Comparing the signal from left and right speakers within different time windows.
The IACC values allow inferences about the quality of stereo imaging. These are influenced by the speaker itself (pair equality) as well as by the acoustics of the room. Both the speaker placement and acoustic measures can be optimized using the IACC.
IACC10: The incoming sound during the first 10ms should reach a value greater than 80% for the best possible stereo imaging. Speakers with a high directivity factor (horn systems, waveguides, etc.) more easily reach these levels due to less interaction of the sound with reflecting or diffracting objects in the first 10 ms of travel.
IACC20: The incoming sound of the first 20ms consists of direct and the first incoming diffuse sound. For the best stereo imaging early reflections should be avoided in the first 20 ms. If these differ between the left and right channel it will decrease the IACC value. Reducing these reflections through appropriate acoustic measures will increase the IACC values.
IACC80: The sound in the first 80ms contains much more spatial information than the first two values.
IACC late: Compares the generated reverberation of the two channels.
NOTE: The IACC value also changes with the use of correction filters. Filters producing lower IACC values may nevertheless produce better perceived audio reproduction. Therefore what is obtained by numbers should always be compared with what is perceived by humans as optimum sound.