A mechanical resonator has an electronically adjustable resonance frequency and is especially adapted to be used as a tunable vibration absorber. The mechanical resonator includes an inertial mass mounted on a free end of a spring, which is secured at its other end to the structure that is to be vibrationally damped. In order to vary the resonant frequency of the resonator, an electromechanical converter such as a piezoelectric element is connected to the spring and/or the inertial mass, and a displacement and/or acceleration sensor provides a sensor signal that is dependent on the respective displacement and/or acceleration of the spring and/or the inertial mass. An electronic control circuit generates an actuating signal based on the sensor signal. The actuating signal is applied to the electro-mechanical converter, which responsively exerts an adjusting force onto the spring and/or the inertial mass. The control circuit may include one or more variable amplifiers, inverters, and phase shifters, to control the actuating signal such that the adjusting force exerted by the electromechanical converter either counteracts or reinforces the bending force exerted by the inertial mass on the spring. In this manner, the effective total spring constant of the resonator can be increased or decreased relative to the inherent spring constant of the spring, whereby the resonant frequency is adjusted.