virtual acoustic reconstruction of ritual and public space

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Stamatis L. Vassilantonopoulos Virtual Acoustic Reconstruction of Ritual and Public Spaces of Ancient Greece

Ancient Acoustics Stamatis L. Vassilantonopoulos, John M. Mourjopoulos Audio Group, Wire Communications Laboratory, Electrical & Computer Engineering Department University of Patras 26 500 Patras Greece Tel: +3061 996474 Fax:+3061 991855 Email: [email protected]

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Stamatis L. Vassilantonopoulos Summary Virtual acoustics can assist the aural exploration and the study of the acoustic properties of famous buildings of the antiquity. Here, examples of such reconstruction of ritual and public buildings of the ancient Greek world are presented and novel findings of their acoustic behaviour are introduced, especially with respect to the modes of speech communication and general functionality. Examples of these auralisations are made available in an electronic address.

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Stamatis L. Vassilantonopoulos 1. Introduction Computer-aided simulation of building acoustics is a well-established tool for the study of the acoustic behavior of such spaces and is often combined with virtual acoustic representation (auralisation) to provide direct aural impression of their response to speech or music signals[1]. This work describes an application of these techniques to buildings of the Greek antiquity which are not currently preserved, but nevertheless are of historic significance and their architectural features have been well-defined in historical sources and through archeological research. The results of this study provide a useful and novel insight on the acoustics of ancient buildings and their functionality for speech communication, hence introducing some new findings which may complement existing historical and archeological evidence. This study is concerned with the virtual acoustic reconstruction of closed spaces, of varying size and function, which were either known through historical records to possess unique acoustic features, or were significant for various public functions. Hence, this work will evaluate and acoustically reconstruct buildings whose acoustic features are -to the best knowledge of these authors- not being described elsewhere, in contrast to the better-documented acoustics of the open Greek and Roman theaters [2], which are usually well-preserved and as such allow their contemporary use for theatrical and music performances and in-situ evaluation and measurement. To complement this study, examples of the virtually-reconstructed spaces are publicly available at the electronic address: http://www.wcl.ee.upatras.gr/audiogroup/AncientAcoustics/Index.html The paper is organised as follows: (i) a brief description of the methodology for the computer-aided acoustic simulations and auralisations is given in Section 2, (ii) Section 3 contains typical examples of spaces with presentation of the results for known acoustic parameters (T30, RASTI, C80, etc.), (iii) finally, some cumulative results and some general conclusions are given in Section 3. 2. Method This work has evolved through the following stages: (a) a selection was made of representative buildings of the Greek antiquity with noted historic significance and acoustic features, (b) a search was conducted to obtain the architectural drawings of these buildings through archeological and other records, (c) acoustic simulations were conducted using well-established computer-aided tool, employing typical positions for source (designated by S) and receiver (designated by R), derived from the collected historical records, (d) auralisations were also produced, based on the estimated building’s impulse response(s), using specially-recorder anechoic speech material with context appropriately chosen from ancient texts of relevance to the simulated space. For the acoustic simulation, a commercially available, geometric acoustic-based program was employed [3]. The program uses 3 different algorithms to derive echograms and acoustic parameters:

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Stamatis L. Vassilantonopoulos (1) Standard ray-tracing with a spherical receiver which estimates sound pressure level

(SPL), lateral energy fraction (LEF2), and most known parameters (D-50, C-80, RASTI etc.),

(2) Image Source Model (ISM), for detailed early reflection calculations, based on first order images of the main source in all reflecting planes and second order sources created by calculating new images in all reflecting planes (except of the previously calculated). This procedure is repeated until the specified maximum arrival-time is reached.

(3) Randomized Tail-corrected Cone-tracing (RTC) for the full response but handling deterministically the direct sound and first order specular and diffuse reflections. With this approach, echograms (impulse responses) for auralisation use can be generated.

For the source, a representative for male speech signal at a level of 70 dB-SPL was specified, having typical speech directivity. Some of the acoustic parameters calculated by the program were: Reverberation time parameters T-15 and T-30, derived from the decays estimated by the algorithm for each receiver. These are derived from straight- line least-square fitting at level intervals –5 to –20 dB and –5 to –35dB respectively. For the examined semi-open spaces, an indicative Reverberation time value was derived, estimated from the responses’ Ts parameter (Center -Time) and assuming exponential decay [5]. Early Decay-Time (EDT), evaluated from 0 to –10dB on the backward integrated estimated echogram. Definition (D-50) and Clarity (C-80), early-to-late energy-ratio based parameters calculated according to their respective definitions [4,5,6] Rapid Speech Transmission Index (RASTI), calculated from the estimated echogram according to [7]. For the auralisations, binaural post-processing of the simulated room impulse responses was employed [1], which were derived from the estimated octave-band echograms at the designated source and receiver positions. The recreated sound is generated by the convolution of the synthesised impulse response with anechoically recorded mono material (in 16-bit, 44.1 KHz Wav-format), the resulting file being produced (in Wav-format and alternatively for efficient internet downloading, in ISO/MPEG Layer III compressed format), suitable for audio reproduction via headphones. 3. Examples of the simulated spaces 3.1 The Acheron Necromancy [ 8 ] 3.1.1. Overview The Acheron Necromancy (“Nekromantion Acheronta”) was situated at the north bank of the Acheron river, in the Epirus region of northwestern Greece and it is considered as the most prominent necromancy of the ancient Greek world. During that era (approximately from the 9th up to the end of 2nd century B.C.), necromancy was associated to religious beliefs according to which, the soul of a deceased person was separated from the body and was led via chasms and caves to an underground world

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Stamatis L. Vassilantonopoulos populated by the “shadow-like” spirits of the dead, having the ability to foresee the future. This belief led many people including famous mythological and historic figures such as Ulysses, Hercules, Theseus, Orpheus, etc. to engage into such rituals . The Acheron Necromancy was constructed as a special-purpose building, from 8th century B.C., on top of rocky hill, over a natural cave which was modified into an underground chamber (Figure 1(a) and (b)), considered to be the palace of underworld king Hades and his wife Persephone. This underground chamber was built from limestone, the roof being formed from 15 arcs (Figure 1(b)). At a later period (approx. at the end of 4th Century B.C.), the overground building was constructed on the ground above this cave. This building was consisting of the main hall of dimensions L15 m x W4.25 m x H3.25 m, and 2 parallel chambers each formed from 3 equally-sized and interconnected rooms (Figure 1(a)). Inner walls were made of limestone blocks, with thickness of 1m, whereas this main labyrinth - like temple (“Hieron”), was enclosed by walls of 3m in thickness, allowing structural integrity (in order to accommodate later extension of the building to a second floor and to support machinery used for movement of the visual spectres) and to assure acoustic isolation from outside sound sources. This main temple whose acoustic behavior will be studied here, was connected to a smaller exit chamber (Figure 1(a)) and was enclosed by corridors, 3 preparatory rooms and a main entrance hall (Figure 1(c)). The Necromancy was destroyed by the Romans (approx. in 167 B.C.) and it was subject of archeological excavations during the ‘50s and 70s , today being only partially preserved by some of its pedestal structure and the underground chamber. The rituals dictated that the visitors of the Necromancy should wait for a number of days before commencing the ceremony, following a strict diet so that they could be prepared for their final visit inside the main building where they were to meet the souls, which were craftily recreated by the priests using models moved by special machines (cranes), uncovered by the excavations and carefully spoken oracles possibly from different positions inside the building. Nevertheless, it is still open to speculations how speech was employed during these rituals. To examine such possibilities, virtual acoustic reconstruction of the main temple was produced for the parameters given in Table I for the most probable source and listener positions. Audio examples of these reconstructions are publicly available at the electronic address: http://www.wcl.ee.upatras.gr/audiogroup/AncientAcoustics/Index.html 3.1.2 Results From the simulations, a number of well-established acoustic parameters were evaluated, such as RASTI, EDT,Τ30, SPL (for a 70dB-SPL speech source),D-50, C-80 and are listed in Table II, for the typical source and receiver positions shown in Figure 1 (a) and (b). The results indicate that both the underground chamber and the overground temple had a characteristic “dry acoustic” response with low reverberance, suitable for speech communication and allowing good intelligibility for most listening positions. From these results, it can be speculated that the extreme acoustic isolation of the main temple with the resulting low level of background noise (argument supported by the historically-documented careful movement of visitor and priests inside the complex), could allow speech or other voiced effects to be communicated even at low levels from

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Stamatis L. Vassilantonopoulos many positions (e.g. from the main hall and / or adjacent rooms), which would lead to a unique audio effect, complementary to the visual effect generated by the models appearing in the total darkness of the space, hence creating the desired feeling of disorientation and isolation. 3.2.The Olympia Echo Hall (Echo Colonnade) [ 9 ] 3.2.1Overview The Echo Hall (“Stoa tis Echous”) was located in the ancient city of Olympia, in the centre of the Peloponnese region, next to the right hand side of the entrance to the famous stadium where the Olympic games were originally held. The hall was constructed at the last half of the 4th century B.C. (approximately in 350 B.C.). It was a long hall (with length 95.93 m), having walls at 3 of its sides and an open side with 44 Doric columns (Figure 2). Originally, the second long-side wall was separating the hall into 2 equal areas, one employed for public meetings, talks and recreational activities (being the space studied here) and the other for preparation of the athletes. The side of the public hall (the west side of the building), had dimensions L95.93 x W10.72 m and maximum height of 8.16 m (Figure 2). It is being referred to in many historic texts as “Echo Hall”, or “Seven-Echo hall”, since as it was mentioned by the well-known ancient writer Pausanias: “…the voice is echoed 7 or more times…”. Today, only the remains of the building’s east-side pedestal and back wall are located at the site of ancient Olympia. To assess how this well-documented unique acoustic character of the hall affected everyday communication, acoustic simulations were undertaken based on the parameters given in Table III for the most probable source and listener positions. Audio examples of this reconstruction are publicly available at the electronic address: http://www.wcl.ee.upatras.gr/audiogroup/AncientAcoustics/Index.html

3.2.2 Results From the simulations, a number of well-established acoustic parameters for this building were evaluated, such as RASTI, SPL (for a 70dB-SPL speech source), D-50, C-80, noting that Reverberation time could only be approximately evaluated (see Figure 4), the hall being semi-open space. These are listed in Table IV, for typical source and receiver positions shown in Figure 2. The unique acoustics of the Echo Hall with the prominent repeated echoes, indicate that this feature (together with the famous wall paintings) might had been exploited as “tourist attraction”, but nevertheless they had a negative effect for speech communication. The results illustrate that intelligible speech communication would be restricted to a maximum radius of 5m around the speaker, but this range could be practically smaller, given the possibility of high ambient noise (the hall being a crowded place). This indicates that groups of up to 300 people (approximately) could be gathered to listen to a speech, provided that no competing sources or noise were present at the hall.

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Stamatis L. Vassilantonopoulos 3.3 The Temple of Zeus [ 10 ] 3.3.1 Overview The Temple of Zeus (“Naos tou Dios”) was located at the ancient city of Olympia, in the centre of the Peloponnese region, being the one of the biggest and most prominent religious temples of Greek antiquity. It was constructed during 470 – 457 B.C. and it is widely accepted to represent the best example of the classic Doric architectural style, having the 6-colunm width and 13-column length outside structure of dimensions L64.12 x W27.66 x H20.25m (maximum height at the middle). These columns had a height of 10.43 m and a diameter of 2.25-2.21 m. The enclosed section of the building, contained within the collonade, consisted of a vestibule (“pronaos”), main temple (with dimensions L28.74 x W13.26 x H14.19 m) and back temple ( “opisthodomos” ) as is shown in Figure 3. The vestibule and main temple were separated by doors of 4.8 m width. The main temple contained a 2-storey collonade and balcony, as well as the famous statue of Zeus (of 12.5 m height, located on a 9.93 x 6.25m pedestal), constructed by the most prominent sculptor of Greek antiquity, Pheidias. The statue was constructed from ivory and was partially covered by golden leafs, and the walls were constructed by shell limestone, covered in places by thin marble plaster, whereas the main temple inner columns were connected by 1m tall similarly plastered barriers. The roof of the building was constructed from wood and was covered by marble blocks. The back temple was separated from the main temple by wall and was employed for speeches made by prominent visitors of the temple, especially during the periods designated for the Olympic games. Today, only the remains of the building’s pedestal are located at the site of ancient Olympia. To assess the acoustic character of the hall, simulations were undertaken for the parameters given in Table V for the most probable source and listener positions. Two different cases were examined, one for the main temple (e.g. as used for ritual ceremonies) and one for the back temple (used for speeches). Audio examples of these simulations are publicly available at the electronic address: http://www.wcl.ee.upatras.gr/audiogroup/AncientAcoustics/Index.html 3.3.2 Results From these simulations, the acoustic parameters for this building were evaluated, such as RASTI, SPL (for a 70 dB-SPL speech source), D-50, C-80, etc. and are listed in Table VI, for the typical source and receiver positions shown in Figure 3. The results illustrate that the acoustics of the main temple were dominated by high reverberance (the RT value being approximated at 3.3 s), but with high degree of diffusion and of lateral reflections generated by the side columns and walls. Additional diffusion could be provided by the statue of Zeus, dominating the back wall, which for obvious technical reasons could not be reconstructed in detail during these simulations. The high level of the EDT parameter relative to T-30 and good Definition values, indicate a positive acoustic character, but clarity and intelligibility were obviously low due to high reverberance, especially for some distance from the source (from 3 m, approximately). These acoustic properties are not very different to a contemporary Christian church of comparable size, clearly being unsuitable for speech

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Stamatis L. Vassilantonopoulos communication, being more appropriate for the performance of hymns or other ritual material. From the results for the back temple that are listed in table VII (being a semi-open space, Reverberation time could be only approximately estimated, as shown in Figure 4), it is evident that speech could be intelligibly produced at this area of the temple. It can be speculated that the radius of intelligibility exceeds the radius of 5m, given that beyond this range, visual and to lesser degree acoustic communication was restricted by the outside perimeter columns of the temple. 4. Discussion and conclusions The results of the study indicate that acoustics were exploited in ancient Greek buildings, although no evidence could be derived allowing speculations whether the acoustics of these buildings were consciously adjusted by the use of specific materials (e.g. absorption). In the examples presented in the previous sections, it has been illustrated how acoustics were exploited to different degree for ritual purposes, e.g. the isolation, low-reverberance and spatialisation as a setting for recreation of the meeting place with the souls of the deceased in the case of the Acheron Necromancy, the high levels of reverberation for the recreation of “god’s greatness” for the case of temple of Zeus, etc. It is also evident that for optimal speech communication, open or semi-open spaces were used (such as the back temple of Zeus, in Olympia), a possibility supported by the low levels of ambient noise, the fair climate and the social organisation. The Olympia Echo Hall, illustrates a rather unusual case where the acoustics of such a semi-open space interfered with communication to such a degree so that they became a noted feature of the building. In Greek antiquity, the acoustics of such open structures evolved into highly sophisticated designs, with the open theaters used for theatrical plays. From the collected data (from the previously described examples, as well as from other simulations, too numerous to describe in this text) it is possible to derive a diagram (Figure 4) showing the general trend of Reverberation Time (RT) vs Volume for the closed public and ritual buildings of the Greek antiquity. In the same diagram, some indicative RT values for the examined semi-open spaces are also given. The diagram reinforces the previously discussed arguments, showing that closed buildings of large volume were not designed for optimal speech communication, the RT value increasing linearly with volume and being not moderated by increase of total absorption, indicating that acoustic treatment was not evidently employed during their construction.

Acknowledgements The authors wish to express their gratitude for the assistance and guidance given by K. Herrmann, Dipl.Ing, Deutsches Archäologisches Institut Athen, and A. Vlachopoulou, Assistant Professor at the Department of History of the University of Ioannina. They also acknowledge the assistance provided by Mr M. Petropoulos, Head of 6th Office of Classical and Prehestoric Antiquities, Patras, G. Mavromatides of the 4th Office of Classical and Prehestoric Antiquities the personnel of the 12th Office of Classical and Prehestoric Antiquities, as well as Dr Andreas Floros and Mr Marios Koutroubas of the Audio Group.

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Stamatis L. Vassilantonopoulos

References [1] Blauert, J.

Spatial Hearing: The psychophysics of human sound localization MIT Press Cambridge USA 1997

[2] Shankland, S.Robert Acoustics of Greek theaters Physics Today 1973

[3] Catt-Acoustic v7.0 User’ s Manual Gothenburg Sweden 1998

[4] Kuttruff, H. Room Acoustics Third edition Elsevier Applied Science 1991

[5] Cremer, L. and Müller, H. A. and Schultz, T. Principles and Application of Room Acoustics Applied Science London 1982 Volume 1

[6] ISO 3382-1985(E) Acoustics Measurement of Reverberation time in Auditoria

[7] Houtgast, T. and Steeneken, H. J M. A Multi Language Evaluation of the RASTI -Method for Estimating Speech Intelligibility in Auditoria

Acustica 54 1984 185-199

[8] Dakaris, S.I. The Acheron Necromancy excavation Greek Archeological Society Records Athens 1964 44-53, (in Greek)

[9] Koenings,W. Die Echohalle Olympiche Forschungen XIV Deutsches Archäologisches Institut Berlin 1984

[10] Gardiner, N.E. Olympia: Its History & Remains Clarendon press Oxford 1925

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List of Tables

Description of space

Number of individual

rooms

Total floor area (m2)

Total volume

(m3)

Surface materials Number of VA

simulations Underground

chamber 1 50.78 242 Floor: limestone plates

Walls: limestone Ceiling: limestone

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Main overground

temple

8 233.74 738 Floor: limestone plates and sand Walls: stone Ceiling: wood

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Table I: Geometric and physical properties of the Acheron Necromancy


Receiver position

RASTI (%)

EDT (s)

Τ30 (s)

SPL (dB)

D-50 (%)

C-80 (dB)

Underground chamber

R1 R5 R9

90.8 88.5 90.4

0.19 0.27 0.19

0.24 0.23 0.24

65.8 61.7 58.0

97.5 97.6 97.9

25.2 24.8 22.8

Main overground temple

R1 R4 R6 R7 R9

77.4 77.3 79.5 75.1 78.6

0.51 0.74 0.56 0.73 0.55

0.53 0.54 0.49 0.49 0.51

63.7 41.1 54.7 42.5 41.1

77.9 29.8 69.3 21.3 61.8

10.0 4.1 8.5 2.5 6.7

Table II: estimated acoustic parameters of the Acheron Necromancy for typical source and receiver positions (mean 125 – 4000 Hz)



rooms

Total floor

area (m2)

Total volume

(m3)

Surface materials Number of VA simulations

Ground floor hall

1 921 7500 (approx.)

Floor: earth, marble Walls: shell limestone Ceiling: wood

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Table III: Geometric and physical properties of the Olympia Echo Hall


Receiver position

RASTI (%)

Ts (ms)

SPL (dB)

D-50 (%)

C-80 (dB)

Echo Hall

(source position: S1)

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10

67.0 61.8 56.8 50.5 50.2 47.8 43.4 44.2 43.8 46.0

49.4 68.2 91.6

124.8 126.3 138.6 179.9 180.3 184.8 171.1

69.2 67.6 65.5 63.3 62.6 62.1 59.2 59.1 58.7 57.0

77.0 68.4 56.4 42.5 43.4 42.4 34.6 35.5 35.4 35.0

6.7 4.9 3.9 1.6 1.5 0.8 -0.5 -0.7 -0.9 -0.2

Table IV: estimated acoustic parameters of the Olympia Echo Hall, for typical source and receiver positions (mean 125 – 4000 Hz)

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rooms

Total floor area (m2)

Total volume

(m3)

Surface materials Number of VA

simulations Main

overground temple

2 381

(complete building: ~1400)

5292

(complete building: ~25100)

Floor: Marble, black stone and mosaic Walls: shell limestone and marble plaster Ceiling: wood Columns: stone covered by white marble plaster

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Table V: Geometric and physical properties of the Temple of Zeus in ancient Olympia


Receiver positions

RASTI EDT (s)

T-30 (s)

SPL (dB)

D-50 (%)

C - 80 (dB)

Main temple

(source

position S1)

R1 R2 R4 R7

50.7 40.7 26.3 24.5

4.2 4.5 4.9 5.0

3.5 3.3 3.0 2.9

71.5 70.1 68.7 67.8

49.8 30.5 14.2 9.0

1.1 -2.1 -5.7 -7.1

Table VI: estimated acoustic parameters of the main Temple of Zeus, for typical source and receiver positions (mean 125 – 4000 Hz).

Description of space Receiver positions

RASTI SPL (dB)

D-50 (%)

C - 80 (dB)

Temple of Zeus –back temple (source position S2)

R12 R13 R14

52.4 37.0 44.2

69.7 61.9 65.3

50.3 12.9 33.3

1.4 -4.3 -1.0

Table VII: estimated acoustic parameters of the Temple of Zeus (open back temple), for typical source and receiver positions (mean 125 – 4000 Hz).

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List of illustrations

Figure 1a: Plan and section of the Acheron Necromancy, main temple (Hieron). (S, R) are typical source and receiver positions for the VA simulations

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Figure 1b: Plan and section of the Acheron Necromancy, underground chamber.

(S, R) are typical source and receiver positions for the VA simulations

Figure 1c: Plan of complete building structure

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Figure 2: 3–D and plan view of the Olympia Echo Hall

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Figure 3:Plan view of Temple of Zeus

0

1

2

3

4

5

6

0 2000 4000 6000 8000 10000 12000 14000

Volume(m3)

Rev

erbe

ratio

n Ti

me

(sec

) 1. Acheron Underground Chamber2. Acheron Overground Temple3. Avaton Temple Epidauros4. Main Temple of Zeus5. Avaton Temple Epidauros (*)6. Olympia Echo Hall (*) 7. Temple of Zeus -Back temple (*)

(*) estimates for semi-open spaces

1

2

3

4

5 6

7

Figure 4: relationship of the estimated Reverberation Time and the building’s volume, for all studied spaces

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List of legends for illustrations Figure 1a: Plan and section of the Acheron Necromancy, main temple (Hieron). (S, R) are typical source and receiver positions for the VA simulations Figure 1b: Plan and section of the Acheron Necromancy, underground chamber.

(S, R) are typical source and receiver positions for the VA simulations Figure 1c: Plan of complete building structure Figure 2: 3–D and plan view of the Olympia Echo Hall Figure 3: Plan view of Temple of Zeus Figure 4: relationship of the estimated Reverberation Time and the building’s volume, for all studied spaces

virtual acoustic reconstruction of ritual and public space

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