If a wave bouncing back to the hole and a wave entering reinfor each other,the strength of the wave will build up progressively to large amplitude. This is a from of resonance. If the amplitude is large enough, even the small fraction of it which leaks back into waveguide through the hole will have a significant effect,as we shall see. By making the size of the cavity variable,the resonant frequency can be varied. So, by adjusting the cavity size to the point where the cavity affects transmission of a particular signal,can be resonated at the frequency of the signal.