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    PV Audio's guide to enclosure types!

    Having seen that some people are still struggling with the fact that not every subwoofer on the market needs to go into a certain type of enclosure, I figured that I’d take some time and briefly go over WHY every subwoofer does not need to go into a certain enclosure. I'll try to write this in as simple a method as possible without confusing anyone with jargon that isn't necessary. I’ll divide this into five parts: sealed, ported, infinite baffle, transmission line and horns. Let’s start with sealed enclosures, shall we?

    What is a sealed enclosure? It’s a closed space behind the woofer which acts as an airspring. As the speaker moves throughout space, the air behind the cone becomes pressurized. This same pressure is what keeps the speaker linear and controlled. The sealed enclosure will typically have a greater transient response and lower group delay than a ported enclosure, but that is merely a rule of thumb and I won’t say that it’s always true. Now, what should you look for in a sealed enclosure? Well, many systems in the world have what is called a Quality Factor, or Q factor for short. This Q factor determines a lot of things: how long a bell will ring after being struck, how long it takes for a charge inside an inductor or capacitor to become neutral and for us, how well a speaker can be controlled by itself and in an enclosure. Using the sealed enclosure as an example, the smaller the box, typically the higher the Q.

    Why? A higher Q (>.5) means that the system is called UNDERDAMPED. What is under damped? That means that it takes a lot longer for something to return to its baseline condition. Hey, that’s kind of hard to grasp, how about an example? Sure thing! Think about the bell I mentioned earlier: if you strike a bell and touch it, what happens? It stops ringing, obviously. Now, let’s assume that the bell is flawless, is radiating in free space with no air or heat or anything to stop it. This bell would therefore continue to ring forever, which means that it has an infinite Q. The higher the Q, the “freer” that something is to oscillations above its equilibrium state. For a circuit, that means that it takes a long time after an impulse is sent to it for it to return to equilibrium, for a speaker, it means the same thing! If you take a speaker with a high Q (called Qts which is a combination of the mechanical and electrical Q factors which I’ll talk about later) and play a tone into it, the more ability that it has to oscillate freely. Since for us that means that the speaker will become uncontrolled easily, you need the control of the sealed enclosure for it to work best.

    The sealed enclosure itself ALSO has a Q which you need to look at. As I said, the smaller the enclosure, the higher the Q. Why? Because as the airspace drops, the speaker must pressurize a smaller amount of air which thusly means the pressure in the box rises. That means that it takes longer for the speaker to return to equilibrium as it has to dissipate that pressure for longer. In the end, that corresponds to a very peaky response because the sound will go from baseline to a gain over baseline, to under baseline to over baseline over and over until the system reaches equilibrium. It’s that simple! The higher the system Q, the more tendency for the sound to be peaky or boomy at certain frequencies near the roll off point since the frequency response is not flat at all frequencies.

    If you make a larger enclosure, then the frequency response becomes flatter to a point where the speaker will actually end up rolling off TOO early. This is just as bad as a peaky response because it can cause what many of you know as bottoming out. What happens is that because there is not enough pressure in the box to keep that high Qts cone under control, then the speaker behaves like it’s just sitting outside an enclosure or in what’s called infinite baffle (I’ll tackle that later). This is BAD for speakers not made for IB applications because it leads to over excursion and subsequently mechanical damage to the speaker. While many speaker companies use various methods to stop bottoming out from happening (special tricks to the ends of the voice coil such that as the speaker is going to clear the magnetic gap which causes excursion, the motor essentially has nothing left to charge up and the cone can’t go any further), if you push it hard enough, it will happen!

    So, how do you tell how large to make the enclosure? Well my friends, you need a few parameters: the Fs of the speaker, the Qts, the Vas, Xmax, Sd and Vd. If you don't fully understand any of those, just ask, because they aren't as simple as you might think (Xmax is not just simply how far the cone can move linearly, it's helpful to know why that's its linear limit ). ONce you ahve those, you're going to want to either use a design program or if you're like me and trust tried and true mathematics, you'll use a sealed enclosure design table. These are are tables with numerous values for the desired SYSTEM Q or Qtc that you want. Some useful Qtc values are:

    Butterworth: 1/sqrt(2) = .707 (flat magnitude response)
    Bessel: 1/sqrt(3) = .577 (flat group delay)
    Chebyshev: > .707 (maximum power handling and efficiency, bad transient response)
    Transient perfect: .5 (critically damped, right on the limit of usefulness for high Q drivers)

    So let's say that you want a Butterworth Q of .707 which has a flat magnitude response. You go to the table, then along the left side of the table, you'll find numerous Qts values for the speaker. Once you find the Qts for your speaker, it's time to choose a value called "alpha". All that alpha is is (Qtc^2)/Qts - 1. Take your alpha value and now you're good to go! THe Vb, or net air space, is the Vas of the speaker divided by alpha, or Vas/alpha. That's it! While there are numerous other things needed to design specific parts to your sealed enclosure, knowing how to pick the correct volume is probably the most important. That's all for sealed, and if you want some more specifics, then let me know!

    -Dave







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    Re: PV Audio's guide to enclosure types!

    I can see this is going to be a GREAT thread.... while alot of this information can be found in many good books on the subject, it seems alot of people don't care enough to read them....

    Fantastic explanation PV, carry on!



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    Re: PV Audio's guide to enclosure types!

    you never replied back to my other pm...hope it didnt get lost...



    http://www.caraudio.com/forums/buyer-seller-feedback/414807-michaellane-feedback-if-i-bought-you-traded-i-sold-you-something.html

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    Re: PV Audio's guide to enclosure types!

    It probably did, people have been PMing me a lot about my design competition. Send it again please? My bad, man.




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    Re: PV Audio's guide to enclosure types!

    Quote Originally Posted by altoncustomtech View Post
    I can see this is going to be a GREAT thread.... while alot of this information can be found in many good books on the subject, it seems alot of people don't care enough to read them....

    Fantastic explanation PV, carry on!
    Ported will be done in about 20 minutes




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    Re: PV Audio's guide to enclosure types!

    Ported Enclosures

    Ahh yes, the bread and butter of the car audio world for reasons most do not understand. Believe it or not, throwing any speaker into a ported enclosure will NOT necessarily make it perform better. To determine what speaker is suited for a ported enclosure, you'll need to be looking at the Qts again. This time, we're going for OVERDAMPED speakers, or speakers with Qts values below .5. Why? Well, these speakers don't tend to oscillate freely and potentially out of control like under damped speakers do, so by placing them in a sealed enclosure, you're essentially doing the woofer's job for it. That is horribly inefficient and a waste of time. Instead, people use ported enclosures to gain some extra output since the speaker can handle it without hurting itself.

    The main precaution surrounding ported enclosures is the fact that you need to design the enclosure around the driver, NOT the other way around. Going out and getting a pre fabricated enclosure is one of the worst things that you can do to a subwoofer. It won't necessarily sound poor, but there is no way it's suited for that specific speaker. How do you know what is suited for your speaker? Good question! You're going to need those tables I talked about earlier, except this time, instead of looking for a Qtc value that you like, you're going to look for a Ql value, or a loss Quality. Ported boxes have losses that you need to take into account. The average loss value is 7, and while this isn't very practical for most people it's the best method: construct the enclosure with QL of 7 in mind, then tune it to the frequency that you calculate (later). Afterwards, try to measure the box for losses and if you're pretty close to neutral, then you're good to go! If not, then here's the rule of thumb: if you have LESS LOSS or a HIGHER Ql, then decrease the Vb. If you have MORE LOSS or a LOWER Ql, then increase the size of the box. Think about it like this: you build a container to hold a cup of water. The higher the quality of the container, the less water that will be draining out. The lower the quality, the more water you'll need to put in and thus you'll need a larger container.

    Now, I know most of you guys are like pffft, I'm not wasting time on that, I'm just going to plug some numbers into a computer and get what I want. Not so fast, slick. You need to understand why your computer is giving you the given values. Your Vb is equal to the Vas/alpha. Alpha in this case CAN be derived, but that formula is going to take too long to derive, so just assume that it's a constant between around .1 and 4 for a normal Ql = 7 enclosure. Your tuning frequency is equal to the driver's Fs * H, where H is the tuning ratio for your given alignment. Now assuming that you have all of this, you can calculate your theoretical port length which is easier to see via a website:

    http://www.diysubwoofers.org/misc/portcal.htm

    I realized about halfway through this that it's quite difficult to have a short theoretical basis on ported enclosures because the science is pretty hairy and you need at least a moderate understanding of physics to fully grasp the concepts. Seeing as most people just want a good enclosure, the quick and dirty steps are to use the tuning and minimum diameter formulas to find those respective parameters, then find the volume that you want by the Vas/alpha formula by looking up alpha in a design table, or calling the manufacturer and asking them. They'll know exactly what it should be and can help you if you need anything else.

    I know this post wasn't very helpful, which is why I added the last part. It's very difficult to go into the theory without you seeing the design tables, and it's even MORE difficult to understand if you want the formulae behind the design tables because they are far too long to write out in one-line text

    I'll make the next post a bit better




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    Re: PV Audio's guide to enclosure types!

    Are you going to cover 4th/6th order bandpasses?



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    Re: PV Audio's guide to enclosure types!

    Very nice guide, worded quite well. Im sure this will be very helpful to alot of fourm members, including myself Thanks.



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    Re: PV Audio's guide to enclosure types!

    Quote Originally Posted by PV Audio View Post
    It probably did, people have been PMing me a lot about my design competition. Send it again please? My bad, man.
    its kool ima eat then pm you. its just a simple aero port tunning question. prolly take you 25 seconds



    http://www.caraudio.com/forums/buyer-seller-feedback/414807-michaellane-feedback-if-i-bought-you-traded-i-sold-you-something.html

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    Re: PV Audio's guide to enclosure types!

    Quote Originally Posted by lambofgood91 View Post
    Are you going to cover 4th/6th order bandpasses?
    No sir. I would rather leave that to someone like 80INCHES or Immacomputer who actually know how to design bandpass enclosures to their maximum possible performance. I don't.




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    Re: PV Audio's guide to enclosure types!

    Quote Originally Posted by lambofgood91 View Post
    Are you going to cover 4th/6th order bandpasses?
    Quote Originally Posted by PV Audio View Post
    No sir. I would rather leave that to someone like 80INCHES or Immacomputer who actually know how to design bandpass enclosures to their maximum possible performance. I don't.

    Bandpass enclosures are like fuzzy logic..... hard to understand without a degree in it.... lol..... but seriously if someone knew how to explain them well I'd be all ears.... I've built a couple but it was only by manufacturer's specs.... they turned out good tho, sounded like bandpass boxes... lol



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    Re: PV Audio's guide to enclosure types!

    Quote Originally Posted by altoncustomtech View Post
    Bandpass enclosures are like fuzzy logic..... hard to understand without a degree in it.... lol..... but seriously if someone knew how to explain them well I'd be all ears.... I've built a couple but it was only by manufacturer's specs.... they turned out good tho, sounded like bandpass boxes... lol
    Indeed. I'm trying to learn more about them myself



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    Re: PV Audio's guide to enclosure types!

    i agree with the bandpass suggestions...i would love to learn more about them.



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    Re: PV Audio's guide to enclosure types!

    Tapped horn enclosures are the shizzzz




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    Re: PV Audio's guide to enclosure types!

    I'll get to infinite baffle tomorrow. That should be the shortest section since it's technically under sealed enclosures.




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