combining both left and right stereo channels on an automotive amplifier into one higher powered mono channel. When an amplifier is bridged, the impedance that the amplifier sees is calculated based upon the output of both stereo channels. Here is a simple formula to help define this:Bridged Mono Impedance = (Y / X)/2; Y = impedance of driver(s) (both drivers should be identical); X = # of drivers in circuit. Based on this formula, hooking up one 4 ohm sub bridged mono would be equal to hooking up two 2 ohm subs in stereo, one to each channel.