For Mac, this is easy.
Step 1: Make sure you have downloaded Soundflower (2ch) for the appropriate version of your Mac.
Step 2: Make sure you have downloaded LineIn, this enables you to hear the channeled audio.
Step 3: Plug headphones into the computer (if it has a headphone port (or jack) as they call it.)
Step 4: Set input to Line In. This channels the sound to your headphones.
Step 5: Set Output to Soundflower (2ch), and then click on Pass Thru.
Step 6: Turn up the output volume, but not too loud, or it can damage your hearing! Do not use in a lightning storm!
Step 7: Test a MIDI using Synthesia. 300k note songs and below will have their lag reduced significantly.
Enjoy Synthesia with less lag!
P.S: This also could be applied to PC users, make sure your PC has a headphone jack.
How to: Less lag with headphones!
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