You can probably look at earlier threads (think earlier Axe-Fx II days) where I've stated this ad-nauseum, but here I go one more time:
Let's get some basic facts down first.
- When you play an amp and cabinet in your room, One probably plays at least around 2m away or more (6 feet away from the cabinet) on average. For a typical guitar cabinet, we call this the "Far field" - we'll get to that in a moment.
- Closer to the cabinet - like when you are mic'ing one, is called the "near field".
- So far so good, right? We have a far field and near field.
- OK, now consider that a typical guitar speaker driver is usually a 3-D cone section with a 10"-12" diameter - it's not a super small point - just keep that in mind as we discuss near-field vs far-field.
What happens in the near field?
- To get a picture in your mind, imagine a microphone near the speaker driver. Now imagine a straight ray arriving at the microphone from every point in the 3-D cone.
- As you'll quickly realize, these "rays" or lines are not all equal in length as shown in this simple illustration:
- All these different waves arriving at the mic contribute to the frequency response (this is why you'll see combing effects, as waves will cancel and reinforce at various different frequencies.
- Now imagine moving the microphone. Now the rays arrive at different ratios than the previous position, changing the frequency response dramatically. This is why people spend so much time finding just the right position for the mic, small movements have dramatic effect on the frequency response.
So then what's the FAR field?
- That's when the difference in length of all those rays are essentially so minute that for practical purposes they all arrive at the essentially the same time, hence the frequency response is not altered dramatically as in the near field.
- For a guitar cabinet and it's typically frequency rolloff at around 5KHz, a player is typically in the far field of the cabinet.
There is also the fact that the directivity typical guitar cabinets is really bad. This means high frequencies fall off rapidly as you move away from the center. This is what causes typical "ice pick" sound as you stand at the same height as the cones in the cabinet. But typical player position is offset at around say30 degrees from center (i.e. cabinet in the floor, pointing at feet).
All of the impulses in the Axe-Fx are near field captures as others have mentioned. This is because the typical studio recording chain, involves near micing a cabinet - this is mostly what you hear on records.
Hopefully now, you understand why this couldn't possibly give you an "amp in the room sound". It's because you don't typically play with your ear right next to the cabinet. You are in the farfield - the frequency response is very different.
To achieve an amp in the room sound however, you need properly captured farfield IRs.
- You may ask, why can't I simply move the microphone back and capture an IR like that?
- Because the reflections from the room will distort the cabinet capture. These are early reflections, and no capturing this is not capturing the "room"- just a few first early reflections which is enough to alter the frequency response of the resulting capture dramatically.
- You need a properly sized room where you can guarantee there will be no reflections for at least the first 20 ms or so, which is enough to capture the frequency response of a typical guitar cabinet
- Why no room though? Because you want the amp in "MY ROOM" sound. You want the contribution of the room you are playing in. So that being the goal, the IR should only capture the contribution from the cabinet alone, nothing else.
How big a room do I need then to properly capture a farfield IR? Let's do some back of the napkin calculations:
- Distance from mic to cabinet ~= 2m
- Speed of sound c = 343m/s
- We need the earliest reflection to arrive 20ms or so after the direct sound.
- trfz = time for reflection free zone > 20ms
- The shortest reflection then should travel at minimum: dmic + c*trfz = 2m + 343m/s*20ms)/1000ms/s ~= 8.9m
- In a room with L, W, H dimensions, placing the cabinet with the microphone dmic away (use 2m) from it, then doing some simple geometry - assume they are placed in the room where the mic and the cabinet are the foci points in an ellipsis where 2a = L.
- The reflection distance in such an ellipsis is then L.
- So L > c*trfz + dmic
- W > 2*sqrt((c*trfz/2)^2 - (dmic/2)^2)
- H > sqrt((c*trfz/2)^2 - (dmic/2)^2)
- L > 8.9m
- W > 8.6m
- H > 4.3m
This means at the very least you need a empty space of 8.9m L x 8.6m W x 4.3m H (in meters) or 30 ft. L x 29 ft. W x 15 ft. H
Now you should see why you won't see any commercial IR offerings that include proper farfield captures. First, the requirements are fairly big (neutral recording mic with calibrated frequency response you can compensate for, proper technique (i.e. Ground plane measurement), room dimensions).
Second the audience size for such IRs seems minimal. The economics probably don't make sense. I thought about renting a gym for some capture sessions but I'm pretty sure the rental fees, equipment, acquiring the cabinet, etc, would not make it economically feasible.
Having said all this, I've heard a properly captured free field IR (At around 30 degree offset, typical of player position) with a CLR (which is a pretty good FRFR) and to my ears that's an entirely convincing amp in the room sound (YMMV) - so yeah I disagree with Fractal on it being impossible to achieve - difficult perhaps, not impossible.