IR Length

[FractalAudio]

@FractalAudio what program are you using to generate the graphs from the or files?

This is very cool stuff.

Matlab.

 

[VidarAus]

Matlab.

Do you do all your algorithm development in matlab?

 

[Jason Scott]

The reflections are slightly delayed in the real world but an ir captures that info the same point in time. these reflections could be the cab itself even when no walls or floor are present. that's what different types of cabs have different response with the same speaker. of course i'm just guessing and not an audio engineer.

In the same medium, all sound waves travel at the same speed. One difference is low frequencies travel longer than high frequencies, generally speaking.

 

[ChainOfThought]

Aren't these artifacts mirroring what typically happens in a studio environment? Maybe the "free space" ones would sound "better". I'm just pointing out that it would no longer be the sound of a mic'd cab as we know it.

looking forward to the results of this thread. Maybe we'll get reflection-free IRs and wall/ceiling/floor distance parameters

I think it’s not actually mimicking a studio because you get the one reflection snapshot and not an actual real time decay of reflections? I feel like that’s some of the basis of what shattered is trying to find a way around, but I’m not really sure.

 

[guitarmike]

Yep, it takes about 3.? meters for a 100hz signal to complete one cycle. How this effects an ir measurement I have know idea. I do know that when I set up a fractal amp using a real cab and then switch to an ir...any ir...I have a bass problem. The opposite is also true.

Please note that I am still on the axeII and this problem is much less prevalent in the latest firmware.

 

[FractalAudio]

Yep, it takes about 3.? meters for a 100hz signal to complete one cycle. How this effects an ir measurement I have know idea. I do know that when I set up a fractal amp using a real cab and then switch to an ir...any ir...I have a bass problem. The opposite is also true.

Please note that I am still on the axeII and this problem is much less prevalent in the latest firmware.

Huh? You're not supposed to use an IR with a real cab. Everyone should know that.

 

[camilovelandiamusic]

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:

I always felt the same way! Actually, a couple of IR's that Ive purchased and really love, have a tiny bit of early reflection on them. It took me a while to realize it. Was checking preamps, interface, etc and eventually when I shortened in the cab block..... Voila!!!

 

[FractalAudio]

I always felt the same way! Actually, a couple of IR's that Ive purchased and really love, have a tiny bit of early reflection on them. It took me a while to realize it. Was checking preamps, interface, etc and eventually when I shortened in the cab block..... Voila!!!

I'm going to work on an auto-correlation display over the coming days and see if that can help in identifying early reflections.

 

[guitarmike]

I get where you're coming from, but think about it: which of the many EQ curves that the speaker produces — even in a reflection-free situation — is the one "inherent to the speaker?" They're all inherent to the speaker. As you move off axis, different cabs change their sound in different ways. Even if you choose one EQ curve as the starting point, you can't apply a standard formula to figure out how the sound will change at, say, 30° off axis. That's why people take multiple captures at different angles when they're shooting IRs.

Of course you are absolutely right, ...when you measure the frequency response with a microphone. But when a play my marshall, I hear all those frequencies responding in a room. Now if I record that sound and make a cab (ir) out of it it may be fine if your listen on headphones or any medium that excludes a room. But as soon as you play through live system, you are doubling up on the room anomalies.

If I do a fast ir at a particular axis, then that is all I have captured if I play that same amp through a speaker cab that only had that single capture, would it sound anything like my real cab?

Now, having said this, let me also say that I can get really good sounds through my Yamaha HS8's. But I always have to manipulate the low frequency spectrum because its always to much. And I usually tweak the hi end also to get things a little more open. So the tools are there to get real guitar sounds, no doubt. But, the fact that every ir (practically) I have ever used requires manipulation to get anywhere near what my real cabs sound like tells me that something is not "perfect" here. I'm not exactly sure what I'm doing to the details of the ir when I eq, but I can't help but suspect that I am removing some of specific detail that ir's tend to have.

It's kind of like eqing a room. If you take an ir and try eqing that room (with an eq) with all the detail that you can see, you might as well be farting in the wind. Your best bet is to average that curve out to something that represents a broader picture. Because that dip you see might only exist at that one spot where you measured and may not represent what the room is mostly doing at all.

What if you took an ir of a cab that included the room and then subtracted just the ir of the room?

 

[guitarmike]

Huh? You're not supposed to use an IR with a real cab. Everyone should know that.

No, I'm just talking about the distance it takes for 100hz to travel one complete cycle. Now whether or not it effects the curve we capture from a cab closer than that distance, I have no idea. Off hand, it seems like we would need a complete cycle but I don't know.

Of course if a switch a preset made with an ir to using a real cab, I turn the ir off, and vice versa.

 

[Rex]

That mic capsule is moving constantly back and forth as it's sending signal, that's a source of modulation/distortion.

The excursion of a microphone diaphragm is measured in thousandths of an inch. That's a tiny fraction of a wavelength at any audio frequency, and its effect is inaudible.

 

[Rex]

What if you took an ir of a cab that included the room and then subtracted just the ir of the room?

How would you capture the IR of a room? You'd need a speaker of some kind, with its own direction-dependent frequency response. That would color your results.

And where would you place the mic and speaker? Your choices would influence the IR capture.


There is no single inherent EQ curve of a room, just as there is no single inherent EQ curve of a guitar cab.

 

[camilovelandiamusic]

I'm going to work on an auto-correlation display over the coming days and see if that can help in identifying early reflections.

Awesome!!! Yeah for this particular IR, it wasn't even evident in clean tones. It was only when adding drives. IRs really are an interesting thing. Do you think there is an advantage to capturing in a dead room

 

[camilovelandiamusic]

Or ISO box for that matter

 

[guitarmike]

How would you capture the IR of a room? You'd need a speaker of some kind, with its own direction-dependent frequency response. That would color your results.

And where would you place the mic and speaker? Your choices would influence the IR capture.


There is no single inherent EQ curve of a room, just as there is no single inherent EQ curve of a guitar cab.

Well, you would need a perfect speaker, or close, located at exactly the same place at the cab with the same perfect mic located ar exactly te same place as the before. Less than perfect solution , eh.

Is there a mathematical relationship between the the curve at the center of the cone and the axis points to the edge?

I really wish I could give a real solution. But I am more interested in the theoretical point I'm very poorly trying to make.

 

[jon]

@guitarmike Serious question-do you understand what an IR is and how it works? Do you understand the speed it is done at? The 'fast IR' you speak of, what speed is THAT done at? At the same 100hz, what time do you think does it take a reflection to happen in say an average sized room?

Also, how do you propose to 'average out' the IR when moving the mic will result in a different IR? What is the 'perfect IR' then and how do you achieve it?

How do you propose to do this 'subtraction of the room IR'? What are you using to take the IR with?

I suspect by researching and answering some of these questions you will have a better understanding of what is going on, and will answer yourself a lot of your questions and concerns.

 

[jon]

Because it doesn't. Even an anechoic chamber isn't as good as using a large space with reflecting surfaces far away.

Awesome!!! Yeah for this particular IR, it wasn't even evident in clean tones. It was only when adding drives. IRs really are an interesting thing. Do you think there is an advantage to capturing in a dead room

This is what Cliff had to say on the dead room, makes sense if you think about it. An large anechoic chamber should help quite a bit though I think?

 

[antcarrier]

Anechoic chambers still reflect low frequency energy. A large space is superior because it allows for an IR to be free of reflections, yet allows it to be windowed to a reasonable length.

 

[guitarmike]

@guitarmike Serious question-do you understand what an IR is and how it works? Do you understand the speed it is done at? The 'fast IR' you speak of, what speed is THAT done at? At the same 100hz, what time do you think does it take a reflection to happen in say an average sized room?

Also, how do you propose to 'average out' the IR when moving the mic will result in a different IR? What is the 'perfect IR' then and how do you achieve it?

How do you propose to do this 'subtraction of the room IR'? What are you using to take the IR with?

I suspect by researching and answering some of these questions you will have a better understanding of what is going on, and will answer yourself a lot of your questions and concerns.

There is no doubt that I have a limited understanding of ir's. I am not an authority at all. But I do understand that in the end, its just a frequency curve. I have enough understanding about tube amps, transformers and speaker cabs to know this relationship is the most complex and dynamic thing going on in the amp. And many will die on the hill that says this is where the magic is.
So,
Just positing some questions and trying to think beyond what seem to be boundaries and into the basic theoretical stuff. All I know is that when I listen to an amp, I am hearing all those axis points, freq curves, ect., interact with the room. If you pick a single one of those points and limit your amp cab to reproducing only that freq curve, how do you think that would sound? If the amp sims in the axe are perfect, and if your frfr system is perfect, and if your room is perfect, then wouldn't that ir you took sound just like your limited amp cab. I also know that there are fairly big differences between axefx presets I tweak with a real cab and an ir. I am speaking of the overall balance of the sound not just nuances. The ir/cab portion of the axefx (I am on the II) appears to be less accurate than a real cab to me. Thus the low cut, hi cut, air, de-phase, ect. It is a great system and you can absolutely get GREAT sounds from it, but I'll bet you Cliff improves it.

I do know that ir's are used to eq rooms by adding the inverse ir as a freq curve.
And if my musings insult the superior intelligence, please accept my apology and ignore me.

 

[DLC86]

Something still causes phaseyness (or lack of clarity) in some irs, even prior to the early reflections, I can hear it. Cliff has made a lot of advances in that area because it used to be worse. The other few modelers with ir loading I've heard have it also (probably more).

Phasiness can also be caused by a lot of other factors, first of all mic position relative to the speaker.
The speaker emits all the frequencies it can emit from every point on the cone, if the mic is on axis all those waves coming from the surface of the cone reach the mic at the same time, if the mic is off-axis some points on the cone surface will be more distant from the mic and those waves will reach it with a slight delay and that causes comb-filtering effects (aka phasiness) and high frequencies roll off.

PS: @NeoSound and @guitarmike , you two should take a look at this thread, it'a long read but it is very educative on the subject and will clear up most of the doubts and questions you have.