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Front Porting vs Rear Porting

Discussion in 'Amps and Cabs [BG]' started by 4Mal, Aug 12, 2003.


  1. 4Mal

    4Mal Supporting Member

    Jun 2, 2002
    Columbia River Gorge
    I'm looking around for one or a couple of cabs to pair up with a Hartke 3500. Some of the cab's I've heard about are rear ported. Makes the cab's smaller, probably keeps the efficiency hi but ... what about the low's that go out the back. Does it actually create excessive bleed into other stage/room mic's (like on the drum kit ...) If you mic, how does it work out.

    I don't mic often but when I do, it's usually a 2 mic setup where the dominant mic is a bit away and out front of the amp.

    So ... any experience with rear ported cabs ?

    I'm posting this over at the Pit as well so if you see it one place or the other ...
     
  2. The placement of the port on the cab isn't important. The only thing you need to be concerned about with rear porting is having enough space behind the cabinet for the port to operate correctly. Most people say a distance of the port diameter or greater from a wall will work fine.
     
  3. BruceWane

    BruceWane

    Oct 31, 2002
    Houston, TX
    >>>>USE THE SEARCH FUNCTION<<<<

    This subject has been discussed endlessly.

    Port placement has no effect on the sound of a bass cab.

    The only thing you might need to consider is using a rear ported cabinet on an extremely small stage. It depends on the size of the port, but generally speaking you don't want to place a cabinet where the port doesn't have at least 6 or 8 inches to breathe.
     
  4. 4Mal

    4Mal Supporting Member

    Jun 2, 2002
    Columbia River Gorge
    I've heard concern over the cab placement... heard that the rear port cabs get a bit lost when playing outside ... get big and boomy when placed in corners. Seems like more variables than I want or can control...
     
  5. There's no need to be concerned. All cabs will do the things you mentioned. Outdoors there aren't as many surfaces for sound to boundary load off so cabs begin to approach free field performance. All bass cabs will get boomy if you put them in a corner because doing that decreases the amount of space the sound from the cabinet needs to fill.
    The frequencies that most ports are tuned to have long enough wavelengths to make port placement irrelevant. These frequencies are also practically omnidirectional.
    Where the port is on the cab is the last thing you should judge a purchase on.
     
  6. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    MArks right. The last time this cam up we even turned a TB'er with rear ported cabs into a lab rat. He reported that there's not difference where the port is.

    As for miking up - just mic the speaker as normal. Nothing changes.
     
  7. 4Mal

    4Mal Supporting Member

    Jun 2, 2002
    Columbia River Gorge
    ummm OK but if those very low freq's are aimed away from the front of the cab, and are reflected off a surface behind the cab, isn't that likely to disturb the phase relationship between ported freq's and non-ported freq's ?

    In a live setting that probably isn't a big deal. In a recording setting that could be a very big deal.
     
  8. I see what you're getting at here but consider this: Low frequecies propagate spherically, so the same situation occurs with a front firing port when the cab is put against a wall or in a corner. Ports don't add any directional characteristics to LF.

    There is a phase anomoly created by the physical distance from the port to the drivers but this exists in both front and rear ported boxes. You can't get a port to be 100% in phase with the actual drivers. Proper cab design can minimize it, but any ported box will have some phase anomoly (group delay) between the drivers and port.
    I'm sure bgavin could explain it better but that's the jist of it. ;)
     
  9. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    This is only part of the picture (I still haven't managed to dig up any data on the other end of the spectrum, i.e. when speakers go from controlled dispersion to omnidirectionality) but you can see from this that it would take a pretty huge speaker/port to 'fire' or 'direct' bass frequencies in a certain direction.

    Speaker/Port Diameter (inches) vs Theoretical Maximum Frequency Before Beaming (Hz)

    0.75" <sp><sp> 18,240 Hz
    1" <sp><sp><sp><sp><sp>13,680 Hz
    2" <sp><sp><sp><sp><sp> 6,840 Hz
    3" <sp><sp><sp><sp><sp>5,472 Hz
    5" <sp><sp><sp><sp><sp>3,316 Hz
    6.5" <sp><sp><sp>2,672 Hz
    8"<sp><sp><sp><sp><sp> 2,105 Hz
    10" <sp><sp><sp><sp>1,658 Hz
    12" <sp><sp><sp><sp>1,335 Hz
    15" <sp><sp><sp><sp>1,052 Hz
    18" <sp><sp><sp><sp>903 Hz

    Note than no bass cab is likely to be tuned to higher than 100Hz.

    In one of his columns in BP magazine Rick Turner explained the theory behind the Grateful Dead's "Wall of Sound" - with this the bass rig consisted of two stacks of eighteen (yes, 18!!!) 15" speakers, and this was a sufficiently large line source that it beamed the bass sound out in a flat 2D plane (not in a narrow 1D beam - that would require a rig 18 speakers wide and 18 speakers high!).

    Basically, what I'm getting at is to fire the bass sound in a given direction rather than it being omnidirectional you need a source that is as large as the wavelength of the lowest bass frequency. That's huge, like 28' for a 4-string and 38' for a 5-string bass. No port is anywhere near this size, hence their output is omnidirectional.

    Alex
     
  10. I have a beaming calculator in the REFERENCE tab of my spread sheet.

    Wave forms are spherical when the wave length is >= 4x the piston diameter. Wave forms are fully in beaming when <= 1x piston diamter. The transition area is between 1x and 4x.

    Bass horns are much more directional. A proper bass horn for 40 Hz is terribly large.. 9 feet to a side for a square mouth, etc.

    As for micing a cab, that would be done strictly for the tone produced by that cab. I prefer DI for bass, but that is personal taste on my part. The port produces nearly all of the radiation at the tuning frequency, so micing would be challenge at best. Especially for cabs tuned fairly high into the usable bass range.

    Keep a rear ported cab a minimum of 1 port diameter distance away from walls or other obstructions.
     
  11. tornadobass

    tornadobass Supporting Member

    Nov 20, 2000
    Iowa City, Iowa
    Endorsing Artist: Black Diamond &amp; SuperSensitive strings
    I think I was the rat lab...used the cab outdoors (with no back wall) this past weekend...sounded fine...plenty o'bottom end.
     
  12. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yes you were. Sorry my memory for names is not the best.....

    Just a couple of points. Andy from Acme explains the front Vs rear porting theory quite well in his website (go to Products page. It's one of the footnote articles at the bottom).

    The other thing about micing a cab is that it's not gonna work too well if you use an SM58 style vocal mic. Even bass cabs that are tuned high go way too low for mics like that - this could be a whole other thread.........