The Effect of Scenery on the Stage Acoustic Conditions in a Theatre

Transcription

1 Department of The Built Environment Building Physics - Acoustics Research group The Effect of Scenery on the Stage Acoustic Conditions in a Theatre Master Thesis Ni Putu Amanda Nitidara Supervisors: Ir. R. H. C. Wenmaekers Ir. C. C. J. M. Hak Eindhoven, July 2014

2 Preface This report is a result from preliminary research for my Thesis Project which was done at the Acoustic Laboratory, Eindhoven University of Technology. Main objective of the Thesis is to developed the design of concert hall for traditional music in Indonesia. The focus of the research presented in this report is about the stage acoustics, following the growing issue on how to create good acoustic conditions on stage for the musicians. Working in this project has enhanced my knowledge in understanding the acoustic in performance halls, especially stage acoustics. It was also a great experience to work in an international environment. I would like to thank both of my supervisors for the assistance and helpful discussion during the work on this project. Hopefully this report can add valuable knowledge and information about stage acoustics. July 2014, Ni Putu Amanda Nitidara 1

3 Abstract A theatre stage is unique compared to the concert hall stage, due to its flexibility in terms of its boundary conditions. A theatre stage is often provided with scenery that allows users to change the stage depends on their needs. Measurements on stage were conducted to analyse the effect of stage scenery to the acoustic conditions on stage. Three variations of stage scenery were investigated, empty stage, absorptive stage, and reflective stage. Comparison study between the three variations were made, by evaluating stage acoustic parametersst early andst late. Theresultsfromtheexperimentshowthatadditionofstage scenery increases the ST early value on the stage since it provides more early reflections. While the ST late was also changes with the changes in scenery, it was mostly affected by the amount of absorption in the stage. Simulation methods to predict the ST early and ST late on stage were also investigated. Image source method and Barron s revised theory used to calculate the ST early and ST late value respectively. The results show that both methods work quite well to predict the behaviour of the two parameters. 2

4 Table of Contents Preface 1 Abstract 2 1 Introduction Background Objective Outline Methods Stage Acoustic Parameters Source-Receiver Positions Measurement Conditions Image Source Method Results and Discussion Measurement Image Source Method Conclusion

5 1 Introduction 1.1 Background Stage acoustics is the branch of room acoustics which focus on the acoustic conditions on stage for the musicians. It is necessary for the musicians to have feedback about the sound they are producing. Musicians need to hear the sound coming from their own instrument, in case of ensemble, they also need to hear the sound from the other team member, and thirdly they need to hear the sound coming back from the hall to get an impression of what the audiences are hearing. The stage acoustic experiment by Gade [1,2] looked for the objective parameters that correlated well with musician s subjective perception of sound on stage. The experiment resulted in the objective parameters Support (ST), Clarity (CS), and Early Ensemble Level (EEL). Later, the Early and Late Support parameters were included in ISO :2009 about measurements of room acoustic parameters in performance spaces [3]. Previous researches that were conducted in Indonesia, investigated the preferred acoustic conditions for its traditional music instrument Gamelan Bali [4,5]. Gamelan is kind of ensemble music that are played by group of musician. The complete Gamelan instruments consist of metallophones, drums, flute, and gong. Gamelan produces loud sound and also quite reverberant, based on this characteristic, indoor performances of Gamelan music should be treated very carefully in order to preserved and enhanced its musical quality. A design of dedicated concert hall for Gamelan Bali was built [6], this design was made based on the acoustic conditions preferred in the hall. Further design on the stage acoustic conditions of this hall is now being evaluated. The objective is to developed and adjust the stage design so it will fit the stage acoustic conditions suitable for the musicians. The stage for Gamelan performances usually came with addition of scenery such as temple in the back of the stage, or panels with illustration (see Figure 1). This scenery is usually used to marktheentrance area forthe performers. It isalsoprovided forthevisual effects, since Gamelan performances usually accompanied by dance performances. The temple can be made out of panels that can be removed and adjusted, or built-in permanently in the rear of the stage (this type usually made by brick). It is necessary to find the effect of this scenery to the acoustics on stage, around the musicians. Therefore preliminary research about the effect of scenery were made. Previous researches on stage acoustic in concert hall showed that the surfaces and boundary conditions (stage risers, canopy, diffusers) on the stage area will affect the stage acoustic conditions [7,8]. In order to achieve further understanding about the stage acoustics of performances hall, an experiment in theatre was conducted. Unlike the stage in concert hall, theatre stage is more flexible in the term of its boundary conditions. A theatre stage is also provided with scenery that allows users to change the conditions on the stage depending on their needs. Backdrop, curtains, drapery, panels, are examples of the stage scenery as shown in Figure 2. 4

6 Figure 1: Indoor performances of Gamelan Bali and dance, in an Auditorium. Gamelan sits on the side of the stage, dancers in the centre of the stage, and there is panels illustrated the temple at the back of the stage. 1.2 Objective Figure 2: Theatre stage scenery. Source by: drapes and stage curtains The objective of this research was to find the effect of stage scenery on the stage acoustic conditions. It was also intended to find which parameters between ST early and ST late that is sensitive to the changes of stage scenery. Measurements in theatre were conducted while changing the stage scenery. Further investigation of predicting the stage acoustic conditions were also performed in this preliminary research. It is necessary to find the good method to predict the stage acoustic conditions, especially for a new-built stage design phase. Image source method was proposed to be the prediction tools. Using the measurement data, validation of this method were performed. 1.3 Outline This report will presented the method and results from the preliminary research about stage scenery and simulation method. The methods of the research will be explained in chapter 2, including the measurement and simulation. The results from the research will be discussed in chapter 3. The report ends with a conclusion. 5

7 2 Methods 2.1 Stage Acoustic Parameters In this research, two stage acoustic parameters were evaluated. The extended parameters of Support (ST) [9] were used for evaluation in various source and receiver distances, see equation (1) and (2). The ST early and ST late are derived from the impulse response on the stage. They are defined as the early and late reflected energy relative to the direct sound energy measured at 1 meter distance. Originally both parameters were only measured at 1 meter distance. The extended parameters however provide the method to calculate Support at various source and receiver distances by introducing a variable time 103-delay. Direct sound delay from the sound to receiver point is taken into account. The lower time limit of 10 ms is used instead of 20 ms in order to provide closer measurement positions to the boundaries up to 2 m. The reference direct sound level was measured separately at 1 m distance. ST early,d = 2.2 Source-Receiver Positions ( 103 delay 10 p 2 d (t)dt 10 0 p2 1m(t)dt ) [db] (1) ( 103 delay ST late,d = p2 d (t)dt ) 10 [db] (2) 0 p2 1m(t)dt Impulse responses were measured in different source and receiver positions on the stage following the measurement grid shown in Figure 3. This measurement grid was developed carefully to match the position of instrument group in the symphony orchestra. Some adjustment to the measurement border and position were made during this research due to the scenery position and dimension of the stage opening. In this experiment 24 m x 11.5 m border were used since that were the position of the tab and backdrop. However, the stage opening dimension is 15 m, therefore to create a condition where all the position were seen from the hall, point 11 and 12 were excluded from the grid. The remaining 5 source positions and 10 receiver positions were measured. Figure 3: Measurement grid and position of source and receiver on the stage [9]. Green line were the border for empty stage and absorptive curtains configuration. Red line were border for reflective panels configuration 6

8 2.3 Measurement Conditions Measurement were conducted in the Parktheater, a hall for theatre performances in Eindhoven. This theatre can accommodate 950 listeners in its 500 m 2 audience area. The hall (audience area) volume is 10,400 m 3. The stage area is quite large, total 1,300 m 2. The stage area consist of main stage, backstage, left-right side stage, and stage tower. The main stage volume is 1,750 m 3 and the total stage volume are 27,550 m 3. Detail illustration of the stage area, division of the main stage and total stage area shown in Figure 4b. The main stage (300 m 2 ) is the part of the stage that audience can see during the performance, while the remaining stage area acts as a support stage. The main stage is connected to the stage tower with almost 30 m height. The stage tower functions as a storage place for the stage curtains, lighting, and installation system. The floor plan and section of the theatre shows in Figure 4. To analyse the effect of scenery on stage, three variations of stage scenery were made: 1. C1 : empty stage without scenery 2. C2 : absorptive stage with stage curtains down 3. C3 : reflective stage with reflective panels put on stage During the empty stage configuration, curtains and drapes were all rolled up. For the absorptive curtain configuration, the backdrop, borders, and tabs were rolled down. In the reflective panels configuration, panels were set in form of orchestra shell, in the same border as the curtains (see Figure 3). The height of the panels were varied from 2 m up to 2.4 m. Figure 5 shows the configuration empty stage and stage with curtains and panels during the measurement. The measurement grid was plotted in the main stage area (green area in floor plan, Figure 4b). For all of the configurations, a complete set of impulse response measurements were made. For each position of source and receiver, multiple measurements were taken with changing the speaker direction in 5 equal angular steps. The source and receiver height respectively are 1.35 m and 1.2 m. Measurements were performed using omnidirectional loudspeaker AE type Pyrite, amplifier AE Amphion, microphones B&K type 4189-A-021, and Dirac 6.0 measurement software. 2.4 Image Source Method The image source method were used to predict the value of ST early in the stage. Using the concept of image source method, the sound waves then be replaced by energy rays in which its behaviour follows that of light rays. For this case, only 1 st and 2 nd order reflections were included due to the early reflection time limit of ST early parameter, only the reflection with time travel below the upper time integration limit are included in the summation. Unlike the measurement, the source and receiver point in the simulation was keep in the center line of the stage with increasing the distances. Using the equation (1) and combined with the inverse square law, the sound level of each reflection from one image is defined by equation (3). The variable d represent the sound 7

9 (a) section (b) floor plan Figure 4: The section and floor plan of The Parktheatre. Blue area indicates the stage area, yellow the audience area, and green the main stage area. (a) Empty stage (b) Absorptive curtains (c) Reflective panels Figure 5: Measurement conditions on stage, variations of the stage scenery. 8

10 path length between the image source and the receiver point. The boundary conditions were also take into account in this method, equation (4) show the calculation for level reduction by sound absorption [10]. L p = 20 log d (3) L a = 10 log ( ) 1 (1 α) (4) The total energy calculated using equation (3) for each image source will give the ST early value for the specified distance. It is important to note that diffraction and interference effect were not consider in this method. However, for 2 nd order reflection of ceiling-floor and floor-ceiling paths, always producing the same sound path length, the sound were assumed to arrive in the same phase therefore 3 db extra was added to the calculation. For the prediction of ST late, formula by Barron s [11] in equation (5) is used. This formula derived from the exponential decay of the impulse response in the stage. Variable V is for the stage volume and T stands for the reverberation time of the room. For the simulation, Sabine s formula of reverberation time is used, see equation (6). Barron s formula predict the ST late as a function of total amount of sound absorption. ST late = ( 10 log ) ( 312 T ) 6 V T (5) RT 60 = V A A = αs (6) 3 Results and Discussion 3.1 Measurement Three variations of the scenery were made during the experiment. Empty stage configuration, absorptive stage with stage curtains down, and reflective stage with reflective panel put on stage in the shape of an orchestra shell. Figure 6 shows the ST early value over the three variations mention above. The graph shows that empty stage configuration (blue line) is having very low ST early value compared with the other configurations. This is due to the massive volume of the stage and also because there is no boundary enclosed the main stage area. As a results, only direct sound were found, and there were no early reflection energy. The addition of stage scenery contributes to changes in stage acoustic conditions. Addition of stage scenery either reflective or absorptive surface increases the ST early value in the stage. This is because the scenery now acts as a boundary for the main stage and provides early reflections to the measurement area. 9

11 It is also shown in the graph that the ST early value have a correlation with the distance of source and receiver, resulting in lower value with increasing distance. This results show good agreement with previous research [9] where the logarithmic correlation of ST early with distance was also found. However, this correlation did not shown in the reflective panels configuration. It seems that the panels distribute the early reflection uniformly. STearly(dB) Empty Stage Absorptive Curtains Reflective Panels distance(m) Figure 6: ST early value with variations of scenery on stage The ST late value is also changed because of the addition of the stage scenery as shown in Figure 7. The absorptive curtains configuration results in the lowest value, while the empty and reflective panels configuration show no significant difference. The stage curtains absorb the sound energy and therefore lower the sound energy of late reflections and reverberation. Unlike the ST early, ST late value was found to have few correlation with distance. Therefore it is sufficient to represent the ST late in single value. STlate(dB) Empty Stage Absorptive Curtains Reflective Panels distance(m) Figure 7: ST late value with variations of scenery on stage 10

12 To further understand the behaviour of the ST early and ST late parameters, we can refer to the impulse responses that were measured on stage, shown in Figure 8. The impulse responses that are shown in the pictures contain only early and late part energy (direct sound 0-10 ms were excluded). Red lines which separate the early and late part were drawn in the position of 100 ms. Generally, the early reflection energy is higher than the late reflection energy. As sound travel from source to receiver, it might hit surfaces around its boundary. As the more surfaces it hits, the more its energy reduced by the absorption of surfaces. Late reflection hit more surfaces and travel more distances than the early reflection, therefore the energy always lower than the early reflection. This concept also explained the ST early and ST late parameters, where ST early measured in one stage will always have higher value than the ST late in that same stage. The early part clearly showed the addition of sound energy by addition of the stage scenery. The late part showed that the reflection energy was very low for the stage with curtains, this is due to the absorption provide by the curtains. The reflective panels and empty stage showed hardly any differences in its amount of energy. Late reflection and reverberation energy, and the amount of absorption is the factor that affect ST late value. (a) Empty stage (b) Absorptive curtains (c) Reflective panels Figure 8: Impulse response on the stage with variations of stage scenery. Red lines separate the early and late reflection part 11

13 3.2 Image Source Method Using the image source method, similar configuration of stage scenery were evaluated. An imaginary box was made representing the main stage area. The border dimension of the box used for absorptive curtains and reflective panels configuration were set the same as the measurements. For the empty stage configuration, the dimension of stage walls was used as the border. The height of the box were set to 15 meter. Each boundary was given estimated absorption coefficients representing the real condition. Absorption coefficient for every configuration are list in Table 1. Table 1: Absorption coefficient for image source method boundaries Configuration Front Back Left Right Floor Ceiling Empty stage Absorptive curtains Reflective Panels The absorption coefficient for the ceiling was set 0.9 (totally absorptive, absorption value of 1.0 is not possible to be used in this formula) since above the ceiling is the stage tower. The front boundary was the opening to the audience area, this boundary was also set fully absorptive since the air was in front of it. For the empty stage configuration, back, left, and right side were also set fully absorptive since it was open air. Floor were parquet on concrete. ST late prediction were calculated using the formula by Barron s shown in equation (3) combined with Sabine reverberation time formula, the results shown in Figure 9. Measurement results as shown in Figure 7 were averaged arithmetically to find the single value ST late. Calculation and measurement were differ by 1.5 db except for the reflective panels configuration which reaches 2 db. The results from the image source method (Figure 10) show similar behaviour as the measurement results. Figure 11 shows in more detail of measurement and simulation results for each configuration. It shows that the simulation results fit the contour and behaviour of the measurement. Generally the overall result from the simulation were higher than the measurements. This might be caused by diffraction and interference effects that were not take into account with this method. Main differences that were found is that the reflective panels configuration in the simulation related with the distance of source and receiver, while that was not found in the measurements. Image source method calculation mention in equation (3) were based on inverse square law, where the sound energy at certain position is depends on its distance and reference sound energy at 1m distance. Using this equation, relation between the parameter value and distance will always be found. 12

14 0 simulation measurement STlate(dB) ,500 3,000 3,500 absorption area (m 2 ) Figure 9: ST late value from calculation using Barron s revised theory compared with the measurement results STearly(dB) Empty Stage Absorptive Curtains Reflective Panels distance(m) Figure 10: ST early value from image source method 13

15 0 simulation measurement STearly(dB) distance(m) 0 (a) Empty stage simulation measurement STearly(dB) distance(m) 0 (b) Absorptive curtains simulation measurement STearly(dB) distance(m) (c) Reflective panels Figure 11: Comparison of simulation and measurement results of ST early over three stage configurations. 14

16 4 Conclusion From this research it can be concluded that the ST early parameter is largely affected by the presence of stage scenery. The scenery acts as an enclosure that provide useful early reflection to the performance area. While ST late value is more dependent on the reverberation time on stage therefore it depends on the amount of the absorption in the stage area. This absorption can also be provided by the absorptive scenery. Overall acoustic conditions of the stage in this theatre is quite good and have sufficient reverberation time for theatre performances. In case of the performances with orchestra in this kind of stage, the use of reflective panels around the musicians is suggested since it will improve the stage acoustics for the musicians. The image source method proved to be a simple method that can estimate the behaviour and conditions of stage acoustic parameters on stage. The calculation formula for ST late also quite good to predict the value of the ST late. These two prediction methods could be useful for the early estimation of stage acoustic conditions especially in the design step of the new built stage in performance places. The experiment explained in this report were conducted using omnidirectional source and receiver. Experiment using dummy head and directional sound source are suggested for future research in order to represent the music instruments as well as human ears. Further research related with stage acoustic and stage scenery will be conducted. The conclusion from this experiment will be used to analyse the stage acoustic conditions in Gamelan Bali Performance Places. Simulation with ISM will also be used for adjustment and fine tuning of the stage design in Gamelan Bali Concert Hall. 15

17 References [1] A. C. Gade, Investigations of Musician s Room Acoustic Condition in Concert Halls. I. Methods and Laboratory Experiments. Acustica 69, , [2] A. C. Gade, Investigations of Musician s Room Acoustic Condition in Concert Halls. II. Field Experiments and Synthesis of Results. Acustica 69, , [3] ISO : Acoustics Measurement of Room Acoustic Parameters Part 1 : Performance spaces. International Organisation for Standardisation (ISO), Geneva, [4] I G. N. Merthayasa, et al, Spatial Factor of Sound Fields for Gamelan Bali COncert Hall. Proceedings of 17 th ICA, Rome, [5] P. Mulita, Studi Psikoakustik Gamelan Bali: Penentuan Waktu Tunda Pantulan Dini Optimum dengan Metode Simulasi Medan Suara (in Indonesian). (Thesis, Institut Teknologi Bandung, Bandung, Indonesia, 1999) [6] N. P. A. Nitidara, I G. N. Merthayasa, J. Sarwono, Modeling and SImulation of Gamelan Bali Concert Hall Based on Objective Acoustic Parameters. Proceedings of Meetings on Acoustics 19, Montreal, [7] E. W. M. van den Braak, C. C. J. M. Hak, H. J. Martin, L. C. J. van Luxemburg, Influence of Stage Risers on Stage Acoustics. Proceedings of Forum Acusticum 2005, Budapest, pp , [8] E. W. M. van den Braak, L. C. J. van Luxemburg, New (Stage) Parameter for Conductor s Acoustics?. Proceedings of the 155th ASA Conference, Paris, [9] R. H. C. Wenmaekers, C. C. J. M. Hak, L. C. J. van Luxemburg On Measurements of Stage Acoustic Parameters Time Interval Limits and Various Source-Receiver Distances. Acta Acustica united with Acustica, 98, , [10] R. H. C. Wenmaekers, L. J. W. Schmitz, C. C. J. M. Hak, Early and Late Support over various distances: rehearsal rooms for wind orchestras. Proceedings Forum Acusticum 2014, Krakow, [11] M. Barron and L-J. Lee Energy Relations in Concert Auditoriums, I. Journal of The Acoustical Society of America, 84, ,