Literally,, back-voltage, is a phenomena found in all moving-coil electromagnetic systems, but for audio is most often used with respect to loudspeaker operation. This term describes the action where, after the signal stops, the speaker cone continues moving (due to inertia), causing the voice coil to move through the magnetic field (now acting as a microphone), creating a new voltage that tries to drive the cable back to the power amplifier’s output. If the loudspeaker does too much of this, the cone flops around unpleasantly. It is not pleasant-sounding. To stop back-emf, the loudspeaker must “see” zero ohms looking backward (a dead short), or as close to it as possible from the output of the amplifier.