Acougamic: Dynamic Acoustical Control

Transcription

1 Acougamic: Dynamic Acoustical Control MOHAMED HUSSEIN, NURUL FARHANAH MUARAT, RAJA ISHAK RAJA HAMZAH, ZAIR ASRAR AHMAD, MAZIAH MOHAMED, MOHD ZARHAMDY MD ZAIN and *NORASIKIN MAT ISA Department of Applied Mechanics, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310, Johor Baharu, Johor MALAYSIA *Dept. of Plant and Automotive Engineering, Faculty of Mech. and Mfg. Engineering, Universiti Tun Hussein Onn Malaysia, Batu Pahat, Johor MALAYSIA Abstract: - This paper presents a preliminary study on variable acoustic technique using acougamics which is the use of origami shapes in acoustical control. The acougamics are acoustic diffusers constructed from a flat sheets folded in origami-like manner. Three acougamics are presented in this paper. The purpose of the study is to investigate the possibility of using acougamics to vary the reverberation time of a multi-purpose hall. Findings from the preliminary investigation reported in this paper are based on the three acougamics being tested according to ISO 354:2003 standard in Measurement of Sound Absorption in a Reverberation Room. The results from the preliminary investigation have shown promising trends demonstrating that the acougamic is capable to be used as a technique to provide variable acoustic of multi-acoustic requirements. Key-Words: - Origami, variable acoustic, multi-purpose hall, sound absorption 1 Introduction Because of economic reasons, the building of multipurpose halls has become apparently common recently [1]. Thus, variable acoustic techniques are necessary in order to accommodate various types of events in the multi-purpose hall, which normally focus on changing the reverberation time according to the required event. This is due to the fact that different types of acoustic events will require different optimum reverberation time, as reverberation time is the crucial parameter to determine the quality of the sound produced. The available techniques on variable acoustics so far can be divided into several ways, namely variable volume, variable absorption, variable scattering and electroacoustic system. Each technique has its pros and cons. Variable volume technique is normally achieved using a movable ceiling, movable shutter system and by coupling the hall with a reverberation chamber to provide adjustable reverberation time [2]. Milton Keynes Theatres, for instance, uses a movable ceiling technique in which its ceiling can be moved in a vertical distance of 10 m and result in a reverberation time range of 1.1 to 1.5 seconds [1]. Another example is Studio Acusticum in Sweden where the ceiling of the hall can be adjusted up to 5m which effectively changes the reverberation volume by 30% [3]. Besides that, the advantage of varying the volume is that it gives a significant change to the reverberation time, and at the same time is able to maintain the sound level inside the hall [4,5]. However, the variability of the reverberation time based on the variable volume is quite costly and sometimes the degree of variability is unsatisfactory [1]. It is also technically more difficult compared to variable absorption technique [6] Variable absorption technique is a most common technique used in the control of reverberation time of multi-purpose halls. The technique is realized through the use of retractable curtains, hinged panels, acoustic banner, adjustable audience seats and/or movable reflectors [1,2,4,5]. This technique has been employed in Fitzwilliam College Auditorium, Cambridge, where the change of reverberation time is accomplished using acoustical absorbing panels and retractable audience seats. The result of the technique produces a reverberation time range of 1.3 to 1.5 seconds which is appropriate for music and speech performance [4]. However, the problem ISBN:

2 associated by variable absorption is it reduces the sound strength and it causes the hall to seem quiet even though the reverberation time is reduced. The effectiveness of this technique also depends on the thickness of the acoustic material which means that high thickness is necessary in order to absorb sound at low frequency [7]. Hence, the limitation of variable absorption is that it is only suitable to be used for sound absorption at low frequency ranges. On the other hand, variable scattering technique, which involves the replacement of flat surfaces with scattered surfaces, has also been tested to improve the sound diffusivity inside a hall. However, the effect on reverberation time is usually not so obvious as compared to the use of variable absorption technique [1]. Variable absorption, variable volume and variable scattering techniques are passive control methods of which the changes of reverberation time are obtained through physical means. Electroacoustic contrariwise is an active control method used to control reverberation by changing the properties of a room electronically using loudspeakers, microphones and recorders in order to produce reverberant halls [8]. However, many musicians are against performing in electronic enhancement environment [1]. This is probably due to the unnatural sound produced. Therefore, this paper proposes another method in controlling reverberation time, which is based on origami concept. This idea is inspired by a parametric survey conducted on the prediction of sound absorption on a periodic rectangular groove structure, where according to the investigation, sound absorption characteristics can be designed by adjusting the parameter of the profile in a suitable way [9]. That means sound absorption properties are dependent on the parameter of the rectangular groove structure, but in that research, the structure is rigid and it would be good if the structure is deformable where the sound absorption properties can be changed according to the deformation of the structure. According to Sabine Equation [10], reverberation time can be varied by changing the volume and the absorption, so to provide variable reverberation time is either by changing the volume of the room or by changing the absorption properties inside the room. 2 Acougamic Acougamic in this paper is the combination of acoustic and origami. Origami originates from Japanese word and it is widely utilized in engineering applications [11]. The concept of origami is by folding a flat sheet to become a three-dimensional structure. In this paper, three types of acougamic are presented as shown in Figure 1 to Figure 3. In order to check the feasibility of the proposed method, the acougamics are tested in the reverberation chamber according to ISO 354. The acougamics are folded using 1200 gsm chipboards and are joined using white glue to form a large specimen. Figure 1: Acougamic 1 Figure 2: Acougamic 2 Figure 3: Acougamic 3 In this paper, the purpose of the investigation is to see whether the acougamics have the potential to provide variable absorptions by changing their geometrical properties. This is because when the acougamics in Figure 1, Figure 2 and Figure 3 are dynamically deformed in the x direction, the geometric properties (value h and x) of the acougamics change as well. 3 Experimental Method In the preliminary study, three acougamics are tested in a reverberation room of Faculty Mechanical Engineering, UTM. The reverberation room has a volume of m 3 and a Schroeder frequency of ISBN:

3 500 Hz. The measurement procedures are according to ISO 354:2003 Measurement of Sound Absorption Coefficient in a Reverberation Room [12]. Measurements are taken in the empty reverberation room, and then again with the presence of the specimens of different configurations as tabulated in Table 1 to Table 3. The excitation inside the reverberation room is carried out using balloon bursts and the impulse responses are recorded by a Solo 01dB Sound Level Meter from two different microphone positions. The microphones are 1.5 m apart and the distance of the specimen to the microphone is 1m. Each measurement is repeated four times and the reverberation times are then averaged for the two microphone positions. The temperature and relative humidity of the measurement were maintained approximately at 26 C and 78% throughout the whole measurement. Table 1: Geometrical Configurations of Acougamic 1 h x z Area (m 2 ) Table 2: Geometrical Configurations of Acougamic 2 h x w z Area (m 2 ) Table 1 until Table 3 represent the geometrical configurations of acougamic 1, acougamic 2 and acougamic 3 respectively. In this study, the surface area of each acougamic is the same, which is m 2. This is because according to Sabine Equation, variation of surface area would affect the absorption inside the room [10], thus in order to investigate the effect of other geometric parameters, the surface area should remain fixed. For each acougamic, the number of geometrical configurations is different due to different structure. Referring to Table 1, when acougamic 1 is dynamically deformed at initial position, which is at configuration 1 to final position at configuration 4, the geometric parameters h and x are varied simultaneously while value z remains the same throughout the process. Table 2 shows that when acougamic 2 is dynamically deformed, the parameter h and x are also varied simultaneously but values w and z remain the same. The same goes for acougamic 3, in Table 3, where the varied geometric parameters are h and x. Table 3: Geometrical Configurations of Acougamic 3 h x y z Area (m 2 ) Results and Analysis The results and analysis of this preliminary investigation are according to 1/3 octave band frequency range, which has been specified in ISO 354. The results presented are in the form of sound absorption coefficient. The main focus is to see whether the acogamics are capable in providing variable sound absorption coefficients when the acougamics are dynamically deformed. Figure 4 to Figure 6 show the results of sound absorption coefficients of several geometrical configurations of acougamic 1, acougamic 2 and acougamic 3. Acougamic 1 has four geometrical configurations followed by acougamic 2, which has ISBN:

4 six geometrical configurations, and lastly acougamic 3 has five geometrical configurations. Figure 4 shows the variation of sound absorption coefficients of acougamic 1 when its geometric properties changes from configuration 1 to configuration 4. At the frequency of 2000 Hz and above, the variations of the results show good trends. Based on the trends, sound absorption coefficients decrease as the acougamic 1 changes its configurations from 1 to 4. The differences on the sound absorption coefficients from one configuration to another seem inconsistent throughout the measurements. Meanwhile, Figure 5 shows the change of sound absorption coefficients from configurations 1 to 6. This clearly indicates that at a frequency of 2000 Hz and above, the sound absorption coefficients decrease as acougamic 2 changes its geometrical configurations, but at configurations 3, 4 and 5, the trends seem unchanged. Similarly, a same trend is also observed at Figure 6 where the sound absorption coefficients slightly decrease as the geometrical configurations of acougamic 3 vary from configurations 1 to 5. Although the results show small differences, the differences between configurations were still noticeable. However, the results of sound absorption coefficients at low and mid frequencies did not show any conclusive trends. This is due to the fact that the wavelengths are very long at low frequencies, thus the thickness of the material should be at least onequarter of a wavelength in order to effectively absorb sound at those frequencies ranges (see Table 4) [13]. Consequently, the results show good trends at high frequencies which are above 2000 Hz, which is probably due to the depth of the acougamic being sufficient to absorb sound at high frequencies. Besides that, the measured sound absorption coefficients are often more inaccurate at low than high frequencies due to modal effects [14]. In addition, since the specimen is quite large, at about m 2, and joined with white glue, there are surface irregularities. This problem then affects the measurement process, especially when taking the value of geometric parameters h, x, w, y and z, as well as maintaining the fixed surface area. The surface irregularity of the acougamic also might contribute to the edge effects which will result in poor prediction of sound absorption coefficients [15]. changes are small. However, this is only a preliminary investigation, and future works in a onefifth scale model of the reverberation chamber will be carried out as to verify the current results. Figure 4: Variation of Sound Absorption Coefficient for Acougamic 1 Figure 5: Variation of Sound Absorption Coefficient for Acougamic 2 5 Conclusion The present preliminary work has shown good trends on the variation of sound absorption for Acougamic 1, Acougamic 2 and Acougamic 3 even though the Figure 6: Variation of Sound Absorption Coeffiient for Acougamic 3 ISBN:

5 Table 4: Wavelengths of the Frequencies in 1/3 Octave Band [13] ¼ of the Frequency Wavelength Frequency Wavelength Acknowledgements The authors would like to thank Universiti Teknologi Malaysia (UTM) and Ministry of Education, Malaysia (MOE) for providing continuous support, especially in term of funding through GUP (Vot:05H30) and FRGS (Vot: 4F524). References: [1] M. Barron, Acoustics for multi-purpose use, Auditorium Acoustics and Architectural Design, Second., London and New York: Spon Press, 2010, p [2] F. A. Everest and K. C. Pohlmann, Adjustable Acoustics, in Master Handbook of Acoustics, 5th ed., McGraw-Hills, 2009, p [3] R. Okvist, A. Ågren, and B. Tunemamlm, Studio Acusticum A Concert Hall with Variable Volume, in Proceedings of the Institute of Acoustics, 2008, vol. 30, pp [4] M. Aretz and R. Orlowski, Sound strength and reverberation time in small concert halls, Appl. Acoust., vol. 70, no. 8, pp , Aug [5] M. A. Poletti, Active Acoustic Systems for the Control of Room Acoustics, Proceedings of the International Symposium on Room Acoustics, ISRA, 2010, no. August, pp [6] W. Chiang, W. Lin, Y. Chen, and H. Hu, Variable Acoustics Design of a Small Proscenium Concert Hall, Asian Archit. Build. Eng., no. May, pp , [7] T. Geometry, Variable Acoustics, Recording Studio Design, Elsevier Ltd, 2012, pp [8] R. W. Schwenke, J. R. Duty, and P. E, NOISE-CON 2010 Electroacoustic Architecture : Is it Green?, Noise-Con 2010, [9] J. Wang, P. Leistner, and X. Li, Prediction of sound absorption of a periodic groove structure with rectangular profile, Appl. Acoust., vol. 73, no. 9, pp , Sep [10] H. Kuttruff, Room Acoustics, Fifth. New York: Spon Press, 2009, p [11] C. Lv, D. Krishnaraju, G. Konjevod, H. Yu, and H. Jiang, Origami based mechanical metamaterials., Sci. Rep., vol. 4, p. 5979, Jan [12] BS EN ISO 354:2003 Measurement of sound absorption in a reverberation room, [13] P. Newell, Sound, Decibels and Hearing, Recording Studio Design, Third Edit., Elsevier Ltd., 2012, pp [14] T. J. Cox and P. D Antonio, Acoustic, Absorbers and Diffusers, Second Edi. Taylor & Francis, 2009, pp [15] J. Kim, J. Lee, Y. Choi, and D. Jeong, The Effect of an Edge on the Measured Scattering Coefficients in a Reverberation Chamber based on ISO , Build. Acoust., vol. 19, no. 1, pp , ISBN: