Yunfeng Qiu,Jingjing Liu,Yue Lu,Rui Zhang,Wenwu Cao,PingAn Hu
Backing materials play important role in enhancing the acoustic performance of an ultrasonic transducer. Most backing materials prepared by conventional methods failed to show both high acoustic impedance and attenuation, which however determine the bandwidth and axial resolution of acoustic transducer, respectively. In the present work, taking advantage of the structural feature of 3D graphene foam as a confined space for dense packing of tungsten spheres with the assistance of centrifugal force, the desired structural requirement for high impedance is obtained. Meanwhile, superior thermal conductivity of graphene contributes to the acoustic attenuation via the conversion of acoustic waves to thermal energy. The tight contact between tungstate spheres, epoxy matrix, or graphene makes the acoustic wave depleted easily for the absence of air barrier. The as-prepared 3DG/W80 wt %/epoxy film in 1 mm, prepared using ∼41 μm W spheres in diameter, not only displays acoustic impedance of 13.05 ± 0.11 MRayl but also illustrates acoustic attenuation of 110.15 ± 1.23 dB/cm MHz. Additionally, the composite film exhibits a high acoustic absorption coefficient, which is 94.4% at 1 MHz and 100% at 3 MHz, respectively. Present composite film outperforms most of the reported backing materials consisting of metal fillers/polymer blending in terms of the acoustic impedance and attenuation.
Objectives: To gain insight into the current practice of hearing protection of Chinese workers and the value of hearing protection device (HPD) fit testing. Design: HPD fit testing was conducted on workers (N = 774) in a petrochemical plant in Eastern China who were on duty during the period of this study. The 3M E-A-Rfit Dual-Ear Validation System was used to measure the personal attenuation ratings (PARs) of a premolded earplug used at the work site. Repeated fit testing was conducted at approximately 6- or 12-month intervals. Wilcoxon signed rank tests were conducted to analyze the pairwise differences between the baseline, postintervention, and follow-up visit PARs, and Mann–Whitney tests were used to compare the PARs obtained by two follow-up groups. Results: The median PAR baseline was 11 dB; significant improvement was shown on the postintervention PARs ( p < 0. 001). No significant difference was shown between PARs obtained during the 6- and 12-month follow-up visits ( p > 0.05). Comparing PARs of follow-up visits with the baseline PAR demonstrated a significant improvement ( p < 0. 001), but revealed a significant decline ( p < 0. 001) comparing with the postintervention PARs. Conclusions: HPD fit testing showed value added as to verify the sufficiency of attenuation. The training along with fit testing showed contributions to improve PARs, maintained effectiveness over time, and assisted in HPD selection. Follow-up is believed to be important to ensure that the HPDs are continually used correctly. There was no significant difference in the sustained effectiveness of the follow-up when observe 6- and 12-month subsequent to intervention.
Effectively attenuating highly penetrative low-frequency noise (LFN, f < 500 Hz) presents a significant challenge. Traditional acoustic insulation materials are circumscribed by the mass law, presenting an inherent conflict between lightweight design and acoustic insulation. Despite the promising potential of PUA aerogels in acoustic insulation, attributed to their lightweight and high damping properties, their low-density results in inadequate areal density. Consequently, it remains challenging to effectively suppress LFN through mass inertia alone. Moreover, lightweight design often reduces mechanical properties, creating a conflict between lightweight, high strength, and low-frequency acoustic insulation. Inspired by the periodic structures of metamaterials, this study fabricates a PUA aerogel with an ordered metamaterial-like structure (density: 0.126 g/cm 3 ). Through self-assembly induced by sodium dodecyl sulfate (SDS), an ordered porous cavity and fibrous topological skeleton were formed. This structure creates an efficient interfacial reflection network that provides dynamic acoustic impedance for LFN attenuation. The average STL is at least 42.6 dB in the 45–500 Hz range and 39.6 dB in the broader 45–6300 Hz range, 16.35 times higher than that of commercial melamine foam of the same 2 cm thickness. Its weight is just 1/770 that of lightweight concrete bricks with equivalent noise reduction performance, and its thickness is only 2/25 of theirs. At 145 °C, the aerogel maintains a temperature difference of approximately 105.05 °C between its surfaces. The compressive strength of the modified aerogels is 2.76 times that of unmodified PUA aerogels. The simple, cost-effective fabrication process and the lightweight, high-strength, and outstanding low-frequency acoustic insulation properties make metamaterial-like PUA aerogels promising for noise reduction in residential, transportation, and industrial applications.
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