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Micro-perforated panel (MPP) sound absorbers are usually made of a thin panel and have narrow absorption bandwidth. This drawback causes the application of MPP to be limited. In this paper, the possibility of realizing wider absorption bandwidth MPP with sufficient structural strength is investigated. For this, multi-MPP (resonator) arranged in parallel to form an inhomogeneous perforation MPP is introduced to widen the absorption bandwidth. The thickness of MPP must be 1.5 times higher than perforation diameter or more in order to have appropriate strength. The characteristics of corresponding absorption coefficients are studied parametrically using theoretical models as thick panels can reduce the MPP's performance. It is found that the absorption bandwidth of thicker panels with inhomogeneous perforation approach can be at least twice times of classical MPP. The problem of reduced peak absorption coefficient in a thick panel can be avoided by keeping the acoustic resistance value around 1± 0.5 Rayls. Compared with homogeneous MPP, inter-resonator interaction exists in the inhomogeneous perforation thick MPP that causes the overall absorption to become higher due to the increasing of the acoustic resistance as well as the shifting of peak resonance following residual acoustic reactance. The measurement results confirm all of the characteristics.

Vibration characteristic of a structure is unique due to mainly its material and geometrical properties. Hence, a variation imposed on either one or both the parameters can cause a different vibration response. This fact can be used to develop an alternative approach in testing the purity of gold particularly when the gold bar (Au) is no longer isotropic material due to the presence of other metals. A fake gold bar is commonly made by filling inside of the bar with other material for example Tungsten or Wolfram (W) which has density nearly the same as gold but its price is cheaper. This can not be identified easily without performing a testing as it has the same appearance as the pure gold bar. The purpose of this research is to develop gold bar purity testing method by observing natural frequencies and Frequency Response Function (FRF) of vibration response. For early development, the vibration response of gold bar and fake one are investigated. For this, numerical calculations are used rather than measurements. The results show that the natural frequencies and frequency response function (FRF) are useful information to detect whether a metal bar is pure gold bar or containing other metals. Higher natural frequency orders are required when the volume of tungsten in the bar reduces as the gap of lower order natural frequencies of pure gold bar and the fake become small. Moreover, the excitation point at corner and central of area of gold bar are suggested to be chosen during purity testing

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This paper discusses in situ measurements of the speech intelligibility of classrooms in a private university in Jakarta. Seven classrooms with and without stepping students seating area are evaluated. Impulse Response method is used to measure the acoustics condition of the classrooms. Their speech intelligibility level is determined using the reverberation times (T30) and definition (D50) conditions. Results show that all the classrooms have a low frequency reverberation problem. These problems lead to lower their Speech Intelligibility Level. It is also shown that stepping student seating area does not increase speech intelligibility level.