Driving serially connected high intensity discharge lamps

A lighting system (1) comprises a plurality of high intensity discharge lamps (9, 39, 6) and an electronic control (2) having a power input (4), an alternating current power output regulator (22), and an alternating power output (3) having an output frequency arranged to be variable within an output frequency range, the output having a first and a second output line. The lamps are connected in series with each other so that the first line is connected to the first electrode (11) of the first of the lamps, the second electrode (12) of the first of the lamps is connected to the first electrode (41) of the next lamp in the series, the second electrode (13) of the final lamp in the series being connected to the second output line. The lamps have an acoustic resonant frequency range and the output frequency range of the control is arranged to be above the acoustic resonant frequency range.