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  • Vent velocity and port size

    Had this question in a discussion over on diyma and I figured people here would know what the answer to this is.

    The discussion was on how audible port noise/chuffing is given a port size.

    The claim was that a 2" port with a 40m/s vent velocity will produce more chuffing/port noise than a 8" port with a 40m/s vent velocity (obviously in two different applications).

    Is it true that vent velocity is more audible on a smaller port than on a larger one, given the same velocity, or does it not make a difference?
    Modding the Lepai T-Amp

  • #2
    Re: Vent velocity and port size

    All I can say is that most port modeling is done under "worst possible/almost sure to never happen" types of conditions.

    For example, people will say "Port diameter is X, power is Y, and vent velocity is Z, therefore chuffing is inevitable". Well, yeah - if you are listening to a sinewave at maximum amplifier output.
    Don't listen to me - I have not sold any $150,000 speakers.

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    • #3
      Re: Vent velocity and port size

      Originally posted by XtremeRevolution View Post
      Is it true that vent velocity is more audible on a smaller port than on a larger one, given the same velocity, or does it not make a difference?
      Perhaps it depends on how hard "it's" driven.
      As a matter of habit I use the largest diameter ( cross-sectional area ) that is practical, and that has worked for me.
      "Not a Speaker Designer - Not even on the Internet"
      “Pride is your greatest enemy, humility is your greatest friend.”
      "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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      • #4
        Re: Vent velocity and port size

        I agree with Johnny and Sydney. I have used ports that showed there could be issues (i.e. greater port velocity than 26 M/s) and I have had absolutely no audible noise. It makes no practical sense to have a 1 cubic foot enclosure with a 4" diameter port that is 3 feet long :D
        Craig

        The lowest possible F3 box alignment is not always the best alignment.

        Designing and building speaker projects are like playing with adult Lego Blocks for me.

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        • #5
          Re: Vent velocity and port size

          Here's some words of wisdon on vent velocity:http://www.eminencedesigner.com/EDTopic14.html The rule of thumb expoused here is 'don't go above 34 m/s. What's not discussed here is the relationship between the vent Velocity and vent diameter. Thus the conclusion I DRAW is there is no relationship. The high speed flow of air over the sharp inlet edges of the port opening causes turbulence and may be the determining factor in the creation of chuffing noise.
          Last edited by carlspeak; 07-19-2011, 10:25 PM. Reason: correction
          Live in Southern N.E.? check out the CT Audio Society web site.

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          • #6
            Re: Vent velocity and port size

            Another characteristic of box-and-port resonant systems (in addition to port "chuffing") is that as with any resonance there is an associated "Q" that determines the rolloff response and the duration of the post-excitation ringing. What is commonly ignored with high "Q" tuned boxes is that the ringing occurs at the box/port resonant frequency, which is not necessarily the same as the tone that excited the resonance. Our common frequency response measurements are not discriminating enough to show the resulting pitch shift . . . it looks like the desired boost in frequency response . . . but (especially if the box "Q" is high) it sounds "wrong" . . .
            "It suggests that there is something that is happening in the real system that is not quite captured in the models."

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            • #7
              Re: Vent velocity and port size

              So based on this post Deward and the post you had in Johnny's thread... what exactly are you trying to pitch???
              Craig

              The lowest possible F3 box alignment is not always the best alignment.

              Designing and building speaker projects are like playing with adult Lego Blocks for me.

              Comment


              • #8
                Re: Vent velocity and port size

                Originally posted by PWR RYD View Post
                what exactly are you trying to pitch???
                "Pitch" ? ? ?

                What's that supposed to imply?

                The deliberate introduction of resonances in speakers, and the inappropriate treatment of resonances where they unavoidably occur, are if not always bad at least something that should be approached with great caution. To get the benefit of a little more boom-boom speaker builders commonly ignore some significant "negatives". In some circuimstances and for some purposes it's a reasonable tradeoff . . . just be aware that tradeoffs include the negatives as well as the positives, and that there may be other, less compromising, ways to get the desired result. Don't believe that the FR curve or the design suggestions in winISD tell the whole story.
                "It suggests that there is something that is happening in the real system that is not quite captured in the models."

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                • #9
                  Re: Vent velocity and port size

                  Originally posted by carlspeak View Post
                  the ratio of edge length to total area of the port is the same and thus the turbulence at the port edges should be the same in each case. Flared port tubes are designed to address the sharp edge issue and thus reduce turbulence.
                  I concur (perhaps obvious, but assuming that the edge profile is the same between both ports). It will take a lot more SPL to excite the volume of air to that velocity in the larger port. The "chuffing" is a result of the turbulent interferences due to the abrupt change from inside the port to ambient space.
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                  • #10
                    Re: Vent velocity and port size

                    Originally posted by carlspeak View Post
                    In the examples you cited, the ratio of edge length to total area of the port is the same and thus the turbulence at the port edges should be the same in each case.
                    circumference varies as r and area varies as r^2, so the quoted statement is false.

                    If one accepts that turbulence varies with circumference/area (not my claim) the 8" port will be better, as circumferece/area varies with 1/r

                    ymmv

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                    • #11
                      Re: Vent velocity and port size

                      Originally posted by rone View Post
                      circumference varies as r and area varies as r^2, so the quoted statement is false.

                      If one accepts that turbulence varies with circumference/area (not my claim) the 8" port will be better, as circumferece/area varies with 1/r

                      ymmv
                      The area that causes chuffing is the surface area exposed to the air, not the cross sectional area of the port. That said, it would be circumference x port length and the ratio would be 1 assuming the port length of each 4" is half that of the 8".

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                      • #12
                        Re: Vent velocity and port size

                        Originally posted by rone View Post
                        circumference varies as r and area varies as r^2, so the quoted statement is false.

                        If one accepts that turbulence varies with circumference/area (not my claim) the 8" port will be better, as circumferece/area varies with 1/r

                        ymmv
                        Agreed. I stand corrected. I got my numbers upside down.

                        Below is a reference to an AES paper. Fig. 12 of that paper shows proof that larger diameter ports can put out more dB before 'distortion is appreciable' [I interpret that as chuffing]. The figure shows a plot of SPL vs vent resonant frequency. What's not clear is what the vent lengths were in the study. Perhaps the full AES paper will yield that answer.

                        H. D. Harwood, “Loudspeaker Distortion Associated with Low Frequency Signals.”
                        J. Audio Eng. Soc. Vol 20, No. 9. (November 1972).

                        Post #11 here http://www.diyaudio.com/forums/multi-way/192191-question-about-reflex-duct-shape.html
                        gives an excellent listing of technical paper abstracts related to vents in loudspeakers. FYI....
                        Live in Southern N.E.? check out the CT Audio Society web site.

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                        • #13
                          Re: Vent velocity and port size

                          Originally posted by kmibb View Post
                          The area that causes chuffing is the surface area exposed to the air, not the cross sectional area of the port. That said, it would be circumference x port length and the ratio would be 1 assuming the port length of each 4" is half that of the 8".
                          Turbulence is related to reynolds number. reynolds number depends on diameter. In reality, the flow in the port is unsteady and not fully developed - so complicated.

                          port length depends on area, an 8" port must be 4 times as long as a 4" port. if your first statement were true, perhaps we should all be using simple ports (wall thickness in length) to reduce this area?

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                          • #14
                            Re: Vent velocity and port size

                            Originally posted by XtremeRevolution View Post
                            Is it true that vent velocity is more audible on a smaller port than on a larger one, given the same velocity, or does it not make a difference?
                            Can you give more background on the question? What is a practical use for the answer? Always using the largest practical port is the right answer. A very long port can start to cause problems with the organ pipe resonance before chuffing becomes an issue.

                            I would say the shape of the port is as important as anything else. Guiding the air smoothly into and out of each end is very important. The Polk "Power Port" is a good solution in this regard. Read their white paper, it is quite informative, if I recall correctly.

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                            • #15
                              Re: Vent velocity and port size

                              I try to keep port velocity down to 15m/s even at full power, no audible chuffing at all.

                              In the design stage I try the smallest port diameter I can use to keep velocity down, Using a larger box to shorten the port to fit inside the box seems to work well if you can spare the space.

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