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Acoustics, Volume 4, Issue 3 (September 2022) – 7 articles

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Article
Control of Acoustic Energy Input for Cleaning of Industrial Boilers
Acoustics 2022, 4(3), 609-636; https://doi.org/10.3390/acoustics4030038 - 31 Jul 2022
Viewed by 365
Abstract
A non-intrusive cleaning method for boiler tubes at Sasol Synfuels power station at Secunda, in the Mpumalanga province of South Africa, is preferred over conventional methods that require boiler shutdown. The elected non-intrusive cleaning method utilizes sound energy waves, produced by an acoustic [...] Read more.
A non-intrusive cleaning method for boiler tubes at Sasol Synfuels power station at Secunda, in the Mpumalanga province of South Africa, is preferred over conventional methods that require boiler shutdown. The elected non-intrusive cleaning method utilizes sound energy waves, produced by an acoustic horn. Due to the nature of sound propagation and the effectiveness required, there is a requisite to control and operate the sonic horn. If the acoustic horn’s sound frequency is too low, it will produce higher sound energy waves that will resonate with the plant’s harmonious frequency and cause structural damage. Conversely, if the sonic horn’s sound frequency is too high, excessive noise levels may be reached and annoy plant personnel. To prevent these undesirable outcomes posed by adopting acoustic cleaning, there needs to be a regulatory system incorporated into the configuration to mitigate vibrations and limit noise. The regulatory system comprises a control system that drives the acoustic horn’s sound frequency as intended through a set point. The designed control system meets the anticipated requirements, such that it has an ideal transient response of 0.562 s, a steady-state error achieved in 1.05 s, with 0.201% overshoot, and most importantly the closed-loop system is stable. Full article
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Article
A Basic Study on the Design of Dotted-Art Heterogeneous MPP Sound Absorbers
Acoustics 2022, 4(3), 588-608; https://doi.org/10.3390/acoustics4030037 - 31 Jul 2022
Viewed by 389
Abstract
Recently, dotted-art MPPs have been proposed in which a designed pattern is made with the holes. In such a case, the MPP becomes heterogeneous in general. However, existing theories used for the prediction of the absorption characteristics of MPPs assume homogeneity. Therefore, the [...] Read more.
Recently, dotted-art MPPs have been proposed in which a designed pattern is made with the holes. In such a case, the MPP becomes heterogeneous in general. However, existing theories used for the prediction of the absorption characteristics of MPPs assume homogeneity. Therefore, the elaboration of a method for heterogeneous MPPs needs to be performed. In previous work, the authors proposed a method to predict the absorption characteristics of a heterogeneous MPP by using synthesized impedances of each part with different parameters; this is called the synthetic impedance method (SIM) in the present paper. The SIM can potentially be used for various heterogeneous MPPs; however, its scope of applicability needs to be clarified. Furthermore, in proposing a design concept of dotted-art heterogeneous MPPs, the condition that would make the designed MPPs fall within the scope of the SIM needs to be determined. Therefore, in this study, in order to clarify the scope of the applicability of the SIM, twelve samples are first prepared, and then measured sound absorption characteristics and predicted ones are compared and examined in terms of prediction errors. The results show that there are two conditions that should be met to produce predictable heterogeneous MPPs: (1) holes are distributed over the entire surface of the specimen, and (2) the hole spacing is constant. Considering these conditions, a design concept for a dotted-art heterogeneous MPP is proposed: two types of holes, larger holes for the pattern and smaller holes for the background, should be used to meet the above two conditions. Case studies with nine prototypes show that the SIM can make predictions for dotted-art heterogeneous MPPs fabricated according to the concept described above. Full article
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Article
Locating Sources of Vibration with Harmonics and Pulse Signals in Industrial Machines
Acoustics 2022, 4(3), 574-587; https://doi.org/10.3390/acoustics4030036 - 29 Jul 2022
Viewed by 238
Abstract
This paper is devoted to a new approach to condition monitoring. The main feature is an application of strain gauge analysis for geometrical locating of vibrating defects. Information about the exact geometrical location of a defect, intensity of excitation and its frequency provides [...] Read more.
This paper is devoted to a new approach to condition monitoring. The main feature is an application of strain gauge analysis for geometrical locating of vibrating defects. Information about the exact geometrical location of a defect, intensity of excitation and its frequency provides accurate diagnostics. The research contains theoretical and experimental parts. Three types of defects are analyzed: defects with harmonic parameters, defects with non-harmonic periodical parameters (pulse periodic signal) and defects with non-periodical parameters (pulse non-periodical signal). For the first type, analysis of micro movements in the equipment is used. The others use triangulation; for detecting time lag of signal approaching in each sensor, an analysis of phase spectrum is used. This method can find sources of vibration/defects with pulse-like signals. An electronic board and computer program for implementation of the proposed method are developed. The electronics measure strain gauge data in real time and transmit it to a computer program. Such an approach gives new information for diagnostics and provides new opportunities for effective defect detection and condition monitoring of various machines and equipment. Full article
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Article
Evaluation of the Oscillation Velocity in the Neck of the Helmholtz Resonator in Nonlinear Regimes
Acoustics 2022, 4(3), 564-573; https://doi.org/10.3390/acoustics4030035 - 19 Jul 2022
Viewed by 379
Abstract
Methods for measuring the acoustic characteristics of orifices have been reviewed. Comparison of three methods for evaluating of oscillation velocity in the neck of the Helmholtz resonator are presented. The first method is measurements in an impedance tube with the two-microphone method, the [...] Read more.
Methods for measuring the acoustic characteristics of orifices have been reviewed. Comparison of three methods for evaluating of oscillation velocity in the neck of the Helmholtz resonator are presented. The first method is measurements in an impedance tube with the two-microphone method, the second is based on measuring the sound pressure in the resonator chamber, and the third is based on direct measurements of bias flow with a Pitot tube. The results of measuring the oscillation velocity in the neck of the Helmholtz resonator are presented, and show that these methods are in good agreement only within linear acoustics, but they lead to different results in nonlinear regimes characterized by high sound pressure levels. Full article
(This article belongs to the Special Issue Resonators in Acoustics)
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Article
One-Way Vibration Absorber
Acoustics 2022, 4(3), 554-563; https://doi.org/10.3390/acoustics4030034 - 13 Jul 2022
Viewed by 449
Abstract
A vibration absorber consisting of a one-dimensional waveguide with a reflectionless termination extracts vibrational energy from a structure that is to be damped. An optimum energy dissipation occurs for the so-called power adjustment, i.e, the same level of resistance and the opposite reactance [...] Read more.
A vibration absorber consisting of a one-dimensional waveguide with a reflectionless termination extracts vibrational energy from a structure that is to be damped. An optimum energy dissipation occurs for the so-called power adjustment, i.e, the same level of resistance and the opposite reactance of structure and absorber. The dimensioning of these impedance parameters on the base of the classic second order “two-way” wave equation provides analytical solutions for a few simple waveguide shapes; solutions for all other waveguides are only accessible via numerical finite-element computation. However, the competing first order “one-way” wave equation allows for an analytical conception of both the known broadband vibration absorber and the “Acoustic Black Hole” absorber. For example, for an exponential waveguide, the two-way calculation shows no resistance (and hence no real wave propagation) below a cut-off frequency, while the one-way wave equation predicts absorption in the whole frequency range. Full article
(This article belongs to the Special Issue Elastic Wave Scattering in Heterogeneous Media)
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Article
Audio Denoising Coprocessor Based on RISC-V Custom Instruction Set Extension
Acoustics 2022, 4(3), 538-553; https://doi.org/10.3390/acoustics4030033 - 29 Jun 2022
Viewed by 509
Abstract
As a typical active noise control algorithm, Filtered-x Least Mean Square (FxLMS) is widely used in the field of audio denoising. In this study, an audio denoising coprocessor based on Retrenched Injunction System Computer-V (RISC-V), a custom instruction set extension was designed and [...] Read more.
As a typical active noise control algorithm, Filtered-x Least Mean Square (FxLMS) is widely used in the field of audio denoising. In this study, an audio denoising coprocessor based on Retrenched Injunction System Computer-V (RISC-V), a custom instruction set extension was designed and a software and hardware co-design was adopted; based on the traditional pure hardware implementation, the accelerator optimization design was carried out, and the accelerator was connected to the RISC-V core in the form of coprocessor. Meanwhile, the corresponding custom instructions were designed, the compiling environment was established, and the library function of coprocessor acceleration instructions was established by embedded inline assembly. Finally, the active noise control (ANC) system was built and tested based on Hbird E203-Core, and the test data were collected through an audio analyzer. The results showed that the audio denoising algorithm can be realized by combining a heterogeneous System on Chip (SoC) with a hardware accelerator, and the denoising effect was approximately 8 dB. The number of instructions consumed by testing custom instructions for specific operations was reduced by approximately 60%, and the operation acceleration effect was significant. Full article
(This article belongs to the Special Issue Acoustics, Speech and Signal Processing)
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Article
Numerical Analysis of the Main Wave Propagation Characteristics in a Steel-CFRP Laminate Including Model Order Reduction
Acoustics 2022, 4(3), 517-537; https://doi.org/10.3390/acoustics4030032 - 25 Jun 2022
Viewed by 591
Abstract
Guided ultrasonic waves are suitable for use in the context of structural health monitoring of thin-walled, plate-like structures. Hence, observing the wave propagation in the plates can provide an indication of whether damage has occurred in the structure. In this work, the wave [...] Read more.
Guided ultrasonic waves are suitable for use in the context of structural health monitoring of thin-walled, plate-like structures. Hence, observing the wave propagation in the plates can provide an indication of whether damage has occurred in the structure. In this work, the wave propagation in fiber metal laminate consisting of thin steel foils and layers of carbon fiber-reinforced polymer is studied, focusing on the main propagation characteristics like dispersion diagrams and displacement fields. For this purpose, the dispersion diagrams derived from the analytical framework and numerical simulations are first determined and compared to each other. Next, the displacement fields are computed using the global matrix method for two excitation frequencies. The results derived from the analytical framework is used to validate the numerically determined displacement fields based on a 2D and a 3D modeling approach. For both investigations the results of the analytical treatment and the numerical simulation show good agreement. Furthermore, the displacement field reveals the typical and well-known characteristics of the propagation of guided waves in thin-walled structures. Since the use of full 3D models involves a very high computational cost, this work also successfully investigates the possibility for model order reduction to decrease the computational time and costs of the simulation without the loss of accuracy. Full article
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