Measurements of the two characteristic lengths of 6 glass wool samples (with flow resistivities between 11,900 and 69,900 Pa S m(-2)) and 6 polyester fibre samples (with flow resistivities between 4100 and 51,000 Pa S m(-2)) have been made. These data have been used to determine the cross-sectional shape factors which are related to the characteristic lengths introduced by Allard. By using the formulas due to Bies and Allard, it has been found that the two characteristic lengths of the glass wool samples can be independently predicted from the glass fibre diameter. In respect to polyester fibre samples, a new relation between the flow resistivity, the fibre diameter and the bulk density has been proposed and examined. The accuracy of the predictions of the non-acoustical parameters has been confirmed by the measurements and predictions of the absorption coefficient using the Delany and Bazley and Allard models. (c) 2006 Elsevier Ltd. All rights reserved.