MCACC — Sound Field Correction Base. With a dedicated microphone, the MCACC function measures the size, level and distance of the speakers and automatically compensates for the difference. The subwoofer audio delay is also set to be in phase with the other channels.
Automatic setting for surround sound (Auto MCACC)
Automatic MCACC measures the acoustic characteristics of the listening room, taking into account background noise, speaker size and distance, and tests for both channel delay and channel level. After setting up the microphone supplied with your system, the receiver uses the information from a series of test tones to optimize the speaker settings and equalization for your particular room.
• Make sure the microphone and speakers are not moved during MCACC auto adjustment.
• Using the MCACC auto setting will overwrite any existing settings for the MCACC preset you have selected.
• Before using Auto MCACC Setup, headphones should be disconnected and iPod USB, XM or SIRIUS Radio mode should not be selected as input source
The test tones used in MCACC auto tuning are one output at high volume
1. Turn on your receiver and TV.
2. Connect the microphone to the MCACC SETUP MIC jack on the front panel.
Position the microphone so that it is approximately at ear level in your normal listening position (use a tripod if possible). Make sure there are no obstacles between the speakers and the microphone.
Article Source: https://www.pioneerelectronics.com/StaticFiles/PUSA/Files/MCACC/2014_MCACC_Ref_Guide_Updated_1209_HR.pdf