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Improving the acoustic working conditions for musicians in small spaces
Applied Acoustics 63 (2002) 203–221 www.elsevier.com/locate/apacoust
Improving the acoustic working conditions for musicians in small spaces X. Zha, H.V. Fuchs*, H. Drotleff Fraunhofer-Institut fu¨r Bauphysik, Postfach 80 04 69, 70504 Stuttgart, Germany Received 20 November 2000; received in revised form 16 February 2001; accepted 20 March 2001
Abstract In the acoustic consulting, testing, design and engineering work of the Fraunhofer-Institute of Building Physics (IBP) the low-frequency end of the noise spectra and the room acoustic conditioning has gained tremendous importance over the years. For solving the long-ranging noise pollution from e.g. exhaust stacks and chimneys, a series of low-frequency sound attenuators with minimum flow resistance were developed. Its first representative was a novel membrane absorber [10] [Ackermann U, et al., Sound absorbers of a novel membrane construction. Applied Acoustics 1998;25:197–215]. Thanks to its slenderness and ruggedness it could also be employed for noise control and reverberation adjustment purposes in relatively narrow enclosures and harsh environments [11,12] [Ve´r IL. Enclosures and wrappings. In: Harris CM, editor. Handbook of acoustical measurements and noise control. New York: McGraw-Hill, 1991; Fuchs HV, Hunecke J. The room plays its part at low frequencies. Das Musikinstrument 1993;42:40–6 (in German). Meanwhile a new type of panel absorber has been optimized for both kinds of application. Its absorption efficiency at frequencies far below 100 Hz could be demonstrated and quantified by a special measuring procedure based on the reverberation of a small rectangular room at its eigenfrequencies [3] (Zha X, et al. Measurements of an effective absorption coefficient below 100 Hz. Acoustics Bulletin 1999;24:5–10). With the aid of this novel tool it is now possible to qualify reverberation rooms and anechoic chambers for frequencies down to 63 and 31 Hz, respectively [9] (Fuchs HV, et al. Qualifying freefield and reverberation rooms for frequencies below 100 Hz. Applied Acoustics 2000;59:303–22). In a companion paper in this same journal [4] [Fuchs HV, et al.: Creating low-noise environments in communication rooms. Applied Acoustics (in print)] appropriate experience is reported in creating low-noise environments in multi-purpose rooms like offices, restaurants, foyers and seminars. A number of representative case studies [5] (Drotleff H, et al. : Attractive acoustic design of multi-purpose halls. 1. Chinese–German Platform Innovative Acoustics 2000, (October, 21–25. 2000)) show ample evidence that the low-frequency perfor-
* Corresponding author. Tel.: +49-0711-970-3320; fax: +49-0711-970-3433. E-mail address: [email protected] (H.V. Fuchs). 0003-682X/02/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0003-682X(01)00024-X
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mance of the rooms has a strong influence on both the amplification of intruding external noise and the development of internally generated noise emanating from communication processes provoked by the users themselves. At work places where producing sound (by voices or/and instruments) is the main or only purpose for their existence, the acoustic qualification of the room at low frequencies turns out to be of the utmost importance, especially when musicians are forced to work in extremely narrow spaces like orchestra pits and rehearsal halls for many hours a day and often under extreme physical and mental pressure. The measures taken and described herein have proven to mitigate if not remove some of the acoustic burden put on musicians employed in states theatres. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Rehearsal rooms and orchestra pits in theatres are work places for professional artists and musicians. Unfortunately these well-trained and highly motivated specialists mostly suffer from extremely high sound pressure levels and have to perform their jobs under severe difficulties concerning the hearing of the sound of their own voice or instrument as well as that of all other members of an ensemble. Factory workers, who are exposed to weighted noise levels exceeding 90 dB(A), are regularly forced to wear ear plugs in order to protect themselves. In orchestra pits and rehearsal halls the ‘‘noise level’’ often reaches much more than 90 dB(A) [1]; a ‘‘Fortissimo’’ may sometimes amount to peak levels of up to 108–123 dB(A)! Can a musician with stuffed ears clearly hear his/her own playing or singing and that of the others? In other words: what kind of room-acoustic environment should instead be created so that adequate working conditions — optimum sound production combined with maximum health protection — can be provided? 1. Acoustic intensity levels must be reduced such that inevitable damage to musician’s ears is diminished: sound development control! 2. Musical communication between musicians and the conductor must be as clear as possible: a necessity for good ensemble playing! 3. Musicians must hear (and control) their own playing exactly: high sound quality! The philosophy about the acoustics in rehearsal halls and orchestra pits has changed over the years. Being work places, they should no longer be built or restored to simulate a specific auditorium for opera or concert performances, not only because of the totally different spatial (i.e. geometrical) conditions but also of the completely different acting requirements. In order to improve the working conditions for artists of the Staatstheater Stuttgart, the Fraunhofer IBP was asked to take measures to improve the acoustical environment within the orchestra pit in September 1994. After this first project, in 1998 the orchestra pit of the Landestheater Flensburg and, during the summer interval 1999, the large rehearsal hall, again at Stuttgart, have been reconstructed successfully. The musicians are now all exposed to much lower sound pressure levels
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and can better perform their hard work in a completely new and considerably improved acoustical environment.
2. Problems and solutions 2.1. Orchestra pit The members of the orchestra of the Staatstheater Stuttgart and the Landestheater Flensburg complained about an unbearable loudness and considerable difficulties to hear each other and themselves when rehearsing or performing in their orchestra pit. When their conductor asked the musicians not to play so loudly (e.g. for a better balance with the singers on the stage), they found it extremely difficult to please him. The acoustical circumstances were typical for most of the orchestra pits all over the world: musicians are forced to sit and work under extremely narrow spatial conditions (Figs. 1 and 2). The acoustically hard boundaries of the pit reflect all sounds unmitigated. In hardly ‘‘better’’ situations, one may find a thin layer of porous absorption material on the walls. Sometimes an attempt is made to improve the working conditions by laying a carpet on the floor. Nothing, however, is normally installed for the absorption of the much more important low frequencies. As an inevitable result a horrible rumbling builds up, which fills the whole orchestra pit as soon as brass or percussion instruments set in. Acoustic consultants seem to hesitate to recommend the introduction of efficient sound absorbers on the walls and ceiling of the pit in fear that the surface covered may be missing for the necessary transmission of sound and vital communication between different groups [2]. Also, there is hardly any space left for the installation of conventional low-frequency absorbers which as so-called ‘‘bass traps’’ tend to become rather voluminous. Contrary to this, IBP favours the application of relatively slim Compound Baffle Absorbers [3] with much the same motivation as for small communication rooms in general [4,5]. In order to test the suitability of such an acoustic treatment, in a first retrofit project in Stuttgart, these newly developed bass absorbers were only provisionally placed within the pit. After a test period, however, the musicians as well as the conductor would no longer like to work without this innovation which turned out to bring about a real relief for the musicians. The same test was made in Flensburg. Only 19 m2 of these newly developed bass absorbers were provisionally hung into the pit. In a subjective judgement, organized by the orchestra, most of the musicians reacted very positively on the effect of these absorbers in their immediate vicinity. 2.2. Rehearsal hall All members of the orchestra and choir as well as the soloists of the Stuttgart States Theatre complained about unbearable acoustic working conditions in their only rehearsal hall by the same arguments as they used in the case of the orchestra pit. After the latter had been treated with the novel low frequency absorbers, the
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Fig. 1. Plan (top) and section (bottom) of the orchestra pit of Staatstheater Stuttgart [6].
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Fig. 2. Photo of the orchestra pit of the Landestheater Flensburg.
instrumentalists even preferred to work there instead of playing in the much larger and more comfortable rehearsal hall proper. This heavily utilised rehearsing room was restored in the early 1960s. Its inappropriate interior lining determined its unsatisfactory room-acoustic features. The floor area is about 350 m2 (Fig. 3). The height up to the concrete roof is 8 m, but a suspended ceiling with a large air space in between wasted about 40% of useful and urgently needed volume (Fig. 4). The ceiling had a large absorbing section exactly over the groups of strings. The rear part of the ceiling on the other hand was reflecting most of the sound from the ‘‘strong’’ brass, percussion and choir groups which are located there, towards the ‘‘weaker’’ instruments (strings and woodwinds). About 30% of the two side and front walls were covered by conical or spherical ‘‘diffusers’’ made of gypsum (Fig. 5). In front of them there were textile drapes intended to control the reverberation time in the hall. With and without such drapes the reverberation time varied by only less than 0.1 s in the mid-frequency range. This may explain why these drapes have hardly been used over the years. This hall has a reverberation time of around 1.0 s. When analysing the impulse responses from 28 pair measuring positions (Fig. 3), severe problems of clarity C80 showed up: too low in general, and ranging from 1.9 dB to 5.2 dB (Tables 1–3). The clarity factor C80, expressed in decibels, is the ratio of the early energy (0–80 ms) to the late (reverberant) energy (after 80 ms), Ð 80ms 2 p ðtÞdt 0 Ð C80 ¼ 10 log 1 ; ð1Þ 2 80ms p ðtÞdt
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Fig. 3. Ground plan of the rehearsal hall of Staatstheater Stuttgart with measuring positions [7].
arriving at a certain receiver position as emitted from a certain source position. From the rear to the front part the clarity is much higher than in the opposite direction (Table 4). This ‘‘one-way’’ peculiarity impeded the communication and hearing among the musicians. Therefore, the following measures were proposed to the client: 1. in order to reduce the overall sound pressure levels: remove the suspended ceiling in order to enlarge the volume from 1700 m3 to 2800 m3 (Fig. 4), increase the room damping over the whole frequency range in order to reduce the reverberation time,
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mount thin (10 cm) and effective (down to 50 Hz) bass absorbers directly under the roof;
2. in
order to avoid reflections between hard walls: remove all ‘‘diffusers’’ and drapes (Fig. 5), mount bass absorber modules on three walls (Fig. 6), hang mid-frequency absorber foils in front of the bass absorbers;
3. in order to improve ensemble playing among the musical groups and communication with the conductor: remove the skewed reflectors in the rear part (Fig. 4, top), install reflectors above the orchestra such as to guide reflections in the right direction (Fig. 4, bottom and Fig. 7) both in order to increase clarity and avoid the ‘‘one-way’’ peculiarity. For readers not familiar with the early-to-late energy parameter C80: a different result for interchanged source and receiver positions does, of course, not contradict
Fig. 4. Cross section of the rehearsal hall before (top) and after (bottom) the acoustic reconstruction.
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Fig. 5. Photo from the choir position of the rehearsal hall before reconstruction. Table 1 Clarity C80 (3), with different measuring configurations before and after the acoustical reconstruction of the rehearsal hall, as averaged for frequencies 500, 1000 and 2000 Hz Measurements
Before
After
Viols to Conductor Viols to Woodwind Viols to 1. Violin Viols to Choir Celli to Conductor Celli to 1. Violins Celli to Choir 2. Violins to Conductor 2. Violins to 1. Violins 1. Violins to Conductor 1. Violins to Woodwinds 1. Violins to Viols 1. Violins to Choir Brass to Conductor Brass to Woodwinds Brass to 1. Violins Brass to Viols Harp to Conductor Harp to Woodwinds Harp to 1. Violins Harp to Viols Drums to Conductor Drums to Woodwinds Drums to 1. Violins Choir to Conductor Choir to Woodwinds Choir to 1. Violins Choir to Viols
5.18 2.80 0.01 1.90 2.78 1.01 3.28 3.50 4.03 4.79 2.26 2.71 0.01 4.06 3.69 3.69 4.05 2.39 2.91 4.15 1.26 3.89 3.94 2.23 2.79 2.46 3.73 3.32
5.85 5.56 4.05 3.72 6.12 4.79 5.15 6.84 7.47 6.55 5.86 4.16 5.12 4.63 5.31 4.82 4.92 5.89 6.89 4.49 4.97 5.19 3.43 4.71 4.24 4.77 5.18 5.84
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Table 2 Clarity C80 (3) (dB) for different measurements from the string sections to the conductor’s position before and after the acoustical reconstruction of the rehearsal hall Source position
Before
After
1. Violins 2. Violins Viols Celli/Basses
4.8 3.5 5.2 2.8
6.6 6.8 5.9 6.1
Average
4.1
6.4
Table 3 Clarity C80 at 1000 Hz in (dB) for different measuring pairs from different instrument groups to the conductor’s position, before and after the acoustical reconstruction of the rehearsal hall in comparison with the recommended value in [8] Instrument group
Before
After
Recommendation
1. Violins 2. Violins Viols Celli / Basses Harp Brass Percussion Choir
4.1 2.5 5.0 2.5 2.0 4.1 4.0 2.8
7.3 6.8 6.0 6.5 4.1 4.1 5.1 5.5
48 48 48 48 14 2 +2
Table 4 Clarity C80 at 1000 Hz in (dB) showing the elimination of ‘‘oneway’’ transmissions in the rehearsal hall before and after its acoustical reconstruction Transmission From
To
1. Violins Choir Viols Choir
Choir 1. Violins Choir Viols
Before
After
0.0 3.7 1.9 3.3
5.1 5.2 3.7 5.8
the reciprocity principle for all individual sound paths between two points in a room. Yet, when a source, e.g. were placed close to a fully absorptive wall and/or ceiling whereas in the other case it were backed by fully reflective boundaries, there is no question that the temporal and spatial energy distributions may considerably differ in the two opposing cases in the close agreement with practical experience.
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Fig. 6. View of the novel absorber modules as installed on the front wall (behind the conductor) of the rehearsal hall. A: air conditioning installations D: door E: elevator.
Fig. 7. Photo of the newly installed reflectors above the orchestra and of the Compound Panel Absorbers mounted under the ceiling after reconstruction of the rehearsal hall.
3. Room-acoustic installations For performing environments as spatially restricted as orchestra pits and rehearsal rooms, IBP pursues a similar strategy as for communication and multi-purpose rooms in general [4,5]:
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1. let the low-frequency part of the music produced not ‘‘fill’’ the room with an ill-defined, poorly articulated ‘‘sound-screen’’ which inevitably chokes up the musicians’ ears for hearing and controlling the more important finer details of the music to be performed, 2. introduce special bass absorbers, e.g. in the form of Compound Panel Absorbers, which are able to ‘‘clear up’’ the low frequency part of the music but nevertheless keeps reflecting the bulk of the energy at the higher frequencies, 3. leave or install, where necessary and possible, acoustically reflecting surfaces above the users/musicians, which support the sound transmission between all communicating groups. Some of the new measures which were carefully derived from profound experiences with numerous projects dealing with speech intelligibility problems [4,5] initially encountered some scepticism among musicians. They feared, e.g. that a massive introduction of low frequency absorbers would weaken the fundamental bass lines which they found so difficult to identify and follow in their stressing work. They were also afraid to loose reflecting surfaces in their immediate vicinity which they thought would help them to hear and control their own sound generation within the overall acoustic chaos in which they usually felt almost drowned if the whole orchestra played something like ‘‘Forte’’ or ‘‘Fortissimo’’. This fear was articulated by the relatively ‘‘weak’’ ensemble players like violins and viols, in particular. 3.1. Orchestra pit After provisional installations for a ‘‘Meistersinger’’ production during the summer interval 1993 which immediately convinced the musicians and the maestro Gabriele Ferro, a few surfaces and niches were found during the summer interval 1994 where a total of about 40 m2 of different Alternative Fibreless Absorber (ALFA)-modules could be mounted on or integrated into the boundaries of that 130 m2 large pit (Fig. 8). Details of this first massive innovative sound absorbing treatment of an orchestra pit were published by Zha et al. [6]. As in preceding restorations performed in acoustically demanding small rooms [4], the main improvements aimed at a frequency region which is normally disregarded by acoustics and ergonomics experts, since it lies well below the range where conventional room-acoustic measurements, evaluations and assessments are performed. Therefore, the immediate reactions and subjective judgements of the many musicians was particularly important after their working environment had been changed so markedly. 3.2. Rehearsal hall Again the philosophy behind the rather radical and uncommon acoustic restoration of the much larger (22168 m) hall was to clear up the ‘‘foggy clouds’’ that immediately ‘‘filled’’ the room as soon as the low-frequencies were excited by bass and percussion instruments. Despite a considerable increase in volume (some 40%), the reverberation time at medium and high frequencies was planned to not exceed
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Fig. 8. Installation of novel Alternative Fibreless Absorber (ALFA) — modules on the front wall (top) and back wall (bottom) of the orchestra pit in Staatstheater Stuttgart [6].
0.8 s with only minor variations with frequency and, most importantly, no steep ascend towards the low frequencies. Details of the acoustic design and layout may be found in [7]. It involved about 260 m2 Compound Panel Absorbers [3–5], 70 m2 of double-layer Microperforated Foil Absorbers hung in front of those low frequency absorbers which were mounted on three walls (Fig. 9).
4. Results 4.1. Orchestra pit Extensive measurements were made after the absorbers were fitted. In the auditorium the reverberation time was, of course, always found unaltered. In the pit itself, however, a very short early-decay time could be identified in Stuttgart after the installations. The ratio between the early-decay and the standard reverberation times were in the range from 0.2–0.3 for different frequencies. Based on this information, one may derive an averaged reduction of sound pressure levels within the pit of the order of 5–7 dB when the sources are assumed to remain constant. In reality, the resulting reduction could be even more, since in the new acoustic environment the musicians are now able to better hear themselves and the other members of the ensemble and need not play louder than necessary for the benefit of an optimum balance between the orchestra and the stage. A great enhancement of clarity has been measured in the pit over the whole frequency range, especially at the lower frequencies. For instance, at 125 Hz between the conductor’s and musicians’ positions in Stuttgart, clarity increased by more than 10 dB for some places in the orchestra, and above 250 Hz, averaged over all positions, by about 5 dB (Fig. 10) [6].
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Fig. 9. Photo and schematic sketch of the novel acoustic lining on a side wall of the rehearsal hall.
The results and reactions of the users in Flensburg were particularly interesting: after the novel absorbers were permanently installed in the orchestra pit (Fig. 2), the musicians were completely satisfied by the obvious improvement of their working conditions. They did not even want to spend any more of their meagre resources for additional acoustic measurements. One of their authorities, however, found that the loudness in the auditorium especially of the violins had now become too low. Before the absorbers were to be removed for this reason, IBP was asked to prove, by objective measurements, whether or not the absorbers had changed the musical performance. The reverberation time in this pit (which is closed by more than 50% by the stage/ceiling) turned out to be but slightly reduced in the frequency range from 125 to 1000 Hz (Fig. 11). Well below the range of frequencies produced by violins, however, the reverberation of the pit could be shown to be dramatically reduced by the installation of the absorbers! Only in this important low-frequency range the sound pressure level in the pit could be shown to be reduced by at least 4 dB with the additional, even more important positive effect of reducing the rumbling which makes it so hard for all musicians to play in an orchestra pit. With the considerably improved clarity for all musicians and the conductor, however, it may well
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Fig. 10. Clarity C80 between the orchestra and the conductor measured as a function of frequency before (- - - -) and after ( — ) the installations shown in Fig. 8.
Fig. 11. Reverberation time measured as a function of frequency in the orchestra pit of Landestheater Flensburg: before (–*–) and after (–&–) the installation of bass absorbers in the pit.
be true that, in the specific performance the authority had attended, the musicians felt they should not play too loud for one reason or another. Measurements of the reverberation time in the auditorium, on the other hand, proved that the acoustical situation there was not changed at all (Fig. 12). In the end, all involved people were satisfied by a demonstration of why the loudness in the auditorium was reduced and how variably the musicians were now able to perform after the right absorbers were installed in the right place. Altogether 37 musicians
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Fig. 12. Reverberation time measured as a function of frequency in the auditorium of Landestheater Flensburg: before (–*–) and after (–&–) the installation of bass absorbers in the pit.
assessed the improvement of the new acoustic environment. According to Table 5 almost 90% found that the working quality has changed for the better. 4.2. Rehearsal hall After the reconstruction the acoustical measurements were repeated as before [7]. Above 160 Hz the reverberation time is in the range of 0.7 0.1 s (Fig. 13). This result agrees with the recommendations for rehearsal halls in [8]. Musicians do not experience the hall as too ‘‘dry’’. Singers feel that they are well embedded within the ensemble and notice a comfortable room response to their voices. From the entirely new room parameters one can easily estimate that the effective sound pressure level has been reduced by more than 4 dB. In reality, sound pressure levels will be reduced even more for the reasons as explained earlier for the pit. Clarity C80 has been considerably raised for nearly all measurement pairs. The variations among all measurements are decreased from 7.2 dB (before: 1.9–5.2 dB) to 3,5 dB (after: 3.4–6.9 dB; Table 1). The C80 average for all strings section positions has improved from 4.1–6.3 dB (Table 2). The acoustical transmission functions between all places and directions are now also more even. From all strings sections to the conductor’s position the values of C80(1000Hz) have risen from 3.7–6.7 dB and are now well within the range favoured in [8] (Table 3). The average transmission to the conductor from all measuring positions has increased by 2 dB. The transmission between several other source and receiver positions has also been improved (Table 1). The value of C80 from the viols to the first violins has changed from 2.7–4.2 dB; to the choir the increase was from 0.0–5.1 dB. The average improvement for C80 of all measurements to the first violins position is 2.7 dB. The worst result before was the transmission from the viols to the choir with a value of
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Table 5 Subjective assessment of the new acoustic working environment in the orchestra pit of the Landestheater Flensburg Subjective assessment
No. of musicians
Substantial deterioration Minor deterioration No change Minor improvement Substantial improvement
1 – 3 22 11
Fig. 13. Reverberation time measured as a function of frequency after the reconstruction in the rehearsal hall without musicians (–*–) and estimated with 100 musicians (–&–) [7].
1.9 dB. Now it is at 3.7 dB. The ‘‘one-way’’ peculiarity has thus been eliminated (Table 4). According to the symmetry of the room, a similar result for the measurement of C80 from the first violins to the viols should be expected as for the ‘inverse’’. Before the reconstruction the values were 2.7 dB (first violins to viols) and 0.0 dB (viols to first violins). After the reconstruction both values are very similar, 4.2 and 4.1 dB respectively (Table 1). The asymmetry of the room acoustic properties both sides of the line conductor–choir has obviously been removed.
5. Conclusions Work places in orchestra pits and rehearsal rooms usually suffer severely from too small volumes and spaces and extremely high sound intensity levels. Steel workers or pilots would under similar conditions have to wear ear plugs or take other noise abatement measures — a solution not very attractive, but a very deplorable necessity
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for numerous musicians working in pits or rehearsal rooms. Musicians rightly expect acoustic engineers to reduce the sound level and, at the same time, let them hear and be heard more clearly. The Compound Panel Absorbers and Broadband Compact Absorbers [4,9] in a novel combination with Microperforated Foil Absorbers makes it possible to treat the problems because of their effectiveness and small thickness, especially at the particularly disturbing low frequencies. In order to achieve a good musical communication, the necessary absorption measures have to be accomplished by well-directed reflection measures well above the musical ensembles, especially in a rehearsal hall.
Fig. 14. Alternative Fibreless Absorber ALFA materials and elements as innovative tools for acute noise abatement and room-acoustic (bold letters) tasks.
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In the Staatstheater Stuttgart, the orchestra pit and the large rehearsal hall have been improved acoustically. Average sound intensity levels could be reduced considerably in both places. The increase of clarity has been verified as a great improvement for hearing and ensemble playing of the musicians. Improving the working conditions by installing novel acoustic modules in the performing environment may, in the end, even raise the sound quality of musical productions in general. These far too late achievements for acoustically tortured musicians forced to work within large ensembles in small rooms was made possible by a new application of novel Alternative Fibreless Absorber ALFA modules developed at the Fraunhofer IBP to supplement or replace conventional acoustic linings employing mainly fibrous absorbers. Fig. 14 lists 12 ALFA family members, at least half of which (those written in bold letters) have proven very useful in various room-acoustic projects [4–7]. This fairly successful research, development and application activity started about 15 years ago with the Membrane Absorber Box (MAB) [10] initially applied (as splitter silencers) in heating, ventilating and air conditioning HVAC systems with high hygiene demands and for other noise control tasks [11]. Their excellent low-frequency performance, their smooth and closed metallic surfaces, qualified these ALFA modules especially for taming booming room modes excited by percussion instruments in concert (Fig. 15) or rehearsal (Fig. 16) environments [12]. In later construction and restoration projects the MAB modules were more and more replaced by the above mentioned and in [4, 9] described CPA and BCA elements for their even broader and more easily adjustable frequency characteristics and, most important in this business, their cost-effective production. The broad and almost enthusiastic acceptance of the retrofit results in long existing environments by the musicians and their conductors have meanwhile encouraged their equally suffering colleagues in Duisburg, Flensburg, Rendsburg, Mainz and elsewhere to ask Fraunhofer IBP to also help them to improve their similar
Fig. 15. Low-frequency MAB absorption screens shielding the sound from different percussion instruments for a concert recording in a music hall of Solitude Castle, Stuttgart.
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Fig. 16. Low-frequency MAB baffles suspended from the ceiling of a rehearsal room in the music school at Waldenbuch.
working conditions in rehearsal halls and orchestra pits. It thus seems no longer questionable that it is the low frequency end of the acoustics, long neglected awfully by acousticians, which is largely responsible for poor communication, difficult ensemble playing and, last but not least, increasing hearing loss in musicians [1] forced into always too small enclosures. How much the bass range may affect the quality of larger auditoria is, of course, quite another topic. References [1] Fearn RW. Hearing loss in musicians. J Sound Vib 1993;163:372–8. [2] Barron M. Auditorium Acoustics and Architectural Design. London: Chapman & Hall, 1993, 311. [3] Zha X, et al. Measurements of an effective absorption coefficient below 100 Hz. Acoustics Bulletin 1999;24:5–10. [4] Fuchs HV, et al. Creating low-noise environments in communication rooms. Applied Acoustics 2001 (in print). [5] Drotleff H, et al. Attractive acoustic design of multi-purpose halls. ACUSTICA (submitted for publication). [6] Zha X, et al. Improving the working conditions in the orchestra pit of the Staatstheater Stuttgart (in German). Bauphysik 1997;118:196–204. [7] Zha X, et al. Room-acoustic improvements in the rehearsal hall of the Staatstheater Stuttgart (in German). Bauphysik 2000;22:232–9. [8] Tennhardt H-P, Winkler H. Investigations for room-acoustic planning of orchestra rehearsal rooms (in German). Acustica 1995;81:293–9. [9] Fuchs HV, et al. Qualifying freefield and reverberation rooms for frequencies below 100 Hz. Applied Acoustics 2000;59:303–22. [10] Ackermann U, et al. Sound absorbers of a novel membrane construction. Applied Acoustics 1998;25:197–215. [11] Ve´r IL. Enclosures and wrappings. In: Harris CM, editor. Handbook of acoustical measurements and noise control. New York: McGraw-Hill, 1991 (chapter 13). [12] Fuchs HV, Hunecke J. The room plays its part at low frequencies. Das Musikinstrument 1993;42:40– 6 (in German).
Improving the acoustic working conditions for musicians in small spaces X. Zha, H.V. Fuchs*, H. Drotleff Fraunhofer-Institut fu¨r Bauphysik, Postfach 80 04 69, 70504 Stuttgart, Germany Received 20 November 2000; received in revised form 16 February 2001; accepted 20 March 2001
Abstract In the acoustic consulting, testing, design and engineering work of the Fraunhofer-Institute of Building Physics (IBP) the low-frequency end of the noise spectra and the room acoustic conditioning has gained tremendous importance over the years. For solving the long-ranging noise pollution from e.g. exhaust stacks and chimneys, a series of low-frequency sound attenuators with minimum flow resistance were developed. Its first representative was a novel membrane absorber [10] [Ackermann U, et al., Sound absorbers of a novel membrane construction. Applied Acoustics 1998;25:197–215]. Thanks to its slenderness and ruggedness it could also be employed for noise control and reverberation adjustment purposes in relatively narrow enclosures and harsh environments [11,12] [Ve´r IL. Enclosures and wrappings. In: Harris CM, editor. Handbook of acoustical measurements and noise control. New York: McGraw-Hill, 1991; Fuchs HV, Hunecke J. The room plays its part at low frequencies. Das Musikinstrument 1993;42:40–6 (in German). Meanwhile a new type of panel absorber has been optimized for both kinds of application. Its absorption efficiency at frequencies far below 100 Hz could be demonstrated and quantified by a special measuring procedure based on the reverberation of a small rectangular room at its eigenfrequencies [3] (Zha X, et al. Measurements of an effective absorption coefficient below 100 Hz. Acoustics Bulletin 1999;24:5–10). With the aid of this novel tool it is now possible to qualify reverberation rooms and anechoic chambers for frequencies down to 63 and 31 Hz, respectively [9] (Fuchs HV, et al. Qualifying freefield and reverberation rooms for frequencies below 100 Hz. Applied Acoustics 2000;59:303–22). In a companion paper in this same journal [4] [Fuchs HV, et al.: Creating low-noise environments in communication rooms. Applied Acoustics (in print)] appropriate experience is reported in creating low-noise environments in multi-purpose rooms like offices, restaurants, foyers and seminars. A number of representative case studies [5] (Drotleff H, et al. : Attractive acoustic design of multi-purpose halls. 1. Chinese–German Platform Innovative Acoustics 2000, (October, 21–25. 2000)) show ample evidence that the low-frequency perfor-
* Corresponding author. Tel.: +49-0711-970-3320; fax: +49-0711-970-3433. E-mail address: [email protected] (H.V. Fuchs). 0003-682X/02/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0003-682X(01)00024-X
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mance of the rooms has a strong influence on both the amplification of intruding external noise and the development of internally generated noise emanating from communication processes provoked by the users themselves. At work places where producing sound (by voices or/and instruments) is the main or only purpose for their existence, the acoustic qualification of the room at low frequencies turns out to be of the utmost importance, especially when musicians are forced to work in extremely narrow spaces like orchestra pits and rehearsal halls for many hours a day and often under extreme physical and mental pressure. The measures taken and described herein have proven to mitigate if not remove some of the acoustic burden put on musicians employed in states theatres. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Rehearsal rooms and orchestra pits in theatres are work places for professional artists and musicians. Unfortunately these well-trained and highly motivated specialists mostly suffer from extremely high sound pressure levels and have to perform their jobs under severe difficulties concerning the hearing of the sound of their own voice or instrument as well as that of all other members of an ensemble. Factory workers, who are exposed to weighted noise levels exceeding 90 dB(A), are regularly forced to wear ear plugs in order to protect themselves. In orchestra pits and rehearsal halls the ‘‘noise level’’ often reaches much more than 90 dB(A) [1]; a ‘‘Fortissimo’’ may sometimes amount to peak levels of up to 108–123 dB(A)! Can a musician with stuffed ears clearly hear his/her own playing or singing and that of the others? In other words: what kind of room-acoustic environment should instead be created so that adequate working conditions — optimum sound production combined with maximum health protection — can be provided? 1. Acoustic intensity levels must be reduced such that inevitable damage to musician’s ears is diminished: sound development control! 2. Musical communication between musicians and the conductor must be as clear as possible: a necessity for good ensemble playing! 3. Musicians must hear (and control) their own playing exactly: high sound quality! The philosophy about the acoustics in rehearsal halls and orchestra pits has changed over the years. Being work places, they should no longer be built or restored to simulate a specific auditorium for opera or concert performances, not only because of the totally different spatial (i.e. geometrical) conditions but also of the completely different acting requirements. In order to improve the working conditions for artists of the Staatstheater Stuttgart, the Fraunhofer IBP was asked to take measures to improve the acoustical environment within the orchestra pit in September 1994. After this first project, in 1998 the orchestra pit of the Landestheater Flensburg and, during the summer interval 1999, the large rehearsal hall, again at Stuttgart, have been reconstructed successfully. The musicians are now all exposed to much lower sound pressure levels
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and can better perform their hard work in a completely new and considerably improved acoustical environment.
2. Problems and solutions 2.1. Orchestra pit The members of the orchestra of the Staatstheater Stuttgart and the Landestheater Flensburg complained about an unbearable loudness and considerable difficulties to hear each other and themselves when rehearsing or performing in their orchestra pit. When their conductor asked the musicians not to play so loudly (e.g. for a better balance with the singers on the stage), they found it extremely difficult to please him. The acoustical circumstances were typical for most of the orchestra pits all over the world: musicians are forced to sit and work under extremely narrow spatial conditions (Figs. 1 and 2). The acoustically hard boundaries of the pit reflect all sounds unmitigated. In hardly ‘‘better’’ situations, one may find a thin layer of porous absorption material on the walls. Sometimes an attempt is made to improve the working conditions by laying a carpet on the floor. Nothing, however, is normally installed for the absorption of the much more important low frequencies. As an inevitable result a horrible rumbling builds up, which fills the whole orchestra pit as soon as brass or percussion instruments set in. Acoustic consultants seem to hesitate to recommend the introduction of efficient sound absorbers on the walls and ceiling of the pit in fear that the surface covered may be missing for the necessary transmission of sound and vital communication between different groups [2]. Also, there is hardly any space left for the installation of conventional low-frequency absorbers which as so-called ‘‘bass traps’’ tend to become rather voluminous. Contrary to this, IBP favours the application of relatively slim Compound Baffle Absorbers [3] with much the same motivation as for small communication rooms in general [4,5]. In order to test the suitability of such an acoustic treatment, in a first retrofit project in Stuttgart, these newly developed bass absorbers were only provisionally placed within the pit. After a test period, however, the musicians as well as the conductor would no longer like to work without this innovation which turned out to bring about a real relief for the musicians. The same test was made in Flensburg. Only 19 m2 of these newly developed bass absorbers were provisionally hung into the pit. In a subjective judgement, organized by the orchestra, most of the musicians reacted very positively on the effect of these absorbers in their immediate vicinity. 2.2. Rehearsal hall All members of the orchestra and choir as well as the soloists of the Stuttgart States Theatre complained about unbearable acoustic working conditions in their only rehearsal hall by the same arguments as they used in the case of the orchestra pit. After the latter had been treated with the novel low frequency absorbers, the
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Fig. 1. Plan (top) and section (bottom) of the orchestra pit of Staatstheater Stuttgart [6].
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Fig. 2. Photo of the orchestra pit of the Landestheater Flensburg.
instrumentalists even preferred to work there instead of playing in the much larger and more comfortable rehearsal hall proper. This heavily utilised rehearsing room was restored in the early 1960s. Its inappropriate interior lining determined its unsatisfactory room-acoustic features. The floor area is about 350 m2 (Fig. 3). The height up to the concrete roof is 8 m, but a suspended ceiling with a large air space in between wasted about 40% of useful and urgently needed volume (Fig. 4). The ceiling had a large absorbing section exactly over the groups of strings. The rear part of the ceiling on the other hand was reflecting most of the sound from the ‘‘strong’’ brass, percussion and choir groups which are located there, towards the ‘‘weaker’’ instruments (strings and woodwinds). About 30% of the two side and front walls were covered by conical or spherical ‘‘diffusers’’ made of gypsum (Fig. 5). In front of them there were textile drapes intended to control the reverberation time in the hall. With and without such drapes the reverberation time varied by only less than 0.1 s in the mid-frequency range. This may explain why these drapes have hardly been used over the years. This hall has a reverberation time of around 1.0 s. When analysing the impulse responses from 28 pair measuring positions (Fig. 3), severe problems of clarity C80 showed up: too low in general, and ranging from 1.9 dB to 5.2 dB (Tables 1–3). The clarity factor C80, expressed in decibels, is the ratio of the early energy (0–80 ms) to the late (reverberant) energy (after 80 ms), Ð 80ms 2 p ðtÞdt 0 Ð C80 ¼ 10 log 1 ; ð1Þ 2 80ms p ðtÞdt
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Fig. 3. Ground plan of the rehearsal hall of Staatstheater Stuttgart with measuring positions [7].
arriving at a certain receiver position as emitted from a certain source position. From the rear to the front part the clarity is much higher than in the opposite direction (Table 4). This ‘‘one-way’’ peculiarity impeded the communication and hearing among the musicians. Therefore, the following measures were proposed to the client: 1. in order to reduce the overall sound pressure levels: remove the suspended ceiling in order to enlarge the volume from 1700 m3 to 2800 m3 (Fig. 4), increase the room damping over the whole frequency range in order to reduce the reverberation time,
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mount thin (10 cm) and effective (down to 50 Hz) bass absorbers directly under the roof;
2. in
order to avoid reflections between hard walls: remove all ‘‘diffusers’’ and drapes (Fig. 5), mount bass absorber modules on three walls (Fig. 6), hang mid-frequency absorber foils in front of the bass absorbers;
3. in order to improve ensemble playing among the musical groups and communication with the conductor: remove the skewed reflectors in the rear part (Fig. 4, top), install reflectors above the orchestra such as to guide reflections in the right direction (Fig. 4, bottom and Fig. 7) both in order to increase clarity and avoid the ‘‘one-way’’ peculiarity. For readers not familiar with the early-to-late energy parameter C80: a different result for interchanged source and receiver positions does, of course, not contradict
Fig. 4. Cross section of the rehearsal hall before (top) and after (bottom) the acoustic reconstruction.
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Fig. 5. Photo from the choir position of the rehearsal hall before reconstruction. Table 1 Clarity C80 (3), with different measuring configurations before and after the acoustical reconstruction of the rehearsal hall, as averaged for frequencies 500, 1000 and 2000 Hz Measurements
Before
After
Viols to Conductor Viols to Woodwind Viols to 1. Violin Viols to Choir Celli to Conductor Celli to 1. Violins Celli to Choir 2. Violins to Conductor 2. Violins to 1. Violins 1. Violins to Conductor 1. Violins to Woodwinds 1. Violins to Viols 1. Violins to Choir Brass to Conductor Brass to Woodwinds Brass to 1. Violins Brass to Viols Harp to Conductor Harp to Woodwinds Harp to 1. Violins Harp to Viols Drums to Conductor Drums to Woodwinds Drums to 1. Violins Choir to Conductor Choir to Woodwinds Choir to 1. Violins Choir to Viols
5.18 2.80 0.01 1.90 2.78 1.01 3.28 3.50 4.03 4.79 2.26 2.71 0.01 4.06 3.69 3.69 4.05 2.39 2.91 4.15 1.26 3.89 3.94 2.23 2.79 2.46 3.73 3.32
5.85 5.56 4.05 3.72 6.12 4.79 5.15 6.84 7.47 6.55 5.86 4.16 5.12 4.63 5.31 4.82 4.92 5.89 6.89 4.49 4.97 5.19 3.43 4.71 4.24 4.77 5.18 5.84
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Table 2 Clarity C80 (3) (dB) for different measurements from the string sections to the conductor’s position before and after the acoustical reconstruction of the rehearsal hall Source position
Before
After
1. Violins 2. Violins Viols Celli/Basses
4.8 3.5 5.2 2.8
6.6 6.8 5.9 6.1
Average
4.1
6.4
Table 3 Clarity C80 at 1000 Hz in (dB) for different measuring pairs from different instrument groups to the conductor’s position, before and after the acoustical reconstruction of the rehearsal hall in comparison with the recommended value in [8] Instrument group
Before
After
Recommendation
1. Violins 2. Violins Viols Celli / Basses Harp Brass Percussion Choir
4.1 2.5 5.0 2.5 2.0 4.1 4.0 2.8
7.3 6.8 6.0 6.5 4.1 4.1 5.1 5.5
48 48 48 48 14 2 +2
Table 4 Clarity C80 at 1000 Hz in (dB) showing the elimination of ‘‘oneway’’ transmissions in the rehearsal hall before and after its acoustical reconstruction Transmission From
To
1. Violins Choir Viols Choir
Choir 1. Violins Choir Viols
Before
After
0.0 3.7 1.9 3.3
5.1 5.2 3.7 5.8
the reciprocity principle for all individual sound paths between two points in a room. Yet, when a source, e.g. were placed close to a fully absorptive wall and/or ceiling whereas in the other case it were backed by fully reflective boundaries, there is no question that the temporal and spatial energy distributions may considerably differ in the two opposing cases in the close agreement with practical experience.
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Fig. 6. View of the novel absorber modules as installed on the front wall (behind the conductor) of the rehearsal hall. A: air conditioning installations D: door E: elevator.
Fig. 7. Photo of the newly installed reflectors above the orchestra and of the Compound Panel Absorbers mounted under the ceiling after reconstruction of the rehearsal hall.
3. Room-acoustic installations For performing environments as spatially restricted as orchestra pits and rehearsal rooms, IBP pursues a similar strategy as for communication and multi-purpose rooms in general [4,5]:
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1. let the low-frequency part of the music produced not ‘‘fill’’ the room with an ill-defined, poorly articulated ‘‘sound-screen’’ which inevitably chokes up the musicians’ ears for hearing and controlling the more important finer details of the music to be performed, 2. introduce special bass absorbers, e.g. in the form of Compound Panel Absorbers, which are able to ‘‘clear up’’ the low frequency part of the music but nevertheless keeps reflecting the bulk of the energy at the higher frequencies, 3. leave or install, where necessary and possible, acoustically reflecting surfaces above the users/musicians, which support the sound transmission between all communicating groups. Some of the new measures which were carefully derived from profound experiences with numerous projects dealing with speech intelligibility problems [4,5] initially encountered some scepticism among musicians. They feared, e.g. that a massive introduction of low frequency absorbers would weaken the fundamental bass lines which they found so difficult to identify and follow in their stressing work. They were also afraid to loose reflecting surfaces in their immediate vicinity which they thought would help them to hear and control their own sound generation within the overall acoustic chaos in which they usually felt almost drowned if the whole orchestra played something like ‘‘Forte’’ or ‘‘Fortissimo’’. This fear was articulated by the relatively ‘‘weak’’ ensemble players like violins and viols, in particular. 3.1. Orchestra pit After provisional installations for a ‘‘Meistersinger’’ production during the summer interval 1993 which immediately convinced the musicians and the maestro Gabriele Ferro, a few surfaces and niches were found during the summer interval 1994 where a total of about 40 m2 of different Alternative Fibreless Absorber (ALFA)-modules could be mounted on or integrated into the boundaries of that 130 m2 large pit (Fig. 8). Details of this first massive innovative sound absorbing treatment of an orchestra pit were published by Zha et al. [6]. As in preceding restorations performed in acoustically demanding small rooms [4], the main improvements aimed at a frequency region which is normally disregarded by acoustics and ergonomics experts, since it lies well below the range where conventional room-acoustic measurements, evaluations and assessments are performed. Therefore, the immediate reactions and subjective judgements of the many musicians was particularly important after their working environment had been changed so markedly. 3.2. Rehearsal hall Again the philosophy behind the rather radical and uncommon acoustic restoration of the much larger (22168 m) hall was to clear up the ‘‘foggy clouds’’ that immediately ‘‘filled’’ the room as soon as the low-frequencies were excited by bass and percussion instruments. Despite a considerable increase in volume (some 40%), the reverberation time at medium and high frequencies was planned to not exceed
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Fig. 8. Installation of novel Alternative Fibreless Absorber (ALFA) — modules on the front wall (top) and back wall (bottom) of the orchestra pit in Staatstheater Stuttgart [6].
0.8 s with only minor variations with frequency and, most importantly, no steep ascend towards the low frequencies. Details of the acoustic design and layout may be found in [7]. It involved about 260 m2 Compound Panel Absorbers [3–5], 70 m2 of double-layer Microperforated Foil Absorbers hung in front of those low frequency absorbers which were mounted on three walls (Fig. 9).
4. Results 4.1. Orchestra pit Extensive measurements were made after the absorbers were fitted. In the auditorium the reverberation time was, of course, always found unaltered. In the pit itself, however, a very short early-decay time could be identified in Stuttgart after the installations. The ratio between the early-decay and the standard reverberation times were in the range from 0.2–0.3 for different frequencies. Based on this information, one may derive an averaged reduction of sound pressure levels within the pit of the order of 5–7 dB when the sources are assumed to remain constant. In reality, the resulting reduction could be even more, since in the new acoustic environment the musicians are now able to better hear themselves and the other members of the ensemble and need not play louder than necessary for the benefit of an optimum balance between the orchestra and the stage. A great enhancement of clarity has been measured in the pit over the whole frequency range, especially at the lower frequencies. For instance, at 125 Hz between the conductor’s and musicians’ positions in Stuttgart, clarity increased by more than 10 dB for some places in the orchestra, and above 250 Hz, averaged over all positions, by about 5 dB (Fig. 10) [6].
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Fig. 9. Photo and schematic sketch of the novel acoustic lining on a side wall of the rehearsal hall.
The results and reactions of the users in Flensburg were particularly interesting: after the novel absorbers were permanently installed in the orchestra pit (Fig. 2), the musicians were completely satisfied by the obvious improvement of their working conditions. They did not even want to spend any more of their meagre resources for additional acoustic measurements. One of their authorities, however, found that the loudness in the auditorium especially of the violins had now become too low. Before the absorbers were to be removed for this reason, IBP was asked to prove, by objective measurements, whether or not the absorbers had changed the musical performance. The reverberation time in this pit (which is closed by more than 50% by the stage/ceiling) turned out to be but slightly reduced in the frequency range from 125 to 1000 Hz (Fig. 11). Well below the range of frequencies produced by violins, however, the reverberation of the pit could be shown to be dramatically reduced by the installation of the absorbers! Only in this important low-frequency range the sound pressure level in the pit could be shown to be reduced by at least 4 dB with the additional, even more important positive effect of reducing the rumbling which makes it so hard for all musicians to play in an orchestra pit. With the considerably improved clarity for all musicians and the conductor, however, it may well
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Fig. 10. Clarity C80 between the orchestra and the conductor measured as a function of frequency before (- - - -) and after ( — ) the installations shown in Fig. 8.
Fig. 11. Reverberation time measured as a function of frequency in the orchestra pit of Landestheater Flensburg: before (–*–) and after (–&–) the installation of bass absorbers in the pit.
be true that, in the specific performance the authority had attended, the musicians felt they should not play too loud for one reason or another. Measurements of the reverberation time in the auditorium, on the other hand, proved that the acoustical situation there was not changed at all (Fig. 12). In the end, all involved people were satisfied by a demonstration of why the loudness in the auditorium was reduced and how variably the musicians were now able to perform after the right absorbers were installed in the right place. Altogether 37 musicians
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Fig. 12. Reverberation time measured as a function of frequency in the auditorium of Landestheater Flensburg: before (–*–) and after (–&–) the installation of bass absorbers in the pit.
assessed the improvement of the new acoustic environment. According to Table 5 almost 90% found that the working quality has changed for the better. 4.2. Rehearsal hall After the reconstruction the acoustical measurements were repeated as before [7]. Above 160 Hz the reverberation time is in the range of 0.7 0.1 s (Fig. 13). This result agrees with the recommendations for rehearsal halls in [8]. Musicians do not experience the hall as too ‘‘dry’’. Singers feel that they are well embedded within the ensemble and notice a comfortable room response to their voices. From the entirely new room parameters one can easily estimate that the effective sound pressure level has been reduced by more than 4 dB. In reality, sound pressure levels will be reduced even more for the reasons as explained earlier for the pit. Clarity C80 has been considerably raised for nearly all measurement pairs. The variations among all measurements are decreased from 7.2 dB (before: 1.9–5.2 dB) to 3,5 dB (after: 3.4–6.9 dB; Table 1). The C80 average for all strings section positions has improved from 4.1–6.3 dB (Table 2). The acoustical transmission functions between all places and directions are now also more even. From all strings sections to the conductor’s position the values of C80(1000Hz) have risen from 3.7–6.7 dB and are now well within the range favoured in [8] (Table 3). The average transmission to the conductor from all measuring positions has increased by 2 dB. The transmission between several other source and receiver positions has also been improved (Table 1). The value of C80 from the viols to the first violins has changed from 2.7–4.2 dB; to the choir the increase was from 0.0–5.1 dB. The average improvement for C80 of all measurements to the first violins position is 2.7 dB. The worst result before was the transmission from the viols to the choir with a value of
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Table 5 Subjective assessment of the new acoustic working environment in the orchestra pit of the Landestheater Flensburg Subjective assessment
No. of musicians
Substantial deterioration Minor deterioration No change Minor improvement Substantial improvement
1 – 3 22 11
Fig. 13. Reverberation time measured as a function of frequency after the reconstruction in the rehearsal hall without musicians (–*–) and estimated with 100 musicians (–&–) [7].
1.9 dB. Now it is at 3.7 dB. The ‘‘one-way’’ peculiarity has thus been eliminated (Table 4). According to the symmetry of the room, a similar result for the measurement of C80 from the first violins to the viols should be expected as for the ‘inverse’’. Before the reconstruction the values were 2.7 dB (first violins to viols) and 0.0 dB (viols to first violins). After the reconstruction both values are very similar, 4.2 and 4.1 dB respectively (Table 1). The asymmetry of the room acoustic properties both sides of the line conductor–choir has obviously been removed.
5. Conclusions Work places in orchestra pits and rehearsal rooms usually suffer severely from too small volumes and spaces and extremely high sound intensity levels. Steel workers or pilots would under similar conditions have to wear ear plugs or take other noise abatement measures — a solution not very attractive, but a very deplorable necessity
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for numerous musicians working in pits or rehearsal rooms. Musicians rightly expect acoustic engineers to reduce the sound level and, at the same time, let them hear and be heard more clearly. The Compound Panel Absorbers and Broadband Compact Absorbers [4,9] in a novel combination with Microperforated Foil Absorbers makes it possible to treat the problems because of their effectiveness and small thickness, especially at the particularly disturbing low frequencies. In order to achieve a good musical communication, the necessary absorption measures have to be accomplished by well-directed reflection measures well above the musical ensembles, especially in a rehearsal hall.
Fig. 14. Alternative Fibreless Absorber ALFA materials and elements as innovative tools for acute noise abatement and room-acoustic (bold letters) tasks.
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In the Staatstheater Stuttgart, the orchestra pit and the large rehearsal hall have been improved acoustically. Average sound intensity levels could be reduced considerably in both places. The increase of clarity has been verified as a great improvement for hearing and ensemble playing of the musicians. Improving the working conditions by installing novel acoustic modules in the performing environment may, in the end, even raise the sound quality of musical productions in general. These far too late achievements for acoustically tortured musicians forced to work within large ensembles in small rooms was made possible by a new application of novel Alternative Fibreless Absorber ALFA modules developed at the Fraunhofer IBP to supplement or replace conventional acoustic linings employing mainly fibrous absorbers. Fig. 14 lists 12 ALFA family members, at least half of which (those written in bold letters) have proven very useful in various room-acoustic projects [4–7]. This fairly successful research, development and application activity started about 15 years ago with the Membrane Absorber Box (MAB) [10] initially applied (as splitter silencers) in heating, ventilating and air conditioning HVAC systems with high hygiene demands and for other noise control tasks [11]. Their excellent low-frequency performance, their smooth and closed metallic surfaces, qualified these ALFA modules especially for taming booming room modes excited by percussion instruments in concert (Fig. 15) or rehearsal (Fig. 16) environments [12]. In later construction and restoration projects the MAB modules were more and more replaced by the above mentioned and in [4, 9] described CPA and BCA elements for their even broader and more easily adjustable frequency characteristics and, most important in this business, their cost-effective production. The broad and almost enthusiastic acceptance of the retrofit results in long existing environments by the musicians and their conductors have meanwhile encouraged their equally suffering colleagues in Duisburg, Flensburg, Rendsburg, Mainz and elsewhere to ask Fraunhofer IBP to also help them to improve their similar
Fig. 15. Low-frequency MAB absorption screens shielding the sound from different percussion instruments for a concert recording in a music hall of Solitude Castle, Stuttgart.
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Fig. 16. Low-frequency MAB baffles suspended from the ceiling of a rehearsal room in the music school at Waldenbuch.
working conditions in rehearsal halls and orchestra pits. It thus seems no longer questionable that it is the low frequency end of the acoustics, long neglected awfully by acousticians, which is largely responsible for poor communication, difficult ensemble playing and, last but not least, increasing hearing loss in musicians [1] forced into always too small enclosures. How much the bass range may affect the quality of larger auditoria is, of course, quite another topic. References [1] Fearn RW. Hearing loss in musicians. J Sound Vib 1993;163:372–8. [2] Barron M. Auditorium Acoustics and Architectural Design. London: Chapman & Hall, 1993, 311. [3] Zha X, et al. Measurements of an effective absorption coefficient below 100 Hz. Acoustics Bulletin 1999;24:5–10. [4] Fuchs HV, et al. Creating low-noise environments in communication rooms. Applied Acoustics 2001 (in print). [5] Drotleff H, et al. Attractive acoustic design of multi-purpose halls. ACUSTICA (submitted for publication). [6] Zha X, et al. Improving the working conditions in the orchestra pit of the Staatstheater Stuttgart (in German). Bauphysik 1997;118:196–204. [7] Zha X, et al. Room-acoustic improvements in the rehearsal hall of the Staatstheater Stuttgart (in German). Bauphysik 2000;22:232–9. [8] Tennhardt H-P, Winkler H. Investigations for room-acoustic planning of orchestra rehearsal rooms (in German). Acustica 1995;81:293–9. [9] Fuchs HV, et al. Qualifying freefield and reverberation rooms for frequencies below 100 Hz. Applied Acoustics 2000;59:303–22. [10] Ackermann U, et al. Sound absorbers of a novel membrane construction. Applied Acoustics 1998;25:197–215. [11] Ve´r IL. Enclosures and wrappings. In: Harris CM, editor. Handbook of acoustical measurements and noise control. New York: McGraw-Hill, 1991 (chapter 13). [12] Fuchs HV, Hunecke J. The room plays its part at low frequencies. Das Musikinstrument 1993;42:40– 6 (in German).