Need some advice from the Sub experts

Discussion in 'Archived Threads 2001-2004' started by Dean Cooper, Feb 6, 2001.

  1. Dean Cooper

    Dean Cooper Supporting Actor

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    I'm finishing up getting the final items for my first DIY sub and I have a quick question.
    Do I want a ported or sealed sonotube sub?
    Here are the variables that go into the answer:
    ---This sub's main purpose will be for HT.
    ---I like deep clear bass with no distortion, loud is good but reference is too loud for me. So if the sub can reach reference its well within my needs.
    ---Size isn't an issue, I have a corner in my HT that’s just dying to be filled with a big 18"-24" by 6' monster.
    ---The driver I plan on using is the Adire Audio Tempest, which looks like its a little better suited for a sealed cab, but (as others have proven) will definitely hold its own in a vented cab.
    ---My room is 15' long 17' wide, has a large opening in the rear and a vaulted ceiling.
    Right now I’m leaning towards the sealed cab because its simple and for a first attempt at something, simple is usually the best way to go IMO. But will I be missing out going with out the extra db at the low end the port would give me? Tell me what you guys think.
    Thanks in advance
    Dean
     
  2. Mark Hayenga

    Mark Hayenga Supporting Actor

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    Given that space isn't a real concern and this is primarily for HT, a vented sub is an easy choice. While they are somewhat more difficult than sealed boxes to get right, I don't think it's an appreciable enough difference to keep from trying one as a first sub. I also think you'll be much happier with the low frequency extension of a vented Tempest. As for the driver, I've listened to / played with it and I think it's fantastic.
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    Mark Hayenga
    www.hayenga.com
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    "Saru mo ki kara ochiru"
     
  3. Greg Monfort

    Greg Monfort Supporting Actor

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    Based on these parameters, two Tempests in a 36"w x 18"d x 48"h 45deg triangle cab stuck in the corner should give all the low end you'll want, with low group delay/distortion, and show off the drivers. Decorate to suit, set a plant, Oscar, Tony, whatever, on top. [​IMG] Screw it to the walls for max shake, rattle, and roll. [​IMG]
    Use the pot in the second VC's ckt tweak to dial them into the room.
    GM
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    Loud is beautiful, if it's clean
     
  4. Brian Bunge

    Brian Bunge Producer

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    Greg,
    You are a SICK man! But I must say, I like this idea!
    Brian
     
  5. Dean Cooper

    Dean Cooper Supporting Actor

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    Greg, that is a sweet idea! you totally turned on a lightbulb above my head [​IMG] I could tape and mud the edges so it looks like part of the wall, and its simple to build too. I should be able to fish the wire through the wall to make it totally seamless. Looks like I can vent the cab later on if I decide to too. Heck if it looks as good as I think it will I might make another for the other corner [​IMG]
    You kind of lost me on the tuning part to my room though, is there someplace that describes this procedure in length? This stuff is right up my alley.
    Thanks,
    Dean
     
  6. Greg Monfort

    Greg Monfort Supporting Actor

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    Brian, so the doctors tell me, while gutting my wallet.
    Deane, if you're willing to make built-ins, build it to the ceiling. The same construction rules apply as for a separate cab, as room walls aren't very rigid unless it's concrete block. At this larger cab volume, you shouldn't need to have to sacrifice one of the driver's VCs to tuning.
    GM
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    Loud is beautiful, if it's clean
     
  7. Dean Cooper

    Dean Cooper Supporting Actor

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    hmm well the vaulted ceiling makes building it there not an option. I was going to build the cab completely out of the novoid ply and then bolt that to the wall through the back walls of the cab. 2 layers for the front plate should prevent any flexing and let me install the subs flush with the front plate. Then tape, mud the seams with the walls and paint the front the same color. I'll most likely put a nice solid oak cap on the top with a little trim to make it look like its supposed to be there. What do you think? Would it bring down the walls I bolt it to with those 2 Tempests shaking in there? Thanks for the help.
    Dean
     
  8. Greg Monfort

    Greg Monfort Supporting Actor

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    Oops! Sorry Dean, didn't mean to misspell your name. I didn't mean for the sub to go all the way to the vault's peak, just to mean height, usually 8ft.
    With the smaller volume, then tuning the Qts with a pot to improve in-room response will probably be needed, and cheaper than a separate EQ.
    Quoting from the Adire site:
    "Think back to playing with magnets. Placed in one
    direction, they attract themselves to each other.
    Reverse the direction, though, and they try to push
    themselves apart. Well, the same basic things are
    going on in your driver. You have one fixed magnet -
    the driver magnet. The other magnet is the voice
    coil (remember, a current passing through a coil of
    wire will create a magnetic field. See our page on
    crosstalk for a description of this).
    So, we have two fields. and guess what: the magnetic
    field of the voice coil flips based upon the polarity
    of the signal sent it! Since audio signals can be
    thought of as combinations of sine waves, we have the
    signal sent to the voice coil constantly changing
    polarity. This causes the voice coil to push or pull
    against the permanent magnet. Presto! Coil moves in
    and out, dragging the cone (and spider, and surround)
    with it!
    So, now we know (in two paragraphs, even!) how a
    dynamic loudspeaker works. What does this have to do
    with dual voice coils? Everything. See, once we
    know how the voice coils make the cone move, we can
    then start to understand how having two voice coils
    will help.
    For example, consider the following: what happens
    when you wire the two voice coils out of phase? that
    is, you feed a signal to one coil, and you feed the
    OPPOSITE signal to the other coil? Well, when one
    coil is trying to push, the other is trying to pull,
    and you end up getting no motion. Note that the
    driver is not trying to "tear itself apart". What's
    happening is that each coil is setting up a magnetic
    field, that happens to be out-of-phase with the
    other. End result is there's NO net magnetic field,
    so there's really no forces in the system!
    Now, what happens when you drive both coils the same?
    Well, you get twice the push! That makes sense...
    The two fields add together, and you get a combined
    push in a given direction.
    What about driving just a single coil, and leaving
    the other open? Well, as you would guess, the push
    is weaker. The system has a peakier resonance (for
    the techie types, Qts increases because Qes
    increases).
    BUT - let's drive one coil, and short the other.
    Guess what? Things change from the original (both
    coils driven) situation, but they also stay the same.
    The driven voice coil is pushing and pulling, as
    normal. But what about the shorted voice coil?
    Well, it's trying to keep things at rest - it's
    trying to resist ANY motion! The net result is the
    overall peakiness of the resonance is the same as it
    was when both coils were driven, even though we are
    only using half the motor (driving one coil). The
    other coil helps "tame" the driven coil, so that the
    system basically behaves the same as before.
    Hey, maybe we can exploit that! Actually, we can...
    Say you want to use our Shiva driver. Say you like
    everything about it (especially the massive Xmax!),
    but for your intended application, you want a higher
    Q. Well, this is the way: drive one voice coil.
    But rather than just shorting the second coil, or
    leaving it open, terminate it with a resistor. The
    result? The Qts of the driver will change from the
    open to the shorted Qts as the resistance is
    decreased. That means you can tune the Q of the
    driver, with nothing more than a potentiometer!
    In fact, for our Shiva subwoofer, operation in this
    mode allows one to literally dial in a Qts from ~0.38
    to 0.80. Now THAT'S flexibility! So a dual voice
    driver is actually amazingly flexible. Much more so
    than usually pushed. Use a dual voice coil driver,
    and you can use a much wider range of enclosures, and
    even have a system that can have a "Qts" knob on it,
    for changing the Qtc of a sealed system on the fly,
    according to your tastes."
    In a separate response on what pot to use, Dan W. said:
    "Digi-key P/N CT2154-ND. It's a single turn, 100 ohm
    5W potentiometer. Runs $3.46 each, in singles. Note
    that Digi-key has a minimum order of $25 to avoid a
    $5 service charge."
    WRT to the construction, it's fine for max house shaking. If only floor coupled it's not quite as bad. Your choice.
    GM
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    Loud is beautiful, if it's clean
     
  9. Greg Monfort

    Greg Monfort Supporting Actor

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    Yeah JG, 'built-in' corner loaded speakers were very common at one time, and still make sense for some designs, such as subs. Mike Bates has four JBL 2235H (the original PA/theater LF bad boy driver) in a sealed 55ft^3 floor/ceiling triangle sub to mate to his HT/music horn system. Not in the Twelve Dancing Shiva league, but should be about all a typically constructed house can stand. [​IMG]
    GM
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    Loud is beautiful, if it's clean
     
  10. Greg Monfort

    Greg Monfort Supporting Actor

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    I was just looking at Mike's latest horns, and noticed that they're vented, and has a stereo pair, so eight 2235H corner loaded. This puts them in the Voice of God category (>130dB/20Hz) WRT home use.
    GM
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    Loud is beautiful, if it's clean
     
  11. Dean Cooper

    Dean Cooper Supporting Actor

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    Holy crap that's big bass. I've had the pleasure of sitting in 165Db of bass in a demo truck ( with ear protection of course ) and it made my vision go blurry after the 135 or so mark from the vibration.
    Thanks for the info on the pot tuning. I'm an Instrumentation engineer so if theres a way to calibrate something I'm all over it [​IMG] (should of seen me with my rpt [​IMG] )
    Dean
     
  12. Brian Bunge

    Brian Bunge Producer

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    Greg,
    Is there a way to actually measure the resulting Qtc of the system using a pot across the second voice coil, or any other subwoofer system for that matter?
    Thanks,
    Brian
     
  13. Greg Monfort

    Greg Monfort Supporting Actor

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    Brian,
    Not AFAIK, but you can calculate it. Remeasure to get the new Qts, then for Vb specified:
    Vab = Yr*Vb
    a = Vas/Vab
    L = (a+1)^0.5
    Qtc' = L*Qts
    Qtc = 1/(1/Qtc'+1/Qa)
    where:
    Yr = 1/no stuffing - 1.2/100% stuffing
    Qa = 10/no stuffing - 5/100% stuffing
    --------
    JG,
    Close. [​IMG] If you drive each 8ohm VC individually with 100W, that's E = (100*8)^0.5, or 28.28V/VC.
    If paralleled, the VC's combined resistance is now 4ohms. W = 28.28^2/4, or 200W.
    If in series, the VC's combined resistance is now 16ohms. W = 28.28^2/16, or 50W.
    Anytime you want to compare drivers with different Re, it's best to convert to volts first. If I had my way, driver sens ratings would only be in xxxdB/2.83V for this reason. It would make it easier for determining what to input into LspCad too. [​IMG]
    GM
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    Loud is beautiful, if it's clean
     

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