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  1. #1
    Join Date
    Jan 2012
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    Oldsmar, FL
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    330

    Default Down-firing port on a bookshelf speaker?

    As long as the base of the speaker is elevated properly, is there any reason not to do it?

    If not, what would be a good rule of thumb for port size vs elevation?

  2. #2
    Join Date
    Sep 2005
    Location
    Chicago-ish
    Posts
    1,832

    Default Re: Down-firing port on a bookshelf speaker?

    That significantly limits placement options since the space between the bottom of the box and whatever it's sitting on forms part of the port volume and must be consistent. Why not just go front-firing slot port?

    I'm not sure I understand your question about port size vs elevation.

  3. #3
    Join Date
    Jan 2012
    Location
    Oldsmar, FL
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    330

    Default Re: Down-firing port on a bookshelf speaker?

    Quote Originally Posted by cjd View Post
    That significantly limits placement options since the space between the bottom of the box and whatever it's sitting on forms part of the port volume and must be consistent. Why not just go front-firing slot port?

    I'm not sure I understand your question about port size vs elevation.
    Sorry...imagine the speaker has stumpy legs on the bottom, kind of like a down-firing sub.

    Relative to port diameter, how tall should they be?

  4. #4
    Join Date
    Dec 2011
    Location
    AZ
    Posts
    836

    Default Re: Down-firing port on a bookshelf speaker?

    You can put a base on the speaker so it wont matter where you place it.


  5. #5
    Join Date
    Jan 2012
    Location
    Oldsmar, FL
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    330

    Default Re: Down-firing port on a bookshelf speaker?

    Quote Originally Posted by Jay1 View Post
    You can put a base on the speaker so it wont matter where you place it.

    That's exactly what I'm referring to, although I think the throat horn may be more about marketing.

  6. #6
    Join Date
    Sep 2005
    Location
    Chicago-ish
    Posts
    1,832

    Default Re: Down-firing port on a bookshelf speaker?

    Quote Originally Posted by BeeTL View Post
    That's exactly what I'm referring to, although I think the throat horn may be more about marketing.
    For the most part, it is. If you build the base below, sure. It's a volume game. I'm not sure there are rules as long as it's enough space. Tuning is a bit tricky (easier to fiddle and measure and adjust than calculate effective volume.) But again, you could just slot port for a similar-ish look and no more or less space taken up.

    On the sub I built with a 3" port, bottom firing, I have 1.25" clearance.

  7. #7
    Join Date
    Nov 2006
    Location
    New Hampshire
    Posts
    6,633

    Default Re: Down-firing port on a bookshelf speaker?

    Quote Originally Posted by cjd View Post
    For the most part, it is. If you build the base below, sure. It's a volume game. I'm not sure there are rules as long as it's enough space. Tuning is a bit tricky (easier to fiddle and measure and adjust than calculate effective volume.) But again, you could just slot port for a similar-ish look and no more or less space taken up.

    On the sub I built with a 3" port, bottom firing, I have 1.25" clearance.
    It is tricky, but yes, you can calculate it so that the section below the duct exit is actually an extension of the duct. And true, the flared section in the pictured cab is purely cosmetic, for the benefit of shoppers who don't know that a port output is omni-directional and think that low frequencies need 'steering'.
    Last edited by billfitzmaurice; 04-16-2012 at 08:15 AM.

  8. #8
    Join Date
    Mar 2010
    Location
    North Texas, USA
    Posts
    3,503

    Default Re: Down-firing port on a bookshelf speaker?

    The general rule of thumb for the end inside the cab is no closer to the wall than 1 diameter. Might want to keep that in mind for your bottom exit.

  9. #9
    Join Date
    Jan 2012
    Location
    Oldsmar, FL
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    330

    Thumbs up Re: Down-firing port on a bookshelf speaker?

    Quote Originally Posted by cjd View Post
    For the most part, it is. If you build the base below, sure. It's a volume game. I'm not sure there are rules as long as it's enough space. Tuning is a bit tricky (easier to fiddle and measure and adjust than calculate effective volume.) But again, you could just slot port for a similar-ish look and no more or less space taken up.

    On the sub I built with a 3" port, bottom firing, I have 1.25" clearance.
    A slot port will not integrate well with the design I'm imagining, as it would add complexity.
    Quote Originally Posted by billfitzmaurice View Post
    It is tricky, but yes, you can calculate it so that the section below the duct exit is actually an extension of the duct. And true, the flared section in the pictured cab is purely cosmetic, for the benefit of shoppers who don't know that a port output is omni-directional and think that low frequencies need 'streering'.
    Are you implying Polk Audio may be less than sincere with their Power Port bass technology PR?


    Quote Originally Posted by fastbike1 View Post
    The general rule of thumb for the end inside the cab is no closer to the wall than 1 diameter. Might want to keep that in mind for your bottom exit.
    I was hoping for half the diameter, but making adjustments will be easy enough.

  10. #10
    Join Date
    Dec 2011
    Location
    AZ
    Posts
    836

    Default Re: Down-firing port on a bookshelf speaker?

    It looks like it extends the length of the port across a second flare. Does that actually reduce noise compared to a standard precision port? Who knows. One of their claims seams fairly accurate at least.

    compact design to mimic the properties of what would be a very long flared port opening
    The power port didn't have anything to do with why I linked that pic

  11. #11
    Join Date
    Mar 2010
    Location
    North Texas, USA
    Posts
    3,503

    Default Re: Down-firing port on a bookshelf speaker?

    Several of the bottom ported builds I've seen look they used 1/2 diameter. Like everthing else in the build, it's a compromise. It will change the build similar to having a rear port up against a wall. We rarely have the perfect place to put a speaker.

    I always think that having speakers in a room, no matter how compromised, is better than not having music in the room.

    Quote Originally Posted by BeeTL View Post

    I was hoping for half the diameter, but making adjustments will be easy enough.

  12. #12
    Join Date
    Sep 2005
    Location
    Appleton
    Posts
    7,560

    Default Re: Down-firing port on a bookshelf speaker?

    Quote Originally Posted by BeeTL View Post
    A slot port will not integrate well with the design I'm imagining, as it would add complexity.

    Are you implying Polk Audio may be less than sincere with their Power Port bass technology PR?


    I was hoping for half the diameter, but making adjustments will be easy enough.
    Top venting works great too.
    1/4" "hardware cloth" around bottom makes it easier to extract HotWheels that "fall" in there.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  13. #13

    Default Re: Down-firing port on a bookshelf speaker?

    Quote Originally Posted by Jay1 View Post
    You can put a base on the speaker so it wont matter where you place it.


    is there a place to buy formed plastic cones like in that base?
    I need that dispersion reflector thing for an omnidirectional outdoor speaker.
    or maybe there is a better standard approach?

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