Indoor Noise Reduction Index with an open window (Part II) C. Buratti * Department of Industrial Engineering, University of Perugia, Via G. Duranti 67, 06125 Perugia, Italy Received 23 August 2004; received in revised form 27 July 2005; accepted 27 July 2005 Available online 19 September 2005 Abstract An index to evaluate indoor noise level reduction with an open window Noise Reduction Index (NRI) was proposed [Buratti C. Indoor Noise Reduction Index with open window. Appl Acoust 2002;63(4):431–51]. The reduction was due to the installation of a false ceiling in the room, thus reducing the contribution of the reverberant field. Experimental data related to two different kinds of false ceiling were compared to the results obtained by an original cal- culation model. Good agreement was found between experiments with two different materials and predictions. The present paper examines six different kinds of false ceiling and arrives at a new validation of the model. Calculations of NRI show good agreement with experimental data: a maximum difference of 1.2 dB(A) was found with a mean difference of 0.5 dB(A) for a wide range of absorption coefficient values. Hence the model represents a reliable instru- ment for indoor NRI prediction, if the acoustic absorption characteristics of materials are known. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Noise Reduction Index (NRI); Buildings acoustic insulation; Open window; Road; Spectrum and L eq(A) measurements; Model validation 0003-682X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2005.07.006 * Tel.: +39 075 585 3693; fax: +39 075 585 3697. E-mail address: cburatti@unipg.it. Applied Acoustics 67 (2006) 383–401 www.elsevier.com/locate/apacoust