A system consisting of a linear power amplifier driving a piezoelectric actuator pair attached to a long viscoelastic bar is analysed. Coupled piezoelectric theory is used, and allowance is made for the dynamics of the amplifier and of the actuators. Formulae are derived for the relation between the input voltage to the amplifier and the normal force associated with extensional waves generated in the bar and for the load impedance constituted by the actuator-bar assembly. It is established that the mechanical work performed on the external parts of the bar at the actuator/bar interfaces is at most equal to the electrical energy supplied by the amplifier. The results are applied to a three-parameter viscoelastic bar and to an elastic bar, and the effects of the cut-off frequency, without load, and the output impedance of the amplifier are examined. For the elastic bar, sharp response minima occur at frequencies that are integral multiples of the inverse transit time through the actuator region. For the viscoelastic bar, the corresponding minima are less sharp and deep. The input voltage to the amplifier required to produce a desired output wave at the actuator/bar interfaces can be determined provided that the spectrum of this wave is not too broad.