Impulse Responses are basically digital copies of Loudspeaker Cabs . That you load into your DAW
Run your guitar sound through them, to get the guitar to sound like they are being played through
A particular Louspeaker Cab that they emulate. For this you need a impulse response player.
There's lots of free ones and responses too. Here's some really good ones by Joey Sturgis.
http://joeysturgistones.com/products...-response-pack
http://youtu.be/juAbnuscL6s
