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Broad Depression in Tweeter Response 7-8kHz

ani_101ani_101
in DIY
I am getting a broad shallow depression in the 7-8khz region for the tweeter FR response, about 4-5 db, and broad. Is this a room artifact, baffle artifact or something else? I recall seeing it on another tweeter response too...

Comments

  • PWRRYDPWRRYD
    Most likely something else.  If you are taking a gated measurement it is not room related.  If your baffle is wider than 2 or so inches then it's not diffraction.  What tweeter on what width/shape baffle?
  • MzissersonMzisserson
    The wavelength is too small to be room or baffle related. How big is the faceplate? Is the tweeter flush or proud? Also, what does it do 15/30 off axis?
    ani_101
  • FaceFace
    Nearfield measurements are in order.
  • ani_101ani_101
    This is the measurement:

    Broad depression from aroun 6k to 8k...

  • ani_101ani_101
    The baffle is 8" in width, this is for an MTM with the tweeter centered vertically on the 8x18 baffle and offset horizontally by 1".
  • jr@macjr@mac
    Crossover frequency?
    I have a signature.
  • ani_101ani_101
    No crossover, this is the raw tweeter measurement, about 1 meter on tweeter axis
  • ani_101ani_101
    This is the speaker. Normally I am used to seeing smooth FR curve, maybe I am able to use a lower gating, it is looking ragged?


  • MzissersonMzisserson
    Tweeter center to fat baffle edge. 1/2 wavelength could be getting destrucgive interference, or reflection off woofers with the equi-distant spacing. I agree with Mike, try a NF meaaurement and see if ir goes away. Also off axis.
  • jhollanderjhollander
    Here's the baffle from Edge

     John H, btw forum has decided I don't get emails
  • jr@macjr@mac
    I had some fuzzers on my mic one time that led to a hair pulling experience figuring out what was wrong with a speaker. 
    I have a signature.
  • MzissersonMzisserson
    jhollander said:
    Here's the baffle from Edge

    Thanks John,
       May be reflections from the woofer cones then. I would definately like to see near field. 
  • ani_101ani_101
    Thanks all, I'll get some near field and off axis in the afternoon once I get back home.  These are all mcm gradient select drivers, 5" AL and mcm silk dome tweeter which were door prizes from Iowa.

    The woofer models close to 40 in 20 liters for a pair based on actual TS, driver cost is just 112$ for a pair of speakers. Hoping this would be a nice bookshelf.
  • jr@macjr@mac
    That depression is real, then. There is a nice copper cap on the motor that suppresses the inductance rise, which also leads to a rising top end response. The baffle effects below that range are causing what appears to be a dip. That tweeter uses the same motor, just different dome material, as the tweeter I used in the MCM 2-way I finished a while back.


    I wouldn't worry about it, just work it into the crossover. There are far worse regions to have reduced output. 

    That being said, the aluminum version offered pretty low distortion above 3K, fair to 2.5k. I bet the soft dome is similar. I am very curious about the little Gradient woofers. I measured T/S on all of the MCM drivers when I had them last fall before taking to Iowa - so I expected to see some nice F3 action from them. Curious about distortion through 3K. 

    Sweet budget project, eat your heart out PE/C-Note. 
    I have a signature.
  • MzissersonMzisserson
    Jr, why do youbelieve it is the baffle effects below that range? They are not low enough to be some divisible raiden of the wavelengths involved. 

    I agree, I dont think it will amount to a hill o'beans sonically, but Just wondering the thought process. 
  • jr@macjr@mac
    Based on John's posted simulation, it presents a gradual rise in response from the dip induced at ~4500 out to ~800Hz, said region the MCM tweeter has a very flat infinite baffle response. The tweeter has a rising response starting around 7500 or so. The dip Ani measured is sammiched right between those two artifacts.
    I have a signature.
  • MzissersonMzisserson
    jr@mac said:
    Based on John's posted simulation, it presents a gradual rise in response from the dip induced at ~4500 out to ~800Hz, said region the MCM tweeter has a very flat infinite baffle response. The tweeter has a rising response starting around 7500 or so. The dip Ani measured is sammiched right between those two artifacts.
    Yep, that makes sense! Sometimes the math doesnt tell the tale, thx for that!
  • ani_101ani_101
    Thanks JR, I'll go on with the rest of the design. The woofers modelled at 37hz f3 for 28-39 liters but was too big. 41hz in 20 liters for 100db is pretty good for me. I'll take the rest of the measurements and distortion anyway.
  • ani_101ani_101
    Tweeter distortion measurements:

    questions, what would be a good crossover point based on the distortion measurements? And can I cross at 2k or would that be a stretch? the FS of the tweeter is around 700. My prelim XO is showing as 2k (posted in another thread).

    in db, 1k - 20k


    in %, 1k - 20k


  • jr@macjr@mac
    2500 would be better - might help suppress that moderate peak in odd order at 3K.
    I have a signature.
  • R-CarpenterR-Carpenter
    I am not very familiar with HOLM. What was the gating?
  • ani_101ani_101
    200HZ, the lowest I could get in room.
  • R-CarpenterR-Carpenter
    edited June 2017
    How many milliseconds? Lower frequency limit is a result of your gating but comes at the price.
    I would gate the MLS so you are limited to 800hz or so and bring the mic closer. .5m would be fine.
    My suspicion is that you are picking up a lot of secondary arrival because the gate is too wide and mic is too far.
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