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Cabinet Nearing completion. More advice needed.

Discussion in 'Amps and Cabs [BG]' started by Magneto, Feb 17, 2005.


  1. First off, thanks to everyone that have helped in my earlier posts. My cab project is close to completion and it's time to decide on a tuning frequency for it. I know I've asked these questions before, but I'm still a bit in the dark.
    Here's what I have:
    Eminence Kappa Pro LF-2
    Cabinet volume: approx. 4.2 cu-ft
    Ports: (2) 3" round PVC

    SBB4 alignment for these drivers shows 2.6 cu-ft Vb, 35hz Fb, with an F3 of approx. 59 hz. This is supposed to yield the tightest bass, correct? It also seems to give the best group delay figures as well.
    My problem: I cannot shrink my cab volume any smaller without adding 4x4 timbers to eat up volume, so I'm stuck with what I have.
    I'm considering tuning somewhere inbetween 35 and 40 hz. Tuning 35 (Fs) gives me lower group delay numbers near the low E note, with an F3 of about 45.3 hz. Tuning 40hz gives me a lower F3 (approx 40hz), but group delay suffers.
    I guess there's no way to really know how this box will sound by looking at the graphs. I don't really know if trying to achieve a lower F3 is extremely important, or a flatter frequency response as shown in the graph.

    Port length and end correction: This is new to me from WinISD beta. I'm going with 3 inch ports, but am confused as to what that end correction means. I'm using 3" PVC pipe (ID 3"), and there will be one free end in the box, and the other end will terminate flush with the speaker baffle. Do I need to make a correction to my port tube length, or do I go with the length shown?

    If anyone can punch in these numbers and advise, it would be a great help.
    Thank you..

    Mag...
     
  2. The Clap

    The Clap

    Jan 5, 2004
    Scottsdale, AZ
    Group delay is a fun graph to look at, but for this application I wouldn't worry about it. People argue about how much group delay is accetable, and how much delay you can have without actually hearing a difference.

    Unless you play lower than a 4 string, I wouldn't tune lower than 40hz.

    Also, note that where you tune is where the port is going to help out by allowing the driver to work at lower excursion levels. If you tune below the used frequency of your bass, you won't get the tuning boost... well.. in the frequency range of your bass
     
  3. Hi, and thanks for the reply. As far as group delay goes, I know that lower is better (of course), but I'm not going to get those SBB4 alignment values with my box. I'm also thinking of tuning near 40 hz, and yes I play a 4 stringer. I've also heard alot of people say that their 5th string low B sounds good even with cabs tuned higher.

    I just cut my vent holes in my baffle. Next is double checking all internal volume displacements, getting the final volume, and then calculating the vent tube lengths. I'm concerned about this end correction figure. Maybe I'll see if WinISD's explanation makes more sense than it did the last time I read it.
    I look forward to hearing more opinions about suggested tunings...

    Mag...
     
  4. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    End correction isn't critical. The theory is that the traditional port length calculation formulas work best if your port hangs half inside the cab and half out. In other words, the air exits the port into free air, nowhere near the wooden baffle of the cab. Some argue that the baffle so close to the mouth of the port make the actual tuning frequency a bit lower that what you were intending.

    But how much lower? Probably no more than 0.5 of a Hz IMO. I wouldn't worry about it. WinISD assumes you're going to have one end of the port against a baffle and adjusts accordingly. That's all it is.

    I don't have that drive rin my WinISD database. I'll have a play with it later when I'm done working.
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Correction, I do have it in my database.

    At 4.2cu.ft, The difference in transient response between 40 and 35 was quite small in this case. And both yield perfectly good group delay calcs. Seriously in you're under 20ms, you've got nothing to worry about.

    I'd tune it to 40Hz to start with, and test it. Ports aren't expensive, so I'd try the 35Hz tuning as well, then go with whatever sounded better.

    At 40Hz, make each port 3.45in long. At 35Hz, make them 5.18in.
     
  6. PeteBass,

    Thanks for explaining that port end correction adjustment. It makes sense. I was playing around with those variables earlier, noticing how the vent length changes as I chose one flange, two flange ends, etc..

    I've done my final calculations, and I'm going with 40hz for starters. I'll cut my port tubes in the morning. The lengths you've provided are the same as what I came up with, so I'm good to go.

    Thanks again for your help. By the way, how did your cabinet project work out for you?

    Mag...
     
  7. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    It's going slowly but I'm in no hurry. The 1st 12" speaker has arrived and I'm trialing in on it's own before I decide what speaker to mate it up with. I'll know more after this weekend.

    Make sure you let us know how this one pans out.
     
  8. The large amount of over-size is going to make your cabinet into an EBS (extended bass shelf) design. This results in a significant amount of increased group delay around the tuning frequency.

    Consider making up a sealed, rigid box (sub enclosure) you can secure inside your cabinet to take up the volume. The right way to build a cab is to use the engineered requirements, and not just a haphazard box that holds the drivers. This is the difference between a tight cab and a boom box.

    Without seeing your design, I'd guess the easiest way to soak up space is by installing an inner backwall panel to reduce the cabinet volume, provided it doesn't interfere with the driver depth. Once in place, you glue/screw/seal it, so it doesn't buzz or leak air. Be sure to calculate the displacement of your volume, and add in another tenth for the port.

    You will always have to do the fine tuning with a signal generator from a computer, and a digital volt meter. Calculations will get you in the ball park, but mine have never landed spot-on without manual adjustment.
     
  9. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I read somewhere (though I can't remember where) that another way to do this is to attach some hard packed styrofoam to the inside of the box. I theory it takes up enough space to make the cab smaller, provided of course it's packed hard enough to stop soundwaves from getting through it, but it weighs a lot less than the timber required to make a sub-cab.

    Is there any merrit in this? I've never tried it. And I haven't quite worked out what type of adhesive to use to hold it in place that won't make the styrofoam disintergrate..........
     
  10. bgavin: I'd like to hear more about how this is done, what type of digital voltmeter is needed, etc.. I've heard you mention this before in other threads, but I can't seem to find instructions.
    Your idea for soaking up some cab volume is an interesting one. My front speaker baffle will be removable, so it's something I can consider if I'm not happy with the sound. Of course, to get down to the 2.6 cu-ft suggested by the SBB4 alignment, I'd have to lose 1.6 cu-ft of volume, and that's alot.
    I do realize that the cabinet should be built around the speaker, and not the other way around. I have plans for better cabs for these two 15's. If I had known this cabinet was going to need so much work, I would have built a new cabinet. Well.. I needed a cabinet to play through, and my budget wouldn't allow buying or building something new.

    Petebass: I do suppose one could use something like you've suggested. It would have to be something light. This cabinet is already going to be heavier than it ever was before.
    I doubt if something like open-cell eggcrate foam would work.

    Anyways.. got my port tubes cut and installed in the baffle. I'll be mounting that board tomorrow.

    Mag...
     
  11. The Clap

    The Clap

    Jan 5, 2004
    Scottsdale, AZ
    I would be VERY Interested to hear specific methods for measuring and tuning speakers/systems with basic tools like bgavin mentions. I've never found a really clear explanation of certain methods online
     
  12. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    That won't work. It will actually have to opposite effect. Sound still gets through that stuff, and it slows if down sufficiently that it takes longer for it to hit the timber wall. As a result, the speaker "sees" a larger cab, not a smaller one.
     
  13. Well.. I got the two 15s mounted in the cabinet and tested it yesterday. My first impressions were very good.
    High end: Well, I expected it to fall short in this area, and it does. I still have the option of adding a tweeter, even though I'll be leaving a hole in the frequency range, but I think it would still brighten things up.
    Mids/upper mids: I'm getting ALOT. I'll need a bit more EQ control over these freqs, but I have a 31 band EQ that I can use.
    Mid-Bass: Sounds pretty full. Not as punchy as I'd like, but I'm running more cabinet volume than ideal for these Kappas.
    Deep Bass: Not lacking anything here! haha.. I tried running as much bass as I could and those speakers didn't have any problem handling it.

    Question: I didn't line my cabinet walls with anything. I decided to try it like this for starters. I'm wondering if lining would help with the harsh mids in any way. The cab does sound a bit boomy. Any ideas? I don't want to run out and buy a bunch of expensive acoustic wadding stuff, but I do have some itchy fiberglass insulation I could try.
    Walmart has some polyfill cotton looking batting that might work.

    All in all things are sounding ok. Now I get to have fun carpeting it. Looking forward to practice on Thursday night. I think the band will be surprised.

    Mag...
     
  14. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    What I recall hearing, was almost a metallic 'clank' type of a distorted sound from the mids reflecting. I found that some batting/insulation behind the drivers cured most of that. The fiberglass may be just fine; just nasty to work with; plenty of glue and long staples.

    Congratulations; it sounds like you have a winner.
     
  15. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I'm almost certain that lining the cab will eliminate the need for the 31 band EQ to control the mids.

    Use whatever you can get your hands on. I've tried everything from cheap fiberglass lining to expensive foam tiles with egg-carton type shapes. To my ears, both did a good job of eliminating standing waves and the cheap stuff sounded the same as the expensive stuff. The only diff is that the whiz bang stuff looks better when you open the cab, or when you look through the cab's port holes - hardly a compelling reason to use it.
     
  16. Thanks for the ideas.. I'll come up with something to line it with. ESP-LTD's remarks sounds alot like what I'm hearing. A harsh metallic sound.

    I'll let y'all know what happens with the lining.

    Mag...
     
  17. Sundogue

    Sundogue

    Apr 26, 2001
    Wausau, WI
    I added some egg crate foam (the black acoustic foam) to the top and back in my cab and it did wonders for the boomy mids. Sounds great now.

    I did have to experiment though. I had some on the sides but it was too muffled. The beauty with using foam is you can easily try different amounts and placements until you get it to sound right.
     
  18. So you just put that foam on the back and top only? That's interesting? And it sounds better than with it on the sides too? That would really work great for me because I have my ports on both sides (about 2 inches from the walls) and wouldn't want my stuffing interfering with those.

    I love the great ideas you people share.. the only problem is that sometimes it gives a person too much to think about? haha.. Thanks..

    Anyone else experiment with lining only a few panels instead of all the way around? I'm curious..

    Mag...
     
  19. Sundogue

    Sundogue

    Apr 26, 2001
    Wausau, WI
    I ended up with it only on the top and sides. I started with the whole inside lined with it. It sounded too dark. I pulled off the foam on the sides, remounted the speakers and it sounded better, but not quite right. I pulled off the foam from the bottom and it sounded perfect.

    Just experiment. I used a staple gun to attach it to the inside. It was all trial and error until I was left with it sounding great on the top and back only. What works for you may be different.
     
  20. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    You might try a foam mattress pad; they usually have a dimpled surface and are 1-2" deep. I've used them successfully before, but I don't think it's going to absorb mids like 3" of fiberglass or polyfill on the back would.