IR Properties

Discussion in 'Digital & Modeling Gear' started by Jay Mitchell, May 17, 2019.

  1. djd100

    djd100 Member

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    Hi Jay, and thanks for taking the time to explain this!

    It's always been my understanding that environmental boundary effects would technically cause comb filtering, and thus peaks and valleys in the freq response across the spectrum etc (assuming dimensions resulting in reflections not being discreet echos etc), is this correct?

    Thanks!
     
    Last edited: May 18, 2019
  2. Jay Mitchell

    Jay Mitchell Member

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    A single specular ("mirror-like") reflection - e.g., a wall or floor reflection - arriving at a similar level to the direct sound will create a very obvious comb filter, with notch frequencies determined by F = (2N+1)*C/2D, where F = frequency, N = arbitrary positive integer, including 0, C = speed of sound, D = difference in length between direct and reflected paths.

    For example, a path length difference of 10 feet yields notches at 56Hz, 170Hz, 282Hz, 394 Hz, etc., etc, with peaks at the arithmetic halfway points between those frequencies. If that is the only reflection in a room - it never will be, BTW - and the reflection is close in level to the direct, the series of comb filters will be dramatic and objectionable.

    A far more common scenario is a collection of reflections and diffractions with dense spacing of path length differences from direct, resulting in a more diffuse collection of reflected energy. The resulting comb filtering is so finely distributed, and the peaks and notches are so small, that it won't be recognizable by humans as comb filtering. Because the lowest response peak in a comb filter sequence is the broadest in percent octave terms, because most interior materials become more absorptive with increasing frequency, and because human hearing neurology has the equivalent of a sliding time window of integration that decreases with increasing frequency, these reflections tend to provide support for lower frequencies. This is why they have orchestra shells. Low brass and string basses get more help from a shell than most of the other instruments.
     
    Last edited: May 20, 2019
  3. djd100

    djd100 Member

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    Awesome, many thanks!
     
  4. Jay Mitchell

    Jay Mitchell Member

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    Here's a log IR^2 graph of a commercially available far field IR of an open back 1x12 cab (AC20). [​IMG]
    The above display is only of the first ~31ms of the IR. Most of this part needs to be clean if you want it to sound like the cab. Note that the average level from ~5ms-12ms stays around -27dBFS and that at no point in the display does it decay to below -40dBFS.

    Now, here is a display of a reflection free far field IR of the same type of cab (a vintage Deluxe Reverb with its original Jensen 12).
    [​IMG]

    Note that, although the charts were created by different programs, the vertical and horizontal scales match. Now, take a look at the decay of the average level in the second chart: it is steep and follows a nearly straight path from ca. 1ms onward. By 10ms, the average level has dropped to almost -50dBFS. This is how open-back cabs - all of them - decay. The first IR contains obvious room reflections beginning in the first few milliseconds. There is no amount of shortening of that IR that will give you the sound of the cab; there will always be reflections. That renders moot the fact that this is a "far field" IR. FYI, the exact conditions of acquisition are undisclosed.

    If this is the kind of "far field" IR that folks have tried, then it's no wonder that they think the concept doesn't work. As with so many other areas in audio and acoustics, getting one thing "right" is, by itself, not going to produce good results. There are lots of things you've got to get right.
     
    Last edited: Sep 8, 2019
  5. Dave Merrill

    Dave Merrill Supporting Member

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    Jay, thanks for all the info in this (and other) thread.

    I really wish I had access to a real far field IR in standard wav format, it would really help me get a sense of what that type of IR is like. I know you need to make a living, but putting a quality sample out there of some well-known cab (or one you like) would simplify conversations like this a lot I think. Theory is theory, but hearing it is hearing it.
     
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  6. vtgearhead

    vtgearhead Silver Supporting Member

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    I would gladly pay for some of Jay's FF IRs in WAV format. They are available for Atomic Amplifier. I do own an AA3, but don't use it enough to make any judgements about the sound. Would really need to load a WAV into my Helix to give it a workout.
     
  7. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    Why not check it out by running the AA in the Helix loop?
     
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  8. Dave Merrill

    Dave Merrill Supporting Member

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    Does anyone have clips of these with any modeller? Ideally also with the smae patch and a std IR?
     
    Last edited: May 19, 2019
  9. vtgearhead

    vtgearhead Silver Supporting Member

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    Yes, that would work. Haven't looked at the AA3 manual in a bit but I believe you can have only the speaker cab emulation active. Thanks for reminding me of the obvious!
     
  10. Ferg Deluxe

    Ferg Deluxe Gold Supporting Member

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    Loving this thread. Thanks Jay.
     
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  11. Jay Mitchell

    Jay Mitchell Member

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    My pleasure.

    So do I.

    None of this will ever make anyone a living.

    It would take a lot more than that. Like a modeler, a guitar, a monitor, picks, strings, etc., an IR is one tool of many that you need to use well in order to create the sounds you're looking for. Since ca. 2009, I have only shared IRs in conjunction with specific presets I had built, at first in the Axe-Fx, then later in Amplifire products. I also posted clips to demonstrate the sounds I was getting and how I used them. Without the presets and the demos, I had found that other folks were often at a complete loss as to how to take advantage of the IRs.

    It certainly is. And none of this is theory. It comprises well established laws of physics, proven mathematics, and properly-acquired loudspeaker response data.

    I've put up many clips over the years. Being device-agnostic at the moment - i.e., not wanting to promote one over the other - and not owning several of the more recently-introduced modelers, I'm not interested in creating demo clips at this time.

    I'll give some thought to sharing a group of IRs for comparison purposes, something I did on occasion more than a decade ago, but I make no promises.
     
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  12. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    Just disable all blocks other than CAB
     
  13. Dave Merrill

    Dave Merrill Supporting Member

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    Apologies for the assumption, I couldn't think of any other reason why there aren't any of these around to hear.

    Seriously Jay? You'd also need two hands, both with fingers, probably attached to a body, which requires air, food, water, yadda... Why are we talking about this again? :)

    So this type of IR requires special handling? I was under the impression that they were a more neutral capture of the sound of the cab. What sort of processing is needed (beyond an amp sim before and probably some sort of verb or other space-evoking thing after)? Would you think they'd be of use to people who don't have your depth of acoustics knowledge, which is probably everyone here?

    Fair enough, hope you can do that. It's just hard to talk about this concept with no reference for what they sound like.
     
  14. Jay Mitchell

    Jay Mitchell Member

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    Because IME a number of the folks who chase "tone" are looking for a magic bullet when there isn't one....

    Well, I guess competent use could be considered "special handling." It certainly can't be taken for granted. It made a difference to a number of folks who ended up liking the IRs of mine that I had chosen to share them along with presets I had built around them.
     
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  15. Chocol8

    Chocol8 Member

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    How are you capturing the far field reflection free? Is the cab mounted on a pole 10-15 meters up or do you have a super well treated room?
     
  16. Jay Mitchell

    Jay Mitchell Member

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    No. Ground plane testing in a large open (but indoor) space.
     
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  17. Dave Merrill

    Dave Merrill Supporting Member

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    What specifically are you talking about with the word "competent"? People plug their amp sim (and maybe other toys pre and post) into an IR, and adjust the amp and/or IR low and hi cut until they like it. What other adjustment dimensions are you (not) talking about that require advanced knowledge?

    Just for the record, no one here would think the less of you for releasing sound samples and/or presets for only one possibly ancient modeler. People (at least me) want to hear the IR(s), not the modeler.

    This is an incredibly frustrating conversation. It feels like you're dangling intentionally vague tendrils of sonic unobtanium that us mortals won't even know how to use. My first stop on such a quest would be the instructions provided, which at this point is this thread, which to put it mildly is sorely lacking in both general concepts and specifics.
     
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  18. nicolasrivera

    nicolasrivera Gold Supporting Member

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    with out any reading other than the OP, my guess out of my experience micing cabs in our studio is that the purple one is the close mic just because when you start placing mics further from the source the bass starts to shift and becomes less prominent, at least in our acoustic controled rooms.
     
  19. Jay Mitchell

    Jay Mitchell Member

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    Here are some more charts. The first pair contains the response of a far field IR that I could use. The second pair contains the close-mic'ed response of the same cab with the mic position chosen so as to produce a similar spectral balance. The third pair contains the response of a far field IR of an open 1x12 cab (not the same cab as the first two) taken by someone else under conditions unknown to me.

    Below is the log IR^2 of the same cab as before (1x12 open back), taken with the same mic at the same distance, but at 20 degrees off axis.

    [​IMG]

    This is its far field magnitude response.
    [​IMG]

    Here's close-mic'ed data for the same cab with the mic placed between the center and edge of the cone.
    [​IMG]

    [​IMG]

    Here is data on the far field IR of an open 1x12, taken under undisclosed conditions.​
    [​IMG]

    [​IMG]

    I've got all three loaded in my modeler and have auditioned them. Because the third IR is not from the same cab/speaker as the first two, some of the sonic differences are due to that. However, the apparent deep - and very audible - suckouts in its response from ca. 200-500Hz and 1200-2kHz are definitely not due to the intrinsic behavior of the cab and speaker. They are caused by early room reflections that were captured in the IR.
    If you'd like to audition these IRs, shoot me a PM.
     
    Last edited: May 20, 2019
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  20. Jay Mitchell

    Jay Mitchell Member

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    Knowing WTF you're doing. Having a clue. That sort of thing.

    I can't account for your feelings. It appears to me that you lack patience, but I could be wrong. You are apparently blissfully unaware of the amount of time it takes to generate the data displays I am sharing in this thread after the work of acquiring the IRs. Here's a helpful hint: if you'd like for me to share some of these IRs, it definitely does not help your case to display the self-entitled attitude that I am presently detecting in your posts.

    FYI, there is an abundance of both "general concepts and specifics" in this thread. If you can't see them, that is not my problem.
     
    Last edited: May 30, 2019

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