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In an anechoic chamber, the flat multi-layer sound absorber is cheaper, easier to install and less complicated in manufacturing than conventional wedge and pyramid absorbers. Therefore, design of the optimum flat multi-layer absorber which has minimum thickness is desirable. In this paper, the genetic algorithm has been employed as an effective optimization tool to determine flat triple layer porous absorber. To obtain a broader range of porous materials, combination of foam and fiber types is used. Theoretical and numerical method (finite element method specifically COMSOL Multi-physics version 4.4) have been used to investigate on the operation of sound absorption correspond to the multi-layer structure. In the first step, mathematical model is verified and finite element method, theoretical and experimental results are compared together for two different samples of structures which show appropriate matching. Furthermore to verify the operation of programmed genetic algorithm, the results obtained from the optimization of flat triple layer porous absorber are compared with others that show accuracy and efficiency of this method. The optimization results indicate that a flat triple layer porous structure can achieve results comparable with quality wedge type structure with overall thickness slightly smaller than a fifth of a wavelength at 80 Hz cut-off frequency.

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Investigation of frequency variations of acoustic impedance can play an important role in identification and optimization of a musical instrument. For a simple tube, the input acoustic impedance can be calculated by analytical methods; for complex geometry objects like wind instrument, however, it cannot be simply computed. Therefore, the impedance is measured for wind instruments. This paper is report of first experiment for measuring the input acoustic impedance of Ney (an Iranian woodwind instrument). For this purpose, a pulse reflectometer device was made. For assurance of correct operation of the reflectometer, in the first step, the input acoustic impedance of a three sections step tube was measured and the results were compared with calculated results using well known formula. the acoustic impedances of a Do-ney for various fingering in six case (from closed all holes to opened all holes) were measured. The results show that, contrary to what seen for flout, the frequencies of minima of the impedance curves have some discrepancies with the frequencies of corresponding playable notes. This may relate to the role of mouth of instrument player in producing tones of ney.