In this paper sound absorption characteristics of three kinds of natural fiber composite is evaluated. The fibers are made from coconut coir, oil palm fiber from empty fruit bunches, and pineapple leaf. The sound absorption coefficient is evaluated using a specimen thickness of 10 mm and 20 mm and two variants of fiber density. A simple impedance tube with two microphones is used in the measurement. The sound absorption coefficient is calculated by the transfer function of the two microphones within 200 Hz to 3000 Hz frequency range. It is revealed that three types of fibers can reduce reflected sound in relatively higher frequency. Denser and thicker of a fibrous material increases the sound absorption of the material and shifts the peak of the absorption coefficient into a lower frequency. Furthermore, pineapple leaf fiber has the highest sound absorption coefficient among others type of fibers. Smaller and uniform fiber diameter has a significant role to absorb sound energy in pineapple fiber. Whereas, coconut coir and palm empty fruit bunches fiber have larger, various and random fiber diameters, that could make the sound absorption coefficient become lower, especially within the frequency range of measurement.

Friction Welding (FRW) is a solid-state welding method. This technology also permits the connecting of dissimilar and similar materials while consuming less electricity than conventional electric welding. Friction welding is frequently used to join a variety of components because it generates high-quality joints and is capable of joining a wide range of materials and their complexity. This research examined the friction welding of stainless steel and pure zinc. The investigation concentrated on the welding parameters, specifically the effect of friction time and provision of preheating on parameters with high joint strength, as well as the mechanical properties, microstructure, and characterization of the joint material. The results of the experimental research indicated that the welding settings had a significant impact on the friction welding process. The tensile strength increased as a result of the reduced friction duration during the welding process, as demonstrated by the experimental findings. The longer the friction period, the more an oxide layer will form on the surface of the metal, preventing the diffusion process and impacting the production of the intermetallic phase for the joint’s strength.

One of sound-absorbent developed nowadays, micro-perforated panel (MPP), effectively reduce reflected acoustic energy after approach the panel based on a Helmholtz-type resonance absorber. Some MPPs modifications have been applied to increase its performance, like the application of double-leaf MPP. Furthermore, the implementation of sound absorption materials made of natural fibers recently has invited more attention to some researcher because it is biodegradable, environmentally friendly, and economical. In this paper, a sandwich panel made of double MPP and natural fiber as sound absorptive material is investigated. The MPPs are made of a transparent acrylic board 1.5 mm thickness, 0.8 mm hole’s diameter with five mm hole distance in front panel, and five mm in the backing panel. Two different types of natural fibers panel; pineapple leaf fiber and oil palm fiber from empty fruit bunches are used in this sandwich. a low-cost impedance tube with double channel microphone is used to measure the sound absorption coefficients. It is found that the sandwich model changes the sound absorption characteristics of MPP by shifting the maximum absorption coefficient into the lower frequency and raising the absorption coefficient of the double MPP following its natural fiber panel absorption coefficient. The total impedance of sandwich gives the effect of decreasing the frequency peak of the absorption, and higher resistance of the fiber panel will increase the absorption coefficient and broaden the frequency peak.

Cellulose is a natural fiber potentially used as a sound absorber material due to its excellent properties, biodegradability, and lower environmental impact than synthetic materials and can be sourced from various plant-based materials, such as wood, Cotton, and Hemp. Which effectively traps and absorbs sound waves. The fibers dissipate the energy of sound waves as they pass through the material and absorb sound energy across a wide range of frequencies. Cellulose can be installed as loose-fill insulation, rigid panel form, composite with other matrix material, or foam. It's important to note that the specific characteristics and performance of cellulose-based sound absorbers can vary depending on the manufacturing process, fiber treatment, and the overall design of the sound-absorbing material. The size of cellulose fibers used in sound absorption can impact their effectiveness. Microfiber and nanofiber cellulose show different sound absorption characteristics. Microfiber has a good absorption coefficient at lower frequencies, and nanofiber cellulose performs better at higher frequency ranges. This paper involves a shortreview study of experimental methods and parameters used to regulate cellulose's sound absorption performance, which seems to be a potential alternative as an acoustic absorber, thereby reducing sustainability concerns related to synthetic materials in acoustics applications.

Squeal noise is generated by an unstable friction-induced vibration in a mechanical structure with friction load. Nonlinear mechanisms like sprag-slip, stick-slip, and negative frictions damping are believed in contributing to generate this kind of noise. However, the prediction of its occurrence still counts on the analysis of complex-linear eigenvalue, which may underpredict the number of unstable vibration modes. The structure also is found to seem to generate squeal noise randomly. In this paper, nonlinear analysis of a squeal noise is investigated. The study is conducted numerically by a simple two-degree of freedom model and an experimental observation using a circular and slider plate with a friction contact interface. The friction force is modeled as a function cubic nonlinear contact stiffness and nonlinear negative velocity function of friction coefficient. It is found that mode coupling instability will occur if the normal contact stiffness and friction coefficient exceed the bifurcation point to generate a couple-complex conjugate eigenvalue and eigenvector. However, when the system is stated linearly stable, instability still can appear because of increasing the nonlinear contact stiffness and coefficient of friction. The instability is affected significantly by relative velocity and pressing force. Both parameters dynamically change depending on the vibration response of the structure. Furthermore, it is also found the stick-slip phenomenon interacted with mode coupling instability to generate squeal noise. It contributes to supply energy to increase the response caused by instability of mode coupling.