An underwater acoustic metamaterial coating—comprising a slit, helical water cavity, stepped metal plate, and rubber—is proposed for broadband low-frequency sound absorption. Finite-element method analysis reveals that the slit facilitates acoustic wave entry, exciting local resonances and promoting mode conversion to enhance shear energy dissipation. The helical water cavity reduces structural stiffness, facilitating low-frequency resonance and amplifying the mode conversion effect within the rubber damping layer. Simultaneously, the stepped metal plate optimizes impedance matching and enhances frictional energy consumption at the rubber-metal interface. This plate promotes wave-mode conversion and thereby enhances viscoelastic energy dissipation in the surrounding rubber, contributing to improved high-frequency absorption performance. Structural optimization using a deep neural network and particle swarm optimization demonstrates that effective absorption is achievable at 80 Hz with a 0.5 loss factor. The average absorption coefficient reaches 0.94 across the 10 Hz to 10 kHz range. This study offers a robust approach for designing high-performance underwater acoustic coating structures through synergistic multi-structural mechanisms.
Winding construction of violin strings may influence subjective impressions of timbre even with comparable constituent materials and the same nominal pitch. However, the effects of winding-related structural differences on these impressions of timbre remain unclear. In this study, a subjective evaluation of timbre was conducted using two commercially available violin A-strings, designated as String-1 and String-2. Material characterization indicated that both strings had comparable constituent materials, including polyether ether ketone (PEEK)-like synthetic multifilament cores and two layers of Al-5Mg alloy flat-wire windings. The two strings differed in the winding configuration, helical geometry, total linear density, and component-wise mass distribution. The participants' musical experience was measured using the Goldsmiths Musical Sophistication Index. Subjective evaluations showed that String-2 was rated as more massive than String-1 regardless of musical experience. These results suggest that even among strings with comparable constituent materials, the contact conditions and mechanical properties associated with differences in the winding structure of strings may influence the subjective impressions of the timbre of a violin, especially in terms of massiveness.
Source directivity is an integral part of room acoustic simulation, affecting not only the intensity and frequency balance of the direct sound depending on source orientation, but also room-acoustic parameters, such as the direct-to-reverberant energy ratio. While the directivity of a loudspeaker is time-invariant, the directivity of other sound sources, such as the human voice, varies over time. Whether these time-dependent directivity changes must be fully considered for a plausible binaural rendering of speech remains an open question, requiring rendering solutions that can reproduce time-varying directivity. This work provides three key contributions. First, it introduces a feature for liveRAZR—a real-time, six-degrees-of-freedom room acoustic simulator—to render sound sources with time-varying directivity from Ambisonics input. Second, it proposes a method to generate speech material with time-varying directivity from monaural recordings using phoneme-dependent directivity filters. Third, it presents a listening experiment comparing fluent speech with multiple time-varying and static directivities regarding transfer-plausibility and perceptual qualities. Time-varying directivity did not significantly improve transfer-plausibility or liveliness and source-orientation clarity ratings compared to static, measurement-based directivity in the tested scenario. However, analytically obtained spherical head model directivity reduced these perceptual qualities, indicating that measurement-based voice directivity is preferable for demanding immersive extended reality applications.
Bimorph ultrasonic transducers are widely used in ultrasonic atomizers, piezoelectric buzzers, and wearable ultrasound monitoring systems. Advancing technologies demand that these devices exhibit higher vibration displacement, improved acoustic impedance matching, and enhanced overall radiation efficiency. To address these demands, an acoustic-black-hole-enabled bimorph ultrasonic transducer (ABH-BUT) is proposed, wherein a power-law thickness taper reduces local flexural wave velocity and focuses vibrational energy. This mechanism significantly enhances vibration displacement, the electromechanical coupling coefficient, and acoustic radiation efficiency. The transfer matrix method calculates the ABH-BUT's resonance frequencies, while a conventional stepped disk bimorph ultrasonic transducer serves as a reference. Vibration modes, electrical impedance, and near-field acoustic pressure distributions of both transducers are evaluated using finite element analysis and experimental measurements. Comparative results demonstrate that, under identical vibration modes, the acoustic-black-hole profile effectively improves the electromechanical coupling coefficient and acoustic radiation efficiency. This study provides a novel design framework for advanced trilaminar piezoelectric flexural ultrasonic transducers.
It is commonly accepted that ocean noise from marine traffic is mostly confined to low frequencies. This article empirically demonstrates that large (length >120 m) vessels equipped with electrically driven “Azipod” propulsion systems generate loud mid-frequency (3–15 kHz) tonal and mid-frequency narrowband and tonal sounds (MF-NTS). The study focuses on six Azipod-equipped tourist vessels operating in the Antarctic Peninsula; it relies on opportunistic passive acoustic measurements collected there during the 2024–2025 Austral summer. The six vessels considered here all radiate MF-NTS, which are interpreted as the acoustic signature of the Azipod (data below 2 kHz are not considered in this study). This signature dominates the ambient sound for several kilometers around the vessel, and can be acoustically detected at ranges exceeding 5 km.
