A driver device for a mist inhaler device, the driver device comprising: a battery ; an AC driver for converting a voltage from the battery into an AC drive signal at a predetermined frequency to drive an ultrasonic transducer ; an active power monitoring arrangement for monitoring the active power used by the ultrasonic transducer when the ultrasonic transducer is driven by the AC drive signal, wherein the active power monitoring arrangement provides a monitoring signal which is indicative of an active power used by the ultrasonic transducer ; a processor for controlling the AC driver and for receiving the monitoring signal drive from the active power monitoring arrangement; and characterised by a memory storing instructions which, when executed by the processor, cause the processor to: A. control the AC driver to output an AC drive signal to the ultrasonic transducer at a predetermined sweep frequency; B. calculate the active power being used by the ultrasonic transducer based on the monitoring signal; C. control the AC driver to modulate the AC drive signal to maximise the active power being used by the ultrasonic transducer ; D. store a record in the memory of the maximum active power used by the ultrasonic transducer and the sweep frequency of the AC drive signal; E. repeat steps A-D for a predetermined number of iterations with the sweep frequency incrementing with each iteration such that, after the predetermined number of iterations has occurred, the sweep frequency has been incremented from a start sweep frequency to an end sweep frequency; F. identify from the records stored in the memory the optimum frequency for the AC drive signal which is the sweep frequency of the AC drive signal at which a maximum active power is used by the ultrasonic transducer ; and G. control the AC driver to output an AC drive signal to the ultrasonic transducer at the optimum frequency to drive the ultrasonic transducer to atomise a liquid. a battery ; an AC driver for converting a voltage from the battery into an AC drive signal at a predetermined frequency to drive an ultrasonic transducer ; an active power monitoring arrangement for monitoring the active power used by the ultrasonic transducer when the ultrasonic transducer is driven by the AC drive signal, wherein the active power monitoring arrangement provides a monitoring signal which is indicative of an active power used by the ultrasonic transducer ; a processor for controlling the AC driver and for receiving the monitoring signal drive from the active power monitoring arrangement; and A. control the AC driver to output an AC drive signal to the ultrasonic transducer at a predetermined sweep frequency; B. calculate the active power being used by the ultrasonic transducer based on the monitoring signal; C. control the AC driver to modulate the AC drive signal to maximise the active power being used by the ultrasonic transducer ; D. store a record in the memory of the maximum active power used by the ultrasonic transducer and the sweep frequency of the AC drive signal; E. repeat steps A-D for a predetermined number of iterations with the sweep frequency incrementing with each iteration such that, after the predetermined number of iterations has occurred, the sweep frequency has been incremented from a start sweep frequency to an end sweep frequency; F. identify from the records stored in the memory the optimum frequency for the AC drive signal which is the sweep frequency of the AC drive signal at which a maximum active power is used by the ultrasonic transducer ; and G. control the AC driver to output an AC drive signal to the ultrasonic transducer at the optimum frequency to drive the ultrasonic transducer to atomise a liquid. <EOS>