IR Properties

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

  1. helixuser1995

    helixuser1995 Member

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    Hello. This is my first post here. I use a Helix. Is the best way to get these IR's to send a Private Message to the OP - Jay Mitchell. (?)

    Thank you.
    Arnold
     
  2. Jay Mitchell

    Jay Mitchell Member

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    You're on to something, but it's not what you think it is.

    Yes. And both IRs are completely free of room reflections.

    That's because the causes of the interference - definitely not room boundary reflections - are different in each case.
     
  3. helixuser1995

    helixuser1995 Member

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    Hello Jay Mitchell. My name is Arnold. Is it OK if I send you a Private Message to get a copy of these IR's for use in my Helix ?

    Thank you.
    Arnold
     
  4. antcarrier

    antcarrier Member

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    On the green line, the significant dip at 1.5k, the bump below 250Hz, and the small, nearly sine shaped bump from 600Hz are all characteristics that I have seen in closed back 4x12s. The smoother lows/midrange of the yellow appear to be the open back, and 1x12s don't scoop between 1-2k as much as 4x12s.
     
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  5. guitarobert

    guitarobert Member

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    We saw that notch around 1.5kHz earlier in the thread on the 1x12 open back. Supposedly it is caused by cancellations from the sound coming from the rear of the cab. It also has the reduced low frequencies below 100Hz. I think green is the open back.
     
  6. helixuser1995

    helixuser1995 Member

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    Hello Jay Mitchell - I just saw Post #40 and have sent you a PM. Thanks, Arnold
     
  7. Yek

    Yek Member

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    I want to thank Jay for sending me the demo IRs.

    "C" is a very pleasing IR which I wouldn't hesitate to use in my clean / edge-of-breakup sounds. It brings back the familiar traditional cab sound, which is addicting.

    I also noticed a subtle and welcome increase of "directness" between attack and output. Perhaps one would expect that from a NF IR more than from a FF IR, because of visualisation (the mind visualises the distance between person and cab), but that expectation simply is a wrong one.

    Apart from the above, the overall characteristics of "C" aren't that much different from the NF IRs I normally use. Perhaps because I always high-pass and low-pass a lot (150 / 5000-6000), and gravitate towards mics such as the Beyer M160, SM7B etc.

    NF IRs are not "wrong", and the fact that they have a lot of top/bottom end isn't wrong either. It's just a property of close-mic'd capturing. But as Hippietim writes, it's a joy to be able to select an IR such as "C", and not having to choose between mics, apply EQ etc., and immediately get the sound of the cab/speaker.

    These are very promising developments. Can't wait for more IRs like this.
     
  8. Jay Mitchell

    Jay Mitchell Member

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    I originally said that rear radiation might be a factor in that notch. However, I later saw that Eminence's 1-meter response data on that transducer - taken with the speaker set into a wall and therefore with no rear radiation - shows exactly the same notch. So, it's characteristic of that model Eminence speaker. Other 12" cone speakers have similar behaviors.
     
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  9. gigsup

    gigsup Supporting Member

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    I went back to read Mark Ganders paper on ground plane reflection measurements.
    The method describes resulting frequency responses from a single speaker in an enclosure, (the horn/tweeter is not drawn in but assumed), as measured by an omni directional mic on the ground. (Initially 1M away, but moved to 2M away for a good reason.)
    The thing I find interesting is the path length from a single 12" speaker has an 'mirror image source' created by the ground plane. So, there is only one path length to consider.

    However, for a 4x12", there are multiple paths to the same mic.
    No matter which way you spin the cabinet, the path lengths to the mic will arrive at different times, resulting in more cancellations.
    Since the cabinet has a 'mirror' cabinet, the baffle will appear a lot larger than the single 12", which I think would enhance the low frequency bump seen between 100-200hz.

    I think that the green graph represents the 4x12.
     
  10. Jay Mitchell

    Jay Mitchell Member

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    I was there for his presentation of that paper at AES. His technique was generally good, the test environment less so - the ground plane was not a plane, and the surface was dirt - and several of his conclusions were incorrect.

    All sources will have an image source. That is the principle on which ground plane testing works.

    No. You would need to have some understanding of the phenomenon of diffraction in order to realize why this is an incorrect statement.

    See above. This is true for any source of sound with nonvanishing dimensions.

    Wrong. If you like, you can aim the cab directly at the mic. In that case, direct sound from all four transducers will arrive synchronously. Given that nobody ever aims their 4x12 directly at their ears, this particular scenario is of no practical interest, but it's one option when you're testing a 4x12. As with any other cab, however, diffracted energy will always arrive later. Diffraction is one reason that speakers are never minimum phase systems.

    In case this hasn't dawned on folks here, multisource cancellations are an integral part of the sonic signature of any cab with multiple drivers. These cancellations are the reason that a 4x12 sounds profoundly different from a 1x12 with the same speaker in it. Ergo, you will need to capture a set of such cancellations in an IR of a 4x12 if you want it to sound like the cab.

    No. The virtual mirror source adds coherently to the real source, which increases the level the microphone sees by 6dB at all frequencies. There is no frequency-selective boost in a ground plane measurement. The response you get from such measurements - when they are done correctly - is the free field response of the cab.
     
    Last edited: Jun 1, 2019
  11. gigsup

    gigsup Supporting Member

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    Then with regard to the ground plane measurement, you did compensate for the angle of the 4x12 cabinet to arrive at identical path lengths to the microphone.
     
  12. Jay Mitchell

    Jay Mitchell Member

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    No. Read my post again. That would place the test mic on axis, and the on-axis response of a 4x12 is of no practical interest.
     
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  13. gigsup

    gigsup Supporting Member

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    I'd imagine most everyone here has the experience of playing through both cabinets.
    As an engineer, mic'ing a 4x12 in the studio is not the same approach to mic'ing a 4x12 in a live situation, in my experience.
    So, we are kind of familiar with walking in circles around our amps.

    The cancellations in the response are what I'm looking for to help explain the differences in the graph.
    If the cabinet is aligned perfectly to the measurement microphone, and the baffle size of the cabinet itself has no effect on the frequency response, are you implying that the diffraction of the cabinet itself explains the differences in the graphs?

    EDIT: I asked if it was aligned "perfectly", which is not encouraged in your last post.
    Still the question of diffraction, does that alone explain the differences in the graph?
     
    Last edited: Jun 1, 2019
  14. Jay Mitchell

    Jay Mitchell Member

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    If, by that statement, you mean the mic is on axis of the cab, then you are not understanding what I have posted here. Yet again: the microphone is not on axis of either cab.
    I never said that. I said that the virtual image of the cab that ground plane measurement creates does not increase the effective baffle size. That is completely different.
    No. Please read what I have already posted, please make sure you understand what I have described, and please stop attributing statements to me that I have not made. Correcting your misreading of my statements has become quite tiresome.
     
    Last edited: Jun 1, 2019
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  15. Jay Mitchell

    Jay Mitchell Member

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    "Mic'ing a 4x12 in the studio" almost never captures the cab's sonic signature in a way the player or audience might hear it. If it did, there would be no purpose in acquiring far field impulse responses.
     
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  16. Jay Mitchell

    Jay Mitchell Member

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    No. Here, let me point out the prominent differences. These are all trivially obvious, but here they are anyway.

    1. The 1x12 cab is an open back type.
    2. The 1x12 cab has one transducer.
    3. The 4x12 cab is a closed back cab.
    4. The 4x12 cab has four transducers.
    5. The transducer in the 1x12 cab is a different make and model than the four identical transducers in the 4x12.
    6. The cabinets have different dimensions.
    7. As with most guitar cabs, there is no acoustically absorptive material inside either cab.
    8. As I pointed out earlier, the microphone is not on axis of either cab.
     
    Last edited: Jun 1, 2019
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  17. vtgearhead

    vtgearhead Silver Supporting Member

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    Jay, would you consider making your 4x12 far field IR available in .wav format? I tried using cab2wav.py to extract it, but results are very suspect.
     
  18. Fatherflot

    Fatherflot Member

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    Jay was kind enough to share the IRs with me and I've been trying them out in my basic little home setup with and Atomic AA3 into a Tech 21 DD.

    The technical discussion of acoustic engineering is far beyond my competence to engage in, so I will confine my comments to my proper sphere --- as an amateur basement "musician."

    There is definitely something going on with these that makes them stand out in subtle ways from my other IRs. Someone said a sense of "directness" and that's what I was thinking. They get my setup a little closer to that feeling of "immediacy" that I get when plugging directly into a tube amp. There is a liveliness in the upper midrange that makes them feel pretty dynamic.

    I liked C the most, then A, then B. C seems like a very versatile and well-balanced IR that could work with almost any amp model.

    Thanks for sharing, Jay.
     
    Last edited: Jun 3, 2019
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  19. DeSelby

    DeSelby Member

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    First off, thank you Jay. ‘C’ it is for me. Not having to tweak the IR is a big plus. I did a comparison to a couple of IRs that I winnowed many, many samples down to and 'C' is a clearer representation of what I expect to hear. I will continue to use a couple of the winnowed IRs though because as Rex Harrison said, I’ve grown accustomed to her face, and also because even with reflections the sheer volume of different speakers and cabs has provided some samples that are very useable.

    Another question. Is it possible to apply the same techniques to acquiring IRs of acoustic instruments and would it yield the same benefits.
     
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  20. gigsup

    gigsup Supporting Member

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    That's an opinion.
    Recordings and live performances have defined the way musicians and audiences hear an electric guitar, one informs the other.

    There is a very good purpose, albeit unrelated to musicians or audiences appreciation of music directly, but indirectly through different technology.
    I believe musicians will depend on these far field impulse responses to re-create what they've come to know and love with regard to existing recordings.
    Keep up the good fight.

    So far, with regard to this graph in particular, I don't see any solid evidence to support the conjectures.
    I'm flip-flopping now.
    I think that perhaps the dip between 1K and 2K represents the frequencies related to the 12" speaker and the cabinet dimensions at those frequencies in an open back cabinet, the path length of waves diffracted from the back of the cabinet 'phasing out' the wave emanating from the front at those frequencies.

    The obvious result in this thread, so far, is most people prefer 'C'. That's encouraging.

    I'm phrasing questions to the physicist in the room, not attributing any statements to him.
    Please, please, don't take any of my mis-understandings personally.
    I'm going to give you some 'likes' so you feel better. Think of them as virtual hugs.
     

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