 [4],which characterizes the relative importance of the earlypart (within 80 ms) with respect to the later part of theimpulse response, and hence provides information on theeffectiveness of sound reflections. For all the combina-tions of SS and M, C80 ranges from 0.5 to 3.2. Theaverage value of C80 in the stalls is 1 . ± . ∼ . ± .
15 s (with iron curtain open), we can com-pare the Margravial opera house with traditional Italiantheatres as well as modern theatres after the work ofProdi et al. [16]. It is interesting to see that the Mar-gravial opera house falls into the group of “regular” the-atres without large hard reflecting surfaces. AlthoughC80 has a clear spatial distribution in the auditorium,the relatively small standard deviation 0 . Conclusions and Outlook
To summarize, room acoustic characterizations are con-ducted for the newly renovated Margravial opera housein Bayreuth. Our investigation suggests that the acous-ics of this opera house matches perfectly that of typical“Italian-style historical opera houses” built between 1637and 1887 [16], representing the belief of the architects on“ideal” theatres [11]. It has a relatively low reverbera-tion time in comparison to modern theatres due to thedecorative interior design and the lack of large hard re-flecting surfaces. It delivers fairly well clarity at least forthe stalls. For the loges and boxes, C80 is expected tobe influence by detailed configurations of seating in theboxes. Therefore, further characterizations in the loges,particularly with the presence of audience are needed tounderstand its acoustic design better.From a technical perspective, this investigation showsthat bursting balloons serve as a more repeatable handysound source in comparison to hand-claps. For recordedsignals with relatively weak signal-to-noise ratio, settingup a threshold for the goodness of fit is valuable forobtaining more reliable and reproducible reverberationtimes.
Acknowledgments
We thank Thomas Rainer from the ‘BayerischeSchl¨osserverwaltung’ and Angela Danner from the pressoffice of Bayreuth University for bringing us the oppor-tunity of conducting the measurement. The detailed in-formation on the history and technical aspects of theopera house kindly provided by Thomas Rainer helpedgreatly in conceiving the original acoustic design of thetheatre. SK acknowledges support by the Elite Networkof Bavaria (Study Program Biological Physics). We arealso grateful to Tobias Eckert, Christoph Schnupfhagen,Nico Stuhlm¨uller, and Michael Seidel for their kind helpin audio recording.
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