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1 Welcome Contents Back 1 Attractive room acoustic design for multi-purpose halls H. Drotleff, X. Zhou 1. Introduction Multi-purpose halls serve various uses, e.g., open plan offices, conferences, exhibitions, speech and music performances. Attractive interior design and comfort are major criteria for the acceptance of such environments. Comfort of a hall consists of climate-, light-, and acoustic aspects. Acoustic comfort is of particular importance where the major use is set on communication. Actual architectural trends emphasise even, smooth and transparent surfaces, for instance glass, concrete and natural stone. Usually these materials are acoustically reflecting. Without further treatment the consequence is a reverberant environment, where among others, a special kind of "party effect" can be experienced. This becomes noticeable with increasing number of people talking in a room. In an unconditioned or wrongly conditioned environment the interference from other conversations makes listening more difficult. In case of unsuitable geometry of the hall, reflecting surfaces may also lead to distinct echoes. The realisation of a desired attractive interior design, however, asks for new acoustic materials which are thin, flexible, even, versatile, transparent and inexpensive. Furthermore, these materials should exhibit proper acoustic absorbing and / or reflecting properties. 2. Acoustic requirements and suitable materials Depending on the use of a hall, an adequate reverberation time is required. For instance, assembly halls and conference rooms require a short reverberation time. In case of conference rooms attention must also be paid to speech intelligibility. Music performances require a somewhat longer reverberation time, but also other quantities such as clarity, lateral fraction etc. must be taken into account. Multi-purpose use should, however, be realised with only one well- Welcome Contents Back 1

2 Contents First Page Back 2 defined reverberation time. A compromise is sought for every single auditorium in the following way: - a carefully chosen moderate reverberation time, - minimum variation over the whole frequency spectrum, - a but small increase towards low frequencies. Mid and high frequency absorption is often inevitably provided in rooms by carpets, drapes, furniture and persons, whereas low frequency absorption is rather scarce. Low frequency problems, down to 50 Hz, in room acoustics have gained importance over recent years [1]. Therefore, effective absorbers for this frequency region, which also enable attractive design, were needed. Following this demand a whole family of novel absorbers has been developed at Fraunhofer- IBP [1], [2]. One of them, the Compound Panel Absorber CPA, aims primarily at the low and mid frequency range. Structure details and absorption coefficient of the CPA are given in [2]. The usual element size of a CPA is 1.5 m x 1.0 m. Although a low frequency absorber, the thickness of such an element is only 10 cm. The Broadband Compact Absorber BCA [2] stands for an extension of the CPA to the high frequency region. The absorption behaviour is also shown in reference [2]. Its thickness increases by 5 to 15 cm. Since both aforementioned absorbers are relatively thin, they can easily be mounted on walls, behind suspended ceilings or below hollow floors as long as sound transmission through these covering construction elements is accomplished. If desired, these absorbers can be hidden behind acoustically transparent layers such as sufficiently perforated gypsum board, timber or metal panels. These layers represent an optical coverage, so that a homogeneous wall surface is obtained. Additionally, these novel absorbers can be integrated in furniture or used as white-, or pin boards. Microperforated Panel Absorbers, Microperforated Foil Absorbers, Microperforated Suspended Ceilings [2] are also perfectly suited for attractive interior design. Since these absorbers can be made of almost any material e.g. acrylic glass, metal, plastic foil or timber they fit into nearly any interior design concept. The absorption features of all types of Microperforated Absorbers are tuneable to the respective requirements merely by choosing its various geometrical parameters. Contents First Page Back 2

3 Contents First Page Back 3 3. Demonstration projects It will be shown by means of already realized projects how acoustic comfort can reconcile with attractive interior design. 3.1 Training Centre A former production hall built in 1903 with a volume of about m³, Fig. 1, should be converted into an open plan office, which is also used as a conference room. The hall was granted the status of a historic monument. The architectural design therefore called for even and homogeneous surfaces which emphasise the original appearance. No conventional acoustic wall treatment with perforated panels etc. was allowed. The ceiling which is equipped with stucco, must not be touched. Since the multi-purpose use emphasises speech performances the required reverberation time in the frequency end between 250 Hz and 2.0 khz was set to be 1.5 seconds ± 10 %. To the lower frequency range, down to 63 Hz, the increase in reverberation time should be as small as possible. The reverberation time measured in the untreated room as depicted in Fig. 2 exceeds 7 seconds. In order to account for low frequency absorption in accordance with the requested architectural design, 58 m² of CPA were installed onto the front and rear walls, Fig. 3 and Fig. 4. Even and homogeneous wall surfaces were achieved by "hiding" these absorbers behind an acoustically transparent panelling. The panel is made of perforated gypsum board covered by a fabric on its view face. The fabric itself was painted in order to imitate the original appearance. An optically opaque and homogeneous wall with a depth of only 12 cm was thus created. Since a low frequency absorber was covered, the perforation of the gypsum board needs not to exceed 20 %. Its estimated transmission coefficient is given in Fig. 5. Additional 54 m² CPA were installed underneath the hollow floor along the walls. Again, the floor was acoustically transparent enough for frequencies below 500 Hz. Onto the historic crane track, which should not be removed, another 32 m² CPA could be installed (Fig. 3). In order to account for absorption in the mid and high frequencies, about 58 m² of BCA were used. A large part of them was installed behind the projection screen (Fig. 3). Up to now, only two axes of the room were treated. Thus, in order to avoid echoes and to adjust the reverberation time at mid and high fre- Contents First Page Back 3

4 Contents First Page Back 4 quencies, the large windows in the side walls were treated with Microperforated Foil Absorbers as mobile roller blinds, Fig. 6. By using a rather small amount of area of the scarce available surfaces, about 20% of the surface were treated, and incorporating innovative absorbent materials in multi-use installations, a reverberation time was achieved which suits multi-purpose uses (Fig. 2). 3.2 Monastery Bronnbach Another historic hall (volume about m³) which over the last decades has been used as a museum is shown in Fig. 7. Additional to this use, it was intended by the user to utilise the room primarily as a concert hall for chamber music and secondarily for speech performances. As is typical for a historic monument, all the surrounding surfaces are rather reflective. Because the resulting reverberation time is of the order of 7 seconds up to 1.0 khz (Fig. 8) the desired use was virtually impossible. A few attempts by the user were not successful. Thus an improvement was necessary. The architectural design again requested even and homogeneous surfaces. Only a small part of the wall surfaces was made available for acoustic treatment. Especially the ceiling exhibiting over the whole area historic paintings and the front wall and large parts of the side walls must not be treated. The original floor is made of stone. The desired reverberation time was chosen to become approximately 1.5 seconds between 250 Hz and 2 khz. Towards the lower frequencies the reverberation time should increase but slightly. Furthermore, for chamber music performances the "clarity" in the audience must lie between 2 db and 3 db. With altogether only 97 m² stationary and movable absorbers of the kind CPA and BCA plus chairs for the audience, the envisaged reverberation time was achieved (Fig. 7 and Fig. 9). Allowing for strong early reflections from the side walls and the ceiling, the "clarity" (C80 (3) according to [3]) was increased to acceptable values between 1.5 db and 3 db (Fig. 10). The absorbers located on the side walls were covered by an acoustically transparent (perforated) metal sheet. For speech performances additional mobile absorbers may be introduced on the front wall. They are covered with the same material. Lean constructions (depth of 10 cm) were thus made to fit into the architectural design. In order to avoid echoes along the longitudinal axis of the room, the rear wall was treated with BCA, which were "hidden" behind a perforated gypsum board (Fig. 7). Contents First Page Back 4

5 Contents First Page Back 5 Altogether about 10% of the surface were treated. The first chamber music performance after restoration, with "Brandenburgische Konzerte" by J.S. Bach revealed a very positive resonance by the audience, the musicians and the conductor. 3.3 University of Freiburg The assembly hall of the University of Freiburg (Fig. 11) is used for academic celebrations. The main purpose of the hall serves speech performances, which are occasionally accompanied by music performances. While the hall was redecorated, the architects found that, with the new design the room-acoustic environment in the hall is not suitable for the main purpose. Thus, after the redecoration was partly finished, an acoustical design was needed which fits into the already fixed architectural design. The hall consists of two connected rooms. Its shape serves as an example for echoes in a standard textbook on room acoustics [4].The volume of both rooms is about m³. In the larger room a 5 cm thick false ceiling made of gypsum was installed. The ceiling exhibits a shape of a half ellipsoid. Between the false ceiling and the boundaries of the main room a distance of about 40 cm is left. In this room large windows are situated on both sides. The side walls, floor and ceiling were thus not available for acoustic treatment. The aimed reverberation time was chosen to be approximately 1.3 seconds in the frequency range between 250 Hz and 4000 Hz. In order to support speech performances the reverberation time should be as even as possible over all frequencies. To account for low frequency absorption 150 m² of CPA (both sides covered with metal sheets) were installed as baffles in the hollow space behind the false ellipsoid ceiling. Furthermore, transparent Microperforated Panel Absorber (ca. 45 m²) were mounted as inner windows on both sides of the main room (Fig. 11). These absorbers are tuned to low and mid frequencies, too. The light transmission into the room is not hindered. In order to avoid echoes along the longitudinal axis of the room, front and rear walls were treated with BCA (ca. 33 m²). For further damping the niches next to the organ were covered with the same absorbent material (ca. 30 m²). The absorbers were hidden behind acoustically transparent but optical opaque panelling consisting of a perforated gypsum board covered with cloth. All the above listed measures do not interfere Contents First Page Back 5

6 Contents First Page Back 6 with the architectural design. Except for the Microperforated Panel Absorber, all the absorbent materials are hidden optically. The resulting reverberation time (including 270 scantily upholstered chairs) is depicted in Fig Office Innovation Centre A hall (volume about m³) which is used as exhibition or assembly hall is shown in Fig. 13. The use of this hall changes rapidly in time. It was found that the hall was hard to use for speech performances as can be seen in the reverberation time chart in Fig. 14. Since the hall reveals already low frequency absorption (due to the relatively light metal walls and ceiling) a fast and really inexpensive solution for mid and high frequencies was proposed and accomplished. With only approximately 160 m² of transparent Microperforated Foil Absorbers on the ceiling (Fig. 13) the reverberation time was reduced to 2 seconds (Fig. 14). The absorber hangs as a single layer approximately 1.5 m down the ceiling leaving sufficient free space for the lights. The achieved acoustical environment allows for speech performances by using electro-acoustical reinforcement. 4. Summary Attractive interior design and comfort are both necessities for well-being in multipurpose halls. A carefully chosen reverberation time with little variation with frequency down to 63 Hz is a first step towards acoustic comfort. Depending on the main use, other aspects such as clarity etc. must also be taken into account. A continuing development approach [5] lead to a family of novel absorbers. It has been shown by means of realized projects that attractive interior design combined with acoustic comfort of multi-purpose halls can be accomplished with innovative fibreless absorbers. The handicap of acoustical design at low frequencies, because of the depth of the absorbers, can be overcome with rather thin and compact absorbers. Additional to the acoustic quality, the aforementioned absorbers exhibit characteristics such as: even surfaces, flexibility, transparency, versatile use. Qualities which assist the designer's task to realize attractive interior design. Contents First Page Back 6

7 Contents First Page Back 7 5. References [1] H.V. Fuchs, X. Zha, X. Zhou, H. Drotleff: Creating low-noise environments in communication rooms. Applied Acoustics (2001) (in print). [2] H.V. Fuchs: Alternative fibreless absorbers New tools and materials for noise control an acoustic comfort. acta-acustica ACUSTICA (2001) (in print) [3] L.L. Beranek: Concert and Opera Halls: How they sound. Woodbury, Acoustical Society of America, 1996, 570 [4] L. Cremer, H.A. Müller: Die wissenschaftlichen Grundlagen der Raumakustik, Band I. Stuttgart, Hirzel Verlag, 1978, 61 ff. [5] H.V. Fuchs: From advanced acoustic research to novel silencing procedures and innovative sound treatments acta-acustica ACUSTICA (2001) (in print) Contents First Page Back 7

8 Contents First Page Back 8 6. Figures Fig. 1: Photo of the training centre, a former production hall before being refurbished into a multipurpose hall Contents First Page Back 8

9 Contents First Page Back Reverberation Time [s] Frequency [Hz] Untreated hall Treated hall Prediction Fig. 2: Reverberation time as a function of frequency measured before and after renovation in the training centre according to Fig. 1 Contents First Page Back 9

10 Contents First Page Back 10 CPA behind optically opaque panelling Historic crane track Projection screen "filled" with BCA Fig. 3: View onto the rear wall and the projection screen Contents First Page Back 10

11 Contents First Page Back 11 CPA Fig. 4: Sketch of the rear wall equipped with Compound-Panel-Absorber in the training centre Contents First Page Back 11

12 Contents First Page Back 12 Transmission Coefficient τ Frequency [Hz] Perforated gypsum board Perforated gypsum board covered by fabric Fig. 5: Estimated transmission coefficient of the perforated gypsum board panelling covered by painted fabric Contents First Page Back 12

13 Contents First Page Back 13 CPA behind optically opaque panelling Microperforated Foil Absorber Fig. 6: Microperforated Foil Absorber as mobile roller blinds in front of the windows of the training centre Contents First Page Back 13

14 Contents First Page Back 14 BCA behind panelling CPA Fig. 7: Monastery Bronnbach. Historic multi-purpose hall used for music an speech performances Contents First Page Back 14

15 Contents First Page Back Reverberation Time [s] Frequency [Hz] Untreated hall Treated hall Prediction Fig. 8: Monastery Bronnbach. Reverberation time as a function of frequency measured before and after renovation Contents First Page Back 15

16 Contents First Page Back 16 BCA CPA BCA Fig. 9: Monastery Bronnbach. Sketch of side walls treated with CPA and BCA modules Contents First Page Back 16

17 Contents First Page Back dB -0.9dB -1.1dB 0.6dB -3.0dB -1.5dB 3.3dB 1.0dB 3 m 2.8dB Fig. 10: Monastery Bronnbach. Measured "clarity" (C80 (3) ) in the audience after acoustical treatment Contents First Page Back 17

18 Contents First Page Back 18 BCA Transparent Microperforated Panel Absorber Fig. 11: Assembly hall of the University of Freiburg Contents First Page Back 18

19 Contents First Page Back 19 3 Reverberation Time [s] Frequency [Hz] Fig. 12: Assembly hall. Measured reverberation time after acoustical treatment Contents First Page Back 19

20 Contents First Page Back 20 Transparent Microperforated Foil Absorber Fig. 13: Office Innovation Centre. View into the exhibition (conference) hall treated with transparent Microperforated Foil Absorber Contents First Page Back 20

21 Contents First Page Back Reverberation Time [s] Frequency [Hz] Untreated hall Treated hall Fig. 14: Office Innovation Centre. Measured reverberation time before and after acoustical treatment Contents First Page Back 21