In gravitational wave astronomy the quasi-normal modes, i.e. the excitations that compose the ringdown part of a two black holes’ coalescence signal, is of fundamental importance. However it has been noticed that this spectrum of modes is extremely unstable under external disturbances, thus potentially hindering future predictions. In this seminar, after a general overview on ringdown and quasi-normal modes, I will talk about how this problem can be understood and treated intuitively using the transfer matrix approach from quantum mechanics.