Let's talk about the Q rating of a vented box (please) :)

Discussion in 'Archived Threads 2001-2004' started by Javier_Huerta, Nov 27, 2002.

  1. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Hi,
    As I go deeper into the mistery of vented designs [​IMG], I have found out something interesting.
    I can make a vented box go flat quite easily. As a matter of fact, WinISD does it for me.
    But in thread after thread, I've seen doing this is not quite as easy.
    I could create a box with a fairly flat looking curve, and then a very steep drop-off. I've been told this is a very bad design idea, since the Q will be too high.
    So I have loaded the three Adire vented designs into WinISD and studied their curves. I have seen they aren't flat at all - they have a slight slope, with no sharp drops anywhere.
    I have tried to simulate my boxes using this criteria. But I do need help.
    For example, in my box, I have a sudden change in slope right in F3. I'd think this is a bad idea. If I have this change in slope, should I just make sure it happens *below* F3? Or should I simply try not to have it?
    Also, is it true a vented box simply cannot reproduce sounds below its F3? I find it hard to believe, since all of Adire's designs have their F3 above 20 Hz.
    Help... please [​IMG]
     
  2. Jack Gilvey

    Jack Gilvey Producer

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  3. Pete Mazz

    Pete Mazz Supporting Actor

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    Keep in mind that at lower freqs room gain will make up for the slower roll off of lower Q designs. In fact, that is why most are designed that way. A sub with a flat FR will exhibit significant gain when placed in a room. So if your sub is flat to 20 Hz on paper, in room it may be as much as 12 dB high. It depends on how much room gain YOUR room has.

    Pete
     
  4. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    I find the Qtc value to be much more important for sealed subs where the rolloff typically occurs in the audible region. For most DIY ported subs the rolloff is so low that its mostly inaudible anyway.

    There's two schools of thought on ported rolloff. One says design a bit of downward slope into the extended bass shelf (EBS) to complement room gains. The problem here is that you likely don't really know how much room gain there is to counter. The other school of thought says design it to be flat without room gain and then you'll always have at least enough low end response - better to EQ it down if necessary than to not have enough. The problem here is you have to spend money and shelf space on an EQ. But once you have the EQ (and measurement equipment) you're more likely to get a flat room response than in the first case.
     
  5. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Jack, your link was exactly the thing I was looking for. I'm beginning to understand box speaker building as an under, over or critically damped system. I hadn't thought about the Q as the ratio of stored vs. dissipated energy. Thanks a lot.

    Ryan, I understand what you are saying. As a matter of fact, I'd suspect you would need one of the Behringer Feedback Destroyers in order to achieve flat response with a sub. I really think there's no other way, there are too many variables involved. Thanks!

    I'm still *very* confused about one thing. I've read about two things. The first one: the -3dB point is the one where a vented box becomes unusable -i.e, the one where it can't reproduce lower frequencies. The second one is that the tuning frequency is the one that dictates the lowest frequency the tuned box can reproduce.

    So, if I tune a box at 19 Hz and the box' -3db point is 30 Hz, and the design is overdamped (as in an EBS), would the box lower frequency limit be 30 or 19 Hz?

    I'm confused about this. Any light shed on the subject will be greatly appreciated!

    Javier.
     
  6. Pete Mazz

    Pete Mazz Supporting Actor

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    The F3 point is simply that....when the output of the system reaches 3 dB below max output, that frequency is it's F3. In sealed enclosures, higher Q systems will have a lower F3. In ported enclosures, the tuning raises or lowers F3. Lower tuning raises F3, higher tuning lowers it.

    In your example, the system is down 3 dB at 30 hz, but you'll have significant output to somewhere below tuning (19 Hz), until you reach a point where the driver unloads and output drops quickly (~24 dB/octave for a 4th order system).

    Pete
     
  7. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Thanks Pete! WinISD's literature says output usually is useless below F3 (I guess they are wrong). I read the article that Jack provided, and it basically said the driver remained usable right to FB, which seems is the correct answer.
    I guess an accurate system would then be either critically damped or overdamped, since an underdamped system would introduce resonances at FB. From what I've found at the web, a +1 dB boost at FB is the highest any box should ever have before it starts to sound badly.
    Again, an educated guess would be to try and start designing a box from what WinISD gives you, and then work out a slightly overdamped solution for an EBS, or a slightly underdamped one for a "boom-box" system.
    I guess I'll buy the Loudspeaker Cookbook now. I'm hooked. [​IMG] Thanks a lot!
     
  8. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    "In ported enclosures, the tuning raises or lowers F3. Lower tuning raises F3, higher tuning lowers it."

    This is only true to a point. At some point higher tuning will raise F3. Imagine tuning to 40Hz versus tuning to 60Hz. Of course the 60Hz will have higher F3.

    I'm thinking that if you reach the point where your tuning frequency and your F3 are the same then you've reached a Q=0.707 alignment. Tuning below this (and keeping all else constant) will be overdamped - Q < 0.707, while tuning above this will be underdamped - Q > 0.707

    Is this correct?
     

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