Is a port needed in a sealed cab that farts out?

Discussion in 'Amps and Cabs' started by plan-x, Mar 20, 2008.

  1. plan-x

    plan-x Supporting Member

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    I have a 1x12 closed back cab with a v-30 that gets muddy in the bass at high volume and just sounds wrong. This didn't happen with the same speaker in an open back combo. My therory is at high volume with max speaker cone movement it creates pressure variances in the cab, thus interfering with speaker cone's natural movement. So I'm considering drilling maybe a 1" hole or so in the front baffle.

    Anyone have any thoughts on this?
     
  2. StompBoxBlues

    StompBoxBlues Member

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    I'm no expert at all, but it makes sense to me. Same thing you are saying put another way, bass is a slower frequency, but especially at higher volumes it makes the speaker move much further in and out, which would be harder if the pressure in the cab (since no venting) would "dampen" it, not let it so easily move.

    I'd have to think about it more, but it might even be that it affects one movement more than another...maybe the "in" movement of the speaker gets dampened most because it is trying to compress, or maybe the out because it creates a buit of a vacuum.

    But, just for argument, say you decide to try it...what considerations does one have to have for the size, location, and design of that actual port?
    I've never read much about how ports need to be "tuned" or if the simply ARE more a function that lets air move in and out, letting the speakers work undampened or if ports also are expected to send out some frequencies to the front, be a part of the overall sound as well?

    I know nothing about ports...
     
  3. rockon1

    rockon1 Member

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    Sealed cabs usually have a tighter bass response but perhaps the internal volume is too small? I wouldnt randomly drill a hole in your cab though. Any type of porting needs to be done correctly in order to function properly. I'd consider making a 1/2 back for the cab first. Or perhaps you might try some acoustic stuffing first though,. By lighly filling the closed cab with poly filling it helps the low end stay more accurate.

    http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-317


    ..
     
  4. bosstone

    bosstone Member

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    I am not sure that using stuffing the cab makes the bass more accurate, it is more that it simulates being more of an infinite baffle and therefor extends the range of bass frequencies.
     
  5. rockon1

    rockon1 Member

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    Anytime Ive tried it it reduces "boominess"-the bass is more controlled and hence I used the term "accurate". It also reduces unwanted resonance from the cab making the bass response more -for lack of a better term-accurate. Same reason it is used in stereo speakers. At any rate short of opening up the back of the cab it might help his situation.Bob
     
  6. StompBoxBlues

    StompBoxBlues Member

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    Is this "acoustic stuffing" something specific (and way expensive) or will other materials do just as well?

    What DOES one have to consider if considering making a port in a closed cab? I have a Marshall 1922 I might think about trying, but am not sure what dimensions, considerations need to be considered.
     
  7. rockon1

    rockon1 Member

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    You can go to a upholstery store and get it cheaper than the link I provided above. It can be poly batting too like I used since my upholsterey store didnt have stuffing. I just 'puffed " it up a bit and lightly filled my 2x12 with it. Fibreglass insulation works well too but isnt very "friendly" to work with.

    As far as porting goes there are "calculators" you can use to figure out the size of a port that can be "googled" on line. The size of the port depends on the internal volume of the cab and the resonance frequency of the speaker. Honestly I got confused about it and gave up on the idea of porting. Maybe I''ll investigate it further one day. Bob
     
  8. beej

    beej Supporting Member

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    At some point John Suhr had mentioned which software he used to calculate the port on the Badger cab- I'm sure you can find it in Google, somewhere.

    Calculating the port size based on speaker characteristics and enclosure size seems like a much better approach than just randomly drilling.
     
  9. StompBoxBlues

    StompBoxBlues Member

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    Okay...thanks for the info.

    From a different angle, supposing one doesn't know the volume, doesn't want to get into the finer calculations of exacting degree on shape, size, location of the port...what then?

    I mean, there must be a "ballpark" rule of thumb port one could try that while not ideal would improve it? Where would you put that and which size, etc.?

    It's like detuned speaker cabs. I tried doing this in a 2 x 12 closed back, just by pulling one of the 12's. Worked pretty nicely. I am certain one could build to acoustic design specs and make it perfect, but this works well enough for me.

    So if I wanted to try making a port in a Marshall 1922 where, and how big would be ballpark enough to maybe improve (and most likely not ruin the sound)?
     
  10. rockon1

    rockon1 Member

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    Dont know. Its my understanding that if I the port is the wrong size(most likely too small) the resosnance frequency of the cab could be below that of the speaker. This senario will likely damage the speaker. You need to keep the resonant frequency of the cab at or above that of the speaker. At least thats my limited understanding of it. Bob
     
  11. teleamp

    teleamp Senior Member

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    Have you tried re EQ'ing your amp, turn the bass down when using the closed back cab.
     
  12. RGB

    RGB Supporting Member

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    I have an old Sonic, Theile ported G112 that came from the factory with what appears to be regular, home type insulation in it. I have a Scumback H75 LHDC 16HP in it and it is a match made in tone heaven! I've had a lot of different drivers in it over the years, but that Scumback just sounds amazing in it.....Like a mini 4x12. Tight, punchy and perfect for small venues as it sounds big at low volume.
     
  13. plan-x

    plan-x Supporting Member

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    Thats probably the solution. Since the porting thing is for rocket scientists. I just love fat bottom amps. They make the rockin world go round.
     
  14. rockon1

    rockon1 Member

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    FWIW -It will cost you 10$ to try the acoustic stuffing from Parts Express. More expenise than an uphostery store but more convenient. Bob
     
  15. guitardr

    guitardr Silver Supporting Member

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    Research Hemholtz, Thiele and time-alignment porting.
    Bag-End & Mesa Boogie have made them in the past.
     
  16. DEMENTED

    DEMENTED Supporting Member

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    For a cheap alternative why not buy a pillow from a department store for a few bucks and use the poly batting in that? Should work, no?
     
  17. phatster

    phatster Member

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    I recently got a port city OS 1-12 cabinet which sounds awesome.Anyway I have another cab that was essentially doing what you explained.I decided to replicate the port city port and it has worked wonderfully!I copied the dimensions exactly and what a difference...easy to do...the bass improvement was very significant,yet the mids remain focused,:munch
     
  18. rockon1

    rockon1 Member

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    Well tell us a bit more then! What are the dimensions of the cab and what port size did it "require"? bob
     
  19. phatster

    phatster Member

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    I'll measure and report back,,,,,,,,,,,
     
  20. phatster

    phatster Member

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    OK...The cab is 18"high 22"wide 11.5'deep......the "port" is a 11/8" slit at the bottom front of the cab.The tricky part is "rounding" the the 90% angles at the back of the cab and at the front which effectively helps push the sound.No need to round the top 90% angle,so your just doing three angles total.I used quarter inch plywood to round the angles,each slant is 5" wide.The bottom angle{front} will require a a much thinner slant.I used screws and calking to make a tight rattle free fit.I hope this is clear. Good luck:BEER
     

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