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ABSTRACT: The sound waves propagate radially and spherically in all direction in absence of obstacles. This spherical propagation can be governed by ambient conditions. In an enclosure the sound waves are reflected and absorbed by its surfaces. The sound propagation of these sound waves is depends on properties, orientation and configuration of surfaces. To improve the acoustical performance of enclosure, sound absorbing materials are used. The sound-absorbing are also applied on the surfaces for reduction of the noise radiated by working machines in the industrial halls. The absorption of sound in the air is caused by a few factors like the viscosity, the thermal conduction with the relaxation processes explained by the theory of irreversible thermodynamics. This factor produced the effect on the increase of the absorption coefficient with a square of frequency and introduces some deviations. The absorption of the acoustic wave depends on the frequency, on the atmospheric pressure and on the relative humidity. The indoor walls of the enclosure should be covered with sound-absorbing material in order to reduce increasing of the sound level inside the enclosure. When sound wave incident on absorbing material it to vibrate the fibers or particle of the absorbing material in unison of incident frequency. This vibration produces thermal effect due to the friction and sound absorption is accomplished by way of energy to heat conversion. The more fibrous a material produces better absorbing effect. The sound absorbing characteristics of acoustical materials vary significantly depends on the frequency. In general low frequency sounds are very difficult to absorb because of their long wavelength. On the other hand high frequency sounds are easily absorbed by these acoustic materials. The material thickness has the great impact on the sound absorbing properties and the composition of the acoustical material determines the material’s acoustical performance. The low frequency performance can be improved by introducing an air space behind an acoustical ceiling or wall panel. In this work attempt has been made to determine the thickness ie layers of absorbing material in the selected frequency range. The acoustic material consists of pure jute and mixed with coconut waste are used to investigate the acoustic performance of small cubical enclosure. The sound absorption coefficients in ultrasonic frequency range should be determined because the manufacturers of these materials do not provide their data for ultrasonic frequencies. The results are discussed with the variation in the intensities of characteristic frequencies of cubical enclosure. Key words: sound waves, enclosure, absorbing materials, noise, acoustical performance and characteristic frequencies REFERENCES  PIERCE, A.D, Acoustics, an introduction to its physical principles and applications, New York, Ac. Soc.Am, 1989.  KUTTRUFF, H., Room Acoustics, New York, Elsevier, 1991.  CEN, European Committee for Standardization, Building Acoustics -Estimation of acoustic performance of buildings from the performance of elements -Part 6: Sound absorption inenclosed spaces, EN 12354-6, 2004.  NILSSON, E, Decay process in rooms with non-diffuse sound fields,Report TVBA-1004, Lund Insitute of Technology, 1992. NIJS, L, P. VERSTEEG, M. VAN DER VOORDEN, The combination of absorbing materials and room shapes toreduce noise levels, Proc. International Congress on Acoustics, Kyoto, 2004  KNUDSEN V.O. (1932), Architectural Acoustics, New York. . SADOWSKI J. (1976), Architectural Acoustics [in Polish], PWN Warszawa–Poznan. . SMAGOWSKA B., MIKULSKI W. (2008), Noise at ultrasonic drill systems – evaluation of health risk [in Polish], Bezpieczenstwo Pracy (Work Safeness), 10, 18–22.
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