An apparatus for preparing hot beverages includes a boiler and a device for inhibiting scale. That device comprises at least one ultrasound transmitter located at the boiler, inside the boiler or in the region of the boiler. The ultrasound transmitter is operatively coupled to the boiler and excites it to oscillate with its natural frequency. In a preferred embodiment, an ultrasound receiver and an amplifier is provided whose input is connected to the ultrasound receiver and whose output is connected to the ultrasound transmitter. The advantage of the invention is that the boiler always oscillates with its natural frequency, so that the sound energy is optimally used and can be kept lower than it would be for the same effect without resonance.