The sound absorption properties of a double-leaf micro perforated panel (DL-MPP) made from a double-layered polypropylene material.

Micro-perforated panel sound absorbers are recognized as an alternative method for noise control, offering advantages over conventional porous absorbers. However, the sound absorption of a single micro-perforated panel (MPP) is limited in its ability to cover a broad range of frequencies, especially in the low-frequency region. Therefore, the primary focus of this study is to investigate the sound absorption performance of a double-leaf micro-perforated panel (DL-MPP) absorber, which aims to achieve high sound absorption across a wide frequency range. This study examines the impact of varying parameters, including perforation ratio, diameter of perforated holes, and air gap thickness, on the sound absorption characteristics of a polypropylene-based DL-MPP absorber. Polypropylene was used as the sound absorption material for this investigation. The DL-MPP absorber consists of two perforated panels installed parallel to a rigid back wall, with an air gap between them. To assess the sound absorption performance, a prototype of the DL-MPP absorber was fabricated and tested using a two-microphone impedance tube method following ASTM E1050 standard. The results demonstrate that the optimal sound absorption performance was achieved by designing the MPPs with a 1% perforation ratio, 1 mm diameter of perforated holes, and a 1 mm sample thickness.


Introduction
The traditional use of porous materials as acoustic absorbers has been replaced by using microperforated panels (MPPs).The concept of MPPs was first introduced by Maa [1], and it involves a panel surface covered with many sub-millimeter holes.Typically, an MPP is positioned with a rigid back wall behind it, creating an air cavity in between.The MPP sound absorber applies the same principle as a Helmholtz resonator, where the sound absorption mechanism primarily relies on the micro-perforations and the resonance phenomenon [2,3].
According to Henrique et.al, [4] the MPP can be classified as a panel absorber, fabricated from metal or plastic.However, Toa et.al [5] have mentioned that the MPP fabricated by metal is tedious and costly since the fabrication process of perforation holes requires advanced technologies, such as laser, jetting, and etching.On that matter, plastic-based MPP has replaced metal-based MPP due to cost issues [6].
Typically, the MPP has been implemented using a single type, wherein its sound absorption mechanism with a rigid back wall is only primally effective at the middle frequencies, exhibiting resonance peak characteristics [7,8].In order to achieve higher acoustic performance in terms of sound absorption coefficient and absorption frequency range, a double-leaf MPP with a rigid-back wall is developed.This absorption type produces two resonators to obtain a much wider sound absorption frequency range than a single MPP absorber [9,10,11].
Therefore, a study on double-leaf MPP with rigid-backing is undertaken.This study investigates the effects of different parameters, such as perforation ratio, diameter of perforated holes, and air gap thickness, on the sound absorption characteristics of a polypropylene-based double-leaf MPP absorber.In this study, the double-leaf MPP is referred to as DL-MPP brevity.

Theory of DL-MPP
The concept of multiple-leaf MPP absorbers is primarily employed in order to obtain a wider absorption range of frequency.Sakagami et al. [12,13], Bucciarelli et al. [14], and Leqi and Tian [15] proposed that the most common form of multiple-leaf MPP absorbers is the double-leaf micro-perforated panel space absorbers (DL-MPP), which are made up of 2 MPPs arranged in parallel, backed by a rigid wall with air cavity.As mentioned previously, the sound absorption is mainly caused by Helmholtz-type resonance.DL-MPP absorbers will literally obtain two resonance peaks, which merge into a broader peak, thus broadening the absorption frequency range.Figure 1 shows the schematic diagram of DL- MPP with a rigid back wall and air cavities, D in between.

Method 3.1 Preparation of MPP
This study chose polypropylene as the primary material used to manufacture MPP samples.Generally, polypropylene was chosen due to its cost-effectiveness and widespread availability, coupled with its impressive mechanical properties, including a high degree of stiffness and impact resistance.To meet fabrication demands, polypropylene materials needed to be in resin form.The general properties of polypropylene resin applied in this study are tabulated in Table 1.Having acquired the overall properties of polypropylene resins, an accurate calculation is necessary to ascertain the precise mass of polypropylene resin needed to fabricate the MPP samples.The mold volume should be considered for the required polypropylene's total mass (m).Henceforth, the total mass of required polypropylene resins is determined by the equation as follows: Where ρ represents the density of polypropylene resin, and V is the volume of mold.The volume of mold could be obtained by using the following formula.
Where r and h represent the radius and thickness of the mold, respectively.Table 2 demonstrates the total mass of polypropylene resins required for 100 mm and 28 mm diameter samples.Once the total mass of the required polypropylene resin has been accurately determined, the resin undergoes a sequence of steps, including hot pressing and perforation, to fabricate a complete MPP sample.The polypropylene resins are distributed evenly into the mold cavity based on the total mass obtained from the calculation to initiate this process.Subsequently, the hot press machine's temperature is adjusted to match the melting point of polypropylene, which stands at 160 o C. Once it reaches this temperature, the mold is placed into the machine for a pressing duration of 10 minutes.Following the pressing phase, the machine cools to a temperature of 40 o C. Finally, 1 mm thick samples are extracted from the hot press machine, and then the perforation diameter is shaped using a mini electrical hand drill to produce a full-formed MPP sample [16].The process details can be elucidated from the flow chart provided in Figure 2. A comprehensive visual representation of full-formed MPP samples is shown in Figure 3. (

Sound Absorption Measurement
The acoustic performance of polypropylene-based DL-MPP is conducted by measuring sound absorption coefficient by employing two microphones impedance tube method, following the ASTM E1050 standard.Figure 5 illustrates the B&K impedance tube Type AFD 1001 and shows the experimentation setup of both low and high frequency impedance tubes.There are two diameter sizes on impedance tubes, 100 mm and 28 mm, representing frequencies ranging from 350 Hz to 2k Hz and 2k Hz to 6k Hz, respectively.The attachment of DL-MPP to the impedance tube was fit according to the respective size.To make it easy for analysis, this study set the air gap between two perforated panels, and the perforated panel to rigid back wall use the same thickness.A data processing procedure is executed to improve the sound absorption coefficients of the investigating acoustic material of DL-MPP.This involves applying the AFD 1001 AcoustiTube Software to analyze an array of data derived from the measured transfer function.

Effect of Perforation Ratio
The parameters of MPPs used to investigate the effect of the perforation ratio on the sound absorption coefficient are shown in Table 3.Meanwhile, the measured sound absorption coefficient of DL-MPP with different perforation ratios is shown in Figure 6.In Figure 6, the MPP samples were subjected to varying perforation ratios, ranging from 0.5% to 1%.Notably, the alteration in perforation ratio exhibited no discernible impact on the shape of the graph.
The obtained results revealed a consistent increasing pattern.There is a significant gain in sound absorption with an increase of maximum absorption from 70% to 86% in low frequency regions due to the increasing perforation ratio of MPP.Moreover, the escalating perforation ratio gave rise to a higher acoustic resonance frequency at which the peak sound absorption coefficient manifested.Notably, the sound absorption peak underwent a shift from 650 Hz to 850 Hz as the perforation ratio increased.This shift can be attributed to the reduction in the cumulative acoustic mass of the perforations, stemming from the heightened perforation ratio.As a result, an increase in the resonant frequency at which the sound absorption peak occurred was observed.Figure 7 portrays the impact of varying perforated hole diameters on the sound absorption performance.A noticeable deterioration in sound absorption is observed as the perforated hole diameter increases from 1 mm to 2mm.The results indicate a significant drop in sound absorption with a reduction of the maximum absorption coefficient of 0.86 to 0.51 for the first resonance peak.Similarly, the sound absorption at the second resonance peak reduces from 0.69 to 0.36 with high frequency regions.These phenomena were associated with widening the perforated hole diameter and acoustic resistance.The acoustic resistance experiences a more increase when the diameter of the perforated holes is reduced to a smaller size, such as 1mm, which results in a high resistance-to-mass ratio.The motion of fluid in the perforated holes is primarily concerned with the viscous and friction effects.The impinged sound waves had been diminished and absorbed due to the viscous and friction effects when the sound wave propagates through the perforated holes.Thus, the MPP absorbers perform well in sound absorption when the diameter of perforated holes is 1mm.However, the acoustic properties are dominated by the acoustical reactance when the diameter of perforated holes is greater than 1mm, whereby the motion of fluid particles is concerned with the inertial effect.The increase in diameter of perforated holes will result in both low acoustic resistance and acoustic reactance.Hence, little acoustic energy is attenuated due to the low viscous and inertial effects on the fluid motions.8 illustrates the sound absorption of DL-MPP with varying air gap thickness.As the air gap thickness increases, the sound absorption peaks shift to a lower frequency range.Specifically, the first peak of the sound absorption coefficient has transitioned from 830 Hz to a lower frequency of 550 Hz, while the second peak of the sound absorption coefficient moved from 2150 Hz to 1950 Hz.Despite these shifts, the sound absorption peaks for all three variations are considered similar due to the minimal disparities between the resonance peaks.The structure of MPP consists of many sub-millimeter openended holes that function as an acoustic mass.Concurrently, the air particles within the air gap serve as an acoustic spring.This interaction creates rises to a resonance.Consequently, the air gap's stiffness eliminates the hole's acoustic mass.This interplay leads to sound absorption peaks as the frequency of sound waves aligns with the resonance frequency of MPPs.The stiffness of the air gap could be reduced by increasing the air gap thickness, which results in shifting the sound absorption peak towards a lower frequency range.

Conclusions
In conclusion, the study successfully fabricates the polypropylene-based DL-MPP using hot pressing and drilling.The investigation into the sound absorption mechanism of these DL-MPP absorbers revealed significant insight into the impact of acoustic properties.The results demonstrated that the sound absorption coefficient was generally correlated to the perforation ratio, indicating the sound absorption peak had also shifted to a higher frequency region.The sound absorption performance deteriorates as the diameter of perforated holes increases.As the air gap thickness increases, the results show the sound absorption peak has moved towards a lower frequency region with a constant sound absorption peak.The best sound absorption performance of DL-MPP is obtained with parameters of 1mm thickness, 1mm diameter of perforated holes, 1% perforation, and 10mm air gap thickness in between.For improvement, the measurement of sound absorption should be carried out by using one impedance tube only with a wider frequency range.Furthermore, the fabrication of MPPs should be done using additive manufacturing technology to avoid imperfection in sample production.

Acknowledgement
Universiti Tun Hussein Onn Malaysia supported this research through Registrar Office, UTHM.

Figure 1 :
Figure 1: Schematic diagram of DL-MPP (MPP1 and MPP2) with a rigid-back wall and air gap thickness (D1 and D2) in between.

Figure 5 :
Figure 5: (a) and (b) Experimental setup for low frequency impedance tube, (c) and (d) Experimental setup for high frequency impedance tube

Figure 6 :
Figure 6: Sound Absorption Coefficient of DL-MPP with different perforation ratio 4.2 Effect of Holes Diameter The parameters of MPPs used to investigate the effect of hole diameter on the sound absorption coefficient are shown in Table 4.Meanwhile, the measured sound absorption coefficient of DL-MPP with different hole diameters is shown in Figure.7.

Figure 7 :
Figure 7: Sound Absorption Coefficient of DL-MPP with different holes diameter4.3Effect of Air Gap ThicknessIn this section, the air gap between MPP1 to MPP2 and MPP2 to the rigid back wall are set to the same thickness to find the effect of the sound absorption.The several air gap thicknesses are tabulated in Table5.The measured sound absorption coefficient of DL-MPP with different air gap thicknesses is shown in Figure.8.

Table 1 :
General properties of polypropylene resins

Table 2 :
Total mass required based on different sizes of samples.

Table 3 :
Parameters of MPP

Table 4 :
Parameters of MPP

Table 5 :
Parameters of MPP Figure.8: Sound absorption coefficient of DL-MPP with different air gap thickness.8Figure