Headphones and micro-speakers and in general small drivers are very sensitive to rocking modes, this problem causes voice coil rubbing which limits the maximum acoustical output at low frequencies and degrades dramatically the reproduced sound quality.The root causes of this problem are small imbalances in the circumferential distribution of the stiffness, mass and magnetic field in the gap.In this paper, a new measurement technique is presented which provides parameters describing the causes of the problem using an advanced modal decomposition and system identification technique.The Imbalance ratio between the symmetrical force generated in the voice coil and the equivalent asymmetrical forces acting on the diaphragm is introduced as a parameter quantifying the root cause exciting rocking modes.The ratio between the mechanical energy associated to the piston mode and the tilting vibration is used to describe how critical the rocking behavior is.Those parameters give clear indications for practical improvements during development and production.The practical application of the new measurement technique to different kinds of electro-dynamical transducers exhibiting the three root-causes is discussed.