Subwoofer EQ test cds

Discussion in 'Speakers' started by Ryan Tsang, Nov 9, 2004.

  1. Ryan Tsang

    Ryan Tsang Second Unit

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    I downloaded the version from Snapbug containing 1/6 octave sine waves. What other test discs are out there that have static tones for easy plotting for freq response?
     
  2. PaulT

    PaulT Supporting Actor

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  3. Kevin C Brown

    Kevin C Brown Producer

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    Here's another test tone generator I got from a tip from BGL:

    http://www.esser.u-net.com/ttg.htm

    I have used this, really easy and simple, and you can demo it for 30 days free.
     
  4. Mike Mitrook

    Mike Mitrook Agent

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    Ryan,
    I used the Rives Audio test CD2. The thing that I like about it is the 2 sets of test tones. The second set is adjusted for the Rat Shack meter (anything to make my life easier)[​IMG] .
     
  5. Brian L

    Brian L Cinematographer

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    Mike,

    I have seen the Rives, and it looks tempting, but what is the frequency spread on the test tones?

    I thought I read that its 1/3 octave. If thats true, its not really optimum for doing EQ work with a sub. For that, you really need fixed frequency sin waves, or you will not be able to get accurate meter readings.....too much interpolation.

    That the tones are pre-corrected for the RS meter is a cool thing, but it just occurred to me that the TTG software mentioned above allows you to burn tones at fixed frequency AND level.

    While I have not done it yet, I could actually burn a disc of sin wavs, pre-compensated in level for the known RS correction factors.

    KCB......did what do you think of that??????

    BGL
     
  6. Bhagi Katbamna

    Bhagi Katbamna Supporting Actor

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    How would you compensate? Do you set the tones above the really low bass at a lower level? Or, do you set the really low bass at lower levels on the test tone disc?
     
  7. Brian L

    Brian L Cinematographer

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    Well, how about this for example.

    Lets say that the RS meter is known to be down 7.5dB at 20 Hz, down 1.5dB at 80 Hz, and flat at 400 Hz (which it is, actually!).

    If is had a test tone at 400 Hz who's level was recorded at -20dbFs, could I not record a 20 Hz tone at -12.5dbFs, and a 80 Hz tone at -18.5dbFs, and would that in essence pre-compensate my test tones, so that I would not need to bias my readings?

    I should think so, assuming one important piece of the puzzle; that the signal levels that TTG lets me set equate to actual acoustic power in the room. In other words, does a 1 dbFs change in signal level equate to 1 dB SPL in room?

    The tones that I have already burned were all done at
    0dbFs. If I set my amp such that a tone recorded at 0dbFs delivered an in room SPL of say 80 dB, would another test tone of the same frequency burned at -10dbFs deliver 70 dB SPL?

    That is the question. And I will certainly cop to being a rank amateur when it comes to this stuff, so there may well be 100 other reasons why this whole plan may be rubbish, but it IS fun to play with!

    BGL
     
  8. Bhagi Katbamna

    Bhagi Katbamna Supporting Actor

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    I did 1000Hz at -20, 10HZ at 0, and so on.
     
  9. Kevin C Brown

    Kevin C Brown Producer

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    Brian- You be busted! [​IMG]

    I actually did think of recording the tones with the Radio Shack compensation in there. But I came up with two problems:

    1) When I do sub calibrating, I don't really have a reference, so using fixed compensation on a test disc won't work unless it's "referenced" somehow to a known level coming out of your speakers.

    2) I use GoldWave as my sound editor. I couldn't figure out an easy way to go back and forth between 0 to 1.0 on the GoldWave loudness scale, and dBs.

    But if you could figure out those two, would be possible.

    Sidenote: my test disc rev 1 is done. Sine Waves at or better than 1/10th octave from 10 Hz to 160 Hz. 40 pts. As soon as I "test" it myself, I might offer it up to people for S&H.
     
  10. Brian L

    Brian L Cinematographer

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    I guess what I could do is burn a bunch of tracks of the same frequency, but at 1 dbFs difference in level between each track (i.e., one at 0 dbFs, one at -1dbFs, one at -2dbFs.....).

    It would then be a simple matter to then play them back in room, and see what the SPL difference is between them.

    Amazing what one will do when they flat out refuse to just buy a damn calibrated mic and use decent PC based software like ETF, SprectraPLUS, etc.!

    My own side note: I ran a cal session with my test disk, using frequencies aligned with my Bijou's frequency centers. Interesting results thus far.

    A couple major league nulls that I had not previously noted, and some good size peaks too....much more work to be done, but I did EQ the main LCR between 80 and 800 (80 and down is BFD territory). Subjectively, it sounded better than I recall. I screened ET the other night, and the music sounded fabulous....better than I recall the last time.

    The combo of BFD'ing the sub and dialing in the front trio a bit better seemed to work well.

    BGL
     
  11. Kevin C Brown

    Kevin C Brown Producer

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    I don't know how dBF's relate either...

    My *next* project is something like 200 Hz to 20 kHz by 6ths or 10ths or something.
     
  12. Bhagi Katbamna

    Bhagi Katbamna Supporting Actor

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    Well I have a parametric EQ so what I will do is first equalize the subs(I have 3) to be within 6dB(+/-3dB) using the RS adjusted tones. Then I will calibrate the sub channel using the old Video Essentials and see what happens.
     
  13. Brian L

    Brian L Cinematographer

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    Well, I think my idea of a CD with RS compensated sine wavs may be valid.

    I burned a test disc with 6 tones, all at 60Hz, but at different levels.

    0dbFs
    -1dbFs
    -5dbFs
    -10dbFs
    -15dbFs
    -20dbFs

    I played then back, starting with 0dbFs. I set my AVR so that this produced 95 dB in room via my trusty RS SPL meter.

    Here is what I measured on playback, without touching the volume knob:

    95dB
    94dB
    90dB
    85dB
    80dB
    74.5dB

    I think we have a winner here, gents. It appears that dbFs and dB SPL have a one to one relationship. I suppose I expected that, only because the loudness scale on my AVR is in dB, with 0dB being reference, and tweaking that normally gives me a one to one correlation in SPL.

    OK, tell me what is I am missing here, before I burn a pre-compensated test disc.

    BGL
     
  14. Bhagi Katbamna

    Bhagi Katbamna Supporting Actor

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    I think you went to too much trouble.
    The test CD is to set EQ levels at different frequencies in a relative(not absolute) fashion(that is relative to each other and doesn't really matter what you receiver volume level is).
    Once that is set, then you can calibrate the subwoofer(or LFE) channel relative to the other channels.
    So I'm not sure what you were trying to accomplish by the above.
     
  15. Brian L

    Brian L Cinematographer

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    I am trying to burn a CD with test tones that compensate in level for the RS Meter's known errors at specific frequencies, which is what the Rives disc does, but it does not use sine waves or have fine enough frequency spacing for accurate sub work.

    Having such a disc would allow me to take readings with the RS meter, and not have to bias them up or down with the correction factors. Its just a convenience thing.

    You are aware of the RS correction factors, right?

    If not, do a quick search in the HTF SW archive...they are posted there.

    BGL
     
  16. Brian L

    Brian L Cinematographer

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    Any one care to give me a hint how I can post a section of spreadsheet?

    I have worked out the relative levels based on the correction factors, but I just posted them manually and the page layout went all to hell...would be easier if I can somehow capture the spread dorectly and post that.

    EDIT: OK...just hit a roadblock. TTG will only allow level adjustments in whole numbers...so, I can do -12dbFs, but not -12.5 dbFs. Based on the published factors, I would be off by .5 or .3 on 15 of the 23 frequencies I am targeting. Dang.

    BGL
     
  17. Ryan Tsang

    Ryan Tsang Second Unit

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    Hey Brian:

    Are you familiar with the Snapbug excel spreadsheet? It's got the RS corr factors built in so you just plug in the raw values and it will graph the corrected measurements. It's sounds like you are using a different program entirely so what you are doing sounds like a worthwhile endeavor....
     
  18. Kevin C Brown

    Kevin C Brown Producer

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    Brian- Post a section of a spreadsheet:

    Do a Print Screen of the PC display, and then just post the jpg view of the spreadsheet.

    What you're trying to do works, but I just think it's easier to add them in a spreadsheet. [​IMG]

    I could do this with GoldWave too, but I'd just need to do some experiments to find the relationship between say 95 dB in my room, volume setting on the pre/pro, and then how that correlates to the 0 to 1.0 volume range I have in Goldwave.
     
  19. Bhagi Katbamna

    Bhagi Katbamna Supporting Actor

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    Yes, I know that. I made a test tone CD with the compensation. I don't get the point of testing it with your receiver to see if there is internal consistancy.
     
  20. Brian L

    Brian L Cinematographer

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    The point was that KCB and I weer debating (well, I was anyway!) if a 1 dbFs level difference in the test tone translated to a 1 db SPL difference when measued in room...thats all.

    But it appears to work fine, so problem solved. Sorry of I was unclear about that.

    Yeah, I kow about snapbug....tons of great stuff, used it when I dialed in the sub with the BFD.

    Just trying to get to where I can take some readings w/o needing to haul out the PC to do the compensation. Just like the Rives Test CD but with sines, and at the frequency settings of my choice. I just HATE having to drag the laptop into the den to tweak my rig.

    BGL
     

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