AV15 slot loaded design

Discussion in 'Archived Threads 2001-2004' started by Vince Bray, Dec 29, 2002.

  1. Vince Bray

    Vince Bray Stunt Coordinator

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    Has anyone seriously considered a slot-loaded design for the AV15? It seems to help with the tuning issues and I can't see the downside. Of course, it does not get the response flat to 20hz like PR would, but PR is not an option and I want to try this anyway.

    gross internal volume 7ft^3 / 198l
    slot on bottom and back 2" x 19" x 48" total length
    net volume after slot ~5.5ft^3 / 155l
    With 750w hits 112db at 20hz, and 116db at 30hz.

    To get to the slot length I took the bottom + back - 2" for the intersection (can't count this twice) - 2" for the opening in the inside of the cabinet (although I think that length would actually contribute). The overall cabinet would be 24" x 22" x 34.5" with double walls and triple front (dry curb weight just short of a honda civic). This tunes to about 18.5hz, f3 at 22.8hz, and max port speed at 750w is around 22m/s. The slot is roughly equivilant to a 7" inside diameter round port. I am also thinking it would be easy to flare the slot on the sides (just flare the short side) which would help with noise, although I think this would not be a problem for this design. Has anyone built a slot design and care to comment on the ups and downs? I must be missing something because this seems way too easy.
     
  2. Dennis XYZ

    Dennis XYZ Stunt Coordinator

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    Sounds good, Vince. Slotted ports are a time-honored method. The only potential problem I see is it makes it harder to install braces to keep the panels from resonating. But you should be able to work around that with a little thought.
     
  3. Bryan Michael

    Bryan Michael Supporting Actor

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    so a slot port os beter than a round port?
    and if so what is the best size for the port?
    i am thinking of a 350l 16hz tuned box for the av15 my self i figured 2 4 in flaired ports would be good but would a slot port be beter?
     
  4. Vince Bray

    Vince Bray Stunt Coordinator

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  5. Vince Bray

    Vince Bray Stunt Coordinator

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    Ummm, and I wonder if my assumption that the port area can be directly compared to the round port area in unibox is correct. This was bothering me but it got me this far...
     
  6. Dennis XYZ

    Dennis XYZ Stunt Coordinator

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    Seas uses slotted ports for some of their high-end monitor kits so they work fine. You might want to try the port calculator in WinISD. It makes the ports shorter for a given area as they start getting wide and thin implying the effective area is a little less than the measured area. Sorta makes sense.
     
  7. Greg Monfort

    Greg Monfort Supporting Actor

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    I have no idea why this double posted, or even posted at all since I hit 'preview', but it never did, so clicked 'stop' and proof read from the 'reply' window. Anyway, here's my 'official' post, which is a bit different:


    >Now I hate Bose as much as anyone. No really they make me sick with their paper cone drivers etc, it is really disgusting how they rip people off.
    ====
    What does paper cones per se have to do with poor sound quality? Some of the best sounding drivers in the world use paper.
    ====
    > However, I have heard some slot-loaded cabinets that had a very full bass response for their size and I wonder if the slots had anything to do with that, more likely it was just the alignment that made it work, and they were 8" woofers.
    ====
    Probably. Square/rectangular/slotted vents have a higher resistance, ergo they will be shorter than a round vent for a given cross sectional area, have a lower Qp/peak output, though the same vent speed.
    ====
    >Ummm, and I wonder if my assumption that the port area can be directly compared to the round port area in unibox is correct.
    ====
    Nope, and tuning their length can be a pain in the drain, but I much prefer their response to a round vent, at least at higher Fbs. If you can accept a little less vent peak output, then it's a viable alternative to go 'square' to shorten it up a bit/improve transient response without increasing vent speed.

    If the vent speed is too high, then increasing its aspect ratio will further reduce it for a given cross sectional area/length. I wouldn't go beyond a 9:1 ratio though unless the cross sectional area is quite large since the vent will become too 'stiff' to be of much use.

    BoxPlot 3.0 is the only program I'm aware of that calculates these with any degree of accuracy, but still will only get you in the ballpark. It's probably 'close enough' if you're not a perfectionist, at least I've been pleased with the results.

    GM
     
  8. Greg Monfort

    Greg Monfort Supporting Actor

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    [​IMG]
     
  9. Vince Bray

    Vince Bray Stunt Coordinator

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  10. Greg Monfort

    Greg Monfort Supporting Actor

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    Please read my edited post for more accurate info. I tend to just jot something down, then go look up info and correct/elaborate as required before posting since my memory's so bad anymore.

    I understand your point about B*** quality WRT paper, just wanted to make sure you only meant their application of it. Kindergarten kids could probably make better diaphragms than the latest? version I saw recently. Sad. Oh well, I have friends who think they're the 'cat's meow' for the bucks (read they like their size/style).
    ====
    >In your opinon is 2"x19" too thin or will it be close enough?
    ====
    Hmm, never tried 9.5:1. The 9:1 limit was published decades ago when they were popular, so it's what I repeat. I did try some 10:1 experiments long ago, and small slits in big cabs might as well have been sealed, but at 30" x 3" it worked really well and sold me on the design. I guess there's some sort of Vb:vent resistance ratio that determines what works, but I've never seen/heard of any formulas or rules-of-thumb beyond the ideal cross sectional should equal driver Sd, and not be
     
  11. Vince Bray

    Vince Bray Stunt Coordinator

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  12. Greg Monfort

    Greg Monfort Supporting Actor

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    Hmm, the vent length is too long in such an acoustically small cab tuned this low, you'll have potentially audible out of BW pipe resonances comb filtering with your CC/mains.
    I have my own rather conservative rules-of-thumb, but DW (Adire) came up with these ROTs awhile back and seems to have found wide acceptance, though I don't believe the max area one applies to large slots when they're basically the bottom of a cab [​IMG]:
    "A way to check for "too long" of a vent is to use the
    following formula:
    MaxLength = 13560 / (20 * Fb)
    Where
    MaxLength is in inches
    Fb is the tuning frequency in Hz
    For area, the following equation would work:
    MaxArea = (H*W)/10
    Where
    MaxArea is in square inches
    H is the height of the panel containing the vent, in
    inches
    W is the width of the panel containing the vent, in
    inches"
    This limits you to ~36.64" and you're up around 53"..... PRs are the only viable option in some designs. Anyway, if you decide to use a long slot vent, let us know how it works out. I've built them with vents up to 96" long, but it works out that you wind up with an acoustic labrynith rather than a ducted port design.
    GM
     
  13. Bryan Michael

    Bryan Michael Supporting Actor

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    can any one moddle me a slot loaded box with 2 av15 and 700l tuned to 16hz. thanks
     
  14. Greg Monfort

    Greg Monfort Supporting Actor

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    BoxPlot says 3.5"h x 31.5"w x 36.75" l.

    GM
     
  15. Vince Bray

    Vince Bray Stunt Coordinator

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  16. Greg Monfort

    Greg Monfort Supporting Actor

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    Hmmm. I know a square port is higher resistance than round, but I would guess square is better than skinny rectangular?
    ====
    Yep, for LF I've built mostly square or rectangular over the decades.
    ====
    > I guess the only benefit then would be making it easier to fold it in the cabinet.
    ====
    Right, I've run them along the bottom and up the back. These were popular back in the '50s, particularly with mono corner designs.
    ====
    > If I shrink the cross sectional area to the roughly the equivilant of a 6" diameter port I can get it down to that length. This seems to be a little paradoxical, noise dicatating a larger area but that in turn lengthening the port (for a given cab volume/tuning) and running up against the combing effects. I have heard of that but forgot all about it. Hmmm.
    ====
    What happens is the pipe has resonances that are independent of the air plug that resonates due to driver/cab loading. The longer the pipe, the lower in frequency they are, ergo the louder they are since acoustic energy is defined by 1/f.
    ====
    >How does this work with T-lines?
    ====
    They are 1/4WL resonators, so the whole line is a resonant chamber.
    ====
    > Is the effect not an issue due to the lack of an internal volume? >This sounds intuitively like a phase issue and may not be a concern with t-lines. I may have to do some reading on this...
    >Thanks for your input Greg!
    ====
    Hmm, not sure what you mean, but the pipe loads at 'x' fundamental dependent on length, with odd harmonics only being developed, which must be damped to keep them from audibly comb filtering between the exit and the driver, and from over modulating the driver from the backside. Here's a nifty site for learning about pipe action. Note that the calculators/formulas are strictly theoretical and don't take end correction or energizing point along the pipe into consideration, so don't try to design one from this info 'cause it will be off enough to be audible. http://hyperphysics.phy-astr.gsu.edu...opecol.html#c2
    Once you have a general understanding of pipe action, go to MK's http://www.quarter-wave.com/ and download/install the Mathcad demo and all his spreadsheets/application docs. Once you get into pipes/horns, etc., and build a few experimentals I'm betting you'll never go back to plain ol' sealed/vented except for subs or bookcase designs.
    I've been building pipe, horn, and tower speakers (he calls them MLTQWTs) for many decades because they were de riguer when I was starting out due to the need for high efficiency. The advent of cheap high power (SS) allowed less efficient drivers to be used, ergo smaller, less effcient cabs became the norm. Unfortunately, to a great extent the sound suffered in direct proportion IMO.
    You're welcome, hope it's of some use at some point.
    GM
     
  17. RayJK

    RayJK Stunt Coordinator

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    Greg,
    I guess I was making a beginners mistake when I built the port for my giant sub box project (a converted closet). I used UniBox to do my calc's and took the port area and figured out what the equivalent rectangular port would be. Going into WinISD it shows a port almost 6" shorter! I came up with a port of 5x24.75x15.25 in Unibox and WinISD shows 5x24.75x9.43 given the same parameters, a major difference!
    If possible could you recommend a rectangular port for a 2500L box with 2 of the below drivers in it? Fb used was 13.5 in Unibox but that doesn't look too good in WinISD. There 14.5hz seems to work better. Just to make things harder for you [​IMG] the port height is fixed at 5" but width is variable,I can go as wide as 24.75" max.
    If this is too much trouble for you can you give me your "best guess" as to which port length I should use?
    FS: 17.5 hz
    RE: 7.32 ohm
    QMS: 11.94
    QTS: .45
    QES: 0.417
    SD: 1159 cm2
    VAS 423.17L
    XMAX: 14 mm
    LE: 2 mH
    Ql: 5-7 estimated.
    Any help would be highly appreciated.
    Ray
     
  18. Vince Bray

    Vince Bray Stunt Coordinator

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  19. Greg Monfort

    Greg Monfort Supporting Actor

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    Ray,

    BP says 6.24"L for 24.75"w x 5.0"h/vent mach 0.03/2500L/14.5Hz Fb. Frankly, the sim says you can cut to any length between 15.25" and 6.25" with little difference in the audible response, though GD in the audible BW will be lower with the longer vent. Have you tried it sealed? Seems like with only a little room gain this would be preferable.

    Just looking at a sim, I sure hope you braced this thing to a fare thee well and sealed it up like a drum.

    What drivers are these, and what's their Pe?

    TIA,

    GM
     
  20. Greg Monfort

    Greg Monfort Supporting Actor

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    >Well I was trying to decide why the port had comb filtering issues that could not be overcome, while a t-line did not suffer these effects. In other words, if a long port is a problem, how is it possible to design t-lines that don't suffer this problem; or how is the problem overcome in the t-line design. Is it the internal damping placed in the line? If absorptive damping works in the t-line, would it work in a port for a sub?
    ====
    Yep, yep. I used to periodically post my thoughts (facts actually) on dealing with high vent mach/transient response issues, but the folks here seem to only worship at the alter of the high Q vent peak SPL 'god', so finally gave up.
    ====
    >OTOH, what is your experience with acceptable vent mach speed? I know there have been some successful designs using a 6" port with a 15" high-excursion driver, but do you have a personal rule for a max speed for somewhat critical listening?
    ====
    Hmm, a loaded question. When I do sims for folks here and on most forums I CYA myself and use 5% mach as the acceptable limit without flares, 10% mach/with, though some pro designers say 20% is fine. For my own designs that require a vent, if a sub it normally gets PRs, and for every other vented design I don't give two hoots n' a holler what it is since I'm going to 'critically damp' them for best transient response anyway. [​IMG]
    ====
    > Have you built a square port with this little area for a high output cabinet?
    ====
    Don't recall, but I doubt it. I'm a big port kind of guy. [​IMG]
    ====
    >Thanks again for the detailed help and responses. This is really making me want to build some cabinets! I usually go with the safe designs due to lack of time but I'm getting the itch... maybe I'll go for a t-line mod for my Audax kit mains.
    ====
    You bet! Good luck with whatever you do. I'm warning you though, you're heading down the slippery slope of hi-def audio addiction, and what a joy ride it is! [​IMG]
    GM
     

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