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Acoustics, Volume 2, Issue 4 (December 2020) – 10 articles

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Open AccessArticle
The Historical Soundscape Analysis of Fatahillah Square, Jakarta
Acoustics 2020, 2(4), 847-867; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040048 - 08 Dec 2020
Viewed by 989
Abstract
Understanding conceptions of the protection of cultural heritage continues to develop until now. Presently, urban historic places are not only comprehended as tangible but also include intangible dimensions. However, the conservation of cultural heritage dominantly still emphasises the visual sense more than any [...] Read more.
Understanding conceptions of the protection of cultural heritage continues to develop until now. Presently, urban historic places are not only comprehended as tangible but also include intangible dimensions. However, the conservation of cultural heritage dominantly still emphasises the visual sense more than any other senses. Thus, this paper addressed several questions on the role of human senses, the historic sonic environments, and the soundmarks of the past in examining a historical area. This paper aims to reveal the relation between sound sources and its predicted sonic environment in historic places over the time. The case study was Fatahillah Square, Jakarta, which has been documented from the 19th century until now. Some methods were carried out such as soundwalk, recalled in memory, and visual analysis. The results show that comprehensive study of multisensorial stimulus can increase a holistic understanding of historic places. Therefore, the protection of historic sites cannot only focus on the object per se, but also it must be considered to be a holistic entity. This research highlights new perspectives in analysing historical areas using combination of pictorial sources and sonic information. Full article
(This article belongs to the collection Historical Acoustics)
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Open AccessArticle
Monitoring the Nonlinear Acoustic Behavior of Fresh Cementitious Materials during the Hardening Process Using Laser Doppler Vibrometry
Acoustics 2020, 2(4), 833-846; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040047 - 30 Nov 2020
Cited by 2 | Viewed by 886
Abstract
A challenge in the field of material science and concrete technology is to associate material properties and behavior with changes in internal microstructure. A fundamental issue for planning, implementation, and quality control of construction projects is the fundamental understanding of the hardening process [...] Read more.
A challenge in the field of material science and concrete technology is to associate material properties and behavior with changes in internal microstructure. A fundamental issue for planning, implementation, and quality control of construction projects is the fundamental understanding of the hardening process in freshly poured cement-based materials. Shortly after the mixing process, cementitious materials exhibit a substantial damping effect on ultrasonic wave propagation along with low-pulse velocity. The way a wave reflects and diffuses in the volume of a solid material can provide important information about internal heterogeneities. An important application of the wave propagation technique is the determination of layers within the material distinguished by unlike elastic properties. During the hardening process, the ultrasonic wave propagation exhibits a noticeable sensitivity at the point of phase change of the material, that is, the critical point at which the state of the suspension transforms from liquid to solid state. After that point, there is a continuous increase of pulse velocity and ultrasonic signal amplitude. The solidification point is of practical importance since at this point the load-bearing capacity of the composite material and its long-term behavior are based on the connectivity of the solid phase. This study aims to develop a nondestructive tool to monitor freshly poured cement-paste composites at early stages, during the hydration process, by means of nonlinear elastic waves. The measurement setup combined a contact ultrasonic transducer with a noncontact optical detection system, in which the principle of operation is based on the Doppler effect. This methodology enabled us to assess the amplitudes of harmonic vibrations of an acoustic wave propagating through the material with a specific fundamental frequency. This enabled the evaluation of important material properties, such as minute changes in the internal microstructure of fresh concrete during hardening, the evolution of nonlinearity parameters that relate to higher-order elastic constants of the material, as well as the ultrasonic wave velocity. Full article
(This article belongs to the Special Issue Acoustical Materials)
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Open AccessArticle
Influence of Swept Blades on Low-Order Acoustic Prediction for Axial Fans
Acoustics 2020, 2(4), 812-832; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040046 - 28 Nov 2020
Viewed by 702
Abstract
The low-speed fans used for automotive engine cooling contribute to a significant part of the global noise emitted by the vehicle. A low-order sound-prediction methodology is developed considering the blade sweep-angle effect on the acoustic predictions of the turbulence-impingement and the trailing-edge noise-generating [...] Read more.
The low-speed fans used for automotive engine cooling contribute to a significant part of the global noise emitted by the vehicle. A low-order sound-prediction methodology is developed considering the blade sweep-angle effect on the acoustic predictions of the turbulence-impingement and the trailing-edge noise-generating mechanisms. We modeled these through the application of a semianalytical method based on Amiet’s airfoil theory, appropriately adapted via a strip-theory approach accounting for rotation and modified to include the blades forward curvature. Sweep was already shown in the literature to reduce the noise emitted by isolated airfoils, but its effect on rotating machines was not yet well understood. In this study, we show that the effect of the sweep-angle is to globally reduce the emitted noise by the fan and to change the sound distribution of the sources along the blade span. Thus, the sweep-angle must be considered not only because it yields a better comparison with experimental results but also because wrong conclusions on the dominating noise-generating mechanisms can be drawn when this effect is not taken into account. The investigation is finally complemented by a sensitivity analysis focusing on some of the key parameters characterizing the acoustic prediction. Full article
(This article belongs to the Special Issue Aeroacoustics of Turbomachines)
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Open AccessCommunication
Improved Source Characteristics of a Handclap for Acoustic Measurements: Utilization of a Leather Glove
Acoustics 2020, 2(4), 803-811; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040045 - 27 Nov 2020
Viewed by 1035
Abstract
A handclap is a convenient and easily available source for room acoustic measurements. If used correctly (e.g., application of optimal hand configuration) it can provide usable results for the measurement of acoustic parameters, within an expected deviation. Its biggest drawbacks are the low [...] Read more.
A handclap is a convenient and easily available source for room acoustic measurements. If used correctly (e.g., application of optimal hand configuration) it can provide usable results for the measurement of acoustic parameters, within an expected deviation. Its biggest drawbacks are the low sound pressure level (especially in the low frequency range) as well as its low repeatability. With this in mind, this paper explores the idea of testing a handclap with a glove in order to assess the effect on its source characteristics. For this purpose, measurements were performed with 12 participants wearing leather gloves. Sound levels were compared with simple handclaps without gloves, and between grouped results (overall A-weighted SPL, octave bands, 1/3 octave bands). Measurements were also performed several times to evaluate the effect on repeatability. Results indicate that the use of leather gloves can increase the sound levels of a handclap by 10 dB and 15 dB in the low frequency ranges (63 Hz and 125 Hz octave bands, respectively). Handclaps with leather gloves also point toward improved repeatability, particularly in the low-frequency part of the frequency spectrum. In conclusion, compared to simple handclaps without gloves, evidence from this study supports the concept that handclaps with leather gloves can be used in engineering practices for improved room acoustic measurements of room impulse response. Full article
(This article belongs to the collection Historical Acoustics)
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Open AccessArticle
An Acoustic Survey of the Cathédrale Notre-Dame de Paris before and after the Fire of 2019
Acoustics 2020, 2(4), 791-802; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040044 - 06 Nov 2020
Viewed by 10143
Abstract
The Cathédrale Notre-Dame de Paris is amongst the most well-known worship spaces in the world. Its large volume, in combination with a relatively bare stone construction and marble floor, leads to rather long reverberation times. The cathedral suffered from a significant fire in [...] Read more.
The Cathédrale Notre-Dame de Paris is amongst the most well-known worship spaces in the world. Its large volume, in combination with a relatively bare stone construction and marble floor, leads to rather long reverberation times. The cathedral suffered from a significant fire in 2019, resulting in damage primarily to the roof and vaulted ceiling. Despite the notoriety of this space, there are few examples of published data on the acoustical parameters of this space, and these data do not agree. Archived measurement recordings from 1987 were recovered and found to include several balloon bursts. In 2015, a measurement session was carried out for a virtual reality project. Comparisons between results from these two sessions show a slight but significant decrease in reverberation time (8%) in the pre-fire state. Measurements were recently carried out on the construction site, 1 year since the fire. Compared to 2015 data, the reverberation time significantly decreased (20%). This paper presents the preliminary results of these measurements, providing a documentation of the acoustics of this historic worship space both prior to and since the 2019 fire. Full article
(This article belongs to the collection Historical Acoustics)
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Open AccessArticle
Effect of Blade Curvature on Fan Integration in Engine Cooling Module
Acoustics 2020, 2(4), 776-790; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040043 - 29 Oct 2020
Cited by 1 | Viewed by 750
Abstract
Two blade curvatures representative of those found in automotive fans are compared. Measured performances are analyzed for forward and backward curved blades, either with or without heat-exchangers placed in front of them. The backward fan demonstrated good efficiency but poor acoustics, whereas it [...] Read more.
Two blade curvatures representative of those found in automotive fans are compared. Measured performances are analyzed for forward and backward curved blades, either with or without heat-exchangers placed in front of them. The backward fan demonstrated good efficiency but poor acoustics, whereas it is the contrary for the forward fan. Investigations are completed by a numerical analysis of the flow in the cooling module. Different integration effects are highlighted depending on the blade curvature, showing variation in pressure, torque and efficiency. Analyses of blade loadings show that the flow is more homogeneous with a forward curved fan and it produces less unsteadiness at the blade tip. Post-processing of detached eddy simulations (DES) shows density fluctuations on the blade wall and confirms the correlation between the large vortical structures and the acoustic sources for both fans. In addition, with the forward fan, the sound propagation is less directed towards the axis of rotation and it yields up to −3.6 dB of sound pressure level (SPL) measured in front of the cooling module. As a conclusion, any choice for a fan must result from a compromise between aerodynamics and aeroacoustics, and the final performances must be carefully checked on the module. Full article
(This article belongs to the Special Issue Aeroacoustics of Turbomachines)
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Open AccessReview
A Review on Sonochemistry and Its Environmental Applications
Acoustics 2020, 2(4), 766-775; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040042 - 25 Oct 2020
Cited by 1 | Viewed by 941
Abstract
Sonochemistry is a significant contributor to green science as it includes: (i) the use of less toxic compounds and environmentally safe solvents, (ii) improvement of reaction conditions and selectivity, (iii) no toxic sludge generation, (iv) reduction in the energy use for chemical transformations, [...] Read more.
Sonochemistry is a significant contributor to green science as it includes: (i) the use of less toxic compounds and environmentally safe solvents, (ii) improvement of reaction conditions and selectivity, (iii) no toxic sludge generation, (iv) reduction in the energy use for chemical transformations, (v) reusability of materials. In water and wastewater treatment, ultrasound is used as an advanced oxidation process to eliminate refractory pollutants. Ultrasound is also used as a very effective sludge pretreatment technology in wastewater treatment plants to facilitate biogas production. Moreover, sonochemical synthesis of nanoparticles has recently attracted great attention as a greener protocol. This paper presents the main applications of ultrasound in environmental remediation and protection. The study also introduces mechanism for the degradation of pollutants from water via sonication in aqueous media and the principle factors affecting the cavitational effect. Full article
(This article belongs to the collection Featured Review in Acoustics Science)
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Open AccessArticle
Angle-Dependent Absorption of Sound on Porous Materials
Acoustics 2020, 2(4), 753-765; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040041 - 16 Oct 2020
Viewed by 1072
Abstract
Sound-absorbing materials are usually measured in a reverberation chamber (diffuse field condition) or in an impedance tube (normal sound incidence). In this paper, we show how angle-dependent absorption coefficients could be measured in a factory-type setting. The results confirm that the materials have [...] Read more.
Sound-absorbing materials are usually measured in a reverberation chamber (diffuse field condition) or in an impedance tube (normal sound incidence). In this paper, we show how angle-dependent absorption coefficients could be measured in a factory-type setting. The results confirm that the materials have different attenuation behavior to sound waves coming from different directions. Furthermore, the results are in good agreement with sound absorption coefficients measured for comparison in a reverberation room and in an impedance tube. In addition, we introduce a biofiber-based material that has similar sound absorption characteristics to glass-wool. The angle-dependent absorption coefficients are important information in material development and in room acoustics modeling. Full article
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Open AccessArticle
Validation of the Lattice Boltzmann Method for Simulation of Aerodynamics and Aeroacoustics in a Centrifugal Fan
Acoustics 2020, 2(4), 735-752; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040040 - 26 Sep 2020
Cited by 1 | Viewed by 1203
Abstract
Due to the fact that legal and market requirements are becoming stricter, fan noise reduction, in addition to energy efficiency, represent a challenge for fan product designers. Most experimental studies are associated with trial-and-error approaches. Therefore, numerical methods are mostly preferable. However, the [...] Read more.
Due to the fact that legal and market requirements are becoming stricter, fan noise reduction, in addition to energy efficiency, represent a challenge for fan product designers. Most experimental studies are associated with trial-and-error approaches. Therefore, numerical methods are mostly preferable. However, the quantitative prediction of the noise emitted by radial fans via numerical simulations remains challenging. The Lattice Boltzmann method (LBM) is a relatively new approach that promises a direct calculation of the aerodynamics coupled with the aeroacoustics. This article presents an LBM simulation of a centrifugal fan using the commercial Lattice Boltzmann Code SIMULIA PowerFLOW of Dassault Systèmes. The simulation model includes both the fan impeller and the spiral housing. In accordance with the experimental setup, the fan was mounted in a test bench to analyze four different operating points. The results of the LBM simulation were validated by experimental measurements. Flow information in terms of pressure rise and efficiency of the centrifugal fan as a function of the flow rate are in a good agreement. Considering the acoustic spectra and the blade passing frequency, the simulation was able to precisely predict the noise of the centrifugal fan. The simulation results are also used to visualize the flow and acoustic field inside of the fan to detect noise-generating flow features. By evaluating the filtered pressure fluctuation in the fluid volume and on the wall, four main noise sources of the centrifugal fan can be identified. Full article
(This article belongs to the Special Issue Aeroacoustics of Turbomachines)
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Open AccessArticle
Assessing the Effects of Noise on Sound Identities of Historical Landmarks
Acoustics 2020, 2(4), 719-734; https://0-doi-org.brum.beds.ac.uk/10.3390/acoustics2040039 - 25 Sep 2020
Viewed by 1257
Abstract
The degradation caused by environmental noise affects the sound identities of several areas, especially at a city level, so there is a need for immediate individual and collective action. This paper focuses on the environmental noise impacts towards the soundscapes of historical landmarks [...] Read more.
The degradation caused by environmental noise affects the sound identities of several areas, especially at a city level, so there is a need for immediate individual and collective action. This paper focuses on the environmental noise impacts towards the soundscapes of historical landmarks located in the city of Thessaloniki (Greece). Furthermore, it offers an insight regarding a new urban quiet area attribute, since it discovers whether elevation contributes to the formation of their sound identity. The first step of this research utilized the opinion of more than 500 residents of Thessaloniki with the aid of a questionnaire, in order to highlight areas of the city’s historic center with unique soundscapes. The resulted soundscapes were studied and analyzed by gathering both quantitative and qualitative data. Finally, these soundscapes were assessed by sound experts in order to thoroughly describe their sound identities. The Roman and Byzantine landmarks studied, being at lower and higher elevation levels, respectively, presented differentiations regarding noise levels and other soundscape characteristics. This paper proposes a new protocol, according to which the factor of elevation is worth considering during the process of identifying and utilizing urban quiet areas. Full article
(This article belongs to the collection Historical Acoustics)
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