Theatrical sound systems are calibrated such that a -20 dBFS pink noise tone will play at 85 dBSPL from the front left, right, and center channels, while the rear channels measure at 82 dB. This is to maintain backwards compatibility with Dolby Stereo analog tracks, which have a monaural surround capability rather than stereo rears. In Dolby Stereo, the entire array of surround speakers is treated as one unit, which together measures at 85 dB. So when 5.1 came along, they essentially split the existing surround in half, with the left and right each playing the same signal at 82 dB instead. When heard together, the combined result of all the surround speakers is at 85 dB as it should be.

What this means is that because the surround channels in movie theaters and post-production stages are deliberately set to play back at a lower level than the front channels, the content of the surrounds will end up being 3 dB louder than it otherwise would be. The mixers will set the level of the surround effects to sound 'correct' to them on the system they are listening on. When this is played on home theater systems, which are calibrated with all channels set equally to 85 dB, the surrounds will end up being 3 dB too loud.

Because of this, a theatrical mix played on a home system must have the rear channels lowered by 3 dB in order to sound the way the mixer originally heard it. 5.1 encoders typically have a selectable option to do this automatically when creating the files for home use, but this was not always the case.