Note, however, that the reversibility principle poses a limitation: the speaker must be
passive (unpowered), without amplifier transitions. In the case of an active (self-
powered) speaker, there is an amplifier between the jack and the speaker, hence the
signal won't be passed from the output to the input side [6]. Since most modern
loudspeakers have an internal amplifier [7], the threat presented in this paper is
primarily relevant to headphones and earphones, and not to the loudspeakers
typically connected to a PC.