Newbie sub eq help

Discussion in 'Archived Threads 2001-2004' started by Steven Hen, Mar 30, 2002.

  1. Steven Hen

    Steven Hen Stunt Coordinator

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    Here's the readings I get running the test tones from the comp setup guide. My polk psw650 sub does absolutely nothing for the 16, 18 and 20 then starts to kick in on the 22 tone. Is this because of the sub itself? I'm totally new too this eq stuff. I just ordered the bfd and a svs 25-31cs+. trying to understand whats going on before the svs comes. Can someone explain what my graph means?
    http://www.villagephotos.com/pubimage.asp?id_=185615
     
  2. brucek

    brucek Second Unit

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    Steven,

    Assuming that you took your readings from your listening position and that your graph is showing the "corrected" SPL meter readings, then I would think you will benefit greatly from equalization.

    The PSW650 is a fairly good sub with a specification of being 3dB down at 28Hz. This would mean that in an anechoic chamber you would likely see a fairly flat response from around 30Hz up to near your crossover frequency, which is likely at 80Hz. Then it would be dropping off according to the crossover slope.

    If you look at your graph from 30Hz to 80Hz, you have a very large peak which should be fairly flat, and could be made so with equalization.

    Your room is the likely cause of this peak. With that peak, the level that you are now calibrating your sub to is greatly influenced by the peak. You are basically listening to what is called "one note" bass. If the response was flat (or with a small house curve) then you would be able to bring up the wholesale level of your sub when calibrating and it would sound much smoother and lower.

    The SVS will likely exhibit the same problem until its equalized, although the SVS sub will go much lower than the Polk. I would not recommend using both subs in your system.

    brucek
     
  3. Steven Hen

    Steven Hen Stunt Coordinator

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    brucek,
    Thanks for the input, I think I'm starting to understand this sub equalization stuff. In reading many of the other bfd posts, I've felt quite overwhelmed by the level of technical speak! Hopefully by the time my new svs and bfd arrive I will have somewhat of a clue on how to set it up.
    I moved my polk sub to a different location. I think the graph looks a little better but still have the peak at 50.
    http://www.villagephotos.com/pubimage.asp?id_=186407
     
  4. Wayne A. Pflughaupt

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    Brucek,

    Haven’t we “been there” with this sub? I think a guy named Angel posted in-room readings with this sub and an SVS a month or two ago, and if I remember the Polk’s response looked a lot like this one, while the SVS went handily down to 16Hz or so.

    We both recommend that he ditch the Polk.

    Do you remember this?

    Regards,

    Wayne A. Pflughaupt
     
  5. brucek

    brucek Second Unit

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    Wayne,
    Yep, I think his name was angel and it was a PSW650 model too. It also seemed to drop off very quickly below 30Hz while his SVS went down to 16Hz.
    I remember you wisely steered him away from trying to integrate the two.... [​IMG]
    brucek
     
  6. Steve Young

    Steve Young Stunt Coordinator

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    What test tones can you use to test your subwoofer output? Is there an mp3 I can burn to disc to run on my system and then measure with the rat shack spl meter?

    Thanks

    Steve
     
  7. Steven Hen

    Steven Hen Stunt Coordinator

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    Steve Y,
    I got my test tones here
    http://www.snapbug.ws/sinewaves/
    They are in mp3, I converted to wav then burnt to cd.
    Yes, my polk psw650 will be retired to another room once my svs arrives.
     
  8. brucek

    brucek Second Unit

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    Steve Y,
    Download this FREE tone generator listed below and save the tones in three second files and burn them on a CDR.
    http://www.nch.com.au/action/index.html
    Scroll down to Freeware and click on TONE GENERATOR......
    It's only necessary to use sixth octave tones.
    Enter the values into the Excel graphs found at this site.
    http://www.snapbug.ws/bfd.htm
    brucek
     
  9. Steve Young

    Steve Young Stunt Coordinator

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    Bruce, thanks, but could you elaborate on that a bit?

    6th octave tones?

    And what frequencies do I have to go through?

    BTW I have a SVS 20-39CS.

    Thanks

    Steve
     
  10. Steven Hen

    Steven Hen Stunt Coordinator

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    Steve Y,

    If you dl that excel file that brucek was referring to, look in the purple column-it lists the freq that you need to use or look at the graph above 16,18,20 etc. The excel spread sheet is a excellent tool as it automatically corrects your raw readings.
     
  11. Cam S

    Cam S Screenwriter

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    Thanks for the link to the sine wave MP3's Steven, I was looking for something like that!
     
  12. brucek

    brucek Second Unit

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    Steve Y,
    To mathematically comply with the logarithmic nature that is representative of human hearing, we use octaves.
    Each octave is a doubling of a frequency. So 20Hz is an octave above 10Hz and 40Hz is an octave above 20Hz. In that regard, the notes within a scale are not equally distributed in frequency. For example, from 20Hz to 40Hz there are 20 (one hertz) steps for the octave. From 40Hz to 80Hz there are 40 (one hertz) steps. The number of steps are getting progressively greater as the frequency increases. So you can't just divide an octave in half and expect the frequency to be half way between. It requires a formula. I won't bother going into it, because it's kinda boring.
    In equal tempered tuning there are considered to be 12 equally spaced notes per octave. It is accepted that if you equalize a subwoofer to 1/6th octave tones, that the response will sound correct even if there are some anomalies between these tones. So we use 1/6 octave tones to do frequency response checks.
    The values on the Excel graph that Steve H mentions on Sonnies site aren't exact center frequencies but are the preferred numbers because the exact 1/6 octave tones are taken to three decimal places and will be different depending on the accepted starting center frequency. These are close approximations or named frequencies and work very well. So while the 1/6th octave below 40Hz may be exactly 35.636Hz, the named ISO frequency is 36... The standard named preferred frequencies I believe is referenced to a starting center frequency of 1000Hz.
    The tone generator I referenced the site for you is very useful in this regard. If I enter a frequency, then every mouse click on the +/- to change the frequency is a 1/12 octave jump. Pretty handy. Enter 40.000Hz and click twice on minus and your 1/6 octave below at 35.636Hz (36). Click twice more minus and your at the next 1/6 octave down at 31.748Hz (31.5). Click twelve times and you're at 20.001hz = one octave below 40Hz..........................get it?
    So if I choose any starting center frequency, say 40.000Hz for example, the exact calculated 1/6 octaves as referenced to the named frequencies on Excel graph would be 15.874 17.818 20.001 22.450 25.199 28.284 31.748 35.636 40.000 44.899 50.398 56.570 63.498 71.274 80.002 89.799 100.796 113.140 126.996 142.549 160.005. The frequencies on the graph are the named frequencies.
    But if I chose 1000Hz as my starting point and calculated all the exact 1/6 octaves below it, these numbers would be slightly different than above because for the ones above I chose 40Hz as my starting frequency. You're right, it is boring - aren't you sorry you asked - just use the values on Sonnies graph...... [​IMG]
    brucek
     
  13. Steve Young

    Steve Young Stunt Coordinator

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    Thanks for the info. One last question though, at what volume level do you set to when performing this test?

    I figure you must set it to a reference level then run the tones? (not too loud of course)

    Steve
     
  14. brucek

    brucek Second Unit

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    Steve Y,

    Well, it's good to start around measured 85dB to 90dB SPL at maybe 40Hz so you have some room to increase and decrease from on the graph. Too low a volume won't properly excite the room.

    It's very important though, once you've set the volume on your processor to not touch it until you're finished your response test or you won't have a reference point. It's wise each time you do a session of measuring and graphing to set the volume first and then carry out a "no-filters" response and graph it. This is your reference. Then apply your filters and do a "with filters" response and graph these results on top of the no filters response. As long as you haven't touched the volume control for these two graphs, then you have a perfect representation of the effect of the filters. If you've touched the volume, then the no-filters graph is not valid as a reference for the filters effects.

    brucek
     

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