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DIY Tempest Sub Project

Discussion in 'Home Theater Projects' started by NathanH, Jun 2, 2003.

  1. NathanH

    NathanH Stunt Coordinator

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    I've decided to embark on a DIY sonotube sub project using the Tempest driver. Before I pull the trigger and purchase all the components, I'd like to get everyone's input on my design.

    Thanks!

    To view my project's status, visit my DIY subwoofer page.



    Tempest Sonotube Subwoofer Design

    Goal: Translate Adire’s vented Adire Alignment specification to a sonotube housing design.

    Driver:
    Downfiring Adire Tempest with downfiring vents

    Tuning: 15.4 Hz

    Sonotube Specifications:

    ·Planned Internal Volume: 214 L (from Adire specification)

    ·Internal Volume conversion calculation to get cubic inches (in3) of volume

    Using inches as the standard measure, convert the suggested volume (214L) from the Adire specification to cubic inches.

    214L*.2642 = 56.54 gallons
    56.54 gallons * 0.00378541 = .214 m3
    .214 m3 / 0.00001639 = 13057 in3

    ·This is the equation to determine the volume of the cylindrical sonotube:
    (Pr2)h = internal volume

    ·I will be using 24 inch sonotube, and need to have 13057 in3
    (P122)h = 13057 in3
    h = 29 inches

    This will leave a little extra volume to accommodate internal volume-consuming components.

    Thus, the inner dimension of the sonotube should be as follows:

    Inner diameter: 24 inches
    Inner Height: 29 inches


    Vent specifications:
    Two 3 inch flared ports 11 inches long (from Adire Audio alignment specification)
     
  2. TimForman

    TimForman Supporting Actor

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    Since you're making everything fire out of the bottom I'm assuming to want to make it look like a table or something. What did you have in mind? What type of material are you putting around the tube?
     
  3. Patrick Sun

    Patrick Sun Moderator
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    I wouldn't use two 3" flared ports. Either go with two 4" wide flared ports, or a single 6" wide port.

    I usually just use 28.3 liters per cubic foot (12"x12"x12").

    You will have to include all of the volume displacement for the vents (treat them like solid cylindrical objects inside the enclosure taking up space). Then, you'll have to adjust the lenght of the 24" wide sonotube accordingly. Also add in another 5 liters for the driver itself. And also account for the amount of MDF thickness on both endcaps that are inside the sonotube and add that to get to the final length of the sonotube. Your 29" long sonotube is going to be a little longer, perhaps by another 4"-6" once you account for everything else.
     
  4. NathanH

    NathanH Stunt Coordinator

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    Well, I'm not 100% firm yet on the aesthetics. I'm thinking that it will become a table-like sonotube with a nice looking round wood top stained the same as my trim in the HT. It will have a matching stand and legs as well. I'm thinking of using material that matches the screen's background, which is slated to be black GOM.
     
  5. NathanH

    NathanH Stunt Coordinator

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  6. Chris A H

    Chris A H Stunt Coordinator

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    Try this on-line tool for calculating precise sonotube sizes....
     
  7. TimForman

    TimForman Supporting Actor

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    Check out Pat's Sonosub III. It's very similar to what you're thinking of doing. Also, I found some indoor/outdoor carpet at The Home Depot that worked very well. It's solid black and not the typical large loop stuff. I have a picture of it here.
     
  8. NathanH

    NathanH Stunt Coordinator

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  9. Patrick Sun

    Patrick Sun Moderator
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    Actually Sunosub II is closer to what's being proposed, though it's probably taller than what Nathan is willing to put up with. Sunosub III was made out of scrap sonotube, and I only used 1 flared 4" wide port, plus I stuffed the hell out of it. The 214L Tempest alignment is probably what Sunosub 2.5 would look like (something in between my 2 Tempest subs).
     
  10. Noed*B

    Noed*B Extra

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    Nathan,

    "I have researched but cannot determine what effect the port size or length has to do with anything in a sub's performance other than to allow the driver to breathe and to avoid port noise."

    A port does not let a driver 'breath'. A port acts as a helmholtz resonator and is actually THE sound producer at and around Fb.

    "Will changing the length or diameter of a port affect the tuning of the sub, assuming you compensate the volume of the sub for the change in port volume appropriately?"

    Yes. You will need to adjust the length if you increase it's diameter (larger port area, = longer for same tuning) Thought this e-mail I sent a friend some time ago might come in handy. It's LOOOONG. But informative.

    "Most of what I know about ports comes from years of trying to build small subs that go low in frequency ( DIY, w/o factory assistance ) and learning the hard way that inadequate port size seriously compromises performance. A 1" ports (to tune low in a small box) in 1.5 cube boxes for 12" woofers a subwoofer does not make ;-)!! That's also how I got started going with passive radiators. I couldn't get low enough in moderate to small box sizes, and even when I got there, ports were still very non linear.

    Joseph D'Appollitto and a guy named Stan Marcewicz schooled me on this for years. You should read the two JBL port studies, they are packed full of useful information.

    Essentially I can boil it down to this though. A port is a sound producer. At and around Fb the port is contributing the majority of the sound the system generates. As with drivers, ports too are non linear in operation. Basically if you move air through a port, it's going to distort. Kinda like PWK's move a woofer cone, it's distorting. Anything you do to compromise the air moving through said port, (IE inadequate area to move the air required by the expected system volumes [too small], asymmetric termination [one in baffle, one just sliced off and dumped inside the box open ended, or a flared / radius exit and just open ended cutoff on the other], or high aspect ratio ports [wide and thin]) you will increase it's distortion.

    I've done power compression tests (before I had access to high dollar equipment) on simple straight ports with square edged termination, which indicated that you almost cannot have to large a port diameter. IE I ended up with ports almost the drivers diameter before they quit contributing more significantly to system power compression than the driver itself did, but then I had those NASTY standing wave problems you and I both experienced in prototypes.

    Obviously using a port with the area of the driver is completely impractical, but it does illustrate the point that ports are significant contributors to distortion, and larger is better than smaller all other things being equal.

    If you were to take everything I know (and am still learning) about ports, and try to distill it into a fairly short to-keep-in-mind list, here's what I'd say.

    1: Do your best to get the largest practical surface area you can in your enclosure with respect to your chosen system tuning. You can almost not have too large a port area. Kind of like it's almost impossible to have too much motor, or too much box size. Obviously in production there is no way you can use enormous ports b/c you are limited in cabinet depth to hit a given tuning, or hit drivers or braces, etc.. so try for the most area you can get. Smaller ports begin to choke off the low end output earlier and earlier as you increase SPL, and can even effectively revert to an equivalent of a leaky sealed systems output at very high drive levels.

    Remember, as you increase the area of the port, two good things happen.

    a: To produce a given sound pressure level, the velocity of air in the port is slower, meaning it's lower in distortion. It's moving slower through the port, and it's entrance and exit speeds create less turbulence and vortex shedding at the mouths of that port. If possible, at the peak SPL you are designing for, try to keep that velocity below about .05 mach. That's the speed at which it begins to increase in turbulence significantly or huff and chuff in straight edged ports.

    b: The surface area of the port wall becomes smaller in ratio to the open ended area of the port reducing viscous drag along said wall.

    Also remember. Just as with ANY given sound producer. The larger it's volume displacement, the greater the SPL it can generate. That's right. A port is like a woofer in some ways. It literally displaces a quantity of air for a given amount of excursion, and that displacement generates SPL. Just as with a driver, SPL is volume displacement divided by frequency.

    2: Do the best you can to avoid high aspect ratio ports (Wide/Thin) when you use non circular ports, by that I mean do NOT use very very wide, but thin ports. Square is waaay better than thin rectangles.

    Remember that a circle has the least amount of internal surface area relative to open ended surface area of any shape. It's THE most linear shape for a ports cross sectional area simply because it's the most economical physical shape.

    As you know the air directly adjacent to the inside wall of the port is barely moving at all, while the air in the center is moving significantly faster. There is a layer that's almost static, then a shear layer [which is one of the biggest problem areas as it becomes significantly thicker and thicker as port air velocity increases], and then the slug that moves the most in it's center (this is a very very simplified view).. The higher the aspect ratio the more significant a percentage of the mouth area that shear layer takes up, Uh.... The more viscous drag there is on the port walls. And the lower in SPL the port begins to chuff and compress at. IE the less efficient it becomes. At lower levels it will measure closer to it's round equivalent with respect to Fb, but as you drive it higher and higher, a high aspect ratio port will behave more and more like a smaller diameter port.

    3: Try to use a flared port, with radiused ends. A flared port, with radiused ends is lower in distortion than a straight sided port, with squared off ends. Almost any amount of flare improves on distortion, and there are certain profiles that are better at lower levels than at higher levels, and vice versa.

    4: If possible, baffle the inside edge of the port, and treat both ends as close to the same as possible so that the difference in impedance as the air enters and exits from one side verses the other, is as close to the same as possible. This reduces second harmonic (asymmetrical) distortion. If you cannot baffle the inside, but one side is radiused and or flared, try to do that on the inside terminus as well.

    As the air inside the port moves back and forth (remember it's not DC, it's AC so it stops and starts moving the other direction) in and out of a port it sees very large changes in acoustical impedance. IE it's not matched well, and as a result suffers a myriad of ill effects. If one side has a significantly Different acoustical impedance, then as the air moves into and out of that end it behaves decidedly different than it does entering and exiting from the other. This contributes to asymmetric distortion (even order) components.

    While even order distortion is often pleasing to the ear at some levels, it is still distortion.
    Hmm.... That isn't all there is to consider when developing a ported speaker obviously. However it's some stuff to chew on, and I hope it helps.
     
  11. NathanH

    NathanH Stunt Coordinator

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    Thanks for all the great info!

    Sounds like bigger is better - even for ports! [​IMG]

    I did some quick calculations and discovered that I can't go over a 6" port without the port needing to be longer than the sonotube. So, it sounds like a 6" tube would be my next size of choice.

    I really like the flared adjustable port kits you can buy in 3" and 4" diameters. Are there any for 6"? I couldn't find any at PE or Adire.

    I'm assuming as well that a non-flared 6" PVC port would still outperform a combination of flared 4" ports.

    I'll revisit my design tonight.
     
  12. Patrick Sun

    Patrick Sun Moderator
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    You won't need to flare the 6" wide port (but it can't hurt to route a flared curve into the endcap MDF for the port opening.
     
  13. Noed*B

    Noed*B Extra

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    Nathan,

    "I'm assuming as well that a non-flared 6" PVC port would still outperform a combination of flared 4" ports"

    A single 6" port has a surface area of 28.27 square inches. A pair of 4" ports have a surface area of 25.13 square inches. Using dual equally treated four" ports will give you close to the same performance as a single 6" with a slight degredation in viscous drag along the port walls, and slight increase in port velocity.

    Dual flared 4" ports will likely outperform a single square edged 6" port at moderate to high SPL, due to the increase in turbulance at the ports entrance and exit. Were I you, I'd happilly use dual easilly obtainable flared 4" ports.
     
  14. Patrick Sun

    Patrick Sun Moderator
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    But remember, it's going to be tricky to fit a 15" driver, and 2 flared 4" wide ports (around 7" in diameter for each) on a 24" wide (inside width) endcap.
     
  15. Noed*B

    Noed*B Extra

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    Fire driver from one end, ports from other end.
     
  16. Patrick Sun

    Patrick Sun Moderator
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  17. NathanH

    NathanH Stunt Coordinator

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    Bought the Tempest driver today. Still putting design details together.

    I'm going to buy the sonotube as well this week if the rain holds off.

    I think purchasing the 2 flared ports may push me over the edge slightly on my budget. Plus, I'm going to have a difficult enough time making one hole let alone two for a vent. So, I will likely go with the 6" PVC pipe. I don't anticipate using the sub at high levels except for the "show-off" sessions with the guys.

    It did occur to me that I could make a sealed design to decrease complexity for this first project, so I may make a trip to the local audio store and determine if I really like the ported vs non-ported sound.
     
  18. NathanH

    NathanH Stunt Coordinator

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    Bought the Parts Express 250W plate amp, dual input terminal cup and 8 tnut sets today.

    Yay!

    Everything should be showing up on Monday!
     

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