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Trouble with my Omega Pro 12 cab

Discussion in 'Amps and Cabs [BG]' started by jock, Jul 2, 2004.


  1. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    My hommade Eminence Omega pro cab sounds a bit muddy on the open E up to the F# on 4-stringed bass. Might have something to do with the not so great group delay at these frequencies. The cab is tuned to 40 Hz and is about 35-40 liters big (SBB4)
    Would it hurt the driver to put it in a sealed cab?
    What would I gain loose?
    I tried it in Winamp and the groop delay disapered almoust completely. So, how will a driver designed for a vented cab work in a sealed cab? Will it hurt the driver?
     
  2. geshel

    geshel

    Oct 2, 2001
    Seattle
    Well, you can see the effect on output it will have in WinISD -- probably a higher rolloff / lower output at 40Hz.

    You've also already noted the g.d. change. That could be why it sounds muddy, yes. More on that later.

    Check out the cone excursion plots for the two options. Notice that, for the vented box, it can handle more power around 40Hz than the sealed one. However, below 40Hz, the sealed one handles more power.

    Running it in a sealed box won't damage the driver unless you push it too hard; which is the same for the vented box, however the limits change at different frequencies between the two.

    I would first do a "taste test" by plugging up the port(s) and playing at a moderate volume. Does that make it sound less muddy? If so, I'd say go for it.
     
  3. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    Thanks. I was thinking about plugging the ports first but wanted to be sure I wouldn´t damage the driver. BTW there is no cone excursion plot in the standard WinISD I have (as far as I know).

    Also what can be used to plug the ports? They are round with an inner diameter of 3,5 cm.
     
  4. geshel

    geshel

    Oct 2, 2001
    Seattle
    Ah, that's right. WinISD Pro has excursion plots, but I'm not sure if it's still available.

    For testing the sound change, you can stuff a pillow or shirt into the ports -- something that fits in pretty tight and makes it so that there's no apparent airflow through the port. It won't mimic a closed box exactly, but I think it will still help you figure out if that is the way you want to go.
     
  5. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    According to WinISD the portdimention wouldn´t cause turbulence but you never know. So you think it will sound thin? :(
    I will try it out hopefully today and repot back.
    Thanks so far!
     
  6. geshel

    geshel

    Oct 2, 2001
    Seattle
    Hmm, two 3.5" ports for a single 12" is a decent amount of port area I'd say.

    Looking at the Omega pro specs, it's definitely better suited for a ported box, but sealed shouldn't be horrible.

    (later) - OK, I just installed WinISD Pro. Yeah, it'll start to rolloff (gently) at about 100Hz. And, if you use EQ to make up for the difference, it will cause the cone movement to go up a lot.
     
  7. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    Right! But there are four of them.
     
  8. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    May I ask why QB3? :eyebrow:
     
  9. geshel

    geshel

    Oct 2, 2001
    Seattle
    Ah! hmm. Yeah, those seem a little small. Of course it is the total area that matters on paper, but in reality as the number of ports goes up and the individual area goes down, they start behaving less ideally.

    Though, I wouldn't expect any of the side effects to cause the sound to be "muddy".
     
  10. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Jock I'm only guessing here. In the past when I've chosen to use more than one port, I've done so to try and get it so each port length the same as the thickness of the baffle? I'm just wondering because at 37.5 litres, tuned to 40Hz, with 4 ports each 3.5 cm in diameter, it looks like each port would have to be 17.5cm in length? How long are your ports?
     
  11. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    My ports are about 15 cm long.
    But Im beginning to think that the mud has more to do with my bass pickup, my living room with rattling furniter or the 60W Behringercombo Im driving it with and not the cab itself. The mud did not go away as I stuffed the ports. The roll off of the lows started a bit earlier and there was quite a difference in response. I might use it that way when I ad a 15" later and use a crossover on the 12 to cut some lows.
     
  12. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    hmmmm....... the porting seems ok, allowing for a slightly different cab volume of course. The group delay at 40Hz should only be about 13ms which isn't too bad. I start to worry when it gets close to 20ms, so I'm also prepared to rule that out as a cause. And with a -3db point in the vicinity of 65Hz, it's not likely to be an abundance os low end that's causing it. So the cab might not be the problem

    If you can get your hands on a different bass and maybe a different amp, it might give you an idea as to which of these might be contributing to the problem.
     
  13. jock

    jock

    Jun 7, 2000
    Stockholm, Sweden
    I will bring it to our rehersal space next week and try it out with my big amp. I think that will make quite a difference.