Abstract: The ultrasound detector for a thin film includes an ultrasound transmitter and an ultrasound receiver. It is generally known that a plastic film introduced into a sound field attenuates the field to a greater or lesser degree. A sound wave of a frequency of e.g. 40 kHz has a wave length of approximately 9 mm in air. When a plastic film of a thickness of, e.g., 0.50-0.10 mm is introduced, the sound field is typically attenuated with 6-20 dB. The the ultrasound detector is formed as a resonator, and the distance between the transmitter and the receiver corresponds substantially to an integral multiple of one half of a wave length. As a result, the obtained sound field is stronger than previously and the attenuation caused by a plastic film between the transmitter and the receiver is higher than previously. Furthermore, the ultrasound frequency is scanned for taking tolerances, such as changes in temperature and humidity of air into account.