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Figuring old school slot ports??

Discussion in 'Amps and Cabs [BG]' started by will33, Aug 8, 2012.

  1. will33


    May 22, 2006
    Here's what I got. Old peavey cab. Ports flare out from the back to the front on the order of roughly 2x.

    Excuse my kindergarten level diagrams.




    As you can (maybe) see...the port opening is 2-3/8" tapering back to about half that, about 1-3/16" at the back, roughly 2-1/2' from the back wall.

    Ignoring the taper for a minute, can I plug this in a program as a 1-3/16" tall port of that width and depth or is it different?

    Assuming I can, these chambers end up to be roughly 2.7-2.75 cu.ft. tuned around 45hz give or take 1 or 2. Or is that way off base?

    Young guy, student, budget matters, as it does to me too. Likely Beta's or CB158's depending. This thing will have a guitar amp doing grindy mids/highs on top of it at least some of the time, and likely most of the time at gigs.

    I'm leaning towards the Beta's, not only because of cost, but to put for the beta bump to put a fatass bottom under that guitar grind. Top amp would be a 100 watt Peavey Windsor.

    Other option is to just stick them in there, maybe do the rice dance to find tuning and adjust by ear, which may not be such a bad idea, but looking for a little more predictability.


  2. "A" for effort!

    Slot port design is one of the portals to Hell.
    Been there, still there, still doing that.

    The taper on the vent is just a red herring.
    The volume of the air mass in the vent does not care about the vent shape.

    You have a slot port defined with two boundaries: left and right cabinet wall.
    A bottom slot has three boundaries: left, right, bottom walls.
    The end correction factor is longer for the three boundary slot.

    Tuning might be done by math, but much more accurate by measurement.
    The end correction factor also changes with the diameter of the slot.
    Smaller diameters (higher velocity) alter the end correction factor.
    Again, empirical testing is the only way to be sure.

    If you have access inside the cab, you can attach rails to mount removable slot panels.
    Take a guess, make a measurement, then adjust.
    There is a tasty calculator where you can plug in your actual tuning, and it will return the effective end correction factor.
    Use this factor to make your final slot length adjustment.
    After all the fiddling is done, glue in the slot panels.

    Note that tuning changes between 1v vs 28 volts.
    It also changes significantly with the addition of even modest damping material.
    In the end, test at operating voltage and intended damping material.
  3. Please tell me you are tuning this cab up for your 400?
  4. zenman


    Jan 30, 2008
    St. Paul, MN
    That's almost more like a transmission line rather than a port. Though to be honest, I don't know the significance of that, except that it resembles a stereo speaker that is buiilt using a transmission line configuration (a type of port) rather than a typilcal slot or round port.

    Where is Bill when you need him?
  5. will33


    May 22, 2006
    @ bgavin. Thank you. I kinda thought the port would still have to function around that smallest opening but didn't know for sure about that or what role the walls would play. Seems we could just line it, load it and adjust to taste from there if needed.

    @Cl400. No, sorry:smug: This is for a young guy here, student on a budget has this cab. Just needs to get some bass going under the guitar part of his rig. Probably a 200-300, no more than 400 watt amp would be running this 215. It's why I'm thinking the Beta in this case wouldn't cause a boom problem, just rather put some midbass/lowmid output under the guitar amp. Could still run this on it's own if he wanted, probably with a little EQ.
  6. will33


    May 22, 2006
    It's not long enoigh to function as any sort of line or horn, at least not at the port frequency. It's more an odd shaped port.

    Yeah, I miss Bill for stuff like this, but Mr. gavin has filled his shoes nicely on this one.
  7. will33


    May 22, 2006
    If we even need to fool with the port, I was thinking I'd run a couple of dividers down the full port depth and make a couple covers you could screw on from the front to adjust it. Something like a tl-606 cab has, but shaped as a wedge to fit this port.

    Are you saying we could instead just block off a portion of the slot entrance at the back of the cab? Leaving the rest of the port full width, would that work?

    May end up adjusting this one by sight. Just meaning turn the amp up, play loud, and see if any port adjustment is needed to keep the cones from flapping too much on the low notes.
  8. You change the tuning by varying the cross sectional area, or changing the vent length.

    If you reduce the slot width, do it uniformly.
    Looking at your drawing, it is probably easier to add another plate to the slot panel itself.
    This would make the height of the slot less, and decrease the cross-sectional area.
    And increase the risk of chuffing.

    You might consider removing this Sunn-clone vent entirely and replace it with a flat block-off plate.
    Install 1-2-3 or ? round vents as needed for a reasonable vent velocity.
    These are MUCH easier to tune.

    Calculate the vent velocity at the power level where WinISD shows the Xmax limit.
    For example, the S2010 in Flat is good for about 21 volts / 55 watts at the upper Xmax limit at 85 Hz.
    Increasing the vent based on the theoretical 150w PE is pointless, unless you plan on overdriving into non-linear distortion.

    My 10" test box for S2010 uses a 4.2" diameter vent, which is MACH 0.045 at 21 volts.
  9. will33


    May 22, 2006
    Thanks again. I was thinking making them uniformly narrower if need be but hadn't thought about just blocking some of the front.

    Yeah, pipes are much easier. Could just cut it out, patch it in and do that. Don't know how much work he wants to put into this. With Beta's and some odds and ends, he'd be into it for about $200. That's about all you could get out of this thing in a sale. Any more and might be better off just finding a deal on some other cab that already works. So, we'll see.
  10. will33


    May 22, 2006
    Guy has a much nicer diagram of it in .pdf but I can't figure out how to turn that into a picture to post. Something about "print screen", MS paint, moving it back and forth. I'm no computer guru with that stuff. It was easier to just draw something and take a picture of the paper laying on the desk.:p
  11. craig.p


    Sep 28, 2008
    New Hampshire
    That's the Peavey 215 cab whose owner posted here a couple weeks ago, yes?

    Slightly OT, it's interesting that his rear port height is much greater than mine, which is about 0.75". Pretty scary, actually. I'm wondering if this is due to a design change or manufacturing tolerances. At any rate, this could account for what different people think of the same cab based on their own individual samples.

    But anyway, the problem with calculating box tuning with a non-uniform height slot vent is that you don't know the exact boundary marker between box-owned air and outside air. You could swag it, but the problem there is if you get it wrong by even a little bit, it could have a huge effect on box tuning.

    So... If it were my project, I'd go one of three ways.

    1. Wedge the port to a uniform 0.75" or 0.5" the full length of the port, depending on what you want for box tuning. Constructing the wedges would be interesting, which is short for "pain in the neck." Same for attachment to the existing angled port wall.

    2. Cut out the existing angled port walls and install new ones installed straight. (Essentially you'd be re-porting the cab with fresh lumber.)

    3. Seal the ports to make it a sealed box. That's probably the easiest solution. Obviously you'll want drivers designed to work in that environment.

    If you choose 1 or 2, verify port velocity before you get started. I'm pretty sure you'd be ok, but best to check. A pair of CA154s in that cab, ported to 0.5", can do 300 watts right at Xmax (at 25 and 62 Hz), and still stay below 20 m/s port velocity all the way down to his low B. I know you're not considering the CA154 but the ones you are considering should model similar.
  12. will33


    May 22, 2006
    Yeah craig p. same one. The measurements at the back where the slot opens into the box are estimations based on measurements I do have and just looking at his diagram. The front height is accurate.

    If he wants to do something with it, it's likely still be listen, adjust, listen, adjust, etc. Or block it off and install round ports. Those I can get close enough via modeling to just say, "use this size pipes, this many and this long" and get it right the first time. The CA154's are nice, as are the CB158's. That sort of stuff would put the driver value beyond the cab value. That's his decision to make. The Betas would put good "oomph" under the guitar half of the rig, handle his smallish/medium size amps, and he might still be able to sell it and break even if he wants to down the road.

    Guy got a good deal on a Hartke cab that works since then, so he may just play that, or flip it for a little profit to help fund this. So, we'll see.
  13. BogeyBass


    Sep 14, 2010
    just a flared slot port.

    if I remember a slot port will behave as if it is 1/2 of the port height longer than it actually is.

    so being the port starts at 1.18 inches and then goes to 2.37
    might have something to do with end factor correction. or the limits that might have been reached for the needed length.

    a smaller longer port can help eliminate out of phase signal from
    leaking out of the port, of course the trade off is higher velocity and possible chuffing. so start small and flare it, simple.

    anyways just think he would need to add extra length in winisd to calculate the actual tuning freq based upon the 1 3/16 calculation.
    but it gets confusing since winisd already automatically subtracts 1/2 port height from length.
    likewise since the opening is roughly 1/2 larger it might just be a simple old school way of including error factor correction without subtracting or adding length to the port. or its a simple flare.

    not saying that is correct just assumption so calculate the port as 1 3/16 and add the difference of 2.37 and 1.18 to the length to get the actual port frequency. around 1.19 inches.

    otherwise just use a 15" speaker you have laying around, and measure the actual impedance curve inside the enclosure to find Fb,
    or just run test signal through it and try to observe at which frequency there is little cone movement. plenty of free signal generators on the net basically just use your computer as a test oscillator
  14. will33


    May 22, 2006
    When I was guessing a middle 40's tuning, I was using 1-3/16 height, 10-1/2 depth and 24 width bit did not include the 1/2 height correction. Now according to craig p's cab, I think my estimates may be off. The cab is in a suburb, not at my house. The guy sent a diagram with most all the measurements needed, but missing those 2. The exact height of the port at the back, and the exact distance from the end of the shelf to the back wall. Those are kind of important.:)

    Sent a request to the owner for those 2 measurements. Yes, I coukd put any speaker in it and find tuning @ 1 watt. Bgavin says these slot things wander in tuning when more power is applied and are also affected by the lining/damping that would be put in the cab. So, it's still a moving target. Maybe easier, or at least more accurate to saw that shelf out, patch in the front and use pipes. Woukd gain a little internal box space that way too.
  15. BogeyBass


    Sep 14, 2010
    just calculate according to both ends.
    to get a rough idea of the range it could be in.
    also calculate in a smaller volume and larger volume to see
    the range it will effect the tuning.

    so if you calculate it at 2.37 inches x 24 with 10.5 depth
    roughly 54 hz add/subtract an inch 56 or 58hz
    and assume a small box 2.8

    calculate it at 1.187 inches x 24 with 10.5 depth
    roughly 42hz add/subtract an inch depth 40 or 43hz

    so assume 42 to 54hz in a small volume

    because the initial opening is smaller 1" or so can only figure your at 40 to 44hz with a smaller box.

    now calculate at a larger 3.8 volume
    those numbers will become much lower.

    so your low range is around 36hz and the high range would be

    all in all falls into my usual assumption that a 15" box is tuned 30 to 50hz and all in all your assumption around 40hz is correct and should be just fine.

    likewise you mentioned betas and I think the Peavey pro low performs similar if not better to a beta, but is much cheaper.
    dont hold me to that, but I know its performance is very similar to one of the american standard 15's and is much cheaper for your budget minded friend.

    I ignored it for some reason, have a feeling it has a little more top end than a beta, not sure about power handling.
    you would have to run it in winisd to confirm of course.
  16. will33


    May 22, 2006
    Thanks for the tip on that Peavey. Will check it out.

    Yeah, as long as the tuning is anywhere in that ballpark, it should be at least workable. Just don't want it unloading on low notes, but this has enough port that should be ok. Some old cabs that just had holes cut for ports were so stupid high anything would fart them.
  17. will33


    May 22, 2006
    I remember that Peavey now looking at it. It would do well as a replacement for PA mains, but the 2.5 xmax isn't quite enough, even for a budget bass speaker. But keep it in mind. It does have other good uses and is cheap.

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