Realizing high-efficiency low frequency sound absorption of underwater meta-structures by acoustic siphon effect

The underwater acoustic siphon effect is proposed in this work, which aims to reveal the basic physical mechanism of high-efficiency sound absorption in meta-structures composed of multiple detuned units. Furthermore, the influence of the area ratio on the underwater acoustic siphon effect is then investigated by finite element simulation (FES) and theoretical calculation. On this basis, a meta-structure with the maximum absorption coefficient of almost 100% and average absorption coefficient of 80% at 600–1400 Hz is achieved. The underwater acoustic siphon effect could provide a better understanding of high-efficiency sound absorption and offer a new perspective in controlling underwater noises.

The Optical Properties of Aerosols at the Summit of Mount Tai in May and June and the Retrieval of the Complex Refractive Index

To study the optical properties of background atmospheric aerosols in East China, we carried out observations of the physical, chemical and optical properties of atmospheric aerosols at the summit of Mount Tai (Mt. Tai, 1533.7 m above sea level) from 13 May to 25 June 2017. The results show that the average scattering coefficient ( σ sca , 550 ) at 550 nm of the aerosols at the summit of Mt. Tai is 40.3 Mm−1, and the average absorption coefficient ( σ abs , 550 ) at 550 nm is 16.0 Mm−1. The complex refractive index of aerosols is a key parameter for aerosol retrieval and modeling. There are few studies on the equivalent complex refractive index of aerosol in the Taishan area. We calculated the aerosol equivalent complex refractive index using the observed aerosol scattering coefficients, absorption coefficients and particle size distribution data, providing more data support for future modeling in this region. The real part (n) of the complex refractive index at 550 nm of aerosol ranges from 1.31 to 1.98 (mostly under 1.50), with an average value of 1.38, while the imaginary part (k) ranges from 0.014 to 0.251 (less than 0.10 for over 95% samples), with an average value of 0.040. The analysis of the n and k of the aerosol average complex refractive index shows that the scattering properties of the aerosols at the summit of Mt. Tai are relatively weak and the absorption properties are relatively strong when compared with those of other kinds of aerosols. The k of the aerosol complex refractive index at the summit of Mt. Tai has strong correlations with the wind speed, temperature, as revealed by the correlation analysis.

Analysis for Combustion Instability and Stabilization Characteristics in a Swirled Premixed Combustor With a Slotted Plate

In this study, new methodologies are introduced to analyze combustion instability in a lab-scale swirled combustor. First, with the help of radial basis function neural network (RBFNN), the flame describing function (FDF) is effectively modeled from a limited number of experimental data. This neural-network-based FDF method is able to generate more refined FDF data in an extended range. In addition, instead of a perforated plate with round holes, a slotted plate is utilized as a stabilization device. In this approach, the acoustic impedance of a slotted plate is modeled by the Dowling approach, and the dimensions of a slotted plate are optimized by simulated annealing (SA) algorithm to get the highest average absorption coefficient in a given frequency range. The present RBFNN-based FDF approach yields the reasonably good agreements with the measurements in terms of the limit-cycle velocity perturbation ratio and resonant frequency. It is also found that a slotted plate optimized by SA algorithm is quite effective to attenuate combustion instability. Numerical results obtained in this study confirm that these new methodologies are quite reliable and widely applicable for the analysis of combustion instability encountered in practical combustion systems.

Utilizing Hollow-Structured Bamboo as Natural Sound Absorber

Studies to find alternative low environmental-impact materials for acoustic absorbers are still progressing, particularly those originated from natural materials. However, most of the established works are mainly focused on the fibrous-type absorbers. Discussion on the non-fibrous-type absorbers is still lacking and this therefore becomes the objective of this paper. Use of bamboo by utilizing its hollow structure to absorb sound energy is discussed here. The normal incidence absorption coefficient was measured based on the length and diameter of the bamboo, as well as different arrangement of the bamboo structure subjected to the incidence sound, namely, axial, transverse, and crossed-transverse arrangements. The trend of absorption coefficient appears in peaks and dips at equally spacing frequencies. For all arrangements the peak of absorption can reach above 0.8. Introducing an air gap behind the bamboo shifts the peak absorption to lower frequency. Covering the front surface of the absorber improves the sound absorption coefficient for axial arrangement by widening the frequency range of absorption also towards lower frequency range. The transverse arrangement is found to have average absorption coefficient peaks of 0.7 above 1.5 kHz. By arranging the bamboo structure with crossed-transverse arrangement, the suppressed absorption peaks in normal transverse arrangement can be recovered.

Preliminary Study on Bamboo as Sound Absorber

Synthetic acoustic materials are known for their poisonous chemical substance to the environment and also the particles which are harmful to human health. Research is now directed towards finding an alternative acoustic absorber made from natural materials. This paper presents the utilization of bamboo, a natural material having hollow structure to act as sound absorber. In an impedance tube test, the hollow path is arranged to face the sound incidence. The result reveals that bamboo having length of 2 cm has average absorption coefficient of 0.95 at frequency above 3 kHz. Performance at lower frequencies can be controlled by adding the air gap behind the system. Introduction of microholes along the body shows no significant effect to increase the sound absorption.

Acoustic Performance Analysis of Bionic Coupling Multi-Layer Structure

The interest of this paper lies in the proposition of using bionic method to develop a new sound absorption structure. Inspired by the coupling absorption structure of the skin and feather of a typical silent flying bird – owl, a bionic coupling multi-layer structure model is developed, which is composed of a micro-silt plate, porous fibrous material and a flexible micro-perforated membrane backed with airspace. The impedance transfer method is applied to calculate the absorption coefficients and analyze the influences of different parameters of each layer on absorption coefficients of this model. Based on numerical simulations, the effectiveness of this proposed model is tested. The average absorption coefficient reaches 0.85 within the frequency range from 200 Hz to 2000 Hz. The significant improvement of absorption coefficients can be mainly due to the Helmholtz effects of the micro-silt plate and flexible micro-perforated membrane, and the combination with porous materials can lead to even better absorption performance in broadband.

Preparation and Performance of Porous Silica of Sound-Absorbing Coating

Using the poly-acrylic acid ester modified porous silica as fillers, the E-44 epoxy resin as matrix, ethylene diamine as curing agent prepared the sound absorption coating, and the sound absorption properties of and wear resistance were tested. The results showed that: the coating wear resistance increased with the addition of filler increasing，and then decreased. The wear mass of 60%filler coating is the lowest. The absorption coefficient increases with the filler increasing, the average absorption coefficient can reach 0.22 of 1000Hz frequency sound.

Mathematical Modeling of In-Cylinder Radiation

This paper develops a multi-zone & multi-dimensional in-cylinder radiant model for internal-combustion engine. The model is based on the radiant temperature formula and takes the quality of total carbon particles and oxidized carbon particles as weight.The model offers analysis of the tendency of average absorption coefficient in various zones under different working conditions. The result agrees with basic characters about radiation.