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  • Port Tube Location

    Does it matter where a port is located? The port is 2"Dx7"L. I could easily fit on the back of the cabinet but I don't want to route a hole in the back of a new cabinet.

    I want to put the port on the front baffle in a PE curved .5 cu ft cabinet. I might have to locate it near a top or bottom corner.

    Does it matter?
    Jazz Wine HiFi

  • #2
    Re: Port Tube Location

    Questions on Vents
    by
    John L. Murphy
    Physicist/Audio Engineer
    Q: "How critical is the placement of the port(s) in an enclosure?"
    To understand the sensitivity of a vented (ported) speaker to the location of the vent it is
    first necessary to understand why it would matter at all. In a vented box type of woofer
    subsystem all of the acoustic out put of the system is from the woofer driver itself at
    frequencies well above the vent tuning frequency of the box, that is, the F(B). At lower
    frequencies, specifically a narrow band of frequencies centered on the box tuning
    frequency F(B), all of the acoustic output is from the vent. Thus the speaker system
    consists of two acoustic sources: the driver and the vent. The total output of the speaker
    system is the acoustic sum of the outputs of these two sources. In order for these two
    sources to sum as intended the two sources must be largely "in phase". It is this phase
    relationship between the two sources that is affected by the location of the vent. The phase
    difference is also affected by the location of the listener.
    What really matters is the DIFFERENCE in the distance of the listener from each of the
    two sources. As long as the listener is the same distance from the driver and the vent the
    space between the driver and the vent doesn't matter. But as a general rule we want the
    frequency response to be constant no matter where the listener is located. Therefore we
    want the vent and the driver located "close" to one another. In this case "close" means
    within a small part of an acoustic wavelength at the box tuning frequency. Now, since
    most vented systems are tuned below 100 Hz that means that the wavelength at the box
    frequency is generally longer than about 10 feet (the approximate wavelength of a 100 Hz
    tone). So as long as your vent is within, say, 1/4 of a wavelength the location of the vent
    will not matter much. If the vent is more than about a 1/4 wavelength then the frequency
    response in the bass range will become dependent on how the listener is positioned with
    respect to the driver and vent.
    So, if your speaker is tuned to 100 Hz you want to keep your vent no more than about 2.5
    feet (1/4 wave) from the driver. But if your box is tuned closer to 50Hz (a more realistic
    figure) then the vent only need be within 5 feet of the woofer. Since this is an easy criteria
    to meet for most systems it seems fair to say that for most speakers the vent placement is
    not critical.
    Below is a table of frequencies and corresponding 1/4 wavelengths.

    Frequency ---Wavelength ---1/4 Wavelength
    200 Hz 5.65 ft 1.41 ft
    100 Hz 11.30 ft 2.83 ft
    80 Hz 14.13 ft 3.53 ft
    60 Hz 18.83 ft 4.81 ft
    50 Hz 22.60 ft 5.65 ft
    40 Hz 28.25 ft 7.06 ft
    30 Hz 37.67 ft 9.42 ft
    20 Hz 56.50 ft 14.13 ft
    10 Hz 113.00 ft 28.25 ft
    The Calipso Line Array System: http://techtalk.parts-express.com/album.php?albumid=9

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    • #3
      Re: Port Tube Location

      Whew!

      Thanks for the info. I learned, for my project, I can put it wherever it will physically fit in the baffle.

      But I learned so more from your very detailed reponse. Thanks again.
      Jazz Wine HiFi

      Comment


      • #4
        Re: Port Tube Location

        Originally posted by jimofoakcreek View Post
        Does it matter where a port is located?
        For the most part, no. But you should avoid having a line of sight view through it to the woofer cone.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          Re: Port Tube Location

          Originally posted by jimofoakcreek View Post
          Does it matter where a port is located? The port is 2"Dx7"L. I could easily fit on the back of the cabinet but I don't want to route a hole in the back of a new cabinet.

          I want to put the port on the front baffle in a PE curved .5 cu ft cabinet. I might have to locate it near a top or bottom corner.

          Does it matter?
          I personally prefer ports in back. Sometimes ports produce a little bit of turbulence, chuffing noises. And sometimes the port may even leak a little bit of upper bass/lower midrange from inside the enclosure. So having the port in back lessens the chance of any of those extraneous noises from being heard.
          Last edited by ; 09-12-2009, 06:24 PM.

          Comment


          • #6
            Re: Port Tube Location

            After considering the alternatives I've decided to put the port on the back. It belongs there. Thanks.
            Jazz Wine HiFi

            Comment


            • #7
              Re: Port Tube Location

              I can't help but wonder if port location would have a noticable effect on the sound due to boundary reinforcement. We all know that the closer you back those speakers up toward the wall, the more bass heavy they get because of boundary reinforcement. If I have a cabinet that's 15" deep and I put the port on the back instead of the front, it's putting that port a lot closer to the wall, so I can't help but think that it's going to have additional boundary reinforcement around the tuning frequency compared to the same speaker with the port on the front.

              Yes/no/I'm an idiot?

              Comment


              • #8
                Re: Port Tube Location

                Originally posted by jim85iroc View Post
                I can't help but wonder if port location would have a noticable effect on the sound due to boundary reinforcement. We all know that the closer you back those speakers up toward the wall, the more bass heavy they get because of boundary reinforcement. If I have a cabinet that's 15" deep and I put the port on the back instead of the front, it's putting that port a lot closer to the wall, so I can't help but think that it's going to have additional boundary reinforcement around the tuning frequency compared to the same speaker with the port on the front.

                Yes/no/I'm an idiot?
                You get boundary reinforcement when the source is less than 1/4 wavelength from the boundary. Typical port radiations extend no higher than 100 Hz, where 1/4 wavelength is 2.8 feet, so the cab would have to be quite large or well away from the wall for the port location to make a difference.
                www.billfitzmaurice.com
                www.billfitzmaurice.info/forum

                Comment


                • #9
                  Re: Port Tube Location

                  Suppose the cabinet was under the floor in the basement or outside, and connected with a 4-8 inch tube. would that make a difference? Or if it was a really long transmission line tube snaking outside and around the building?

                  Marlboro
                  The Calipso Line Array System: http://techtalk.parts-express.com/album.php?albumid=9

                  Comment


                  • #10
                    Re: Port Tube Location

                    Originally posted by billfitzmaurice View Post
                    For the most part, no. But you should avoid having a line of sight view through it to the woofer cone.
                    I own a Dr Hsu Subwoofer for my home theater and it has the port directly behind the sub.

                    Comment


                    • #11
                      Re: Port Tube Location

                      Originally posted by PapaEcho View Post
                      I own a Dr Hsu Subwoofer for my home theater and it has the port directly behind the sub.
                      "Woofer" and "subwoofer" are different. The midrange is rolled off in the subwoofer, so it is not the same issue as it is with a driver that extends higher in frequency.
                      Click here for Jeff Bagby's Loudspeaker Design Software

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                      • #12
                        Re: Port Tube Location

                        thanks jeff, that makes sense, the higher freq would escape the port?

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