This part of IS0 6721 describes a pulse propagation method for determining the storage component of the complex tensile modulus E’ of polymers at discrete frequencies typically in the range 3 kHz to 10 kHz. The method is suitable for measuring materials with storage moduli in the range 0,Ol GPa to 200 GPa and with loss factors below 0,l at around 10 kHz. With materials having a higher loss, significant errors in velocity measurement are introduced through decay of amplitude. The method allows measurements to be made on thin films or fine fibres and long specimens, typically tapes of 300 mm x 5 mm x 0,l mm or fibres of 300 mm x 0,l mm (diameter). This method may not be suitable for cellular plastics, composite plastics and multilayer products.