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IR Length

FractalAudio

Administrator
Fractal Audio Systems
Moderator
I've been experimenting with IR length lately and keep finding that I like a shorter length. So I gave some thought to it and I think the reason is that a shorter IR trims off the early reflections.

As I stated over at The Argument Gear Page:
A 1024 sample IR is over 20 ms. If there is a wall 5 ft. away that puts a reflection smack dab in the middle of the IR. The Redwirez IRs you can see (and hear) the ceiling reflection pretty clearly (ceiling was probably about 8 ft). Using a shorter IR removes that reflection.

I've actually been turning down the IR length lately on my personal patches, typically 512 samples, as I find it makes the IR "clearer". While the push in the industry has been towards longer and longer IRs I'm not sure that's a good thing unless you are careful with your IR capture to ensure that you aren't capturing reflections.

Some IRs, particularly ported bass speakers, may need the longer length to capture the low end with sufficient detail but the average guitar cab is probably fine at 512 or even less samples. Heck, prior to the original Axe-Fx some products were even using 128 samples.

The next time we go into the studio to shoot IRs I'm going to lay the speakers on their backs to try to eliminate the floor reflections. Or maybe just lean them back, say, 45 degrees.
 

jon

Fractal Fanatic
I too have been under the impression that longer IRs were better, but this is definitely food for thought! Makes total sense, let the experimentation begin!!

AKA there goes the rest of my Saturday evening!
 

Shaw

Inspired
That all certainly makes sense.

Ever consider a ”modeled” IR? Start with essentially a 30 band EQ... it certainly makes sense to remove reflections from the IR to clean it up (unless of course, you’re specifically trying to capture the room.

... or is it possible to add the ability to truncate a loaded IR in the Cab block? That would seem like a good middle ground...

jut spit-balling here.
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Here's another enlightening graph. This is a plot of the raw IR vs Min-Phase:
4x12 Brit 70s GB 313 A_raw_vs_mp.png

As you can see they are virtually identical. I delayed the MP version by 22 samples so that it is aligned. I've long maintained that a guitar speaker is essentially a minimum-phase device and that the benefits of MPT'ing the data far outweigh any minuscule differences in the IR phase response (the magnitude response is identical).
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
That all certainly makes sense.

Ever consider a ”modeled” IR? Start with essentially a 30 band EQ... it certainly makes sense to remove reflections from the IR to clean it up (unless of course, you’re specifically trying to capture the room.

... or is it possible to add the ability to truncate a loaded IR in the Cab block? That would seem like a good middle ground...

jut spit-balling here.
The Cab block already has the ability to truncate the IR. That's what this is all about. Scroll down to the bottom of the menu and you can set the IR length.
 

Randalljax

Fractal Fanatic
I should add that for this particular IR I believe the ideal length is 512 samples. 256 is too short as there is significant energy beyond that. 1024 is too long as there is a reflection around 550.
So different IRs could and would likely reveal different result ?
Like a 212 open back
Vs Mesa 412 w V30

Ps your playing with the cab block algos again perhaps ?
 

Shaw

Inspired
JESUS!

You lot think of everything! I wasn't sitting in front of my AxeIII when I typed that, but as soon as I did, I thought: "I bet that's already there, and have just never though to try it..."

At least I was correct once today. :cool:
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
So different IRs could and would likely reveal different result ?
Like a 212 open back
Vs Mesa 412 w V30

Ps your playing with the cab block algos again perhaps ?
No, it depends on the room the IR was captured in. An open back cap will also have the added possibility of a reflection off the back wall coming back through the cabinet.

No, I'm not playing with the cab block algorithms. There's nothing to play with really. I just noticed that I'm instinctively turning the IR length down lately and wanted to get to the bottom of why I thought it sounded better. I believe this is why.
 

iaresee

Moderator
Moderator
I’m guessing there’s no problems with reduction in low frequency data if you truncate because it’s all close mic’ed content anyways? (I know so little about the physics of sound)
 

zenaxe

Fractal Fanatic
I’m confused... wasn’t Ultrares about making IRs longer, essentially? Or does memory not serve?
Also, I thought you had said earlier this year you were into raw (non-MPT, right?) IRs? So this is a reversal of that stance?
Sounds like you are doing a 180 on some of the previous recommendations or maybe I am just not following.
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
I’m confused... wasn’t Ultrares about making IRs longer, essentially? Or does memory not serve?
Also, I thought you had said earlier this year you were into raw (non-MPT, right?) IRs? So this is a reversal of that stance?
Sounds like you are doing a 180 on some of the previous recommendations or maybe I am just not following.
Yup.
 

Shaw

Inspired
I’m confused... wasn’t Ultrares about making IRs longer, essentially? Or does memory not serve?
Also, I thought you had said earlier this year you were into raw (non-MPT, right?) IRs? So this is a reversal of that stance?
Sounds like you are doing a 180 on some of the previous recommendations or maybe I am just not following.
And that brings me back to question about the possibility of a "Modeled IR" -- zero reflections.
 
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