Applied Acoustics最新文献

Accurately recognizing sound states in the production line of small electric motors is of great importance for manufacturers to carry out quick repairs and ensure high quality deliveries. Since the number of normal samples is much larger than the number of abnormal samples in practice, resulting in unbalanced data, which poses huge challenges to traditional detection methods. To overcome these difficulties, this study presents a morphological dictionary learning-based sparse classification (MDL-SC) combined with audio data augmentation method for small electric motor state recognition under unbalanced samples. Firstly, audio data augmentation methods such as adding background noise, pitch shifting, time stretching and combined augmentation are investigated for augmenting the number and diversity of samples. Secondly, morphological dictionary learning is proposed for characterizing transient sounds of small electric motors and enhancing the discriminative feature learning capability of the dictionary. Finally, the minimum reconstruction error strategy is relied upon to establish automatic recognition of small electric motor states. Three small motor datasets with unbalanced ratios are established in the experiments to verify the effectiveness of the proposed MDL-SC, which has higher recognition accuracy under unbalanced conditions compared with traditional dictionary learning based sparse classification (DL-SC), k-nearest neighbors, support vector machines and convolutional neural networks. This study can provide some theoretical implications for the later development of online detection of small electric motors or other types of electric motors.

Guided wave ultrasound offers the ability to inspect slender structures from a single transducer location. However, the propagation of guided waves is more complex than the propagation of bulk waves used in conventional ultrasonic testing, and the experimental equipment required is more sophisticated. Typical equipment is expensive and may limit research and industrial application. This paper investigates what can be achieved by using low-cost instrumentation for laboratory measurements. The application is the measurement of guided waves in rock bolts (cylindrical rods) at relatively high frequencies (5 MHz). The dispersive wave propagation requires the use of narrowband excitation signals. The use of a bench-top waveform generator and oscilloscope is compared to the use of a compact and low-cost USB oscilloscope, which includes an arbitrary waveform generator. The introduction of a pre-amplifier mitigated the poorer noise performance of the USB oscilloscope, and comparable results were obtained from the two setups. It is demonstrated that waves propagated 9 m in a free rock bolt, with 10 dB/m attenuation, could be detected and analyzed with a measurement setup costing approximately USD 1110.

Effective analysis of ship underwater acoustic signals requires accurately capturing and distinguishing subtle differences between various types of signal features. This paper introduces a multi-objective feature extraction method based on intrinsic mode decomposition and statistical parameterized cepstral coefficients, aimed at identifying different ship signals. Firstly, the original sample signals are preprocessed and converted into multiple frame signals. Each acoustic signal frame is then decomposed into intrinsic mode functions (IMFs) using variational mode decomposition (VMD). Cepstral coefficients are extracted from each IMF, and the statistical parameter features of each IMF are integrated to enhance the differentiation of various types of ship-radiated noise. These features also form unique “fingerprints” for each ship type, facilitating identity accurate authentication. The performance of the proposed method is evaluated using both K-nearest neighbors (KNN) and support vector machine (SVM) classification models. Experimental results demonstrate that the synergy between the proposed method and SVM significantly outperforms KNN, effectively distinguishing between 12 types of signals, including 11 ship-radiated signals and background noise, achieving an accuracy rate exceeding 89% across 1000 random tests. This method significantly increases the number of classifiable ship targets, demonstrating its considerable potential in distinguishing various underwater acoustic signals.

This paper presents a developed time-domain model for the reflection of spherical waves in an absorber-like surface. The model is made to enable evaluation of measurement methods for assessing the sound absorptive properties of traffic noise barriers in direct sound field, it is thus called the Direct Field Absorption (DFA) model. In this study, the DFA model is applied to the in-situ SOPRA method for quick sound reflection index measurements on road noise barriers. The DFA model results in an impulse response (IR), based on a theoretically derived impedance of a certain absorber. The DFA IR is entered to the SOPRA formula to calculate the reflection index, $R{I}_Q$, of the absorber, which is subsequently compared to the measured $R{I}_Q$ of a wall fitted with the absorber in question. If the results are similar, it is reasonable to assume that the SOPRA measurement results are valid. But if there are any significant differences between the DFA and SOPRA sound reflection indices, possible reasons for them should be examined.

The first results are encouraging, showing that the DFA model can be a valuable tool to evaluate the results of reflection measurements. Furthermore, the DFA model could be useful for estimating the sound absorptive performance at lower frequencies of a noise barrier limited to its spatial extent in width and height, i.e., when it is physically impossible to measure. Further studies are necessary though, since the conclusions of this paper are based on only one kind of absorber.

Acoustic metamaterials manufactured from natural materials can exhibit exotic characteristics that cannot be found in nature materials through the design of geometric structures and resonance effects, and acoustic metamaterials can be used to achieve sub-wavelength acoustic imaging as the acoustic metamaterial lens (AML). Currently, the AMLs based on the Fabry–Pérot (FP) resonance principle are all with single effective length hole, which is directly related to the thickness of its structure. They can only achieve acoustic imaging in a relatively narrow frequency range, and there is no AML which can achieve broadband sub-wavelength acoustic imaging. In this paper, a kind of broadband acoustic metamaterial lens (BAML) is introduced. While maintaining a constant overall thickness of the AML, a design which is composed of the supercell consisting of a cross distribution of different effective length holes is proposed. This design enables the AML formed by these supercells to encompass multiple operating frequencies corresponding to various holes, thereby broadening the frequency range of acoustic imaging. Finite element simulation and imaging experiments were conducted, and results demonstrated that the BAML consisting of multiple effective length holes has the ability to enhance bandwidth of acoustic imaging. Compared to AML with single effective length hole, it offers a broader range of imaging frequencies, this design offers a general method of bandwidth enhancement. BAML has potential application value in ultrasonic imaging, sound absorption, intelligent thermal control and medical diagnosis.

Engine cooling fan noise is a relevant issue for manufacturers. It is well known that both the operating point and testing environment can affect the noise generation mechanisms and, consequently, the measured noise may change. Therefore, these aspects are investigated on a reference industrial fan, for which experimental data exists, using high-fidelity numerical simulations based on the lattice-Boltzmann method. Two operating conditions, namely the free blowing and the maximum efficiency ones, and three testing environments are analyzed: (i) a conventional semi-anechoic room, (ii) an ideal free field environment, and (iii) a testing environment resembling an anechoic aeraulic facility. For cases (i) and (ii) no pressure difference across the fan is imposed, while, for case (iii), a pressure difference across the fan can be imposed. For the latter, the impact of a fully reflective and fully absorbing wall separating the two regions upstream and downstream of the fan is analyzed. At free blowing conditions, the flow over the blades is largely separated. When the blade passes through a blockage region, because of the presence of a honeycomb-like structure needed for structural purposes, it experiences a prominent loading hump. The far-field noise, at a listener located along the axis of rotation, is therefore highly tonal, with a clear peak at the blade passing frequency tone. When the same fan is tested in a free field environment, it is found that there is a difference in the acoustic pressure at higher harmonics of the blade passing frequency due to the presence of flow recirculations in the anechoic room. Placing a thin wall across the fan increases the mass flow rate, for a given rotational speed, which results in a more severe flow separation over the blades and, therefore a higher tone prominence at the blade passing frequency. If the thin wall is modeled as a sound-absorbing wall, there is a drop of the overall sound pressure level of about 2 dBA. When the fan is tested at its maximum efficiency, i.e., nonzero pressure difference across the fan, it is found that the blockage effect is less relevant. The main noise generation mechanism is the back-flow vortex induced by the pressure difference across the fan interacting with the blade tip leading edge.

Narrowband active noise control (NANC) systems can effectively eliminate periodic disturbances at its error sensors, but sometimes the error sensors are inconvenient to be placed at target locations for a long period. Remote microphone technology (RMT) can tackle this problem by moving quiet zones to the target areas. Nevertheless, the RMT-NANC system has significantly higher computational complexity than conventional NANC systems, particularly for multichannel systems. This hinders their implementation in real-time systems with limited computational resources. The additional computational burden stems from multiple convolutions involving the estimated global high-order secondary paths and observation filters when estimating the virtual error signals. To address this problem, a low-complexity parallel local RMT is proposed based on the narrowband filtered-x least mean square (PLRMT-NFxLMS) algorithm in this paper. Using a two-step local modeling approach, multiple local modeling low-order filters for both secondary paths and observation filters are constructed during the training stage. In the subsequent control stage, virtual error signals are estimated in parallel for different frequency components using these filters instead of global modeling filters, thereby alleviating the substantial computational cost arising from convolutions between lengthy vectors. Moreover, a filtered-error structure, termed the PLRMT-NFeLMS algorithm, is introduced in the proposed algorithm to further reduce the computational complexity. A comprehensive analysis of computational complexity is provided to demonstrate the superiority of the two proposed algorithms. Extensive simulations and real-time experiments were conducted to validate the feasibility and practicability of these proposed methods.

Open-plan offices are common in the tertiary sector, yet occupants often complain about noise, particularly from co-worker conversations. This issue can differently affect normal hearing people and those with presbycusis. This study therefore examines the impact of mild hearing loss (the onset of presbycusis) on performance in open-plan offices, focusing on the effect of irrelevant speech. An analysis of the decrease in performance on serial recall task as a function of speech-to-noise ratio was carried out with young, normal-hearing subjects under two auditory conditions: with and without a hearing loss simulator, as well as with hearing-impaired elderly subjects. Participants were exposed to five speech-to-noise conditions and silence. Subjective intelligibility was also measured. The results showed a minor, non-significant difference in decrease of performance between normal-hearing and hearing-impaired participants. The hearing loss simulator produced results comparable to those of the older group, validating its efficacy.

While the volume of air traffic continues to rise, there are concurrent shifts in aircraft fleets and advancements in technology aimed at enhancing aircraft efficiency, sustainability, and minimizing noise footprints. These simultaneous developments contribute to the complexity of forecasting airport noise associated with future air traffic. This study presents various scenarios of technology adoption for the years 2030 and 2040 at a European airport, namely Dublin Airport, encompassing changes in fleet distribution. Aircraft are classified into different generations to reflect their technological advancements. Projections indicate an anticipated growth in the wide body fleet at Dublin Airport from 2023 to 2040, while the shares of narrow body, regional jet, and turboprop fleets are expected to decline. Each forecast year showcases three distinct technology uptake scenarios, with noise contour lines at $L_{den}$ =

Topological waveguide states in condensed matter physics systems show significant potential for applications, such as high-capacity information communication and energy regulation of complex physical fields owing to the phenomenon of large-area wave transport. The lack of tunability design for sonic crystals makes it difficult to change the structure and function of existing topological waveguide states transport structures after sample fabrication, especially in terms of the freedom of width and tunability on the transport path, and thus hinders further engineering applications of topological waveguide states. To address these challenges, the design method based on nested difference theory to achieve topological phase transitions in sonic crystals is proposed, which in turn leads to the construction of fully reconfigurable acoustic topological waveguide states and explores their potential applications. Firstly, a reconfigurable nested scatterer structure is designed, which is split into a snowflake-like core and an outer shell. This scatterer is demonstrated to be able to break the spatial inversion symmetry and realize the transition of sonic crystals between three valley topological phases by only changing the nesting mode of the nested outer shell. Based on reconfigurable sonic crystals, the fully reconfigurable acoustic topological valley-locked waveguide states are constructed. Simulations and experiments further confirm that such waveguides are characterized by high-capacity transport, acoustic focusing, and robust transport over large inflection corners. In addition, we explore the potential applications of reconfigurable waveguides for acoustic channels and acoustic logic gates. The precise distribution of the waveguide states energy is achieved in the acoustic channel by adjusting the rotation angle of the channel. Moreover, the logical computational relationships of the transport structure of the topological waveguide states are determined using different acoustic excitation modes. The excellent transport properties generated by fully reconfigurable acoustic topological waveguide states will have potential applications in both field enhancement and energy harvesting. This provides the possibility for the development of novel acoustic devices capable of modulating waves in complex physical scenarios.