I built my first sub, now it's tweaking time

Discussion in 'Home Theater Projects' started by SamK, Jun 6, 2003.

  1. SamK

    SamK Auditioning

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    After juggling my options for way too long, I decided to just build a sub and see how it goes. Here's what I did:

    I bought a Dayton 12" DVC ($90 at ebay) and a 250W PE amp with no boost. I figured with a good driver and amp I can experiment with different setups.

    I built a 2.5 ft^3 sealed box. That was the largest box that would fit in my preferred sub location. The plan was to try it sealed, and if that didn't work, add a port.

    Using a RS SPL meter and the RS correction tables from the net, I measured the room response from 20 to 85 Hz. It is within 3 Db from 24Hz all the way up, except for a serious peak at 35-45 Hz and a smaller valley at 65-70 Hz. There is a significant drop from 24 to 20 Hz.

    Question: are those RS correction tables accurate? I hear a significant drop in loudness from 34 Hz down, but maybe it is my hearing? The adjusted RS values are pretty flat to 24 or 25.

    I should also mention that my peaks and valleys are nothing like the ones I expected based on the room simulation software I had experimented with. Is that normal? I understand that rooms are hard to model, but I expected at least some correlation...

    Anyways, now that the sub is working my question is: What next?

    - Add some boost at 20Hz? (4 Db looks about right.)
    - Add a port (tuned to 21.5 Hz)? Wouldn't that just add a bump to my reasonably flat in-room response?
    - Buy a BFD and equalize away (but that's a lot more $$$ than some resistors or a port tube)
    - Leave well enough alone?
    - Forget my current box and build a sonosub. [​IMG]

    I should add that this sub is primarily for movies, but I think accurate reproduction of music is important.
     
  2. TimForman

    TimForman Supporting Actor

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    Build the sono-sub.
     
  3. Pete Mazz

    Pete Mazz Supporting Actor

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  4. SamK

    SamK Auditioning

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    What are the symptoms of a leaky box?

    The reason I ask is that when I run a quasi-anechoic measurement I find that the low frequency falloff is much steeper than predicted by WinISD: [email protected] and [email protected] less than predicted values, after applying the RS SPL corrections.

    I was careful when cutting and glueing the box, but did not make any additional effort in sealing it. Will caulking boost my lower bass?

    I plan to finish the box this weekend, so this may be a good time to seal it as well.
     
  5. Jonathan M

    Jonathan M Second Unit

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    I wouldn't trust the RS meter down low - even with the "corrections" file. It has serious rolloff in the electronics, and who knows what the mic is doing. Some trust it, however.

    Can you do an impedance measurement of the box? If so, this will tell you fairly quickly if you have a leak, as you'll see a small second peak as well as the usual resonance peak.

    How do you like the sound? Do you have provisions to add a port without completely messing up the box? If so, then you could add a port and then if you don't like it later, you can always reseal it up.
     
  6. Pete Mazz

    Pete Mazz Supporting Actor

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    Is the box stuffed? If not, I would recommend ~3 lbs polyfill stuffing.

    Did you use some sort of seal around the drivers flange? Test for leaks using a 20 Hz signal and listening for "burps". If you can hear 20 Hz tones, go lower. Very small leaks can be detected dangling some tissue paper around the seams.

    A leaky box would give you a 3rd order rolloff instead of 2nd, but it would take a lot of small leaks to have such an effect. I would worry more about in-room response, personally. What are the nearfield responses below 50?

    Pete
     
  7. Jonathan_D

    Jonathan_D Stunt Coordinator

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    Your perception of the loudness of bass frequencies also starts to drop dramatically below 60hz. IOW, the lower you go the more SPLs are required for you to perceive the sound as the same "loudness". Of course, as pointed out above, the fact that you have a peak at 35-45hz probably exacerbates the situation (though a peak at those frequencies is usually described as "punchy" or "fat" bass).

    Taming room effects on bass is a tricky problem. Get a BFD! [​IMG]
     
  8. SamK

    SamK Auditioning

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    Jonathan,

    I don't know how to test impedance, but I do have a multimeter, if that is of any help.

    I like the way the sub sounds, much better than my Onkyo SKW-110, but then that is not saying much. On the other hand, I want to be sure I'm getting the best for my investment. More of a tweaker's attitude than anything else. If I had bought this sub I would not consider doing anything with it, but since I built it, I should be able to make it perfect!

    I have room for a 3" port, a 4" would be problematic, but I might be able to fit one with a 90 degree bend.
     
  9. Wayne A. Pflughaupt

    Moderator

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    Sam,

    The only reason to go with a ported box is if you are interested its sonic attributes. A ported box will not change the response deviations that the room introduces. I’m with Peter: Keep the sealed box if music is important to you.

    The BFD is your best bet for smoothing room response. Any sub project should budget for one if there is any interest in getting the best performance.

    Personally I wouldn’t be overly concerned about the RS meter’s inaccuracies. If it were all that bad, HT buffs would have ditched it long ago and found something better. The meter’s (read mic?) response deviations are symmetrical in nature, and the correction values are accurate enough to get the job done. All you’re looking for in plotting room response is a point of reference. You can be plus or minus a dB or so and still get a good idea of the overall picture. Besides, any final tweaking should be done by ear, not by numbers, further minimizing the need for clinical accuracy.

    Regards,
    Wayne A. Pflughaupt
     
  10. SamK

    SamK Auditioning

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    Pete,

    The box is partially stuffed with about 1 lb pollyfill. I can try more stuffing...

    I sealed the driver with rope caulk. I just removed the driver today and let me tell you that rope caulk made a beautiful seal. I think I'm going to seal all the internal seams with rope caulk. Easy enough, I have plenty of the stuff and it can't hurt anything. At worst it might eventually dry and fall off.

    I also think the problem is my room response. Nearfield, the response is:

    Freq. Raw Measurement RS Corrected
    60 Hz 76 Db 77.5
    55 75.5 77
    50 75 76.5
    45 74 76
    40 73 75.5
    35 68 71
    30 62 65.5
    25 59 64
    20 50 57.5

    In room:
    Freq. Raw Measurement RS Corrected
    60 Hz 87 Db 88.5
    55 87 88.5
    50 88 89.5
    45 89 91
    40 92 94.5
    35 89 92
    30 86 89.5
    25 83 88
    20 77 84.5
     
  11. SamK

    SamK Auditioning

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    Jonathan_D,

    I agree, the BFD is the only thing that will help with the room response. I just wish I could KNOW what kind of response I'm getting. Perception is not a good tool, and I'm getting surprising results with the RS meter, so I guess I will just have to tune to what I like and forget measurements?
     
  12. SamK

    SamK Auditioning

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    Wayne,

    I'm happy with the sealed sound. I just wanted to know if porting or a boost would be better for flattening the low end response.

    Looking at my response curve, and trusting the RS corrections, I would need about 4DB @ 20Hz to get flat response (ignoring room peaks.)

    OTOH, the low end may not be the worst of my problems, and the boost may be just a bandaid while a BFD would be the cure.
     
  13. Jonathan_D

    Jonathan_D Stunt Coordinator

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