what is a null spot in a freq resp curve?

Discussion in 'Archived Threads 2001-2004' started by gomez_a, Feb 28, 2002.

  1. gomez_a

    gomez_a Stunt Coordinator

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    Can someone please explain what is a null spot in a freq resp curve is? Can a parametric EQ fix it?
     
  2. Bob Christensen

    Bob Christensen Stunt Coordinator

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    Gomez: A null is a “low spot” on a frequency response curve. It represents a frequency range that is (for one reason or another) under-represented. This can be due to a mechanical factor (enclosure design, driver choice) or electronic (cross-over), or due to room interaction (phase interaction, standing waves, etc). Yes, parametric EQ typically can help. But this is typically at the expense of attenuating the high points on the frequency response, rather than boosting the null frequencies themselves.
     
  3. Steve_Ma

    Steve_Ma Second Unit

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    Gomez, I'm glad you asked the question. Bob, Great answer. Can you (or someone else) give me a layman's explanation as to why boosting a null spot decreases the headroom?

    Also, is it true that null spots are generally considered to be less problematic than peaks with regards to bass response?

    I am plannning on buying a BFD soon, and I know I'll have to deal with this.

    --Steve
     
  4. Bob Christensen

    Bob Christensen Stunt Coordinator

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    Steve: In my experience, it is MUCH easier to cut a peak out, rather than EQ out a null.
     
  5. Bob Christensen

    Bob Christensen Stunt Coordinator

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    Actually, I will temper my comment above with the fact that many people subjectively find a peak in the 30-80hz range pleasing ot the ear in a sub. So personal tastes are also a huge factor.
     
  6. gomez_a

    gomez_a Stunt Coordinator

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    uh, ohh, then I'm not too encouraged with my sub's response curves: http://ncstatesman.tripod.com/angelswebsite/id1.html . . . The bottom graph (yellow curve) looks like it has some 7 to 10 dB dips at 45, 56, & 71 Hz . . . which looks like nulls to me
    Thank you for explaining nulls!
     
  7. Bob Christensen

    Bob Christensen Stunt Coordinator

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    Pull down the nodes at 16 and 36hz, and it looks like it shouldn’t be too bad. The 36hz node is likely either a room gain issue, or maybe a crossover issue. You could try moving the sub into a different location (or taking near-field readings) to see if it goes away (suggesting a room interaction issue).
     
  8. Steve_Ma

    Steve_Ma Second Unit

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    Thanks Bob, but I guess I am still wondering why boosting a null compromizes the over all sub performance? (are we talking about headroom here?)

    Does it require that much more power to boost a null as opposed to cutting a peak or something?
     
  9. Bob Christensen

    Bob Christensen Stunt Coordinator

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    Steve: As tempting as it appears, it is frequently a backwards approach to try to boost nulls with the EQ. The nulls are there because of some “real issue”. This real issue is doing something to swallow up certain frequencies. Simply heaping on more wattage into the same frequencies will likely not help. Think of it as shining a flashlight up into the night sky. You don’t see anything because the light is “swallowed up” by the sky. Point a search light up into the same night sky and you will likely see nothing as well.

    EQing typically works best when you cut gain to the peaks, which typically respond rather well. You can often bump up the overall power to the sub then and see a dramatic improvement.
     
  10. Steve_Ma

    Steve_Ma Second Unit

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    Light shines on great marble head.

    Thanks Bob,

    --Steve
     
  11. BruceD

    BruceD Screenwriter

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    Nulls/peaks are frequently caused by specific locations of a listener and/or sub within the room. In other words move either one and the nulls/peaks change.

    This simply means the waves of low frequency sound interact with the walls of the room and this interaction changes when you move your seat, stand-up, or move the sub.

    So if one is going to use a parametric EQ, a specific corner of the room will probably make the most peaks, but those peaks can then be cut with the EQ to give you a smoother bass response.

    Peaks don't actually tax the headroom capability of an amp, they are only specific frequencies that have been re-inforced by reflections from the room's walls. On the other hand pumping more power (EQ boost) into a null does tax the amp's headroom capability (gives it less reserves to deal with instantaneous peaks).

    Headroom is essentially the capability of the amp to respond to instantaneous loud source material without distorting. I would describe the ability to play loud without distortion as a "wide dynamic bandwidth" (large difference between soft and loud music).
     
  12. Wayne A. Pflughaupt

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    Bruce, I usually like your posts on this subject – this is a good one too, but I have part company with you in one regard.

    I like the way Bob described this:
     

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