Methods for measuring a resonance frequency of a tool set in ultrasonic vibration during the machining of a workpiece, involving radiating a working signal with a working frequency into a tool holder comprising a tool by a generator to produce the ultrasonic vibration of the tool; after the start of the machining of the workpiece, radiating a test signal with a test frequency varying by the working frequency and a lower power than the working signal power into the tool holder by the generator; generating a sensor signal from the ultrasonic vibration of the tool by a sensor apparatus arranged in the tool holder; reading out the sensor signal by a read-out apparatus; splitting the sensor signal into a frequency spectrum involving a main frequency and an auxiliary frequency by an analytical apparatus; determining the main frequency from the working frequency and the auxiliary frequency from the resonance frequency.