Development of Louvered Noise Barrier with Changeable Sound Insulation from Waste Tire Rubber and Investigation of Acoustic Properties
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1. Introduction
The expansion of cities leads to the continuous site development of vacant spaces. Due to the increase in the price of land, commercial, residential, and industrial buildings are built within close proximity to each other. Different heating and ventilation equipment is used to create comfortable working and living conditions. This equipment emits large amounts of air and causes noise as a result. According to the World Health Organization data, approximately 20 percent of the population of the European Union suffers from a level of noise that medical experts identify as unacceptable. Acoustic comfort problems can be solved by installing noise barriers. However, ventilation equipment cannot be covered as it must cool down and take in or release air into the environment. Therefore, this noise prevention measure is not suitable for such cases. The increasing standardization constraints of noise values have led many manufacturing and commercial companies to look for effective solutions to keep the prescribed noise levels in the environment.
The aim of the work is to create an innovative, effectively sound-absorbing structure with changeable sound insulation, which would comply with the principles of circular economy and be used to reduce the sound level produced by equipment that requires high air permeability (heat pumps, generators, fans, etc.). Waste tire rubber is to be used for the design of the structure.
2. Materials and Methods
All experimental studies were conducted in accordance with standard laboratory measurement methods. Environmental factors, including temperature, humidity, pressure, and other ambient conditions, were controlled and considered during measurements, thereby minimizing their influence on the results. Additionally, laboratory equipment underwent regular calibration to maintain accuracy, ensuring reliable and consistent measurements. To minimize variability between samples, meticulous attention was given to sample preparation. Factors such as sample size, shape, and surface were precisely checked and selected to ensure measurement accuracy.
The length and width of the plastic cartridges are both 265 mm and the thickness is 30 mm. The plate is perforated with 4 × 4 mm square holes with a gap of 2 mm between them, and the plate thickness is 2 mm. The recycled tire rubber plates of different thickness are rigidly inserted into the 3D-printed plate and covered with the perforated plates from the top and bottom.
2.1. Methods for Identifying Sound Absorption and Sound Transmission Loss in a Material by Using an Impedance Tube
To determine the acoustic parameters of the material, the sound absorption and sound transmission loss in the material are under analysis. The ratio of the sound energy absorbed by a material α represents sound absorption. Different materials absorb and reflect sound waves differently. The sound absorption of the material depends on the thickness, density, and porosity of the test sample.
The impedance tube indeed presents several limitations. While it typically operates within a specific frequency range, this range, although often broad, might not encompass all relevant frequencies, especially for materials exhibiting significant frequency-dependent absorption properties. In our case, the impedance tube’s measurement range was 160 to 5000 Hz. Moreover, the impedance tube measures absorption coefficients under normal incidence, which means that it might not accurately depict the absorption behavior of materials under oblique angles of incidence. Additionally, the accuracy of measurements can be influenced by the thickness of the tested material. For thin materials, the tube’s measurements might not fully capture their absorption properties, while for very thick materials, limitations in the tube’s length and the wavelengths of incident waves might hinder the full characterization of absorption.
Sound transmission loss in a material (DTL) is an important acoustic property, showing how much sound is lost as waves propagate through a given substance. The density, thickness, and air resistance of the test samples have the greatest impact on sound transmission loss. A commonly used method to determine sound transmission loss is through the impedance tube and it is called the four-microphone method.
2.1.1. The Method of Measuring Sound Absorption by Using an Interferometer
Three samples of each rubber granulate plate are examined. The results are presented in the 1/3 octave frequency range, ranging from 200 to 5000 Hz.
2.1.2. Methods for Identifying Sound Transmission Loss in a Material by Using an Interferometer
Setup for measuring the frequency range 1000–5000 Hz: four microphones labeled 1, 2, 3, and 4 are positioned accordingly. The distance between microphone No. 1 and No. 2, denoted as X12, is set at 20 mm. The distance between microphone No. 2 and the sample, indicated as X2S, is 60 mm. Likewise, the distance between microphone No. 3 and the sample, labeled as X3S, measures 40 mm. Finally, the distance between microphone No. 3 and No. 4, identified as X34, is 20 mm. The setup for measuring the frequency range 160–1000 Hz is similar to the previous setup. The distance between microphone No. 1 and No. 2, denoted as X12, is extended to 120 mm. The distance between microphone No. 2 and the sample, designated as X2S, remains at 60 mm. Similarly, the distance between microphone No. 3 and the sample, labeled as X3S, is maintained at 40 mm. The distance between microphone No. 3 and No. 4, identified as X34, is adjusted to 120 mm.
The formed matrix should satisfy these conditions: T11 = T22 and T11T22 − T12T21 = 1.
The sound transmission loss adequately demonstrates the ability of a material to isolate sound, specifying the characteristics of the material in terms of its porosity, reflectivity and sound absorption. Three samples of each rubber granulate plate are examined. The results are presented in the 1/3 octave frequency range, ranging from 160 to 5000 Hz.
2.2. The Method of Determining the Acoustic Parameters of the Structure in the Sound-Absorbing Chamber
The wall separating the chamber rooms is equipped with a 1 m2 opening in which a 1.0 m × 1.0 m sample is rigidly installed.
In the sound reception room, the microphone is permanently installed, and in the sound transmission room, the omnidirectional speaker creates a constant diffuse sound field; its spectrum is uninterrupted in the selected frequency range. By way of experiment, the sound level is measured using a one-third octave band filter. The measurement duration at each microphone position and in the one-third octave band is at least 60 s.
The plates are rotated from 0, 15, 30, and 45 degrees, every 15 degrees. The following parameters are measured:
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Average sound pressure level without installation of the structure in the provided opening;
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Average sound pressure level in the sound reception room after the installation of the test structure;
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Equivalent reduction in sound pressure level across the frequency band.
First, during the experiment, the sound level was measured without installing the test structure. Then, the structure was installed and the sound pressure level was measured. The result of the sound pressure level after the installation of the structure was subtracted from the result obtained by measuring the sound pressure level prior to the installation of the structure. Thus, graphs of the sound pressure level decrease in the whole frequency band were obtained. The same principle was used to measure the equivalent sound pressure level, and an equivalent sound level loss was obtained.
Insertion loss was calculated as the difference in the sound pressure level measured before and after the louvers were installed. The result was submitted in graphs of insertion loss. One microphone position was used to measure insource and receiver rooms.
During the experimental measurements, the omnidirectional loudspeaker “Brüel & Kjær The OmniPower Sound Source Type 4292-L” was used as a sound source. White noise was used for sound attenuation testing. Sound insertion loss was measured from 800 Hz to 8000 Hz using 1/3 octave band filtering. During A-weighted insertion loss LAeq measurements, the LAeq of the source was 95.0 dB. The spectrum involved ranged from 160 to 8000 Hz.
During the experiment, structures with different fractions and thickness of recycled rubber granulate filler were tested. During the research, the sound level reduction of each structure was measured with a change of tilting angle of the louvers from 0 to 45 degrees. First, the louvers were filled with 21 mm thick rubber granulate plates in the 2–8 mm fraction. The perforated plates were filled with 30 mm thick rubber granulate (4 to 8 mm fraction of rubber granules) to improve the acoustic properties of the structure. During the research, measurements of the sound level reduction throughout the frequency band and the equivalent sound level loss were made. The distance between the plates in the horizontal position was 86 mm for a structure consisting of 15 cartridges and 65 mm for a structure consisting of 27 cartridges.
3. Results
This section presents the results of an interferometer-based study of the acoustic properties of rubber granulate samples with nine different parameters. After the interferometer measurements of sound absorption and sound transmission loss in the material, two rubber granulate plates with the best performance were selected and used as the sound absorbing material in the test structure. The sound level loss and the equivalent sound level reduction were identified in the sound absorbing chamber by using the test structure. The result is presented with the estimation of the dependence on the tilting angle of the louvers and on the position in relation to the sound source.
3.1. The Results of Measuring Sound Absorption by Using an Interferometer
3.2. The Results of Identifying Sound Transmission Loss in a Material by Using an Interferometer
The presented results show that the samples which were the most effective in reducing sound transmission were No. 4, No. 1, and No. 8. Sample No. 1 (160 Hz to 500 Hz) was significant in the low frequency band. The sound transmission loss (DTL) in this sample ranged from 9.67 dB to 10.89 dB. The results of the samples No. 4 and No. 8 at 500 Hz were 9.77 dB and 8.98 dB, respectively. In the medium frequency range (500 Hz to 1600 Hz), these samples were marked with a good consistent result and at 1000 Hz they reached 13.28 dB, 11.94 dB, and 10.54 dB, respectively. In the high frequency band (1600 Hz to 5000 Hz), most of the sound was lost as the sound waves propagated through sample No. 4, and at the 4000 Hz limit, the sound loss (DTL) was equal to 15.39 dB. The results of the samples No. 1 and No. 8 at the level of the same frequency were 13.15 dB and 10.74 dB, respectively. The thickness and density of the rubber tire granulate were proved to have the most significant impact on sound loss in the samples. In this case, density determines the amount of free air gaps in the rubber granulate. This factor affects the porosity, with the increase in which sound loss during the propagation through the test material shows worse results.
The interferometer measurements of sound absorption and sound transmission loss in the material showed that the best results in the analysis of sound absorption (α) and sound loss (DTL) were obtained from samples No. 1 and No. 8. For this reason, rubber granulate plates which met the sample parameters were selected for the use in the structure of the louvers and for further testing in the sound absorbing chamber.
3.3. The Results of the Testing of the Acoustic Parameters of the Structure in the Sound-Absorbing Chamber
The louver’s structure assembly was installed in the sound absorbing chamber. The sound level loss throughout the frequency band and the equivalent sound level loss were determined by tilting the louvers at different angles directed to and from the noise source.
The use of 27 rubber granulate plates with a thickness of 30 mm (4 to 8 mm fraction) and a density of 0.772 g/cm3 when tilted away from the noise source resulted in a maximum sound level loss of 17.3 dB (in the 8000 Hz range); when the louvers were tilted in the direction of the noise source, the result was 17 dB. The estimation of the efficiency of the louvers consisting of 15 plates with the use of the same type of rubber granulate plate proves that the efficiency was significantly lower and the maximum sound level loss was 8.0 dB (at 3150 Hz). The use of 27 rubber granulate plates with a thickness of 21 mm (2 to 8 mm fraction) and a density of 0.757 g/cm3 shows that the maximum sound pressure loss—11.7 dB—was observed at the frequency of 8000 Hz. Meanwhile, the maximum efficiency of the louvers consisting of 15 sound-absorbing plates was noticed in the frequency band of 2500 Hz with the value of 6.3 dB (while tilting the louvers towards the noise source).
A direct dependence of the result on the tilting angle of the louvers and on the number of rubber granulate plates was identified. The highest values throughout the frequency band were obtained while using the louvers with 27 rubber granulate plates compared to 15, regardless of their thickness. This is due to the smaller air gap between the louvers, which reaches 65 mm in the horizontal position, while when using 15 plates, the distance is 86 mm. The increase in the number of plates and the tilting angle in the test structure also expands the absorbing surface area, which results in the higher efficiency of the structure.
This consistent pattern can also be observed in structures using the following rubber granulate plates: thickness—21 mm; fraction—2–8 mm; and density—0.757 g/cm3. In conclusion, it is most appropriate to fill the louvers with 30 mm thick, 4–8 mm fraction, 0.772 g/cm3 rubber granulate and use a structure consisting of 27 plates, thus increasing the absorbing surface area and reducing the air gap between the louvers. Experiments have shown that the structure is most effective in suppressing high-frequency sound (in the range of 3150 to 8000 Hz). The maximum sound level reduction was 17.3 dB (in the 8000 Hz frequency range), and the maximum equivalent sound level reduction (LAeq) was 7.3 dBA. In summary, the results obtained by investigating the acoustic properties of the samples with the use of an interferometer corresponded to the results obtained in the sound absorbing chamber. Thicker and larger fractions of rubber granulate plates demonstrate higher efficiency. It was found that the type, number, thickness, and tilting angle of the rubber granulate plate have the most significant impact on the structure efficiency.
5. Conclusions
An innovative sound-proof structure has been developed. Waste tire rubber was used to produce it, thus presenting a solution for the waste management problem. The structure could possibly be used indoors and outdoors, is easy to install, and is suitable for reducing noise generated by equipment that requires a large amount of air flow to function.
Interferometer measurements of nine different rubber granulate plates revealed the sound absorption and sound transmission loss of the samples. The best results in the analysis of sound absorption (α) and sound loss (DTL) (throughout the entire frequency band) were obtained in the case of a sample with a thickness of 30 mm, a fraction of 4 to 8 mm, and a density of 0.772 g/cm3, and a sample with a thickness of 21 mm, a fraction of 2 to 8 mm, and a density of 0.757 g/cm3. The 30 mm thickness sample had a maximum sound absorption ratio (α) of 0.75 and a sound loss (DTL) of 14.00 dB. The 21 mm thickness sample reached a maximum sound absorption ratio (α) of 0.71 and a sound loss (DTL) of 11.51 dB. The results were taken into consideration and the rubber granulate plates corresponding to the parameters of the samples were used in the structure of the louvers.
The structure efficiency was investigated in the sound absorbing chamber. The best result of sound absorption was found to be provided by a structure consisting of 27, 30 mm thick, 4–8 mm fraction, 0.772 g/cm3 density rubber granulate plates, with a space of 65 mm between the louvers in the horizontal position and a tilting angle of 45 degrees, with the louvers directed towards the noise source. The maximum sound level reduction observed was 17.3 dB (in the 8000 Hz frequency band), and the maximum equivalent sound level reduction (LAeq) was 7.3 dBA. The highest efficiency of the construction was achieved by attenuating high-frequency sound (from 3150 to 8000 Hz); therefore, it could be used to reduce noise from such equipment as centrifuges, compressors, pumps, and turbines which often generate sound in the higher frequency range. Also, louvered barriers can be used for noise reduction from cooling systems, cooling towers, fans, and HVAC systems.
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