Acoustics

Near-seafloor acoustic field characteristics are essential prior knowledge for near-seafloor underwater acoustic engineering. Scholte waves are a crucial component of the near-seafloor acoustic fields. These fields, when considering Scholte waves, are susceptible to seafloor relief. However, open-source bathymetric datasets generally lack sufficient precision. Therefore, acoustic field simulations using open-source data can contain significant errors or even introduce erroneous propagation characteristics. The spectral element method (SEM) is an example of exploring the influence of an inadequate spatial-sampling rate and sea-depth precision on acoustic field simulations. In this article, appropriate methods for topographic processing are presented. The results indicate that the terrain can be corrected using cubic spline interpolation in cases of an inadequate spatial-sampling rate. Where there is insufficient sea-depth precision, this study proposes a terrain processing method. The first step involves sequentially determining the interpolation points for the rising and falling edge, depressions, bulges, and horizontal segments. Then, it adopts cubic spline interpolation. The SEM examples effectively verify this effect. Given the limited research on terrain correction in acoustic field simulations, this study introduces a low-complexity method that can effectively support exploring acoustic fields affected by seafloor terrain. Full article

Enhancing the acoustical quality in learning environments is necessary, especially for hearing aid (HA) users. When in-field evaluations cannot be performed, virtual reality (VR) can be adopted for acoustical quality assessments of existing and new buildings, contributing to the acquisition of subjective impressions in lab settings. To ensure an accurate spatial reproduction of the sound field in VR for HA users, multi-speaker-based systems can be employed to auralize a given environment. However, most systems require a lot of effort due to cost, size, and construction. This work deals with the validation of a VR-system based on a 16-speaker-array synced with a VR headset, arranged to be easily replicated in small non-anechoic spaces and suitable for HA users. Both objective and subjective validations are performed against a real university lecture room of 800 m³ and with 2.3 s of reverberation time at mid-frequencies. Comparisons of binaural and monoaural room acoustic parameters are performed between measurements in the real lecture room and its lab reproduction. To validate the audiovisual experience, 32 normal-hearing subjects were administered the Igroup Presence Questionnaire (IPQ) on the overall sense of perceived presence. The outcomes confirm that the system is a promising and feasible tool to predict the perceived acoustical quality of a room. Full article

After the Council of Trent (1545–1563), the Catholic Church undertook a profound renovation, which affected the spatial configuration of the churches to adjust to the spirit of the Counter-Reformation. The acoustic cultural heritage in these spaces have been studied by different researchers, proposing the joint analysis of 66 Catholic churches from the Baroque period. This study delves into the global characterisation of the sample and establishes correlations between geometric and acoustic parameters. From the acoustic analysis, it is clear that the central floor typology, as opposed to Latin cross churches, presents better average values of musical clarity in relation to their volume. The analysis of the relationship between acoustic and geometric parameters, when the sample of churches is discriminated by typology, allows for the establishment of appropriate correlations for Latin cross floor plans, single naves and basilicas, but not when the analysis is carried out for the entire sample. These correlations are a tool that allows us to evaluate acoustic parameters not measured in situ in Catholic churches of the Baroque period in a predictive way as a function of other measured acoustic or geometric parameters. Full article

This study explores the effects of trees on the acoustic and thermal environment in addition to people’s responses to trees in different contexts. Through field measurements conducted during the summer of 2023 at the campus of the Southwest University of Science and Technology in Mianyang, residents’ neutral points were locally found to be 52.2 dBA (acoustic) and 23.8 °C (thermal). Further, at their maximum, the trees were able to reduce heat stress by 4 °C (indicated by the physiologically equivalent temperature—PET) and the noise level by 10 dBA (indicated by the A-weighted sound pressure—LAeq); this was achieved by trees with a crown diameter of 20 m. Subjective acoustic and thermal responses varied depending on the context. Acoustically, their neutral LAeq values toward the sounds of traffic, teaching, sports, and daily life were 46.9, 52.5, 51.0, and 52.7 dBA, respectively. Thermally, pedestrians’ neutral PET values were 24.2, 26.1, 22.3, and 25.1 °C, respectively, under the same conditions. These phenomena might be a consequence of the effects of sound frequencies. Future urban forestry research should focus on planting for environmental quality improvement. Full article

In the realm of underwater active target detection, the presence of reverberation is an important factor that significantly impacts the efficacy of detection. This article introduces the improved permutation entropy algorithm into the analysis of active underwater acoustic signals. Based on the significant difference between the improved permutation entropy in the frequency domain and the time domain, a frequency-domain-improved permutation entropy detection algorithm is proposed. The performance of this algorithm and the energy detection algorithm are compared and analyzed under the same conditions. The results show that the spectral entropy detector is about 2.7 dB better than the energy detector, realized via active small target signal detection under a reverberation background. At the same time, based on the characteristics of improved permutation entropy changing with the length of processed data, the short-time Fourier transform is integrated into frequency domain entropy detection to obtain distance and velocity information of the target. To validate the proposed methods, comparative analysis experiments were executed utilizing actual experiment data. The outcomes of both simulation and actual experiment data processing demonstrated that the sliding entropy feature detection method for signal spectrum has a small computational complexity and can quickly determine whether there is a target echo in the receive data. The two-dimensional entropy feature detection method for short-time signal spectra was found to effectively mitigate the impact of reverberation intensity and while enhancing the prominence of the target signal, thereby yielding a more robust detection outcome. Full article

Galling wear, a severe form of wear, is a known problem in sheet metal forming. As the wear state is not directly observable in closed tribosystems, such as in industrial sheet metal forming processes, indirect tool wear monitoring techniques for inferring the wear state of the tool from suitable signal characteristics are the subject of intense research. The analysis of acoustic emissions is a promising technique for tool condition monitoring. This research has explored feature selection using t-tests, linear regression models, and cluster analysis of the data. This analysis has been conducted both with and without the inclusion of control variables, friction, and roughness to discriminate between the behavior of the acoustic emissions during different stages of galling wear. Scratch testing at slow sliding speed (1 mm/s) has been used to produce the galling wear between a tool steel indenter and aluminum sheet at 10 N applied load, for which the acoustic emissions were recorded. The bursts of the acoustic emission signal were processed and investigated to observe how the bursts changed with increasing galling damage (increasing material removal and transfer). Novel parameters in the field of galling wear have been identified, and novel models for observing the change in galling wear have been identified, thus furthering the development of acoustic emissions analysis as a non-invasive condition monitoring system, particularly for sheet metal forming processes. Full article

Nonlinear acoustic effects become prominent when acoustic waves propagate through an orifice, particularly at higher pressure amplitudes, potentially generating vortex rings and transferring acoustic energy into the flow. This study develops and validates a predictive theoretical model for acoustic behaviour both within and outside an orifice under linear conditions. Using transfer matrices, the model predicts the external acoustic field, while finite element numerical simulations are employed to validate the theoretical predictions in the linear regime. The experimental setup includes an impedance tube with a plate and orifice, supported by a custom-built system, where a loudspeaker generates acoustic waves. A single microphone is used to measure acoustic particle velocity and characterize the phenomenon, enabling the identification of the onset of nonlinearity. The experimental data show good agreement with the linear theoretical predictions. This work represents the first observation of nonlinear effects in a free-field environment within a semi-anechoic chamber, eliminating reflections from external surfaces, and demonstrates the efficacy of a purely acoustic-based system (speaker and two microphones) for evaluating speaker velocity and the resulting velocity within the orifice. Full article

We are examining the behavior of resonance frequencies and their response to variations of material parameters such as thicknesses, masses, and bulk velocities for certain Rayleigh–Lamb acoustic modes in a multilayered structure. The treatment is based on recent explicit analytic solutions that have allowed us to explore the entire parametric space using dimensionless ratios. This exploration has revealed a complex parametric dependence of the phase velocities and their mass loading response. Specifically, for the fundamental flexural modes in a bilayer, we have shown that both quantities change in a strongly non-monotonic way with thickness, density, or bulk velocity ratios. Even in the regime of thin coating, commonly encountered in acoustic sensing applications, we have found important differences from previously known results, e.g., that response to loading may switch its sign multiple times when the velocity of the deposited material is increased. We have also discovered that the fundamental dilatational modes can be highly effective in stabilizing resonant frequencies against even large variations of the thickness or mass of the exposed layer. This property is demonstrated in an explicit form by the derived expression for the mass coefficient of frequency for an arbitrary number of layers. Full article

This paper presents a decoupled modal reduction method for the steady-state vibration analysis of vibro-acoustic systems characterized by non-classical damping. The proposed approach initially reduces the order of the coupled governing equations of the vibro-acoustic system through the utilization of non-coupled modes, subsequently employing the complex mode superposition technique to address non-classical damping effects. By leveraging non-coupled modes, this method circumvents the need to solve for coupled modes as required in traditional modal reduction techniques, thereby diminishing both computational complexity and cost. Furthermore, the complex mode superposition method facilitates the decoupling of coupled governing equations with non-classical damping, enhancing computational efficiency. Numerical examples validate both the accuracy and effectiveness of this methodology. Given that modal decomposition is independent of frequency, an analysis of computational efficiency across various stages further substantiates that this method offers significant advantages in terms of efficiency for computational challenges encountered over a broad frequency range. Full article

Ultrasound B-scan images are traditionally formed from the envelope of the received radiofrequency echoes, but the image texture is dominated by granular speckle patterns. Longstanding efforts at speckle reduction and deconvolution have been developed to lessen the detrimental aspects of speckle. However, we now propose an alternative approach to estimation (and image rendering) of the underlying fine grain scattering density of tissues based on power law constraints. The key steps are a whitening of the spectrum of the received signal while conforming to the original envelope shape and statistics, followed by a power law filtering in accordance with the known scattering behavior of tissues. This multiple step approach results in a high-spatial-resolution map of scattering density that is constrained by the most important properties of scattering from tissues. Examples from in vivo liver scans are shown to illustrate the change in image properties from this framework. Full article

In this paper, we present a novel approach for text-independent phone-to-audio alignment based on phoneme recognition, representation learning and knowledge transfer. Our method leverages a self-supervised model (Wav2Vec2) fine-tuned for phoneme recognition using a Connectionist Temporal Classification (CTC) loss, a dimension reduction model and a frame-level phoneme classifier trained using forced-alignment labels (using Montreal Forced Aligner) to produce multi-lingual phonetic representations, thus requiring minimal additional training. We evaluate our model using synthetic native data from the TIMIT dataset and the SCRIBE dataset for American and British English, respectively. Our proposed model outperforms the state-of-the-art (charsiu) in statistical metrics and has applications in language learning and speech processing systems. We leave experiments on other languages for future work but the design of the system makes it easily adaptable to other languages. Full article

The Stockholm Public Library was realized in two distinct phases of construction in the 1920s and early 1930s, and remains a well-known work in twentieth-century architecture, with a heritage status today. While previous studies have focused on the library’s architectural design, particularly its lighting, acoustics were also an important aspect of the building’s design and construction. This study marks the first detailed investigation of the library’s architectural acoustics, with a suite of standard measurements performed to assess and characterize the library’s historical room and building acoustics. Reverberation time measurements in the library’s reading rooms yielded results of about 1.5–2 s for frequencies associated with speech. A significantly longer reverberation time of 5–6 s was measured in the library’s central rotunda, confirming a prominent acoustic issue in the library, where appropriate heritage discussions are needed in the future as the library undergoes a major renovation in the coming years. A comparison of the measured airborne and impact sound insulation of the 1920s and 1930s reading room ceilings also yielded interesting results. While the materials in library’s two construction periods are notably different, the airborne sound insulation performance of the 1920s and 1930s floors or ceilings was comparable and in line with contemporary standards. Impact sound insulation results from the 1920s and 1930s floors, however, differed significantly, with the latter displaying a relatively poor performance. Flanking transmission effects related to historical construction details and deviations from archival plans were investigated and discussed. This work emphasizes the practical and academic importance of conducting on-site measurements, and the close mutual development of modern architecture, construction, and architectural acoustics. Full article

This paper describes the use of an electric drive as an acoustic actuator for active noise cancellation (ANC). In the presented application, the idea is to improve the noise, vibration, harshness (NVH) characteristics of passenger cars without using additional active or passive damper systems. Many of the already existing electric drives in cars are equipped with the required hardware components to generate noise and vibration, which can be used as compensation signals in an ANC application. To demonstrate the applicability of the idea, the electrical power steering (EPS) motor is stimulated with a control signal, generated by an adaptive feedforward controller, to reduce harmonic disturbances at the driver’s ears. As it turns out, the EPS system generates higher harmonics of the harmonic compensation signal due to nonlinearities in the acoustic transfer path using a harmonic excitation signal. The higher harmonics impair an improvement in the subjective hearing experience, although the airborne noise level of the harmonic disturbance signal can be clearly reduced at the driver’s ears. Therefore, two methods are presented to reduce the amplitude of the higher harmonics. The first method is to limit the filter weights of the algorithm to reduce the amplitude of the harmonic compensation signal. The filter amplitude limitation also leads to a lower amplitude of the higher harmonics, generated by the permanent magnet synchronous machine (PMSM). The second method uses a parallel structure of adaptive filters to actively reduce the amplitude of the higher harmonics. Finally, the effectiveness of the proposed ANC system is demonstrated in two real driving situations, where in one case a synthetic noise/vibration induced by a shaker on the front axle carrier is considered to be the disturbance, and in the other case, the disturbance is a harmonic vibration generated by the combustion engine. In both cases, the subjective hearing experience of the driver could be clearly improved using the EPS motor as ANC actuator. Full article

In this paper, we utilise optimization methods to determine a frequency-independent phase shift such that two phase-shifted versions of a signal can be summed and the resulting amplitude spectrum is unchanged. A phase difference between two signals is thus defined, which remains constant for all frequencies within a given range. For the intended purpose of this study, we set the frequency range to the audible human hearing range of 16 Hz–20 kHz. We found that a new 3-stage filter method provides a variable phase shifter (i.e., ϕ = 0–360°) without the need for additional amplifiers. In addition, we present a new method that reduces the number of filters necessary, improving both the accuracy and efficiency of current techniques. Full article

This paper aims to conduct an acoustic analysis through noise measurements of a hybrid propulsion system intended for implementation on a drone, from which the main noise sources can be identified for further research on noise reduction techniques. Additionally, the noise was characterized by performing spectral analysis and identifying the tonal components that contribute to the overall noise. The propelling force system consists of a micro-turboshaft coupled with a gearbox connected to an electric generator. The propulsion system consists of a micro-turboshaft coupled with a gearbox connected to an electric generator. The electric current produced by the generator powers an electric ducted fan (EDF). The engineturbo-engine was tested in free-field conditions for noise generation at different speeds, and for this, an array of microphones was installed, positioned polarly around the system and near the intake and exhaust. Consequently, based on the test results, the acoustic directivity was plotted, revealing that the highest noise levels are at the front and rear of the engine. The noise level at a distance of 1.5 m from the turboengine exceeds 90 dBA at all tested speeds. Spectral analyses of both the far-field acoustic signals (measured with a polar microphone array) and the near-field signals (microphones positioned near the intake and exhaust) revealed that the primary contributors to the overall noise are the micromotor’s compressor, specifically the gas dynamic phenomena in the fan (BPF and 2× BPF). Thus, it was determined that at the intake level, the main noise contribution comes from the high-frequency components of the compressor, while at the exhaust level, the noise mainly originates from the combustion chamber, characterized by low-frequency components (up to 2 kHz). The findings from this study have practical applications in the design and development of quieter drone propulsion systems. By identifying and targeting the primary noise sources, engineers can implement effective noise reduction strategies, leading to drones that are less disruptive in urban environments and other noise-sensitive areas. This can enhance the acceptance and deployment of drone technology in various sectors, including logistics, surveillance, and environmental monitoring. Full article

Understanding students’ acoustic evaluation in learning environments is crucial for identifying acoustic issues, improving acoustic conditions, and enhancing academic performance. However, predictive models are not specifically tailored to predict students’ acoustic evaluations, particularly in educational settings. To bridge this gap, the present study conducted a field investigation in a university library, including a measurement and questionnaire survey. Using the collected personal information, room-related parameters, and sound pressure levels as input, six machine learning models (Support Vector Machine–Radial Basis Function (SVM (RBF)), Support Vector Machine–Sigmoid (SVM (Sigmoid)), Gradient Boosting Machine (GBM), Logistic Regression (LR), Random Forest (RF), and Naïve Bayes (NB)) were trained to predict students’ acoustic acceptance/satisfaction. The performance of these models was evaluated using five metrics, allowing for a comparative analysis. The results revealed that the models better predicted acoustic acceptance than acoustic satisfaction. Notably, the RF and GBM models exhibited the highest performance, with accuracies of 0.87 and 0.84, respectively, in predicting acoustic acceptance. Conversely, the SVM models performed poorly and were not recommended for acoustic quality prediction. The findings of this study demonstrated the feasibility of employing machine learning models to predict occupants’ acoustic evaluations, thereby providing valuable insights for future acoustic assessments. Full article

This study delves into the effectiveness of two time integration techniques, namely the adaptive implicit–explicit (imp–exp) and explicit–explicit (exp–exp) methods, which stand as efficient formulations for tackling intricate systems characterized by multiple time scales. The imp–exp technique combines implicit and explicit procedures by employing implicit formulations for faster components and explicit calculations for slower ones, achieving high accuracy and computational efficiency. Conversely, the exp–exp method, a variation of explicit methods with sub-cycling, excels in handling locally stiff systems by employing smaller sub-steps to resolve rapid changes while maintaining stability. For both these approaches, numerical damping may be activated by adaptive time integration parameters, allowing numerical dissipation to be locally applied, if necessary, as a function of the considered discrete model and its computed responses, enabling a highly effective numerical dissipative algorithm. Furthermore, both these techniques stand as very simple and straightforward formulations as they rely solely on single-step displacement–velocity relations, describing truly self-starting procedures, and they stand as entirely automated methodologies, requiring no effort nor expertise from the user. This work provides comparative studies of the adaptive imp–exp and exp–exp approaches to assess their accuracy and efficiency across a wide range of scenarios, with emphasis on geophysical applications characterized by multiscale problems, aiming to establish under which circumstances one approach should be preferred over the other. Full article

Objectives: The purpose of this study was to evaluate and compare the noise levels in current incubator models and a prototype designed to improve acoustic comfort in neonatal incubators. Methods: Tests were carried out on three different models of incubators and a prototype called Neonatal Incubator Acoustic Comfort Class (NICA). The tests measured both internal and external sound pressure levels under laboratory conditions. The noise index has been taken as the A-weighted equivalent continuous sound pressure level (LAeq,T) for a time interval of 1 min. Results: The results obtained show variations between the different models of incubators, although, overall, they are high values (around 56/60 dBA). The results prove that premature newborns under normal conditions of using these incubators are exposed to noise levels above international recommendations. The new incubator design minimizes noise generation and generates noise levels lower than international recommendations. Conclusions: The results obtained from the prototype (NICA+) show the effectiveness of the proposed design in improving acoustic comfort in neonatal incubators. The data show that the noise levels generated by the prototype under normal operating conditions are significantly lower than international recommendations. Full article