This part of ISO 11093 specifies a method for the determination of the flexural modulus by using experimentally measured natural frequencies in the free-free mode of transverse vibration of cylindrical paper and board cores, which meet the following criteria:- internal diameter:50 mm to 350 mm;- minimum wall thickness:0,02 × internal diameter or not less than 2,0 mm;- minimum length of core:8 × internal diameter.NOTE For the determination of the flexural modulus by the three-point method, see ISO 11093-7.