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Rear ported cabs?

Discussion in 'Amps and Cabs [BG]' started by basss, Oct 27, 2001.


  1. basss

    basss

    Aug 27, 2001
    NYC
    The idea of rear ports on a cabinet seems kind of hit and miss to me. It seems the sound would vary widely depending on where the cabinet is in the room. Sometimes we don't have a choice of where we put our cabs. Ive never used a rear ported cab but I'm wondering just how hit and miss they are. Whats your experience with rear ports? Are you screwed when you can't put them against a wall?
     
  2. Steve

    Steve

    Aug 10, 2001
  3. basss

    basss

    Aug 27, 2001
    NYC
    OK Thats an interesting article. Thanks Bikertrash82! But what if the rear (or side) port is jammed up against a wall? It seems like a front ported cabinet would be more consistent in situations like this. Is the acme article comparing two differently ported cabinets under acoustially neutral conditions or is this guy taking into account real world situations (port jammed against a wall)?
     
  4. Nothing works correctly when jammed into a wall. Bass, somebody's head, your Johnson, etc.

    The rule of thumb is have 1 port-diameter clearance. More won't hurt, but absolute minimum is 1 diameter. No matter where the port is located.
     
  5. basss

    basss

    Aug 27, 2001
    NYC
    Does that mean that the port will respond the same at any distance as long as it is at least this distance away from a wall?
    P.S. My Johnson works just fine when jammed against a wall thank you very much:eek:
     
  6. I had a drummer complaining to me about his bass player's rear ported Hartke Combo. (The combo was placed slightly to the front of him when I saw them). He reckoned he got too much bass (is this possible? :) ) in his direction and struggled to hear himself on vocals.

    I've never used one, but thought you might like to hear his opinion. (The bass player was happy with it by the way)
     
  7. Steve

    Steve

    Aug 10, 2001
    I used to be a die-hard front porter. It just made more sense to my brain. When lower back issues forced me to buy the lightest 4x10 and 2x10 I could find that I thought would do the job I ended up at Acme. Being a rear port hater I discussed the issue with Andy at length. He likes about 6" of clearence.

    Just for you, last night at the gig I shoved my cabs
    as close to the wall as I could get 'em. There was about 2" of space to allow for not binding the cord or connector. I played it loud enough to get the air really moving and the drummer and guitar player b*tching about it.

    Bottom line, No Audible Difference.

    I really don't think rear ports are any more affected by acoustic coupling with surrounding archectural surfaces (walls) than front ports. I also don't think the lack of walls, as in playing outside, makes any difference either.

    I've never felt like my low end got "screwed" under any conditions. I wouldn't let a rear port stop me (or you) from buying an Acme, old SWR or anything else that might be rear ported. It's just a brain game and really doesn't matter. IMHO, YMMV, and I'm sure others will disagree.

    As far as the drummer griping about getting too much bass, any cab will sound like crap if you're so far off axis that you're behind it, and who cares what a drummer thinks anyway? ;0)

    You might also want to consider Mr. Gavins clearence formula for your johnson as well. That kind of damage can haunt you later in life...
     
  8. basss

    basss

    Aug 27, 2001
    NYC
    Thanks!

    Thats really helpful and it makes sense!
     
  9. I have tests in mind to determine any measurable differences in port location and also driver position.

    I plan on using one of my 1x15 subs placed out on a school grass yard as an outdoor, no-walls environment. If I can get 100 feet of power cable, a computer, and power amp out there, it should be easy to take measurements at say 10 feet while sweeping through the low bass frequency range.

    My 1x15 subs are tuned exactly at 31 Hz. I expect to see zero deviation on the dB meter while measuring and turning the cabinet in 90-degree increments. I also expect no change at 31 Hz with the front-mounted driver and port facing 180-degrees away from the dB meter. In other words, I expect to find the sound wave at 31 Hz to be completely spherical.

    It will be interesting to see how much higher than 31 Hz is when the sound becomes more directional. Again, I believe I might get as high as 80 Hz before the dB meter shows any loss from directional beaming. This test will be done with the drivers facing 180-degrees away from dB meter.

    I can also cut a second port in the rear of my 1x15 subs identical to the front port. My pair of 1x15 are just working prototypes, so nothing is lost by hacking them up a bit. Then I can choose to either use the front or the rear port (never both) and measure those as well. At 31 Hz, I expect to find zero measurable difference.
     
  10. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    Acoustics of the venue have - in my experience - by far the greatest influence on sound. Standing waves, resonating stage floors and the like often make speaker size, port location, etc. irrelevant.

    However cab location is very important, because coupling effects with floor and walls have great influence on low end response. e.g. raising the amp on a chair really helps if there's too much bottom, placing the amp in a corner improves low end performance dramatically.

    Matthias
     
  11. Steve

    Steve

    Aug 10, 2001
    Most certainly you are absolutely correct.
    But do you think a rear ported box is more affected
    by those environmental variables?
     
  12. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    I don't think so - at least as long as you keep a minimum distance to the wall.

    Once I had major troubles with a cab with rear ports: Bass was much too loud behind and beside my amp and to weak where I was standing, but there was no way to change the setup. I too suspected those rear ports, so I even tried to close the ports by stuffing T-shirts into them :eek: - no effect at all.
    And I remember this small club where I played my Ampeg SVR-215 which has one backfiring speaker (working only below 300Hz) - even 'worse' than rear ports one might think - and had the best sound ever - don't ask me why...
    I'm really desperate about acoustics...

    Matthias