1. Introduction
Sustainability has become a recent issue in the global era. The building industry should integrate the efforts to sustain the environment among the challenges on climate change, ozone depletion and CO
2 emission. One issue on the building material industry sector is of how to produce recycled materials [
1,
2]. Renewable Energy, material re-use and recycled material are the efforts being made in green building movements, meanwhile the use of recycled material should consider both economic and environmental aspects. Furthermore, the Green building movements provide considerable points for buildings that use recycled material. Some types of building materials that usually use recycled materials including concrete, wood, ceramics, and wall panels while the waste material that has often been used comes from natural resources such as wood waste, forests, and marine resources.
One of the abundant sea resources is seashells served for culinary business [
3,
4]. As those studies conducted by Setyowati et al. stated, that each culinary business using seashells in its menu could produce as much as thirty kilograms seashells as waste of the business process per day, so there are actually tons of shell waste produced by these culinary businesses [
3,
4]. Even more the seashells can be taken freely from coastal areas when there is a tide. The shells observed in this research are from three types of species namely
Anadara granosa Linn (blood clams),
Perna viridis Linn (green mussels), and
Placuna placenta Linn (oyster) [
3,
4]. In the previous research, it was reported that a polymeric composite having a thickness of 30 mm made of
Placuna placenta Linn shell had a very good sound absorber performance with a peak at 0.47 on the medium frequency of 2.25 kHz. The polymeric composite from this species has a better ability to absorb sound in a wider frequency range than the other species [
3]. Furthermore, the performance of transmission loss composite materials from this kind of shell ranges between 53 and 58 dB. The polymeric composite materials made of shells are good for both interior and exterior building materials [
3].
The use of shell materials as diffusers and absorbers is possible, but they need treatment and improvement through additional resonators, coated fibrous panels, back cavities and any treatments such as those by Lee and Chen, Bravo et al. and Takahashi et al. in their studies [
5,
6,
7]. Since it has the function as the protector of the mollusk the seashell is naturally tough. It containing cementeous compounds, minerals, is naturally hard, and has a good mechanical strength that is required for shelter purposes. The mechanical and chemical properties have been studied in various methods for many different purposes, including development of low strength materials, concrete, composites while also been used as biocomposite for dental applications. Research was conducted by Fombuena et al. and Odusanya et al., who studied seashell filler reinforced with unsaturated polyester (at 30 wt% and 10 wt% respectively) related to the flexural and hardness properties, but it did not consider the acoustical properties [
8,
9]. The results showed that the flexural strength, as well as the hardness and impact properties of the SFRP with 10 wt% seashell flour reinforcement, were largely improved. Research on seashell waste as a material was widely observed by Fombuena et al., Teixeira et al., and Li et al. [
9,
10,
11], but most of the studies were not for an acoustical material.
As reported in the literature, seashells and mortar based composites are mostly hard and dense so are acoustically reflective. Further improvement is needed to get a higher performance especially for absorbing sound wave energy. This research discussed acoustical behavior improvements of seashell filler reinforced polyester which is a continuation and refinement of previous research on seashell concrete [
3]. An experimental study referring to the ASTM E-1050 standard method was conducted on a 30 mm thickness polymeric composite by Setyowati et al. It found that composite containing
Placuna placenta Linn powder as its raw material had a sound absorption coefficient of 0.47 for the frequency range above 2.25 kHz. This figure indicated a low performance with regard to the ability of controlling the high frequency noise. Continuing on, the applications of seashell waste in buildings were discussed in studies by Setyowati, et al. [
12,
13,
14]. Further testing by using four microphone impedance tube techniques showed that the same sample had a sound transmission loss ranging from 53 dB to 58 dB with a spand frequency up to 6.3 kHz. Contrary to its sound absorption performance, it indicated the high ability on reducing energy of transmitted sound waves propagated through the material.
Unlike the topic of conventional porous and soft absorber materials, this research highlights non-porous material made of seashell waste composed of unsaturated polyester with treatment to improve the acoustical behavior associated with surface morphology, which has rarely been published elsewhere relating to acoustic-themed publications. Concerning an architectural term, this research described the application of SFRP panels with waffle texture, side-tailed cavity and front-tailed cavity in the final section of this paper. A surface modification technique is usually applied to improve the performance of sound silencers and sound diffusers. In relation to research purpose, the micro perforated plate (MPP) has been mostly used in many reports and publications such as studies conducted by Bravo et al. and Yahya et al. [
6,
15,
16]. There are many approaches and techniques proposed by other researchers. Vigran and Meng et al. tried to fulfill the needs in acoustics, especially in the low frequency range. These examples involved the use of an MPP multi-layer with air banks between the layers [
17,
18]. A research on MPP also used a micro perforated panel called QRD (Quadratic Residue Diffuser). The QRD is a perforated panel combined with extended long-thin resonator tubes that could increase its acoustic performance to low and medium frequencies without changing the design of the panel [
16]. The research of multiple-duct perforated tube resonators was also analyzed by Kar et al. related to the improvement of acoustic performances. The result was that this material could be used in commercial automotive mufflers [
19]. Similar to the MPP, multi layer absorbers supported by the Helmholtz resonator and compartment cavities were developed by Ayub et al. and Kidner and Hansen [
20,
21]. The design of absorber is an approach that gives benefits not only in the absorption shift to a lower band, but also in improving MPP performance in the medium–high frequency range.
The behavior of sound waves touching the surface of a material experiences reflection, absorption, and the loss of sound due to the occurring viscous damping [
19]. Sound absorption improvement is conducted as a noise control strategy in building components. In this research, an observation was made of the acoustical performance in seashell waste that was composed of polyester materials and catalysts. Some researchers conducted a material test on natural material composite shells, such as coconut shells and seashells, using a different method and approaches. Several researchers observed the thermal insulation and tensile strength of composite shell materials [
8]. This research has a different approach because it combines the treatment of sound absorption improvement with the study of the surface morphology of the materials. On the other hand, some researchers observed the effective change of half wavelength to be the quarter wavelength that improves the acoustical performance of a material. The occurring viscous damping on the multilayer structure also has an effective role in improving sound absorption performance [
5,
16].
Moreover, the morphology of seashell based composite was also investigated since it has a strong relation with the density and its acoustic impedance. It determines the response of materials due to sound waves propagation and acoustical disturbances within. Many studies found that the density factor will affect the acoustic performance on both sound absorption and sound transmission loss. Acoustical response of wool board was reported by Hua and Enhui (2017) [
22], while glass fiber and rockwool materials and high desinty bio-polymer foam (HDBP) were reported by Wang and Trong (2001) [
23] and Latief et al (2014) [
24] respectively. Another research by Nandanwar et al (2017) [
25] showed that lower density fiber board posed a better sound absorption performance compared to similar board with a higher density. In regards to the use of cavity inclusion, Narang (1993) showed that the addition of a cavity to the metal frame could improve STL to the limit of 10 dB [
26], while a similar result was reported by Ko et al (2007) who proposed that aluminum foam material could have high sound transmission loss if it was treated with the addition of a cavity to the structure [
27]. The related reports on the acoustical properties of seashell based sound diffuser elements have rarely been published. So this research not only performed acoustic treatment but also described the application of SFRP panels as an acoustic component on the building.
2. Materials and Methods
The shell waste was obtained from the sea food culinary sector in the Java Province, particularly in the city of Semarang and its surrounding coastal regions. The waste was cleaned, separated based on species, and then thoroughly dried to get rid of all dampness. After drying for 3 days, the dry shells were ground using Hammer Mills. The next process was filtering the material with a 250 micron strainer in the Material and Acoustic Laboratory. The three kinds of ground shells, in the form of flour, were weighed using an electronic scale based on particle weight percentage. This flour mixture was then added to polyester resin composite. The improved polyester composite syntheses were divided into three variations based on the three species of seashells observed. The polymer comprised polyester resin (200 ml), seashell filler (125 g), and catalyst (methyl ethyl ketone peroxide; 10 ml). This composition was manually stirred for 3 minutes under ambient temperature until a homogeneous mixture was obtained. Given the three kinds of shells observed, this research produced three types of dough according to shell type. The three types of dough were poured into a cylindrical mold with a diameter of 30 mm and 6 variants of samples were obtained from the three types of shells (see
Table 1).
Circular specimens measuring 30 mm in diameter were measured to obtain their initial weight (gram), diameter (d), radius (r), and thickness (t). The volume of the specimens was calculated as
. After their volumes were determined, the densities of the samples were calculated using Equation (1):
where
is the density of material in grams per cubic centimeter,
m is the initial weight of a specimen in grams, and
V is the volume of sample in cubic centimeters. Eighteen samples of SFRP were weighed and the average densities of them can be seen in
Table 2.
Each shell species sample has an average density for each thickness of 15 mm and 30 mm. The
Anadara granosa Linn species Shell FRP has the highest density of the three species of shells with 1.541 and 1.730 g/cm
3 for thicknesses of 15 and 30 mm, respectively (see
Table 2). The lowest density shell species is
Placuna placenta Linn with 1.353 and 1.431 g/cm
3 for 15 and 30 mm, respectively.
Many studies found that the density factor will affect the acoustic performance on both sound absorption and transmission loss. Research by Wang and Torng [
23] and Latief, et al. found the increment of density could increase the sound absorption coefficient in low frequencies [
24]. Different results were released in research by Nandanwar, et al. where low density fiber boards possess a higher sound absorption coefficient than high density fiber boards [
25]. For research on the effect of density on Sound Transmission Loss performance (STL), two studies conducted by Ko, et al. and Narang identified that additional cavities on the metal material could improve the STL performances [
26,
27]. The porosity properties of the material were observed using a Scanning Electron Microscope (SEM) to analyze the differences of the surface morphology of the three shell species. The content of carbon in the shells flour was also observed in relation to the acoustical performances of either sound absorption or transmission loss (STL).
A transfer function using the impedance tube technique with two microphones referring to ASTM E-1050 was conducted to measure the sound absorption coefficient. Each sample was treated in three different ways as listed in
Table 1. The B & K 4206 impedance tube with a small tube dimension used in this experiment had a sound frequency range up to 6.4 kHz. The effect of the addition of resonators towards sound absorption on the 30 mm diameter material sample was investigated in an acoustic laboratory. The method of treatment was to drill the sample to form a quarter wave length resonator structure. The resonator diameter was 3 mm, while the number of resonators was either 4 or 8 as listed in
Table 1. The additions of a 10 mm back cavity and porous front layer were applied to the SFRP sample to determine their effects on sound absorption. The front porous layer was made of commercial dacron lining.
The impedance tube B & K 4206 was connected to four LAN-XI B & K Analyzer channels and was fully controlled by the computer. All experiments were controlled by B & K material testing software. In the experiment, the impedance tube power amplifier disseminates the random noise as the sound source along the tube before touching the surface of the test specimen (see
Figure 1). Two ¼-inch 4187 B & K microphones capture both incident and reflected waves before they are decayed by using transfer function analysis. The decomposed sound energy and the process of decay must be entirely associated with the absorption performance of the sample.
The frequency response function H
12 and reflection coefficient R can be expressed as follows,
and
with
P1 and
P2 being the sound pressure levels captured by the two microphones in position one and two respectively;
k is the wave number while
h and
s represent the distance from the first microphone to the sample and the respective distance between microphones. The coefficient of absorption α can be calculated by:
Several theories have been developed to determine the STL of a material. One research was conducted by Tenenbaum, R., A. and Magalha, M.B.S., who observed the character of compound walls related to the STL This study performed theoretical and analytical studies on how to quickly calculate the loss of transmission in compound wall construction. Compound walls with multiple layers of absorber had more effective acoustic performance than single walls of the same thickness. The theoretical model used provided an easy way to evaluate transmission loss and guided the procedure to determine the optimal material choices to be used in the partition [
29]. The distance between two microphones is assumed to be the same to simplify the STL equation, so the sound transmission loss can be calculated by Equation (5) below,
where
;
is the transfer function of sound pressure at position 1 and 2;
is the transfer function of sound pressure at position 3 and 4;
is the ratio between the auto spectrum in both the upstream tube and the downstream tube.
Figure 2 illustrates the schematic diagram of sound transmission loss measurement. Four microphones of B & K 4187 were located in the upper and lower tubes used in this investigation. The white noise was driven by a spectrum analyzer of B & K 3160-A-042 and amplified with a power amplifier of B & K 2716C generated by a hard speaker mounted on the upper tube. The sound captured in the microphone was then processed in the module of B & K 3160-A-042. In this experiment, a lower tube type was used to achieve a high frequency test for soundproofing. Specimens with thicknesses of 15 mm and 30 mm were used for the anechoic termination in the lower tube.
4. Conclusions
This research produced some conclusions relating to the acoustical performance and the treatment effects on sound insulation improvement in the material structure of seashell composites. Related to sound absorption improvement, it was found that the addition of resonators and fibrous layers in the material structure effectively improves the acoustical performance at low frequencies; even though in a narrow frequency range. In an improvement effort, the additions of resonator combinations, fibrous layers, and cavities gave a positive effect in increasing sound absorption at high frequencies with a wide frequency range. Moreover, the combination of all three also affects the increase in sound absorption value; mostly above 0.2 at a frequency above 2.0 kHz.
In combination with the addition of a material structure with resonators, fibrous layers, and cavities, it was found that the 30 mm composite shell had better acoustical performance at high frequencies compared to the 15 mm composite shell. On the observation of the sound transmission loss, the addition of resonator holes decreased the transmission performance of the material structure because the occurring half wavelength propagation process is quickly transmitted to the opposite surface. The shell composite with a thickness of 15 mm has a better transmission loss performance compared to the shell composite having a 30 mm thickness because the rapid transmission process disappears on thinner panels rather than on thick panels. The thick panels will store sound energy longer than the thin panels. Continuing, the Placuna placenta Linn filler composite with an 8-hole treatment and 30 mm thickness has the highest coefficient value, as much as 0.97, which is not much different from the Placuna placenta Linn filler that is 15 mm thick. This is comparable to the SEM analysis showing that Placuna placenta Linn has relatively small, regular, and solid pores compared to the other two shell types. Furthermore, composites with Placuna placenta Linn filler 30 mm thick have an STL value of 54.5 dB and 58.0dB for the composites with a thickness of 15 mm. This shows that composites with 15 mm thickness have good ability in eliminating sound.
In the end results, the Placunas composites were treated by the side-tailed cavity and front-tailed cavity where these composites have a quite good performance at middle to high frequencies and reach wideband frequencies and peaks of 0.85 on sound absorption coefficient especially for the 15 mm Placuna composite with front-tailed cavity. This research enriches the discourse about materials made of seashell waste which can be recycled into acoustical building materials. Although naturally tough, the acoustical performance of these materials was successfully improved with the front tailed cavity treatments. By using ATSM methods, these materials were proved of their acustical behavior and could be considered as diffuser–absorber in buildings. Finally, the side-tailed cavity and front cavity treated placuna composites have qualified performances on sound absorption and might be used as diffuser–absorbers in interior buildings application. Due to the tight times on manufacturing the materials for scale-up, it is suggested to measure and observe the materials in the an-echoic chamber to ensure the reverberation time performance in further research. Regarding the development of the material for implementation in buildings, research on the material quality improvement should be conducted in collaboration with the related manufacture as the third party.