The resonant frequency of an example transducer can be adjusted by changing the effective mass of a backing mass using a tuning module. The tuning module includes a electrical source, a switch, and an electromagnetic coil connected in series as an electrical circuit. The electromagnetic coil is mechanically attached to the backing mass, and is disposed within a reservoir of a magneto-rheological fluid enclosed within a casing. When the switch is closed, the electrical source applies a voltage and current to the electromagnetic coil, and induces a localized magnetic field within the magneto-rheological fluid. In response to this localized magnetic field, the magneto-rheological fluid increases in viscosity, assumes properties comparable to a viscoelastic solid, and become affixed to the electromagnetic coil. As the electromagnetic coil is mechanically attached to the backing mass, the solidified magneto-rheological fluid increases the effective mass of the backing mass. As a result, the resonant frequency of the transducer is altered.