Impulse Response Quality - No difference between 200, 500, low, high, etc?

Discussion in 'Digital & Modeling Gear' started by Jimmy_Rage, Jan 31, 2019.

  1. Jimmy_Rage

    Jimmy_Rage Member

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    Been messing a lot with Impulse Responses using a load box of late and I definitely have my own leanings, so I thought let me test out some of the stuff you hear regarding impulse response quality, such as there is no difference between 200 and 500, high quality and low quality, etc.

    Here are a couple of tracks. One uses 200 ms impulses, the other uses 500ms impulses. I am attaching the tracks as wav files from dropbox because Soundcloud really compresses the tracks and makes them sound exactly the same.

    https://www.dropbox.com/sh/ep36taa0grp3os4/AADRkwbXUq0ikGCBiP6qbMNGa?dl=0

    In each track, I go through the various quality modes offered in the Nadir loader - Low, Normal, High and Extreme.

    Methodology: Have set my amp settings to taste, then set up my reactive load and IR loader. I play some single note stuff in each of the quality settings and then some power chords in the same way. Both the single notes and power chords are double tracked with the idea that it would be easier to tell the difference between the various settings.

    Anyone want to guess which track is 200ms and which is 500ms? Also, anyone want to take a guess at the quality setting of the IR loader (the same order on both tracks).

    Better to listen with monitors, there's definitely a difference. Louder is better for making that out. You may be able to tell the difference better with the chords than the single note stuff, just skip to the 58-second mark. I apologise for all sloppy playing. There's no noise gate on there as well.

    I'll post the answers about which track is which and the order of the quality settings later.
     
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  2. Jimmy_Rage

    Jimmy_Rage Member

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  3. Lele

    Lele Member

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    What impulse response did you use?
     
  4. ColdFrixion

    ColdFrixion Member

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    I'd recommend clean tones, as you miss a lot of subtleties when using these sorts of distorted tones.
     
  5. frthib

    frthib Member

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    Most of the impulse noticable information is in the first 50-100ms (if I remember clearly) .. The rest is some kind of subtle trails who are overpowered when the next sample "wave" starts

    [​IMG]
     
  6. cliffc8488

    cliffc8488 Member

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    The energy of the speaker itself is contained in less than 50 ms. Anything beyond that is room reflections. Therefore any differences that you may hear are room reflections.

    The question then becomes do you want room reflections. Some say yes, some say no. One approach is absolutely no room reflections and then you add them with room simulation. The other approach is to use longer IRs with reflections in them. Both are valid approaches.

    Close mic'ing minimizes room reflections as the direct path is much shorter than the reflections path and sound pressure decreases by the square of the distance.
     
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  7. dlc86

    dlc86 Member

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    Can't there be any "cab reflections" (assuming that's the right term to describe what I mean) beyond 50ms too?
     
  8. frthib

    frthib Member

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    It will depend of the room. In a anechoic chamber, there will be 0 reflections.
    In an empty "hard surface" room, there will probably much more information beamed back to the mics by the hard surface beyond the first impulse

    Interesting vid by Ola in a anechoic room. The sound is really weird
     
  9. frthib

    frthib Member

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    Thanks for the info btw. very appreciated
     
  10. dlc86

    dlc86 Member

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    Yes that was clear for me, I wasn't talking about room reflections. Interesting video though.
    What I meant is, room reflections aside, are there any reflections happening inside the cab that contribute to the sound of that cab and which could last longer than 50ms?

    PS: IIRC when the ultrares format was introduced in the Axe fx, Cliff talked about this as one of the reasons to extend IR lenght.
     
    Last edited: Feb 1, 2019
  11. frthib

    frthib Member

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    Good question.. I guess there could be more complex reflexions than the sound coming directly from the back of the cab but those must not be very loud compared to the original impulse and there will be some phasing cancellation weirdness happening..

    But the cab "vibration" is a sound on it's own at the same time and that vibration must be longer than the original impulse... At that point, it's way out of my league/comprehension.. Maybe @cliffc8488 can take the lead here
     
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  12. Jay Mitchell

    Jay Mitchell Member

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    This has come up from time to time over the past decade or so, and the same misguided opinions get put forth every time. Here are some hard facts. None of this is opinion.

    Most, but not all, hardware-based convolvers - for example, modelers that use IRs for cab sims - can only process IRs of 20 milliseconds in length. A few double that to about 40 milliseconds. When you load a longer IR into such a device, it necessarily gets shortened at some point in the loading process. The best way to reduce the length is by windowing the IR (gradually reduce the level to zero at the limit the processor can convolve). A less desirable technique is to truncate it.

    If you are using an IR to simulate a guitar cab, it can sound like the cab only if there are no room reflections in it (and a number of additional conditions are met). There is nothing to be gained by using more than 20 milliseconds, assuming that any leading silence has been removed. The first 20 milliseconds of the IR of a guitar cab contain all the audible details of the cab. Bass cabs are another matter and merit their own discussion for several reasons.

    Most IRs of guitar cabs contain room reflections in the first 20 milliseconds of the IR. When you shorten such an IR, what remains does not sound in any way like the room in which the IR was taken. Room reflections in this early time window create additional tonal colorations that are not present in the sound of the cab alone.

    Acoustic spaces - even very small ones - produce reflections that take seconds to die out. An IR that is half a second or shorter in length cannot produce a convincing illusion of any acoustic space. I posted a demo here some years ago that definitively bore that out.

    From the above, a few conclusions are obvious:

    1. To convincingly simulate a guitar cab, a short (~20ms) IR with no room reflections is all you need.

    2. To simulate an acoustic environment ("room") a long (more than a second) IR is an absolute necessity.

    3. Given that most commercially available cab IRs have room reflections in the first 20 milliseconds, adding length cannot possibly make them sound more like the cab.

    Cab IRs of 200ms and 500ms definitely sound different from the same IR shortened to 20ms. They do not sound more like the cab from which they were taken, however, nor can they possibly do a good job of capturing the room in which they were acquired.
     
    Last edited: Feb 1, 2019
  13. Jay Mitchell

    Jay Mitchell Member

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    No. There are none that last longer than 20ms.
     
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  14. frthib

    frthib Member

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    So is there a point to have/use/make a 500ms guitar cab impulse since the room information will not be adequately captured? Is it usefull for a mic placed 20 feet or more from the cab to simulate distance?
    What would explain the sound difference between the 20ms and the 200ms IR? The bass "waves" are more accurately captured ?

    I'm really curious and I know you are a specialist in this subject
     
  15. Jay Mitchell

    Jay Mitchell Member

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    No. None at all.

    Mic distance is important, but not to "simulate distance." The response of the cab is completely different with a mic placed close to the grille than with the mic placed at a realistic playing distance. Practically speaking, two meters is far enough away for guitar cabs. The response of the cab won't change at greater distances than that.

    Edit: In case you are misunderstanding, I'll point out that the "time of flight" (the time it takes for sound to travel from the cab to the mic) is always removed from cab IRs. Ergo, greater mic distance does not require a longer IR.

    Much greater energy from room reflections. The IR won't sound like the room, but the reflections will be audible. IRs between about 30ms and 1000ms are of little value. If you could capture a longer guitar cab IR with no room reflections - none of the IR sellers can - it wouldn't sound different from the same IR shortened to 20ms.
     
  16. MixMinisterMike

    MixMinisterMike Supporting Member

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    And yet just a few months ago there was a guy arguing with me that he could absolutely tell the difference between 2048 and 1024 IRs when sent to FOH (even though he was using a real guitar cab onstage, 1024 to FoH was not good enough)! :rolleyes:
     
  17. Lele

    Lele Member

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    Thank you a lot, @Jay Mitchell, for recapping the main points about short and long IRs!
    And - playing live - I confirm that I can hear little difference between 1024 and 2048 samples IRs (with my Helix), and when there is any audible difference it's not a matter of better or worse sound, but just a little different sound (maybe due to the early reflections).

    But now I'm curious...
    Without entering into details, what is the suggested length for bass cab IRs? I mean the equivalent to the 20-23ms of guitar cab IRs? (if length is one of the main point for bass cabs!).
     
    Last edited: Feb 1, 2019
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  18. Jay Mitchell

    Jay Mitchell Member

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    There's no easy answer to that. The length of an IR determines how much frequency detail can be represented. At the low end, a cone speaker in a cab is a highpass filter. A 20-ms IR can capture highpass behavior that would affect the sound of a speaker used for guitar, but it might (I stress might) not capture highpass behavior that would affect the sound of a bass guitar. Many bass speakers cut off at 80Hz or higher and would be well-represented in a 20-ms IR. It is possible that some bass cabs cut off lower - the fundamental of low E on a bass is 41Hz - and would require a longer IR to capture their highpass response. How great an audible effect the added length would actually have is an open question. It's not a subject I've explored in detail yet.
     
    Last edited: Feb 1, 2019
  19. Jimmy_Rage

    Jimmy_Rage Member

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    So the clips were 1- 500ms, 2 - 200 ms, 3 - 500ms, 4 - 200ms.

    The order of each section was low, normal, high and extreme IR quality.

    @Jay Mitchell , you said that lower notes may require added IR length.

    In that regard, the recordings I made are all with a tuning of C#.

    I also have an eight string tuned to F#.

    It's very subjective, but I can definitely hear an improvement in the quality of the 200ms recordings when I set the IR loader to extreme quality, which basically means the whole length of the IR is taken into account.

    With the 500ms recording, I think at low resolution, it is quite comparable to the 200ms recording at extreme quality.

    Hike up the 500ms recording to normal quality, etc, and I "think" I hear a better sound, but perhaps it is room reflections.

    All subjective, but it's pertinent to note that there could be benefits from longer IR lengths when it comes to lower tunings.

    Not that it's written in stone...

    I'd encourage people to check out the recordings and see if you can hear a difference in quality between 200ms and 500ms as well as the various quality settings in Nadir.
     
  20. Jay Mitchell

    Jay Mitchell Member

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    No. I said nothing of the sort.

    You are barking up entirely the wrong tree here. The difference between shorter and longer IRs is the total contribution of room reflections. There are no 200-ms cab IRs with no room reflections. There are only a handful of 20-ms IRs with no room reflections. With all commercially-available IRs, the longer the IR, the higher the proportion of total energy comes from room reflections. You are not hearing "improved resolution."
     

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