Sound Absorption Properties of Waste Pomelo Peel

Abstract

To solve the issue of environmental noise pollution and promote the resource recycling of waste pomelo peel, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) are used to systematically characterize the microstructure and chemical composition of waste pomelo peel. It was found that waste pomelo peel has a porous network structure, which is conducive to the improvement of sound absorption performance. Waste pomelo peel/polycaprolactone (PCL) sound-absorbing composites are prepared by the hot-pressing molding process, and the single-factor analysis method is adopted to explore the effects of seven factors (waste pomelo peel mass fraction, composite density, composite thickness, hot-pressing time, hot-pressing pressure, hot-pressing temperature, and thickness of rear air layer) on the sound absorption performance. Through process optimization, under the optimal conditions, the average sound absorption coefficient (SAC) of the composites reaches 0.54, the noise reduction coefficient (NRC) reaches 0.57, and the maximum SAC reaches 0.99, with the sound absorption performance reaching Grade III. This study not only provides a new idea for the preparation of porous sound-absorbing composites but also opens a new path for the high-value utilization of waste pomelo peel resources.

1. Introduction

Noise is an irregular vibration emitted by a sound source, which interferes with people’s work, life, and rest [1]. As technology evolves rapidly, noise pollution has become increasingly severe, making it an urgent matter to reduce the harm it causes. Consequently, sound-absorbing composites have emerged. Sound-absorbing composites are mainly divided into two categories: porous sound-absorbing composites and resonant sound-absorbing structures. Sound absorption materials are generally divided into two kinds: porous sound absorption materials and sound absorption resonators [2]. Specifically, resonators performed well in absorbing low-frequency sounds but were less effective in the mid-high frequency range. In contrast, porous sound-absorbing composites demonstrated outstanding sound absorption performance in the mid-high frequency range. Since the hot-pressing method to prepare porous sound-absorbing composites offered advantages such as a short process flow and simplicity in technology [3]. As a result, the preparation of porous sound-absorbing composites by hot-pressing had become the focus of the research on sound-absorbing composites.

Pomelo, with its unique commercial value, was widely cultivated. However, as pomelo was primarily consumed fresh, a significant amount of waste pomelo peel was discarded. Most of the waste pomelo peel was either incinerated or directly disposed of in landfills, and it caused environmental pollution. Only a small portion of the waste pomelo peel was reprocessed to create new products. Gao et al. [4] synthesized two kinds of nanocellulose from waste pomelo peel for Pickering stabilizer. Wan et al. [5] used waste pomelo peels to make cellulose aerogel and further carbonize it to stop bleeding quickly. Chen et al. [6] used dietary fiber from waste pomelo peels as the base composite and prepared edible packaging films. Qi et al. [7] prepared a new type of composite for efficient photocatalysis, and they used N-doped carbon quantum dots derived from waste pomelo peel and BiOBr. These studies had a certain impact on the recycling of waste pomelo peel. However, the preparation process was complex and not conducive to the treatment of a large amount of waste pomelo peel, and there were certain requirements for the freshness of the waste pomelo peel. Argun et al. [8] found that waste pomelo peel had a dense cellular porous structure and was rich in cellulose, hemicellulose, and lignin, which could be used as a filler for preparing three-dimensional composites. Liu et al. [9] extracted oil adsorbent from waste pomelo peel and modified it into carbon aerogel, which not only increased the adsorption performance but also preserved the pore structure of waste pomelo peel to the greatest extent. Imran et al. [10] used waste pomelo peel as a precursor to prepare a three-dimensional porous lignocellulose carbon aerogel (C-GPA) with high electrical conductivity. This demonstrated that waste pomelo peel could be processed into a three-dimensional porous structure. Kolya et al. [11,12] chemically treated coconut shells and sugar maple leaves to retain as much cellulose as possible and removed lignin and found that a high cellulose content and the porous structure could enhance the sound absorption performance of composites. Liu et al. [13] used Juncus effusus with a porous network structure as raw material, prepared it into a micro-perforated panel, and adopted a sandwich structure to achieve broadband absorption. Alagarsamy et al. [14] prepared coir fiber non-woven felts from waste coir fibers containing cellulose and then formed sound-absorbing composites with perforated wooden boards through compression molding. The maximum sound absorption coefficient (SAC) reached 0.9, and the noise reduction coefficient (NRC) reached 0.762. This indicated that the use of waste pomelo peel with a cellulose-containing porous structure to prepare sound-absorbing composites was beneficial for the improvement of sound absorption performance. Polycaprolactone (PCL) powder exhibited properties such as thermoplasticity, energy efficiency, biodegradability, and excellent biocompatibility [15]. PCL has a melting point of 60 °C and a glass transition temperature of −60 °C. Using it as the matrix, the waste pomelo peel can be mixed evenly [16,17]. This demonstrated that the hot-pressing method could be employed to combine waste pomelo peel with PCL to fabricate sound-absorbing composites. This approach not only addressed the issue of noise pollution but also reduced the waste pomelo peel. Additionally, the process was characterized by its simplicity and short production cycle.

2. Experiment

2.1. Experimental Raw Composites and Equipment

Waste pomelo peel (from orchards in Pinghe County, Zhangzhou City, Fujian Province, China) and PCL (Perstorp 6500, Skåne County, Sweden) were used as raw composites. The following instruments and equipment were employed for characterization and processing: SpectrumOne-B Fourier Transform Infrared Spectrometer (FT-IR, PerkinElmer, Waltham, MA, USA), D/max-3B X-ray Diffractometer (XRD, Rigaku, Osaka, Shimadzu, Kyoto, Japan), JC-FW400A Small Universal Crusher (Qingdao Juchuang Jiaheng Co., Ltd., Qingdao, China), and JSM-6460 LV Scanning Electron Microscope (SEM, JEOL, Tokyo, Japan). MP2000D Precision Analytical Balance (Changzhou First Textile Equipment Co., Ltd., Changzhou, China), QLB-50D/Q Flat Vulcanizing Press (Wuxi Zhong kai Rubber & Plastic Machinery Co., Ltd., Wuxi, China), φ100 mm Circular Steel Mold (Dalian Nasta Mold Co., Ltd., Dalian, China), 200 × 200 mm Steel Backing Plate (Dalian Nasta Mold Co., Ltd., Dalian, China), JC-FW400A Small Universal Crusher (Qingdao Juchuang Jiaheng Co., Ltd., Qingdao, China).

2.2. Experimental Methods

In this study, waste pomelo peel/PCL sound absorption composites were synthesized via hot-pressing molding technology. The waste pomelo peel was crushed and sieved to obtain a raw composite of 1–2 mm, which was evenly mixed with PCL. The mixture was poured into the mold, and the whole was placed in a plate vulcanization pressure-forming machine that had been preheated to a specific temperature. After the hot-pressing ended, the preform was taken out, solidified at room temperature upon cooling, and waste pomelo peel/PCL sound absorption composites were obtained. The flowchart for the preparation of sound absorption composites is presented in Figure 1.

2.3. Test Indicators

FT-IR spectroscopy was employed to acquire the spectrum of waste pomelo peel, which enabled the exploration of its composition and content, as well as the analysis of its chemical structure and sound absorption mechanism. XRD technology was utilized to obtain the diffraction pattern data of waste pomelo peel, and this investigation explored the correlation between its sound absorption performance and aggregation structure. The microstructure and surface morphology of waste pomelo peel were observed via scanning electron microscope to explore its morphological structure [18].

The SAC was used as the evaluation index of sound absorption performance, and the SAC of the composites is measured by the transfer function method. The test instrument was the SW422/SW477 impedance tube produced by Beijing Prestige Co., Ltd. (Beijing, China), and the test standard was the GB/T 18696.2-2002 Measurement of Sound Absorption Coefficient and Acoustic Impedance in Acoustic Impedance Tubes—Part 2: Transfer Function Method. This method only simulated normal incidence of sound waves. The SAC test was carried out at 80–6300 Hz frequency to obtain the sound absorption curve [19].

An important index for evaluating sound absorption performance is SAC (α), and the calculation formula is shown as Equation (1).

$$\alpha ={\displaystyle \frac{{\alpha }_{125}+{\alpha }_{250}+{\alpha }_{500}+{\alpha }_{1000}+{\alpha }_{2000}+{\alpha }_{4000}}{6}}$$

(1)

Another measure of sound absorption performance was the NRC calculation formula as shown in Equation (2).

$$NRC={\displaystyle \frac{{\alpha }_{250}+{\alpha }_{500}+{\alpha }_{1000}+{\alpha }_{2000}}{4}}$$

(2)

According to GB/T 18696.2-2002, the sound absorption performance grade was according to the NRC as the index, and the sound absorption grade of the waste pomelo peel/PCL sound absorption composites was divided according to the value of the NRC. The division standard was shown in

Table 1. Classification table of sound-absorbing performance.

NRC	>0.8	0.8 > NRC ≥ 0.6	0.6 > NRC ≥ 0.4	0.4 > NRC ≥ 0.2
Level	I	II	III	IV

.

3. Results and Discussion

3.1. Structural Characteristics of Pomelo Peel

3.1.1. Chemical Structure

Figure 2 was the infrared spectrum of waste pomelo peel. It could be seen from the figure that there were a variety of characteristic peaks in the waste pomelo peel. These peaks reflected the unique vibration frequency of various chemical functional groups in the waste pomelo peel. Through the analysis of the characteristic peaks in the figure, it could be seen that the strong characteristic peak generated by the infrared spectrum of the waste pomelo peel at 3224 cm⁻¹ was the O–H stretching vibration characteristic peak, which was the characteristic band of all cellulose [20].

This was a sugar characteristic peak that represented the stretching vibration of C-H at 2923 cm⁻¹, and this peak indicated that it conformed to the general structural characteristics of sugars. The characteristic peak at 848 cm⁻¹ indicated that the polysaccharide contained an α-glycosidic bond pyranose. The characteristic peak at 670 cm⁻¹ represented the O–H out-of-plane bending vibration, and this representation indicated that the waste pomelo peel had the basic characteristics of cellulose [21]. The results indicated that waste pomelo peel was a natural polymeric cellulose compound that consisted of numerous polysaccharides.

Analysis showed that waste pomelo peel was rich in cellulose, whose macromolecular structure was a straight-chain polymer without branches. The basic unit of cellulose was the oxygen six-membered ring (also known as the glucose ring). As shown in Figure 3, which presented the chair conformation of the oxygen six-membered ring structure of β-D-glucose [22], these basic units were interconnected through 180° rotation and by glycosidic bonds. When the acoustic energy acted on the fiber molecular chain, the internal vibration of the macromolecular chain segment caused the oxygen six-membered ring single bond to rotate internally, to continuously overcome the external resistance to do work, and to dissipate the acoustic energy.

The content of cellulose in waste pomelo peel was high, and its macromolecular chains maintained a stable structure through hydrogen bonds and van der Waals forces and contained pores. When the sound wave entered, it dissipated due to the pores, and the hydrogen bond restricted the movement of the chain segment. Moreover, under the action of the sound wave, the vibration of the main chain drove the movement of the hydroxyl group so that the hydrogen bond underwent reciprocating changes to produce more internal friction and the sound energy was converted into heat energy or other forms of energy consumption and finally played a sound-absorption role. This process is similar to the dissipation of sound waves through resistance and viscous forces in pores. The sound energy dissipation mechanisms of porous composites mainly include viscous-thermal dissipation and pressure diffusion dissipation [23].

3.1.2. Aggregate State Structure

Figure 4 showed the XRD pattern of waste pomelo peel, which could characterize the relationship between the sound absorption performance and the aggregation structure of waste pomelo peel. The waste pomelo peel had three diffraction peaks at the diffraction angles of 2θ of 15.7°, 21.2°, and 34.4°. Among them, the weak peak at 2θ = 15.7° represented the amorphous region of cellulose, 21.6° represented the 002-plane diffraction peak of the intensity of the crystalline region, and the diffraction peak at 34.4° corresponded to the 040-crystal plane that belonged to the cellulose I type [24] with a crystallinity of 30%. The propagation of sound waves in the molecular structure of the fiber was completed by the axial direction of the molecular main chain, the atomic vibration on the molecular chain, and the deformation of the bond. When the crystallinity of the fiber was low, the arrangement between the macromolecular chains was relatively loose, the porosity between the molecular chains was high, and the distance between the molecules was large [25]. Therefore, the interaction between the molecules was weak, and the molecular chain was easier to move. This resulted in more friction and collision during the propagation of sound waves and thus achieved better sound absorption.

3.1.3. Morphological Structure

To explore the relationship between the sound absorption performance and the morphological structure of the waste pomelo peel, the SEM was used to obtain the SEM image of the morphological structure of the waste pomelo peel, as shown in Figure 5.

It could be seen from Figure 5 that the waste pomelo peel had an obvious pore-like network structure. The pores were closely arranged, their size was large, and some pores were connected to form channels. Thus, the connectivity of the pores was improved, and this improvement resulted in an interconnected porous structure [26]. Sound waves entered the interior of the composites. Multiple refractions and reflections of the sound waves occurred. Heat conversions were continuously carried out. Thus, the sound absorption performance was achieved. The sponge tissue of the waste pomelo peel was shrunk and twisted because of drying and dehydration. The pore structure and fibrous tissue were partly damaged. As a result, the pores became uneven with various shapes and sizes. The layered structure and porous structure of the waste pomelo peel were well preserved as a whole. The layered structure and porous structure of the waste pomelo peel conformed to the principle of porous sound absorption.

3.2. Sound Absorption of Waste Pomelo Peel/PCL Sound Absorption Composites

3.2.1. Effect of Waste Pomelo Peel Mass Fraction on Sound Absorption

Under the conditions of composite density of 0.44 g/cm³, composite thickness of 1.0 cm, hot-pressing time of 20 min, hot-pressing pressure of 10 MPa, and hot-pressing temperature of 120 °C, the effect of the mass fraction of waste pomelo peel on the sound absorption performance of the composites was investigated. After the test, the SAC curves of different factors were shown in Figure 6. The average SAC and NRC of the composites with different factors were calculated and are shown in

Table 2. Average SAC and NRC of the composites with different factors.

	Waste Pomelo Peel Mass Fraction (%)				Composite Density (g/cm3)				Composite Thickness (cm)				Hot-Pressing Time (min)
	40	50	60	70	0.29	0.34	0.39	0.44	1.0	1.5	2.0	2.5	15	20	25	30
αmax	0.73	0.75	0.82	0.70	0.87	0.87	0.85	0.84	0.87	0.92	0.82	0.74	0.90	0.92	0.95	0.90
αavg	0.28	0.31	0.32	0.42	0.35	0.37	0.34	0.32	0.37	0.41	0.41	0.38	0.40	0.41	0.44	0.39
NRC	0.21	0.25	0.29	0.40	0.27	0.31	0.30	0.29	0.31	0.38	0.49	0.49	0.38	0.38	0.49	0.49

.

It could be seen from Figure 6 that the sound absorption performance of the composites increased with the increase in the waste pomelo peel mass fraction in a certain range. In the low-frequency region, the waste pomelo peel mass fraction had no significant effect on the SAC, and the sound absorption performance was poor compared with the mid-high frequency. It could be seen from Table 2 that the sound absorption performance was the best when the waste pomelo peel mass fraction was 60%. Because the waste pomelo peel had a porous structure. When the content of waste pomelo peel was small, the number of pores decreased. Due to the decrease in waste pomelo peel content, the content of PCL increased, the PCL inside the composites polymerized, and the connectivity of the pores became worse. The number of acoustic wave friction in the pore was reduced, the probability of acoustic wave reflection and refraction was reduced, the propagation path was shortened, the loss was reduced, and the sound absorption performance was reduced. However, with the increase in the waste pomelo peel mass fraction, the content of PCL decreased. The structure of waste pomelo peel experienced excessive extrusion and deformation. The porous structure was damaged. The quantity of effective pores diminished. Consequently, the sound absorption performance deteriorated.

3.2.2. Effect of Composite Density on Sound Absorption

The effect of composite density on sound absorption performance was investigated under the conditions of waste pomelo peel mass fraction of 60%, composite thickness of 1.0 cm, hot-pressing time of 20 min, hot-pressing pressure of 10 MPa, and hot-pressing temperature of 120 °C.

It could be seen from Table 2 that the sound absorption performance of the sound-absorbing composites with a composite density of 0.34 g/cm³ was more excellent. When combined with Table 2 and Figure 6, it could be observed that as the composite density increased, the low-frequency SAC increased more significantly. This was because with the increase in material density, the contact area and bonding tightness of waste pomelo peel fibers during the hot-pressing process were improved, and this improvement resulted in a denser internal structure and reduced porosity of the sound-absorbing composites. Due to the weak penetration ability of low-mid frequency sound waves, the sound waves could not pass through the compact internal material, so the sound absorption performance decreased. However, the reduction in porosity also led to fewer reflections and refractions inside the composites. When high-frequency sound waves passed through, their frictional loss decreased, and this decrease resulted in reduced acoustic energy consumption and decreased sound absorption performance. With the increase in density, the content of waste pomelo peel and PCL per unit volume increased, and this increase led to a tighter combination between waste pomelo peel and PCL. Therefore, the sound absorption performance of the waste pomelo peel/PCL sound absorption composites with a density of 0.34 g/cm³ showed a downward trend.

3.2.3. Effect of Composite Thickness on Sound Absorption

The effects of composite thickness on sound-absorption properties were investigated under the conditions of mass fraction of waste pomelo peel 60%, composite density 0.34 g/cm³, hot-pressing time 20 min, hot-pressing pressure 10 MPa, and hot-pressing temperature 120 °C.

It could be seen from Figure 6 and Table 2 that the sound absorption performance of the sound absorption composites with a thickness of 2 cm was the best. When the thickness of the composites was small, the airflow resistance was large and the penetration was small. As a result, the sound absorption performance was poor. However, as the thickness increased, the airflow resistance decreased. The resistance of air through the unit composite thickness decreased. The air penetration was large. Thus, the sound absorption performance was improved. At the same time, the increase in the propagation distance of the sound wave in the sound absorption composites led to an increase in friction loss. And the sound absorption performance was also improved [27]. As the thickness increased, the pore channel became longer. However, the excessive thickness of the composites led to the closure of the pore channels inside the composites. The reduction in the number of pores affected its sound absorption effect.

3.2.4. Effect of Hot-Pressing Time on Sound Absorption

The effects of hot-pressing time on the sound absorption performance of the composites were investigated under the conditions of a waste pomelo peel mass fraction of 60%, composite density of 0.34 g/cm³, composite thickness of 1.5 cm, hot-pressing process parameters of hot-pressing pressure of 10 MPa, and hot-pressing temperature of 120 °C.

From Table 2, it could be seen that the sound absorption performance increased with an increase in time in a certain hot-pressing time, and the SAC values reached the maximum when the hot-pressing time was 25 min. From Figure 6, it could be seen that the hot-pressing time had a great influence on the high-frequency sound absorption performance. Because in the appropriate hot-pressing time range, PCL melted and was uniformly dispersed between the interior of the sound-absorbing composites and the waste pomelo peel, forming a good adhesion effect that helped to stabilize the internal structure of the composites. If the heating time was too short, PCL might not be fully melted and uniformly dispersed, which led to aggregation, which led to a decrease in porosity and poor sound absorption performance. If the heating time was too long, there was no new adhesion between PCL and waste pomelo peel, which damaged the porous structure of waste pomelo peel and reduced the sound absorption performance of sound-absorbing composites.

3.2.5. Effect of Hot-Pressing Pressure on Sound Absorption

Under the conditions of waste pomelo peel mass fraction 60%, composite density 0.34 g/cm³, composite thickness 1.5 cm, hot-pressing time 25 min, and hot-pressing temperature 120 °C, the effect of hot-pressing strength on the sound absorption performance of waste pomelo peel/PCL sound absorption composites was investigated. The test results showed the average SAC and NRC of the composites with different factors in

Table 3. Average SAC and NRC of the composites with different factors.

	Hot-Pressing Pressure (MPa)				Hot-Pressing Temperature (°C)				Thickness of Rear Air Layer (cm)
	8	10	12	14	100	110	120	130	0	1	2	3
αmax	0.85	0.95	0.95	0.84	0.82	0.87	0.95	0.92	0.95	0.97	0.99	0.99
αavg	0.39	0.44	0.41	0.40	0.40	0.39	0.44	0.40	0.44	0.48	0.49	0.54
NRC	0.36	0.49	0.38	0.39	0.40	0.38	0.49	0.44	0.49	0.50	0.50	0.57

.

It could be seen from Table 3 and Figure 6 that the sound absorption performance was better at 10 MPa. Within a specific range of hot-pressing pressure, the sound absorption performance improved with an increase in pressure, particularly for medium-to-high frequencies. This was because higher pressure promoted the flow of PCL and its coating on waste pomelo peel, thereby influencing the pore quantity and size of the composites. When the pressure was too low, insufficient penetration and flow of PCL between waste pomelo peel particles resulted in excessively large pores and high acoustic transmittance, and this result deteriorated sound absorption. Conversely, excessive pressure damaged the porous structure of waste pomelo peel. This damage reduced pore connectivity and the number of effective pores due to enhanced PCL penetration, and this reduction consequently decreased the composite’s sound absorption performance [28].

3.2.6. Effect of Hot-Pressing Temperature on Sound Absorption

Under the conditions where the mass fraction of waste pomelo peel was 60%, the composite density was 0.34 g/cm³, the composite thickness was 1.5 cm, the hot-pressing time was 25 min, and the hot-pressing pressure was 10 MPa, the effects of hot-pressing temperature on the sound absorption properties of waste pomelo peel/PCL sound-absorbing composites were investigated.

As indicated in Table 3, the sound-absorbing composites achieved the highest SAC and optimal sound absorption performance at a hot-pressing temperature of 120 °C. It could be seen from Figure 6 that the sound absorption performance of the composites increased with the increase in temperature in a certain temperature range. Because the melting point of PCL was low and its melting temperature was 59–64 °C. When the hot-pressing temperature was 100 °C, PCL could not fully flow because of the low temperature. As a result, the uniformity of the composites decreased. Also, the waste pomelo peel could not be completely wrapped, and thus aggregation occurred. When the temperature was low, PCL could not fully flow, and this led to the aggregation of PCL. When the temperature rose to 120 °C, the hot-pressing temperature was more suitable. PCL was fully melted and had good fluidity. It was mixed more evenly with waste pomelo peel. Then, more stable pore structures were formed. These stable pore structures increased the loss of sound waves and improved the sound absorption performance [28]. Moreover, high temperature destroyed the porous structure of waste pomelo peel. Consequently, the internal pores of the composites were reduced and unevenly distributed, and the sound wave transmission was reduced. The internal structure of waste pomelo peel resulted in a reduction in its sound absorption performance.

3.2.7. Effect of the Thickness of Rear Air Layer on Sound Absorption

Under the conditions of waste pomelo peel mass fraction 60%, composite density 0.34 g/cm³, composite thickness 1.5 cm, hot-pressing time 20 min, hot-pressing pressure 10 MPa, and hot-pressing temperature 120 °C, the influence of the thickness of the rear air layer on the sound absorption performance of waste pomelo peel/PCL sound-absorbing composites was investigated. SAC curves of different rear air layers were shown in Figure 7.

It could be seen from Table 3 that the maximum SAC reached 0.99 when the rear air layer was 2.0 cm and 3.0 cm, but the average SAC and NRC were higher when the rear air layer was 3.0 cm. Figure 7a demonstrated that the adjusted thickness of the rear air layer significantly influenced the sound-absorbing composites in low frequencies, while it minimally affected high-frequency absorption. This phenomenon occurred because the rear air layer cavity and the internal composites cavity formed a resonant absorption structure [29], which enhanced low-mid frequency performance. Essentially, an increase in the thickness of the rear air layer extended the composite’s effective thickness and promoted the gradual dissipation of sound wave energy through the vibration and friction of air molecules during the penetration of low-frequency waves. As the frequency decreased, the wavelength increased. The increase in the thickness of the composites promoted the coefficient of the composites for low-mid frequencies. Therefore, the sound absorption performance of the composites for low-mid frequencies was improved.

3.2.8. Optimal Process of Waste Pomelo Peel/PCL Sound-Absorbing Composites

After optimization experiments, the following were determined as the optimal process conditions. The measured density was 0.34 g/cm³, with the hot-pressing time set at 25 min, the hot-pressing pressure at 10 MPa, the hot-pressing temperature at 120 °C, and the thickness of the rear air layer at 3.0 cm. Under these optimal technological parameters, the average SAC reached 0.54, the NRC was 0.57, and the maximum SAC value achieved 0.99. The tabular data classified the composites as a Grade III sound absorption performance material, and the SAC curve under the optimal conditions was illustrated in Figure 7. The sound-absorbing composites exhibited sound absorption performance within the frequency range of 500 Hz to 6300 Hz. Figure 7c presents a comparison of sound absorption performance between waste pomelo peel sound-absorbing composites and several typical biomass sound-absorbing composites. Considering the three indicators of the material thickness, NRC, and α_max, the composites prepared in this study exhibit particularly excellent sound absorption performance, with the NRC reaching 0.57 and the α_max reaching 0.99, and the material thickness being the smallest compared with other materials [19,30,31,32].

4. Conclusions

In this study, the waste pomelo peel was used as the reinforcing phase, and PCL was used as the matrix. The sound-absorbing composites were successfully prepared by the hot-pressing method, and the optimum process conditions were determined: material density 0.34 g/cm³, hot pressing time 25 min, pressure 10 MPa, temperature 120 °C, and rear air layer thickness 3.0 cm. Under these conditions, the SAC of the material was 0.54, the NRC was 0.57, and the maximum sound absorption coefficient was 0.99, which belonged to the class III sound absorption material and had good sound absorption performance. Due to its lightweight and environmentally friendly characteristics, the composite material had potential application value in scenarios that required lightweight and environmental protection, such as building interior walls, ceiling sound-absorbing layers, and office, home environment, and vehicle interiors.