An apparatus for applying negative pressure to a wound, the apparatus comprising: a source of negative pressure configured to provide negative pressure via a fluid flow path to a wound dressing; a driving circuit configured to supply a driving signal to the source of negative pressure to cause the source of negative pressure to provide negative pressure, the driving signal having a driving signal magnitude and a driving signal frequency; and characterised by a controller configured to, while the source of negative pressure is maintaining negative pressure under the wound dressing within a pressure range, iteratively and at an operating frequency: determine the driving signal magnitude detected at a first time and the driving signal magnitude detected at a second time subsequent to the first time, compare the driving signal magnitude detected at the first time and the driving signal magnitude detected at the second time, in response to the driving signal magnitude detected at the first time being less than the driving signal magnitude detected at the second time, operate the driving circuit to increase the driving signal frequency, and in response to the driving signal magnitude detected at the first time being greater than the driving signal magnitude detected at the second time, operate the driving circuit to decrease the driving signal frequency. a source of negative pressure configured to provide negative pressure via a fluid flow path to a wound dressing; a driving circuit configured to supply a driving signal to the source of negative pressure to cause the source of negative pressure to provide negative pressure, the driving signal having a driving signal magnitude and a driving signal frequency; and characterised by a controller configured to, while the source of negative pressure is maintaining negative pressure under the wound dressing within a pressure range, iteratively and at an operating frequency: determine the driving signal magnitude detected at a first time and the driving signal magnitude detected at a second time subsequent to the first time, compare the driving signal magnitude detected at the first time and the driving signal magnitude detected at the second time, in response to the driving signal magnitude detected at the first time being less than the driving signal magnitude detected at the second time, operate the driving circuit to increase the driving signal frequency, and in response to the driving signal magnitude detected at the first time being greater than the driving signal magnitude detected at the second time, operate the driving circuit to decrease the driving signal frequency. determine the driving signal magnitude detected at a first time and the driving signal magnitude detected at a second time subsequent to the first time, compare the driving signal magnitude detected at the first time and the driving signal magnitude detected at the second time, in response to the driving signal magnitude detected at the first time being less than the driving signal magnitude detected at the second time, operate the driving circuit to increase the driving signal frequency, and in response to the driving signal magnitude detected at the first time being greater than the driving signal magnitude detected at the second time, operate the driving circuit to decrease the driving signal frequency.