Why does a larger enclosure give you more output?

Discussion in 'Subwoofers' started by shadyJ, May 20, 2013.

  1. shadyJ Audioholic Ninja

    shadyJ
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    Like the title says, when everything else is equal, a larger cabinet permits more output from subs, but I never really understood why that is. What does the extra enclosure volume do to make the sub get louder?
  2. markw Audioholic Overlord

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    What, google doesn't work for you?

    I notice that you frequently post seemigly simple questions that require deep study to understand the answers and don't do any work on your own to answer the question. Why is that?

    If your overly simple statement were true, an infinite baffle would be the end all and be all, but it isn't. Here's a good primer on speaker enclosure theory. Here's another general description of speaker enclosure types.

    I'm sure you can find more detailed information if you try. Madisound has some excellent books on this subject if you wish to delve deeper into tis subject.
    Last edited: May 20, 2013
  3. shadyJ Audioholic Ninja

    shadyJ
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    I was hoping for a succinct answer from someone who understood the physics. I did some google searches but didn't turn up anything helpful. The Wikipedia entry, for instance, does not explain why a larger cabinet gets louder. Your links explain the various types of enclosures which is interesting, but they do not answer my question.

    I'll put the question in another way. Take for example the Outlaw subs, the LFM-1 Plus and EX. Same driver, same amplifier. The only thing different are the port sizes and cabinet sizes. I understand the ports, to an extent, and their resonance, and I get that larger ports will produce more output. However, in my understanding the port's output is limited to the lower frequencies of the sub's tuning frequency, around the resonant frequency of the ports. However Outlaw claims that the EX has 4 dB more output across the board, not just at the tuning point. From what I gather, that is due to the larger cabinet. I don't understand why volume of the cabinet makes a difference here. If I had to take a wild guess, I would say this is because the larger cabinet has less air air pressure and therefore less air resistance and so the driver doesn't need as much current to oscillate. I haven't found anything that confirms that though, or maybe I did but didn't see the answer because I didn't understand that jargon and didn't recognize the answer.

    Also, I don't just ask questions without attempting to find an answer on my own first, and I don't think any questions I asked had a readily apparent answer to a non-expert, although I will admit the post that started this thread should have been more specific.
  4. Irvrobinson Audioholic Ninja

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  5. Steve81 Audioholics 5-O

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    I don't know about that, but I can give you a succinct answer from someone who slept in a Holiday Inn Express once (actually that's a lie too, but whatever). Suffice it to say, optimal box volume is determined by the T/S parameters of a given driver, with Qts, Vas, and Fs being the critical variables. Needless to say, because of that you can't just keep increasing the box size and expecting ever greater output. I mean, you can take the driver from the Hsu STF-1 and put it in a massive enclosure, but at the end of the day it's never going to touch a VTF-15H for output.
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  6. markw Audioholic Overlord

    markw
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    I'd say Steve put it about as simply as it gets. Your statement greatly oversimplifies the situation and, most likely, you DID read the answer but didn't understand it. Some concepts take a bit of learning before one can even undersand the answer to some quetions. Again, Madisound has some good books on this subject if you're really curious.
  7. shadyJ Audioholic Ninja

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  8. shadyJ Audioholic Ninja

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    I realize that at the end of the day its all about which unit is going to displace more air, and that a typical 8" is never going to move as much air as a typical 15", but from what I have read (and understood) a larger enclosure allows the same driver more output. Is this due to a greater amount of inner air pressure creating more resistance in a smaller cabinet? But yes, it seems like a large cabinet isn't always the best idea for the sake of frequency response, and/or the character of the sound.
  9. markw Audioholic Overlord

    markw
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    As Steve said, there is an optimum size for an enclosure and ports/loading can only do so much. That's the law of physics at play.

    You can appeal to a higher court if you want, but I'm not going to get involved in your games.

    There are books out there that will explain it to you in the depth you think you're capable of and software out there if you want to play with it.

    Maybe you can explain to us why, using your logic, an infinite baffle is the best enclosure for your application.
  10. shadyJ Audioholic Ninja

    shadyJ
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    I am not trying to challenge the laws of physics, I am just trying to understand them from a layman's perspective, ie a non-physicist or non-engineer. I just need the jargon dumbed down a shade.
  11. Irvrobinson Audioholic Ninja

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    The problem is with the question in your original post, where you say "everything else is equal". If everything else is equal, and the sub design was well optimized overall, then simply increasing cabinet volume is not advantageous. That was what Monkish was trying to explain in the thread I pointed to.

    If you what you really mean to ask is, if we compare two subs of identical design, where one has twice the volume of the other, with construction parameters optimized for the cabinet volume, each with drivers optimized for the cabinet volume, will the larger sub produce greater output? The answer is yes, the larger sub will produce greater output. Of course, the two subs won't have the same drivers.
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  12. lsiberian Audioholic Overlord

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    You are mistaken.

    The larger the box the lower the tuning. The lower the sub goes. The smaller the box the higher the tuning the louder it plays.
    A simple modeling program that xmax limits your design can easily demonstrate this.

    As Irving stated your scenario isn't possible.
  13. Swerd Audioholic Ninja

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  14. Irvrobinson Audioholic Ninja

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    Only my mother calls me Irving. ;)
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  16. shadyJ Audioholic Ninja

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    It looks like the answer is that bigger isn't necessarily better with respect to cabinets, and that output vs enclosure volume depends on the driver attributes. So my original question was a gross simplification.
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  17. markw Audioholic Overlord

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    What more needs be said. Now, read that book that was suggested.
  18. Swerd Audioholic Ninja

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    If you put the same woofer in larger and larger enclosures, you don't vary the overall output, but you can vary the low frequency roll-off behavior of the woofer. The shape of the roll-off curve changes, as well as a unitless value (a ratio) we call Q. In a sealed cabinet, it's called Q[SUB]TC[/SUB].

    See figure 3.3 here
    Loudspeakers: For music recording and reproduction - Philip Newell, Keith Holland - Google Books

    Curve A is with a 7 liter (L) cabinet, B with 14 L, C 28 L, etc. As the cabinet gets larger the bass response goes deeper, but the loudness is lower. Also note how the shape of the curves change as the cabinet volume gets larger. Curve A and B have raised humps, C is pretty flat until it begins to drop quickly, and D, E, and F all go deeper but start dropping off at higher frequencies. Q[SUB]TC[/SUB] is a shortcut to talk about the curve shape in the roll-off region. For curves A and B, Q[SUB]TC[/SUB] is >1. For C, Q[SUB]TC[/SUB] is about 0.7, and for D-F it will be lower approaching 0.5.

    Now scroll down and look at figure 3.4. It shows transient response curves with various different Q[SUB]TC[/SUB] values resulting from different cabinet volumes. Those curves show loudness vs. time after a single short impulse is sent to the speaker. With a Q[SUB]TC[/SUB] lower than 1, the woofer stops responding fairly quickly. But with a Q[SUB]TC[/SUB] greater than 1.0 you see the price you pay for getting that louder bass (shown in figure 3.3). The bass rings on longer and longer as Q[SUB]TC[/SUB] gets larger. This make for muddy bass sound and speakers that lack clarity.

    So to design a cabinet for bass response, you have to choose a balance between loudness, bass depth, and little or no ringing (also called damping the bass).

    Ray Alden's book walks you through the math needed to understand this. Once you go through this, you'll see that there are several plug-in formulas that allow you to use 3 different TS values to predict the bass roll-off curves. You don't have to manually do the arithmetic to calculate this, there are web sites with built in calculators that do this. But they all assume you understand the background, and unless you teach yourself by reading Ray Alden, you'll remain confused.
    Last edited: May 20, 2013
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  19. gmichael Audioholic Spartan

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    Does this mean that you are going to stop flirting with him?
  20. shadyJ Audioholic Ninja

    shadyJ
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    Thanks for the info Swerd, good stuff. It definitely helps fill out the picture for me. After some googling, it seems that the most preferred Qtc is .707. Am I right in thinking that not all commercial subs go by .707? Take the FR of the Cadence CSX15 mk2, for example, that looks like the FR of a high Qtc.

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