to the drawing plane of 120 mm the depth t of slot 5 and the diameter d of the hollow cylinder 4 are 10 mm the hollow cylinder 4 has a vertically running breath b with respect to the drawing plane of 300 mm which extends over the total body 1 immersed in the flow in accordance with fig1 on assuming an ideal helmholtz resonator the resonance frequency f r of the cavity resonator 46 is equal to f r v s a t v o for the formula v s is the velocity of sound v o is the volume of the hollow cylinder 4 a the area of the cross section of slot 5 and the opening 6 respectively and t the depth of the slot 5 on calculating from the above given values v o is equal to 23 600 mm 3 and a is equal to 60 mm 2 this results in a resonance frequency f r of approx 5 400 hz if one considers the hollow cylinder 4 as lambda 2 resonator in which the width b corresponds to half the wave length of the first fundamental oscillation the fundamental resonance frequency f gr of the cavity resonator 46 is equal to 2v s b 550 hz and the harmonic vibration of the cavity resonator shows frequencies of n 550 hz all these resonance frequencies lie within the frequency range of the instability frequency kelvin helmholtz instability or the first subharmonic of the instability frequency of the free shear layer respectively with the reynolds 39 number being in the range of re 10 5 to 10 6 the instability frequencies.