I have an amazing -20dB dip from 4KHz to 20KHz! Why?

Discussion in 'Speakers & Subwoofers' started by Dave Dahl, Sep 7, 2003.

  1. Dave Dahl

    Dave Dahl Stunt Coordinator

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    Hi all,

    I was messing around with a computer based spectrum analyzer with a radio shack SPL meter hooked into it. I have discovered that from 20Hz to 4KHz everything was, while not completely flat, what I had expected. What I did not expect is that starting at 3.5KHz to 4KHz there is a 20dB drop in output. From there to 20KHz it is reasonably flat.

    What on earth can be causing a fourfold decrease in volume above 4KHz? My room is rectangular with dimensions of 251" long and 176" wide. I have a drop ceiling and have arranged my speakers using the standard Cardas method.

    While I know the subwoofer cannot be to blame I performed the test with and without it. I also have bookcases with staggered books at the the early reflection points. I have performed the measurements with and without them only to get the same result.

    This is driving me a bit crazy. Any ideas...



    Thanks!

    Dave
     
  2. Brian L

    Brian L Cinematographer

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    IIRC, the RS meter response drops off a ton above 10K. Take a look at the response charts in instructions.

    Don't have an explanation for the 3K to 10K drop, however.

    I would think if you had problems in that range, your ears would be telling you that there seemed to be a blanket over your tweeters.

    Do your ears concur with your meter? If not, it could just be a problem with the meter.

    BGL
     
  3. Wayne A. Pflughaupt

    Moderator

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  4. RichardHOS

    RichardHOS Second Unit

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    Hmm... a blown tweeter? [​IMG]
     
  5. Dave Dahl

    Dave Dahl Stunt Coordinator

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    Good point Wayne,

    I have experienced a sharp drop off starting at 3.5 KHz, stabilizing at 4KHz and from there remaining stable all the way out to 20KHz.

    The graph looks something like this:

    _______________
    ***************-
    ****************-
    *****************-
    ******************------------------------------

    -Dave
     
  6. Rick_Brown

    Rick_Brown Second Unit

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    Treble control turned way down?
     
  7. Brian L

    Brian L Cinematographer

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    I will bet a shiney new nickel that this is a measurement issue.

    Again, are your ears in agreement with what you are measuring? That kind of a FR will definetly be audible, assuming your ears are working above 3K.

    BGL
     
  8. DarrenAlan

    DarrenAlan Stunt Coordinator

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    Ummm -- you bought Bose?

    Just kidding.
     
  9. BrianWoerndle

    BrianWoerndle Supporting Actor

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    3.5khz - 4khz is usually in the range of the crossover. You could have a bad crossover that is cutting out the tweeter.
     
  10. Danny Richie

    Danny Richie Stunt Coordinator

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    Sounds like it could be a cancellation problem by an out of phase problem.

    It is in the right region for a reverse null dip but that is a pretty narrow range of only 500Hz your describing. Typically it is much broader than that. You could be limited by your measurement gear though.

    Just to see what effect it has you might consider pulling out the tweeter on one of them and flipping the polarity on it then taking another measurement.
     
  11. John Garcia

    John Garcia Executive Producer

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    Disconnect one speaker altogether, and see if you have the same dip, then try the other by itself. Have you tried swapping speakers from one side to the other?
     
  12. Dave Dahl

    Dave Dahl Stunt Coordinator

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    So, it turns out that I finally went to work (I work at Tweeter) and did an A/B comparison between my Haydns and the ones at the store. Lo and behold, my treble is muted compared to the store floor models.

    So I figure that something's up with either the crossover or tweeter and sent both of them into service.


    Wish me luck!

    Dave
     

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