Acoustic Suspension vs Infinite Baffle??

Discussion in 'Home Theater Projects' started by Daniel_AVR, Dec 19, 2003.

  1. Daniel_AVR

    Daniel_AVR Extra

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    Hi,

    In my thought to build my sub, I'm in the "educate yourself" stage.

    I talk about sealed enclosures sytems here

    A slight passage of "The loudspeaker cookbook" need a bit more precision.

    "....air suspension system exist between values of (ALPHA) FROM 3-10. values of (alpha) less than that venture into the realm of the infinite baffle."

    If I take a look at correspondant tables for various Qtc, it become evident that it's nearly impossible to acheive a Butterworth (Qtc of .707) or Bessel (.577) and less with most popular 12" woofer, as you need a Qts of .35 or less, without dropping into the infinite baffle world.

    Any modeling also show that the cone excursion values increases as the Qtc decrease, as a result of also decreasing max power handling.

    So what's the picture? To build a enclosure with a Qtc of .707 I'll need to find a woofer with a Qts of .35 or less? an it's getting worse if I try to build with a Bessel alignment (Qtc of .577) I'll need a woofer with a Qts of .28 or less???

    If I want to use maybe a Titanic MKIII 12" or a Stryke AV12 MKII in a Butterworth alignment (.707) system will end in the infinite baffle world as their Qtc are .41 and .452.....

    So why PE recommend a 2 pi3 box for their Titanic MKII....??? in a .69 Qtc.... Way into an I.B. aligment!! Qts of .41, (Qtc of .707 give an alpha of 1.9735)

    What are the implications of setting a driver in I.B. spec. world in a sealed box?? Others than dramatically reduce their power handling to stay into their cone excursion limit?

    Am I right into figuring the whole portrait?

    Daniel
     
  2. ThomasW

    ThomasW Cinematographer

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    Well there are a couple of termology issues here. Acoustic suspension ( = sealed box), is also frequently called an 'infinite baffle', but in reality it's not 'infinite'. So to avoid confusion when discussing 'portable' subs let's not use the term infinite baffle, instead use sealed box or acoustic suspension.

    With a 'true' infinite baffle (let's say a 'box' at least 10+ times larger than the driver's Vas) the system Qtc is the driver's Qts.

    For 'small' sealed box (acoustic suspension) systems; placing the driver in the box instantly raises the "Q". And the smaller the box the more the "Q" is increased. So it's better to start off with a driver having a very low Qts, if the goal is to have a low Qtc from a 'small' sealed box. If you start with a driver having a 'relatively' high Qts and want a low Qtc system the box will need to be quite large.

    The implication of putting an IB spec driver ( =Qts ~.7) in a small sealed box is the creation of a very high Qtc system

    Finally yes as the box size increases to a 'true' infinite baffle size, the power handling is related to the excursion limits of the driver's suspension. With smaller sealed boxes, the power handling is primarily related to the thermal limits of the voice coil.
     
  3. Daniel_AVR

    Daniel_AVR Extra

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    Tomas,





    Ok I like rules of thumb.
    I figured to calculate driver volume + amp vol. and then add this total "lost" volume to the box construction to reach wanted Qtc instead.

    In fact where it hurt, at first I don't want a small car box in my living room and neither a 1.1 Qtc as I'm more music that HT, I figured a 2.5 to 3.5 pi3 box with a Qtc of something between .65 to .8, but I'm learnig limitations now. Driver with an higher Vas puts me near where I want to be(like the original Titanic 12") but still venture into the infinite baffle realm according to Dikanson. So power and excursion limitations will let me with a poor max SPL.....compromises....compromises.....

    Should I build 2? WAF will probably put a fence here....

    thanks

    it's a fascinating world

    Daniel
     
  4. Brian Bunge

    Brian Bunge Producer

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    Daniel,

    I would suggest using the Adire Audio DPL12 for a sealed sub around 3ft^3. It will give you a Qtc ~ .6 if you stuff it with around 3lbs. of polyfil. It will also probably be pretty flat to 20Hz in room.

    http://www.adireaudio.com/diy_audio/...dire/dpl12.htm
     
  5. Daniel_AVR

    Daniel_AVR Extra

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    You're right Brian, the DPL12 models awesome in a near 3 ft3 sealed box....it was my first choice at first, In fact I nearly ordered it....

    But after a few question here I decide to educate myself a bit....I have the time actually, and it's pretty funny.

    And I found a used Hypex HS500 with 720W on 4 ohms....it open whole new horizon on my project.....knowing that, the DPL12 could choked my project a bit by it's cone excursion limit vs newest drivers, as a result a relatively modest max power capacity vs my amp. + I prefer durability of a butyl suspension over a foam one....

    Maybe I'll end with my first choice who knows....but 2 of them....

    Did you ever try the DPL12?

    Thanks

    Daniel
     
  6. ThomasW

    ThomasW Cinematographer

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    The newer generation of foam surrounds are very long lived.

    look at the Maelstrom. It's 4 ohm driver, very efficient, low "Q" so it's happy in a very small sealed box. Same cost as a pair of DPL12's, lots more displacement....
     

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