Bass Peaks and Dips

Discussion in 'Archived Threads 2001-2004' started by James Edward, Mar 14, 2003.

  1. James Edward

    James Edward Supporting Actor

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    Last night, I decided I was unhappy with the bass in my system. It seemed I had a lot of low bass(under 40Hz)but much less upper bass. Using test tones from the MoFi Sound Check CD and my SPL meter confirmed my suspicions. Plenty of bass at 40Hz and below, and slight dropoff at 50Hz, and 6-8db down at 80 and 100Hz.

    My tone controls for the receiver seem to be centered around 80Hz, so I was able to smooth things out by raising my bass tone control to +8, and lowereing the sub level 2 db. Fronts are set to small, so the control won't affect much below 80 Hz.

    But I know that this is only affecting my two fronts- the tone controls only affect the front L/R. Others on the forum have used equalizers to smooth out their response- do those equalizers work up to the 80-100Hz levels, and if so, is it advisable to have the sub compensating for the dropoff at those levels?

    I have a square, acoustical nightmare for a room. Please advise. If more info is needed, let me know.
     
  2. James Edward

    James Edward Supporting Actor

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    I guess I'll help this along. Should I be looking at the Behringer Feedback Destroyer in my situation?

    If it helps- Sub is SVS 25-31PC Plus, tuned to 20 Hz, receiver is NAD T752, speakers are PSB 4T's, 8C, 1B's.
     
  3. jeff morris

    jeff morris Agent

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    I'm considering one of two options currently available. One is the 1/3 octave eq that SVS offers and the other is the BFD. I suppose each has its own pluses and minuses. I can appreciate the BFD and how parametric eq'ing can be more precise in a given situation, like with narrow banded problems of peaks or dips, while the broadband eq being used to adjust more for 'house curves', which seems to be what you might be going after.
    Has there ever been a discussion on this board about the benefits of either??
    I'm first going to try some hand plots with my RS meter and try and get a general picture of how bad/good my room actually is. Perhaps some of the other members can speak of their own experiences with eq'ing.



    jeff[​IMG]
     
  4. Frank Carter

    Frank Carter Screenwriter

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    It seems like you're suffering from some room cancellations. A BFD usually makes a significant difference. The first thing you need to do is plot your response to see what the FR looks like. I can email you an excel sheet with where you just enter the number and it graphs it while automatically entering the correction values for the RS SPL meter.

    For tones I use Stryke's Basszone Test CD 1. If you're CD doesn't have the correct freq's for the graph you can creat your own and burn them on a cd with the NCH tone generator.
     
  5. RichardHOS

    RichardHOS Second Unit

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    I would not suggest a graphic EQ like the one SVS offers. Mode induced peaks and nulls can be very narrow in bandwidth. 1/3 is certainly not good enough, and I wouldn't even be that comfortable with 1/6 octave.

    The BFD on the other hand fits the bill perfectly.
     
  6. jeff morris

    jeff morris Agent

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    Thank you Richard for your explanation, which is alot clearer than what I thought I posted. Perhaps the combination of the two peices together, the BFD to flatten the responce and then the graphic eq used like 'tone controls' to set the sound to one's preference.

    jeff[​IMG]
     
  7. James Edward

    James Edward Supporting Actor

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    OK- If I get the BFD. or something similar, how do I run the test tones? Specifically:

    1. The test tones are on a CD. Does this mean that I should use my '5 channel stereo' mode to send tones to all the speakers?

    2. If above is correct, do I then take the readings at the various frequencies with the SPL meter, and begin to make my eq adjustments to smooth out the response?

    3. If all speakers are set to 'small', with a 80Hz x-over point, should I try and eq much above the 80Hz level with the eq in the sub signal path? Or am I effectively out of the sub's range?

    4. I have adjustable crossover points- 40, 60, 80, 100, 120.
    If I'm using an eq, does it make any sense to use anything other than the usual 80Hz?

    Thanks for the replies.
     
  8. Wayne A. Pflughaupt

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    James,

    It’s best to take your readings (from the test CD) with the mains on. That way their interaction with the sub can be taken into account when you equalize.

    Keep in mind that a sub’s crossover does not act like a “brick wall.” So, the equalizer can indeed effect the sub above the crossover point. There’s really no reason to change your crossover point.

    Regards,
    Wayne A. Pflughaupt
     
  9. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    Maybe you've already thought of this, but my first guess would be that you need to check the phase setting of your sub. If the sub is out of phase with the mains, then you will get cancellation in the crossover range (which is what you're describing -- 8dB drop around 80Hz). If your sub has a phase switch, try flipping that to the other setting and taking new readings. If you're using a subwoofer connected to an external amp, simply reverse the positive and negative wires to switch phase. If your sub has a continuously variable phase dial, try the opposite setting from where you're at now and then fine-tune.

    You have to keep in mind that even though you may have positive hooked to positive and negative to negative, the acoustics of the room can reverse the phasing of a speaker, so it's important to check for it. My room does this same thing. At 0 degrees phase, the upper bass sounds anemic. At 180 degrees, it comes alive (and gives much flatter frequency response).
     
  10. James Edward

    James Edward Supporting Actor

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    I checked the phase after jeremy suggested doing so, but the output went down even more at 80 Hz. It was also lower at the other frequencies, so phase is out as my problem.

    I am able to smooth things out quite a bit with equalization, but, as usual, other questions have arisen.

    1- The meter seems to jump around quite a bit- I have been using the higher swing of the meter as my db value. Is this correct? I am using c-weighting and slow on the meter.
    How should I read the meter?

    2- My readings, with the meter corrected values, are as follows:

    Hz Db
    20 81.5
    25 80
    31.5 75
    40 75.5
    50 76
    63 71.5
    80 71.5

    Can they be off this much? 10 db's?

    Thanks again...
     
  11. Jim A. Banville

    Jim A. Banville Supporting Actor

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  12. PaulT

    PaulT Supporting Actor

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    That's a pretty nice room curve [​IMG]
     
  13. Wayne A. Pflughaupt

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    James,

     
  14. James Edward

    James Edward Supporting Actor

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    OK- I appreciate all the replies- and I have an ART 355(2 channel version 351) available.

    This will be my last question:



    Does this mean I should try and keep the 20 and 25Hz output at about the same level as 31.5? I think that's what you're saying, I just don't want to misinterpret...
    And is it OK to try and up the level at 63 and 80Hz? they seem awfully low.

    Thanks everyone.
     
  15. Wayne A. Pflughaupt

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    Don't worry James, there is no quota on questions. [​IMG]

     

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