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SUMMARY
This study confirms the best specification of hole length, hole width first, hole distance of geometry micro-perforated panel(GMP), and discusses the application of geometry micro-perforated panel in the ceiling type of continuous, discontinuous and acoustic baffle with different factors, such as the form of air layer, air-back cavity style, the layer number of air layer, the arrangement form, and the height. CNS-9056 Acoustics-Measurement of Sound Absorption Coefficients in a Reverberation Room is referred and the methodology of measurement in this study.
The results show the geometry micro-perforated panel(GMP) performs the best in hole length of 0.4mm, hole width of 0.04mm and hole distance of 3mm. The NRC and αw are both 0.75 in air-back cavity of 30 cm. The highest peak value of sound absorption coefficient is 0.97 with 200Hz of low frequency.
The geometry micro perforated panel(GMP) under the air-back cavity performs best with the single layer of geometry micro-perforated panel(GMP) of 30cm. However, the single layer of geometry micro-perforated panel(GMP) is suggested putting in the indoor ceiling without air-back cavity.
Keywords: geometry micro perforated panels, continuous type, discontinuous type, Sound absorber panels; sound absorption coefficient
INTRODUCTION
The micro perforated panel was developed from the research of general theory and design of micro perforated panel absorbers by Maa, Dah-You in 1996. It generates resonance of sound-absorbing by the structure of stiletto. In practice, it does not need to be added the materials with multiple holes, and is suitable to be using in places with high temperature and humidity because of water-repellent and high temperature resistant characters, such as glass wool, rock wool, etc. (Jiang, Zhon-Gjie, 2007)
In the research of geometry micro perforated panel which was published in 2007 by Jiang, Zhon-Gjie. In the process of manufacturing, he punched holes instead of abandoning the vertical piercing way, observing the shape of holes from the side of dissecting and discovered these holes were like geometric shapes which were different from general micro perforated panels with straight shape. Hence, it was named as geometric micro-perforated panel (GMP).
Through experimental result, he found that the performance of sound absorption was significantly impacted by changes of piercing rate. Therefore, the piercing rate can be controlled by the hole length, the hole width, and the hole distance indirectly. (Jiang, Zhon-Gjie, 2007).
The changes of piercing rate has significant impact to elevate the sound absorption performance of GMP and makes bandwidth of sound absorption wide by comparing with straight micro perforated panels. The overall performance of sound absorption is elevated apparently.
When adding geometry GMP to general indoor ceiling, GMPs need close air layers in order to elevate the performance of sound absorption, the ceiling are covered with micro-perforated panels with continuous type. Nevertheless, if the type of air layer is changed from close to open, the sound absorption performance of the GMP may be changed.
Materials and Methods
This study discusses three types of ceilings which are flat roof (60cm×60cm×2.5cm with basic unit and single layer GMP), discontinuous (60cm×60cm×5cm with basic unit and double layer GMP) and baffle (180cm×20cm×5cm with basic unit and double layer GMP) by using hole length of 0.4 mm, hole width of 0.04mm, and hole distance of 3mm as a standard specification.
CNS-9056 acoustics measurement of sound absorption rate in the reverberant chamber is referred to and stereo laboratory of the department of Architecture from National Cheng Kung University is the testing place with floor area of 32.8 m2, volume of 171.6 m³, the testing capacity is 10.8 m2.
Results and Discussion
1.Comparison of structure of GMP:
Through the comparison of the experimental results, two sound absorption performance of GMP the same which are 0.3-2P and 0.4-3P respectively. P 0.4-3P is chosen to be the standardized specification in the future due to consideration of standardized specification and the yield rate of producing.
2.Continuous type ceiling:
A.Thickness of back air layer:
The experimental result shows that the bandwidth of sound absorption of 0.4-3p GMP increases and the trend of sound absorption moves from high frequency to low frequency when the thickness of back air layer increases.
B.Type of back air layer:
The experimental results show that the situation is obvious between low frequency of 100 Hz and high frequency of 630 Hz. The biggest value of difference is over 0.3. There is no significant difference above high frequency of 1000 Hz.
C.Comparison of the numbers of layers:
When the structure of layer transforms single into double, and the coefficient of sound absorption maintains over 0.8 with the low frequency between 200 Hz and 2000 Hz, those make up the part of declining with high frequency of 1000 Hz of the single layer. The sound absorption performance apparently elevates only when the geometry micro-perforated panel is added to aluminum board, the high frequency is from 200Hz to 2000Hz, and the low frequency is 80 Hz.
3.Discontinuous type ceiling:
A.Content of the structure:
Double side of GMP is better than single side’s. There is no difference under the low frequency of 200 Hz and significant difference over the low frequency of 200 Hz. The biggest difference of sound absorption coefficient is over 0.25.
B.Comparison of Height change:
GMP of double side is taking to do change of height. With the change trend of height and the move of low frequency, there is apparently significant difference in the mid-low frequency with the height of 5cm. However, there is no big difference in the high frequency. Moreover, the sound absorption performance is the best with the height of 40 cm.
C. Form of arrangement:
When the numbers of GMPs is arranged to be unmovable, the result shows there is no significant difference with overall sound absorption performance. The sound absorption performance of arrangement of dot form is different only in mid frequency of 630 Hz and the biggest difference is over 0,05.
4.Sound absorber panels:
A.Sound-absorber the structure:
Double - sided GMP on each frequency was significantly superior to the one-sided GMP, the most significant differences at high frequencies, sound absorption coefficient difference of 0.3 or more.
B.reflecting surface between the distance:
GMP done using double-sided reflective surface 0cm snapping back wall and back wall from the highly reflective surface 20cm, found snapping back wall there is a significant difference when 0cm in the low to intermediate frequency 630Hz 200Hz reflective surface, the difference between the highest absorbing coefficient of 0.3 or more.
C.Spacing between the sound-absorber panels:
Discussion on distance should be controlled when the same amount, when the distance between the absorbing body and absorbing body in 30cm, at intermediate frequency 315Hz to 5000Hz with the distance of 10cm, the most significant difference, the maximum difference of 0.1 or more.
Conclusion
When decorating ceiling, there is no limit to add GMP. The best performance of GMP is continuous single layer. Performances or material cost are all the most effective. However, if there is limitation with other demands of models or ceiling decoration and no complete air back layers, the GMP of double layer is the best to be the unit of decorating, and then we can do different decoration type of ceiling according to different demand.
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