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Fibres have evolved over eons from simple textile yarns to complex fibre-optic communication systems. Nevertheless, a key premise has remained unchanged: fibres function as static devices or sensors, incapable of controllably changing properties at arbitrary frequencies. Enabling electrically-modulated fibres could pave the way for unprecedented technological capabilities. Previous approaches have focused on refractive index modulation1-4, non-linear optical mechanisms realized in silica glass fibres5-8, and electro-actively modulated polymer fibres9. These implementations are inherently limited by simple geometries, short fibre lengths and high driving fields. Here we lift these limitations through the composition of an internal phase that is simultaneously crystalline and non-centrosymmetric. Tens of metres of fibres containing a thin ferroelectric polymer layer spatially confined and electrically contacted by internal viscous electrodes are thermally drawn entirely from a macroscopic preform. The viscous state of the draw enables fibre cross sections of variable symmetry and resulting acoustic wave fronts. Heterodyne optical and acoustic measurements substantiate the piezoelectri