By means of a non-uniform interaction among partons we have introduced a variable elasticity and density of mass into the Nambu-Susskind string. These functions characterize the string. There is a very general class of strings, related by reparametrizations, which are equivalent to the homogeneous dual string model. Further we generalize Veneziano amplitudes. States are much less degenerated than in conventional dual models. Four and five-point amplitudes are explicitly computed showing correct Regge behaviour.