A method and apparatus for determining the resonance frequency for a vibration welder (12) are described. The vibration frequencies at a predetermined vibration level and on both sides of the resonance frequency are derived and are then used to determine and operate the vibration welder (12) at the resonance frequency. In one embodiment the vibration frequency of the vibration welder (12) is swept up from one side of the resonance point and the vibration amplitude is monitored and a first frequency at a particular vibration amplitude is determined. The same sweeping is done from the other side of the resonance point and a first frequency at a particular vibration amplitude is determined. The same sweeping is done from the other side of the resonance point and a second frequency determined for the same vibration amplitude reference level but on the other side of the resonance frequency. The two measured frequencies are then combined to yield the resonance frequency which can then be used to operate the vibration welder (12).