open back cab vs. oval port

Discussion in 'Amps and Cabs' started by fr8_trane, Jan 13, 2008.

  1. fr8_trane

    fr8_trane Member

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    The oval port cab is trendy these days but I wonder what, if any, tonal benefit it provides over the traditional open back cab. My uneducated guess is that it might provide more bass since it could act like a bass reflex panel that vibrates sympathetically with the speaker since it is a single piece of wood. Of course if this is true and the port size was not tuned properly for the cab it might make the low end worse (muddy, boomy, etc.)

    Discuss:munch



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  2. Miles

    Miles Member

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    It's probably going to result in a very slight difference and it is probably more aesthetic than anything. But, if you have less open space, you'll probably have more cabinet thump. So, it would probably be a slightly happier medium between a closed back and an open back. But again, it is going to be a slight difference. I would imagine different cabinet woods/thickness being a much larger factor in difference than open vs. oval back.
     
  3. FFTT

    FFTT Member

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    As far as acoustics, don't most studios avoid 90 degree corners to help sound distribution?
    So I guess some of that same pincipal applies rounding the rear port opening.
     
  4. TRW

    TRW Member

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    That might make a difference if the port was of any significant length, but typically in loudspeakers thats more of a flaring in the duct anyway and not just round or oval. Those cabs aren't made with thick enough walls for the oval thing to make any difference IMO. Cut a rectangular port the same size and I bet you couldn't hear a difference.

    Acoustically, 90degree angles can worsen/cause modal issues, but more often than not studios avoid 90degrees and parallel surfaces to reduce flutter echo and room colouration in the mid-range and HF...not really modal colouration.

    Good old fashioned rectangles are far easier to compute for modal response...things gets complicated very quickly with irregular shapes. Anything round or spherical is a nightmare acoustically (generally in listening spaces) unless employed on a small scale to provide diffusion/scattering.

    However that is off-topic now!

    There are many loudspeakers out there with rectangular ports, from hifi right through to ported and transmission line studio monitors like Focal and PMC.

    The oval backed cab 'might' sound a little tighter if the port is smaller than a traditional open-backed cab. I'd say a lot of it is aethestics, although there are cabs which are much more open than the one you posted above, in which case the oval version might change rear sound distribution, sound tighter and not quite as dispersed, but still rich due to rear sound interference.

    Smaller ports that are relatively tuned make quite a difference in my experience though. My ducted ported cab is killer. Love the look of the oval port though.

    -T
     
  5. quinnamps

    quinnamps Member

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    To anyone who really feels there is no potential difference in tone really needs to plunk down on a sheet of 3/4" BB and start cutting and experimenting.
    It is easy to cut ovals. Easy to make a pinched sounding cab, easy to cut one that is too loose and spills it all out. Sadly I think most cabs I see with this are doing nothing more than cutting ovals. There is no experimental phase. Ovals are easy, great sounding ovals are not...
     
  6. CitizenCain

    CitizenCain Member

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    Any tips on how to determine a great sounding oval?\ other than trial and error? Anything you might have found that's pretty general across the type?
     
  7. TRW

    TRW Member

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    Do you think the cabinet resonances and perceived tone sound subjectively different when the ports are exactly the same size/volume but one is rectangular and the other is a rounded rectangle (referred to here as 'oval')?

    Surely the most difference comes from port size and placement on the cab?

    -T
     
  8. quinnamps

    quinnamps Member

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    Read whatever you can about why Dumble did it. As far as I can tell he was the first to do it with a purpose. Anything that came before was all about looks. This is as far as my understanding of it is. If there is some work there that predates I'll stand corrected.
    As for general rules. Most I have heard that weren't too hot were either really tiny and working more as a real port, or too large to be working any better than a rectangle cut across the back.
    Experimentation is the very best to hear these differences. Once you can key in one the differences through empircal study then it gets a lot easier to narrow in on what your ears tell you.
    I can save some cutting though. Think of cutting with a largish rectangle in the middle of a ply, about big enough to hold a piece of wood with an oval cut into it right there. Run a strip of tolex just in the inside lip of this rectangle. Cut you some wood pieces that will fit in that hole nice and snug against the tolex rim you added. Cut a crapload of them and start working on some oval holes on those smaller pieces. Try 'em out live by swapping them in and out. The tolex should hold it tight enough so that you can hear the actual differnecs without needing to completely remove a baffle.
    Find where things go bad in either direction and then focus on the middle ground.
    Then when you get that somewhat sorted you can start messing with having the center dead on to the speaker, offset, angled, etc. The biggest factor though seems to be the size more than anything else. When they are really getting right you'll really hear it coming into focus. Like when you first started understanding tuning a guitar and how the waves beat. Same type of experience really.

    Well to have a similar rectangle port would require it not to go to the end of the cab. I have never seen a rectangle port. I have seen cabs with a top baffle and a bottom baffle but never one (offhand anyway) that had an actual solid piece of wood with a rectangle cut into it's middle as a baffle.
    So I cannot comment at all on if a rectangle of similar size would indeed sound the same or not. However I will say that when working right there is a lot of sound coming out of there and the hole itself become a vehicle for the vibrations. Observation of nature would seem to point to the oval as being the better choice. Whether that bears out here, to me, is unknown.
    So purely speculation but I would tend to think the oval would have the slightest edge as regards tone. That said who knows till they try?
    Aside from that it is my own personal feelings that ovals look better too.

    Perhaps but again I know of no one who has actually tried the same experiments with both shapes. I would again have to err on the side of nature and think that rounded is a better surface for waves to move around.
     
  9. TRW

    TRW Member

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    Yeah thanks for the response, that's fair enough - if I build a Ceriatone ODS sometime next year I may have a play with this and do some tests in the Anechoic chamber at work.

    I agree ovals looks better and are probably easier to cut! 90degree angles are a bitch to saw out neatly!

    Tom
     
  10. CitizenCain

    CitizenCain Member

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