The distortion factor is a harmonic distortion. In other words the ratio of distortion to input signal is not constant. This distortion increases in proportion to the input signal. Distortion occurs, for example, when a voice coil extends beyond its homogeneous magnetic field, and, on the other hand, the surround might limit the cone excursion. Cut-off effects of this kind occur as a result of clipping, similar to the same effect with amplifiers. This clipping causes harmonics which become audible as distortion. The extent to which distortion is audible will depend on the kind of music. If the sound is a pure tone, a distortion of as little as 1% will be heard quite clearly. On the other hand, with some kinds of music, especially low frequencies, even 5-10% will hardly be perceived as interference.