Expanded perlite is a porous, lightweight, fire resistant and moisture retaining material with sound and thermal insulation properties. In this research, acoustical behaviour of plates made of expanded perlite was studied experimentally. Since these plates are used for sound absorption the acoustical parameter selected for this study is "sound absorption coefficient". Preliminary experiments indicated that moisture reduced the sound absorption coefficient on plates and there is not much significant difference between the dry and 50% humid conditions. However, there is a significant difference in acoustical properties for the 50% - 95% humid conditions. Thus, this interval was studied in detail. A number of expanded perlite plates having different mixtures were prepared and tested. It was observed that coating of expanded perlite particles with sodium silicate increased moisture resistance, and that the addition of mineral fibre into the mixtures increased the strength and sound absorption coefficient of the plates.