I am very new at this so, bear with me. What is an “IR”? Where do I get them? How do they get into the GPDI/IR? How do I know which one I am using? Does the GPDI/IR show-up on my computer? If so, does it connect thru the USB?
Consider the a speaker cabinet in a room with a microphone in it. Consider that the “input” to this configuration is the voltage on the input jack to the cabinet, and the “output” is the voltage at the XLR output of the microphone. Normally to record, we’d connect the output of our amplifier (the speaker outputs) to this input, and we’d record the microphone output.
It turns out that, abstractly speaking and in the case where the speakers/cabinet/air are operating in the linear regime and you don’t move them, and under the same ambient conditions, that this configuration of speaker cabinet and room and microphone form what is called a linear time-invariant transformation from one time varying signal (the input to the speaker cabinet) to another time varying signal (the output of the microphone). There are numerous equivalent ways of describing a linear time-invariant transformation, and one of them is its impulse response. The impulse response is simply the output of the configuration when the input is an impulse. An impulse, in layman’s terms, is a pulse which has a particular total time-averaged power but is infinitely narrow in its duration. The impulse response gives you all the information to construct the output of the configuration (i.e. the microphone output signal) if you already know the input signal (the input to the speaker cab).
Thus, if you have the impulse response and also have a representation of the input (the input to the speaker cab, i.e. the tap-out signal from between the amp and the speaker load) then you can convolve those two things together and get the same output you would have gotten if you actually had the cab in the room and mic’d it.
tl;dr it’s a speaker/microphone emulator.
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Someone is flexing their mathematics degree… 
(I learnt something 
)
@JCramer you must have messed around with graphic EQs and how it change the sound. Well, imagine a graphics EQ with about 500–1000 sliders. Imagine how many different ways that can change the sound. An IR is like a settings file for the position of all those sliders. Set them one way and it sounds like a 4x12, set them another way it sounds like 1x12… etc.
Like @choalla says, you can actually measure this file using a microphone and a cab. It becomes the blueprint for the character your cabinet imposes on the amp. Why they became so popular is because you can drastically change the sound of any preamp easily with a simple IR file (usually in .wav format), and it sounds incredibly realistic.
OK. Where do I find these IR files? Is there a site that has them? (I TOLD YOU…I am new and don’t know shyt.
I think I downloaded these before Impulse Response Download - Overdriven.fr
…I tried to de-math my post as much as possible but such is the curse…
I started collecting impulse response files maybe around 2007 or 2008 when I discovered readily available convolution plugins. I stopped collecting them around 2016 when I got my house and started collecting microphones instead. I haven’t looked back. I have a PL/IR but have been very happy with the eight stock IRs so I haven’t experimented with using them with others. I have been experimenting with making my own but I’ll probably only use those in my DAW out of convenience.
Back when I was collecting them, I would normally find free ones from word of mouth on others forums. Recording gear oriented forums (such as gearslutz – I’m sorry, – gearspace) I remember being a good resource. Free ones would usually be stuff that people would create themselves or free demos/samples from entrepreneurs who were selling packs. Those types of things are probably still around and useful, but I’m nowhere as plugged in as I used to be. “free impulse response guitar” is a probably a useful web engine query but I have no idea how far that will get you.
My recollection is that any given impulse response file is going to be very hit/miss. The main reason being that the IR is just one piece of the tone, and whoever created it did so in the context of whatever other gear (guitar/amp/effects) they were using, which is probably different than yours. Which all means that finding a good set of “useful” and free IRs is an uphill battle, even for an enterprising individual, simply because you have to wade through so much muck to get there.
Which is probably why I was so very impressed that the stock IRs on the PL/IR all sounded so great and achieved a great and useful balance/mix of timbres even across only 8 IRs.
OK…Where did FRYETTE get their IR files?
Steve sent a van full of cabs to Michael!
Thank you. 
