Re-tweaking my DIY center channel speaker

Discussion in 'Archived Threads 2001-2004' started by Patrick Sun, Aug 6, 2001.

  1. Patrick Sun

    Patrick Sun Studio Mogul

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    Now that I've gotten my SunOnes and SunTwos squared away (in my mind, at least) this past Friday and Saturday (until I get another opinion on their sound later), I figured I'd take another stab at my Center channel speaker beast I created about 4-6 months ago.
    I asked one of my friends what he thought of the sound quality of it about 3 months ago, and he thought it sounded a little funny, but once you got used to it, it started to sound okay. Well, that just wasn't music to my ears, so I ordered some capacitors a few weeks ago so that I could be flexible in my CC tweaking/tune-up.
    I had nothing to do Sunday, so I decided to give it a shot using the lessons I've learned since, plus the perspective I've gained by tweaking the other 2 sets of speakers. I was sort of amazed at the speed of my tweaking this time around. Then again, I didn't "document" every tweak/change in an orderly fashion, but I knew where I was, and where I wanted to be. I did make notes of each change, but didn't produce graphs upon graphs with each change
    this time around.
    I was bummed that my plans to replace the tweeter with the Morel MDT30s that I used in my other speakers were thwarted by the existing tweeter hole being too big! Doh! So I decided to press on with the original tweeter (from my old DIY speaker project). (after tweaking, I can't really find any reason to feel too bummed about not being able to use the Morel tweeter - I'm using an old Phillips AD11600/T8 fabric tweeter).
    I used a more systematic approach this time (the previous efforts had me changing something and then taking measurements of the whole frequency response and then deciding if the change was beneficial - I
    had no idea if the individual filter slope were actually working as designed). I broke the tuning down into 3 phases:
    measuring the current individual drivers with their current filters in place, graphing their response, and then making
    changes to get the filter slopes to cross at the desired frequencies.
    So once I was happy with the woofer low pass filter, I moved on to the midrange high pass filter, got the integration of it with the woofer filter where I wanted, and then moved onto the integration of the midrange low pass filter and the tweeter high pass filter. Sounds simple, eh?
    I can't believe I didn't do this the 1st time around, and I was tweaking pretty darn blindly.
    I can't show the "before" situation yet (left it at home - will post it later tonight), but I can show the "after" frequency response down below. I started with the woofer response. I knew I wanted to slope off the parallelled
    woofers' response around 500-700Hz (to try and get that baffle step loss minimized), and then use my midrange from around 500-3000Hz, and then let the tweeter take care of the rest.
    Let just be nice and say that my initial attempt at the crossover was pretty marginal.
    Once I plotted the individual response of the driver/filters, I knew why my friend said the CC sounded weird: the tweeter was padded too much, and the midrange was carrying too much of the load on the high end, and IIRC, the woofers were playing too high up the road (around 1200Hz).
    After about 5 hours of tinkering with the filters, seeing if the textbook filters were doing their things (or if I noticed that the filters were "delayed" in when the slope started to kick in, I would calculate a new filter with the break frequency that was lower by 1/3 or 2/3 of an octave to get the filter to cross over as desired), I got something that started to sound "normal" to me, with slightly more tweeter output, and was able to shape the midrange output to covered the region I wanted it to cover.
    I think it took 2 changes to the woofer filter, 5 changes for the midrange (for the high pass and low pass filtering), and another 3 for the tweeter filter.
    What also helped was when I got my other speakers measured on the LAud setup, I was able to produce a table of RS SPL meter measurement correction values for my particular meter that gets me in the ballpark. I put these corrections values (what I needed to add or subtract from any given frequency's SPL reading - since I use 1/3 octave test tones, it's a simple matter to standardize on the corrections) in an Excel spreadsheet, and it gave me a
    "better" idea of the frequency response, especially in the high frequencies after 3KHz. Believe me, it's a lot closer
    than the correction values I was using before (which led me to pad my tweeters too much - usually 3dB too much).
    Just keep in mind that for the low end (60-150Hz) I have this weird room mode that always accentuates the bass response, it's not as bad as it sounds/looks once the speaker is put in its place above the TV. The extended ridge in the 1KHz range is actually beneficial once the speaker is in place as well, so that's why I did not take steps to pad down the midrange. It's not all that "flat-line" pretty, but without serious measuring gear and filter design software, it gets most of the job done. So without further ado...
    SunCC frequency response plot
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  2. Hank Frankenberg

    Hank Frankenberg Cinematographer

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    Congrats - you've got a fairly flat response curve there. If you believe your RS meter corrections are correct, you might post them. I've wondered about the correction table that has been posted for the last couple of years. Has anyone compared a new RS meter to a calibrated lab-grade mic lately to verify the correction table values?
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    [Edited last by Hank Frankenberg on August 06, 2001 at 12:42 PM]
     
  3. Patrick Sun

    Patrick Sun Studio Mogul

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    Hank, thanks. When I get back home, I'll dig them up and post them (but I hope they don't become "the gospel" because it's really more representative of my meter, and of course YMMV).
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  4. Patrick Sun

    Patrick Sun Studio Mogul

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    Here's the "before" frequency response when I started yesterday. As you can see it was a bit messy (too much of the woofers were working past 1KHz, the midrange kicked in too late, and the tweeter was padded too much).
    And here's what I used for correction values for my own RS SPL meter for values from 2KHz (I did a comparison from a LAud frequency response plot, and compared it to what I measured with the SPL meter and made the necessary correction values to get the same vales for a home made SPL meter plot):
    2.0KHz 0
    2.5KHz 0
    3.2KHz -3
    4.0KHz -6
    5.1KHz -7
    6.4KHz -6
    8.1KHz -1
    10.2KHz +7
    12.9KHz +13
    16.2KHz +18
    20.0KHz +25
    Please don't think this will be the same for your SPL meter, it most likely won't, but maybe it'll get you within the ballpark because of the major swings after 2KHz in the readings.
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  5. Hank Frankenberg

    Hank Frankenberg Cinematographer

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    Patrick, thanks for posting you graphs and your updates. I see your point about over-padding the tweet and the xover diff between your woof and mid.
    Quote:
    "Please don't think this will be the same for your SPL meter..."
    Exactly my point and what's been bothering me about the correction chart that's been out for a couple of years. I know that with "budget" circuitry and component tolerances/changes, etc, that a meter three years old may have a differenct response than a brand new one. I think we should use our RS meters as indicators of relative differences between speakers measured at one point in time, and not use the numbers as absolute measurements, and certainly not use the numbers to compare to others' speakers. If my speaker A is x dB under my other speakers, I should adjust A up x dB and forgiddaboudit.
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  6. Patrick Sun

    Patrick Sun Studio Mogul

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    Okay, here's one more attempt at getting my DIY center channel speaker to sound better.
    When we last visited, the response looked like this:
    [​IMG] There is a little dip around 500Hz, but nothing too critical. I'll try to do the individual driver responses when I get some time/energy/motivation. Heh.
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  7. Hank Frankenberg

    Hank Frankenberg Cinematographer

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    Lookin' good, Patrick! If the mids sound natural, you're 95% home, right? Would you kick in your bass management at 70 Hz?
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  8. Patrick Sun

    Patrick Sun Studio Mogul

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    Thanks! Well, if my receiver would allow me to mess with the subwoofer output's frequency cutoff, I would, but I can't at this time. But even so, that's just for running this speaker in the small mode, but I might be tempted to run it in the large mode (even if it only goes down to around 50Hz).
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  9. Patrick Sun

    Patrick Sun Studio Mogul

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    Okay, finally measured the individual drivers at 1 meter away (couldn't do the 3m measurement too easily), so here it is:
    Final? CC frequency response graph
    It looks like the midrange doesn't do much, but it does fill in what I need in the 700-2000Hz region, so I can't say it's a total waste given the layout of the center channel front baffle. The hump down under 100Hz isn't too problematic, and nothing to get concerned about (especially if I run the speaker in small mode).
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