Abstract: An apparatus is provided for determining the natural frequencies of a longitudinally oscillating band excited by a transmitting oscillator, and whose oscillations are received by a receiving oscillator and evaluated. At least one of the exciting oscillator and the receiving oscillator is a sandwich oscillator having metal end masses. The oscillations are transmitted longitudinally along the pre-tensioned length of the band disposed between the transmitting oscillator and the receiving oscillator. An attachment device for attaching the band to the respective oscillators includes a roller which is spring-biased to rest in a groove formed in one face of each of the respective oscillators.