Study the effect of shape on acoustic performance of micro perforated absorber at low frequencies
Micro-perforated absorbers are one of the structures that are widely used nowadays. The sound absorption mechanism is performed by viscous energy losses in the cavities on the plate. This paper examined the effect of the surface shape on the micro perforated absorber performance at low frequencies (less than 500 Hz). The three-dimensional finite element method was used to predict the absorption coefficient of this group of adsorbents. Also, the results obtained from the shaped absorbers were compared with the flat micro perforated absorbers. After validating the numerical results, six different designs were defined as the surface shape of the micro perforated plates in the COMSOL Multiphysics, Ver. 5.3a software. The results reflected the fact that the factor of the surface shape can be used as a contributing factor in some frequencies. In general, the dented or concave shapes provide better outcomes than other flat designs and shapes and the convex or outward shapes bring the weakest results.
Keywords: Micro Perforated Absorbers, surface shapes, Low Frequency, Absorption Coefficient
The progress of the industry over the past 60 years has been accompanied by an increase in pollutants such as air and water pollutions, visual, and noise pollutions. Although noise pollution is an integral part of urban life, meanwhile, the impact of excessive exposure to sound can cause problems, one of the most common is hearing loss . According to the World Health Organization, the number of people suffering from hearing loss all over the world has increased from 120 million in 1995 to 250 million in 2004 . Continuous and long-term exposure to sound pressure levels can lead to quantitative and qualitative disruptions in communication with individuals, resulting in the lack of proper and effective understanding of the warning signs. Other side effects may include increased risk of accidents , increased stress, increased blood pressure ,increased heart rate , psychological injuries [6, 7], distress, sleep disturbances, and increased cardiovascular diseases . To prevent this problem, an adaptation needs to be made between the standards of sound and technology, transportation, work sites and recreational facilities, which can be done in various ways such as upgrading the machinery, vehicles replacement, design and construction of new indoor environments or noise control protocols.
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Noise control can be done in a variety of forms such as controlling at the source, controlling at the receiver, and controlling in the sound propagation path. When the goal is to control the noise in the path of propagation, the issue of reduction in the reverberation time comes to the focus of attention. Reducing the reverberation time includes the control of noise transferred through the air, known as the airborne noise. One of the methods of noise control is the absorption phenomenon. Acoustic materials are typically designed for noise absorption. The passive absorbers are effective at mid and high frequencies, where the ear has a high sensitivity. However, the noise control in the spectrum of low frequencies seems still problematic. This type of sound occurs in a range of frequencies that is less diminished by walls or other structures. It can also mask higher frequencies and travel longer distances just with a bit reduction. Other features of low-frequency noise are the creation of resonance in humans and the development of mental and, to some extent, physiological responses . These complications include tinnitus, headache, increased level of cortisol secretion, increased stressful reactions, respiratory impairments, sense of discomfort, and complaints [10, 11]. The equipment and devices involved in generating low-frequency noise may include the followings: Internal combustion engines, compressors, fans and blowers, power transformers, gearboxes, ventilation devices, computer network facilities, control rooms in different industries, diesel engines, office work environments, roads and highways traffic, sewage pipelines path, ionizing beam production equipment, pumps and washing machines, boilers, refrigerators, and cooling towers.
The perforated plates have been successfully used so far as absorbers in many buildings [12, 13], medical equipment , and mufflers [15, 16]. A micro perforated absorber typically contains a perforated plate, which is placed with at distance from a rigid wall. The sound absorption mechanism is performed by viscous energy losses in the cavities on the plate. When the size of the cavities is reduced, these class of absorbers gain a high resistance and low reactance, which provides them the required conditions to become suitable absorbers with a broad absorption spectrum. This structure provides a better performance than other resonance structures . However, it provides a weaker performance in comparison with the porous material both in terms of frequency band width and the absorption rate. Many studies have been done on effective factors aimed at increasing the acoustic performance of this category of absorbent materials in recent years. Some of these studies include compartmentalizing the space behind the plate and creating spaces with different depths, the use of two consecutive perforated plates, the use of multi-layer micro perforated plates [20 ,21], and the use of absorbent materials behind the plate . In some studies, the resonance of the same micro perforated structure has been used, especially at low frequencies [23, 24].
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The surface shape of these materials is one of the factors affecting the acoustic behavior of the absorbent materials. Few studies have looked at the apparent shape of the adsorbent materials. For example, Chens examined the effect of the shape of porous absorbers behind the micro perforated plate. He examined the simple, semicircular, concave, and triangular shapes and concluded that the form of porous absorbers definitely affect the absorption coefficient at some frequencies . Easwaran et al. studied the sound reflection coefficient from the foam edges using the Galerkin finite element method . Kang et al. investigated the rates of adsorption coefficient and sound transmission loss in the panel composite structure by the finite element method . The results of both studies revealed that the porous materials with a edged shape improve the rates of absorption coefficient and the transmission loss in some frequency bands. Also, Tsay et al. conducted a research on exploring the acoustic absorption rate of polyurethane foams with a pyramidal surface geometry. Different dimensions of pyramidal geometry of polyurethane foam with different vertex angles were analyzed, which results showed that the highest absorption coefficient has been obtained at a vertex angle of 29°. According to the conducted research, it seems that few studies have focused on the appearance of absorbers, particularly the micro-perforated absorbers. Therefore, in this study, we evaluated the effect of the surface shape on the performance of micro-perforated absorbers using the finite element method .
The theoretical part of the finite element method
To explain the acoustic performance of the perforated plate, we assume that the sound wave is emitted from the sound source with a “tetha” angle and creates an azimuth beta angle in impacting the plate. A part of the acoustic energy of the wave is dispersed and a part of it is absorbed by the micro-perforated plate. We used the finite element method in the frequency domain to simulate the acoustic performance. The computing range includes the space behind the micro-perforated plate, the micro-perforated plate itself, and a virtual channel. The sound field in the back space and the air passageway is satisfied by solving the Helmholtz equation.
Fig. 1. Theoretical model of the MPP absorber
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