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Preliminary IMD measurements for subwoofers

Discussion in 'Speakers & Subwoofers' started by Ilkka R, Apr 4, 2006.

  1. Ilkka R

    Ilkka R Second Unit

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    Steve's question is a good one and that is one reason why original 70 Hz was chosen. 30/65 Hz could be possible, but 2f1-f2 yields a 5 Hz component.

    I think the lowest possible would be 30/71 Hz.
     
  2. RogerW

    RogerW Auditioning

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    There probably isn't a perfect answer to this. The two tones chosen have to be within the range that most subs can reproduce without undue distress. Ideally, none of the IMD components should coincide with a harmonic of either of the excitation tones (a problem with 30/70). In practice, some of the IMD components are always liable to end up on top of each other, but if we are primarily interested in Total IMD% this should not matter too much. We don't want any IMD components below 10Hz or we can't measure them as accurately as we would like. And we don't want any IMD or HD components to be so close together in frequency that our spectrum analyzer lacks the resolution to discriminate between them. Finding a pair of frequencies that can fulfil all that lot is quite a tall order. [​IMG]
     
  3. Edward J M

    Edward J M Cinematographer

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    30/53 yields the following IM components:

    83
    23
    7
    76
    113
    136
    166
    46
    189
    143
    129
    37
    16
    99
    196
    219
    67
    182
    173
    242
    249
    69
    272
    226
    152
    14
    295
    203

    And the following harmonics:

    60106
    90159
    120212
    150265
    180318
    210371
    240424
    270477
    300530

    +++++++++++++++++++++++++

    30/65 yields:

    95
    35
    5
    100
    125
    160
    190
    70
    225
    155
    165
    25
    40
    135
    220
    255
    55
    230
    185
    290
    285
    105
    320
    250
    200
    10
    355
    215

    60130
    90195
    120260
    150325
    180390
    210455
    240520
    270585
    300650

    +++++++++++++++++++++

    30/72 (suggested by Ilkka via email) yields:

    102
    42
    12
    114
    132
    174
    204
    84
    246
    162
    186
    18
    54
    156
    234
    276
    48
    258
    192
    318
    306
    126
    348
    264
    228
    24
    390
    222

    60144
    90216
    120288
    150360
    180432
    210504
    240576
    270648
    300720

    +++++++++++++++++++

    While S/T SpectraPro is capable of 0.008 Hz resolution, the settings we're considering using provide a 1.5 Hz spectral line resolution, which should be capable of handling a few of the close ones.

    Ed
     
  4. Warren_Sc

    Warren_Sc Second Unit

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    Subwoofer C is the AVS-12 model Illka was banned for marketing. [​IMG]

    ...Sherv... pass the popcorn... I found you guys...
    [​IMG]
     
  5. Warren_Sc

    Warren_Sc Second Unit

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    Wouldn't it be possible to just hold one tone (say 30hz) and move the other tone around (changing its frequency) and see which combinations give the highest level of IMD?

    PS This is really great stuff.
     
  6. Edward J M

    Edward J M Cinematographer

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    Yes, this is possible. SpectraPro has a multi-tone generator which can output up to 10 tones simultaneously, or you can pick/choose which ones. It would only take a few seconds to check all of them.

    However, which gives the higher %IMD and which gives what subjectively might be perceived as the more offensive IMD are two different things.

    For example, with harmonic distortion, we find higher orders more audible/offensive than lower orders, and we also find odd orders more offensive than even orders.

    The 30/53 case is interesting because it generates a lot of "in-band" and "near-band" IM components, with many of them falling in the
     
  7. dave alan

    dave alan Second Unit

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    This one looks the most interesting to me, also, FWIW.

    Dave
     
  8. Robb Roy

    Robb Roy Supporting Actor

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    Ilkka,

    Great work! It's nice to see some hard numbers on the subject. I've been so busy with 80+ hour work weeks that I haven't been following things too closely, but allow me to express my surprise that this got you banned at AVS. When did AVS become a front for the Flat Earth Society?

    -Robb
     
  9. Ilkka R

    Ilkka R Second Unit

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    Thanks Robb.

    AVS changed since they became the AV Scientology instead of AV Science. [​IMG] Luckily we still have other good forums.
     
  10. Edward J M

    Edward J M Cinematographer

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    Looks like we lost some good posts during the HTF server/database problems - that's too bad. [​IMG] Anyone willing to repost would be appreciated.

    Anyway......I ran some IMD tests over the long weekend on a review subwoofer. I did 30/53 and 30/72, both at 95 dB and 100 dB.

    This particular subwoofer struggled to hit (and didn't quite make) the 100 dB mark. This is because its 10% THD limit and its output compression limit at 30 Hz are both just under 100 dB (2 meters ground plane). It can play louder than 100 dB at 53 Hz and 72 Hz, but 30 Hz was the limiting factor.

    After looking at the spectral display for both and listening to the tones being played, I've got to swing toward 30/72 as having a more uniform distribution of IM components across the pass band, including a large-ish one at 12 Hz and a few between the two test tones too. Subjectively it also sounds more objectionable to my ears, but YMMV of course.

    I'll have to calc IMD for both data sets to see which set of test tones provides a higher value. It will also be intersting to calc the combined THD from both tones to see if THD is higher than the IMD. At first blush just by glancing at the spectral display, I suspect (as does Ilkka who has seen this data) that this is indeed the case.

    If the "higher THD than IMD" trend holds over time for several tested subwoofers, it will serve to deconstruct the notion that a woofer can have low THD but high IMD....and it will also allow THD to be used with confidence as a proxy for IMD - at least on a straight percentage basis. It would not address the subject of harmonic/IM weighting for relative audibility or offensiveness, though.
     
  11. Warren_Sc

    Warren_Sc Second Unit

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    Hi-

    What's the latest?

    Thanks,
     
  12. Ilkka R

    Ilkka R Second Unit

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    My next measurement session is due to happen quite soon (in 2-3 weeks), so after that we will be having loads of comparable IMD data for many different subs. Including both low end and high end subs.

    The fundamentals will be 30 Hz and 72 Hz. I will probably be testing IMD at three different levels, 90 dB, 100 dB and 110 dB. Maybe even higher if the subwoofer can do it.
     
  13. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    >>>The fundamentals will be 30 Hz and 72 Hz. I will probably be testing IMD at three different levels, 90 dB, 100 dB and 110 dB. Maybe even higher if the subwoofer can do it
     
  14. Ilkka R

    Ilkka R Second Unit

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    Yes, naturally it will be 2 m GP. And I wouldn't be so sure about the other one... [​IMG] There aren't many, but there are some.
     
  15. Edward J M

    Edward J M Cinematographer

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    Wow - 10 dB is a big jump (a 300% increase in acoustic output). If the sub craps out between 10 dB intervals, maybe you could fall back on 5 dB intervals.
     
  16. Ilkka R

    Ilkka R Second Unit

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    It is a big jump, but I would like to cover as large amplitude range as possible, but I don't have time to use very small steps...for example 90, 95, 100, 105, 110 dB. But we'll see...
     
  17. Edward J M

    Edward J M Cinematographer

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    It sounds like a massive undertaking. Better bring the gas grills and plenty of food for a big cook-out. [​IMG]

    Take plenty of pics - looking forward to seeing/reading everything. [​IMG]
     
  18. Warren_Sc

    Warren_Sc Second Unit

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    I am looking forward to it...

    ...just make sure all of that festive stuff does not effect the noise floor...
    [​IMG] [​IMG]

    Btw, if possible there should be time allowed to one test within your intervals to reduce potential questions. For example, maybe the test is 90, 100, 110, then if one frequency craps out you can squeeze in a 95 or a 105. Then again, I'll take whatever you can do. Thanks!
     
  19. Lee-c

    Lee-c Second Unit

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    Interesting tests. [​IMG] Have the results of any of the outdoor tests come in yet? [​IMG]
     
  20. Ilkka R

    Ilkka R Second Unit

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