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  • Optimum vent velocity?

    Simming my new sub build (new enclosure, same ACI SV10/Dayton SA250 amp) and with a 4' port it needs to be over 40" long, however with a 3" port is half that. The difference is that the 4" peak velocity is 24ms at Fb, the 3" peak velocity is 42ms at Fb. Is the 3" port going to produce too much noise with that velocity, or would I be okay? I intended to front port it, but rear porting is always an option (my choice merely based on aesthetics).

    Also, I am taking this sub from an f3 of 37hz to an f3 of 19.5hz, and adding a few db to my max volume. Niiice. (build thread to come)

  • #2
    Re: Optimum vent velocity?

    I think WinISD uses Mach 0.15 as a "limit". That'd be 2000 ips, or 170 fps, or a pinch over 50 mps. In reality, there are many factors that determine the "chuffing" limit. A 4" PPort can blow a LOT higher velocities than a square ended 4" id round chunk of PVC w/out sounding bad. I generally prefer pointing the port away from the listening position, as well. Also, you might chuff mightily with a 30Hz test tone, but never notice it in music or HT playback, depending on what else is going on.

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    • #3
      Re: Optimum vent velocity?

      Originally posted by dustinbagby View Post
      Simming my new sub build (new enclosure, same ACI SV10/Dayton SA250 amp) and with a 4' port it needs to be over 40" long, however with a 3" port is half that. The difference is that the 4" peak velocity is 24ms at Fb, the 3" peak velocity is 42ms at Fb. Is the 3" port going to produce too much noise with that velocity, or would I be okay? I intended to front port it, but rear porting is always an option (my choice merely based on aesthetics).

      Also, I am taking this sub from an f3 of 37hz to an f3 of 19.5hz, and adding a few db to my max volume. Niiice. (build thread to come)
      That is way too high!

      My ROT is maximum port air velocity of 17 m/sec (= 56 ft/sec).

      Destructive port compression affecting the output occurs well before "chuffing" becomes evident.

      That said: Everything is a trade-off. Pick your best compromise.
      “I cried because I had no shoes until I met a man who had no feet”

      If we all did the things we are capable of doing, we would literally ASTOUND ourselves - Thomas A. Edison

      Some people collect stamps, Imelda Marcos collected shoes. I collect speakers.:D

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      • #4
        Re: Optimum vent velocity?

        Chris is right on about not hearing chuffing with real world signals in many cases. The thing a high port velocity will do is cause port compresssion. The shape of impedance and FR plots change quite a lot at high port velocities. It's kind of unavoidable in home sub boxes where low Fb, low Vb and relatively high displacement combine.
        Loren Jones

        http://techtalk.parts-express.com/sh...-sound-drivers

        http://techtalk.parts-express.com/sh...for-live-sound

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        • #5
          Re: Optimum vent velocity?

          Great minds think alike. We were young at the same time.

          Originally posted by thekorvers View Post
          That is way too high!

          My ROT is maximum port air velocity of 17 m/sec (= 56 ft/sec).

          Destructive port compression affecting the output occurs well before "chuffing" becomes evident.

          That said: Everything is a trade-off. Pick your best compromise.
          Loren Jones

          http://techtalk.parts-express.com/sh...-sound-drivers

          http://techtalk.parts-express.com/sh...for-live-sound

          Comment


          • #6
            Re: Optimum vent velocity?

            Originally posted by lorenmjones View Post
            Great minds think alike. We were young at the same time.
            Same answer - different choice of words.:D .............. Young was long ago.:eek:
            “I cried because I had no shoes until I met a man who had no feet”

            If we all did the things we are capable of doing, we would literally ASTOUND ourselves - Thomas A. Edison

            Some people collect stamps, Imelda Marcos collected shoes. I collect speakers.:D

            Comment


            • #7
              Re: Optimum vent velocity?

              I meant typing, not young.
              Loren Jones

              http://techtalk.parts-express.com/sh...-sound-drivers

              http://techtalk.parts-express.com/sh...for-live-sound

              Comment


              • #8
                Re: Optimum vent velocity?

                Originally posted by lorenmjones View Post
                I meant typing, not young.
                I am guessing you use word prediction?
                “I cried because I had no shoes until I met a man who had no feet”

                If we all did the things we are capable of doing, we would literally ASTOUND ourselves - Thomas A. Edison

                Some people collect stamps, Imelda Marcos collected shoes. I collect speakers.:D

                Comment


                • #9
                  Re: Optimum vent velocity?

                  Dustin, we all want our port velocities to be Mach .001 in WinISD, but you need a chimney to make that work! Compromise, compromise, compromise. Give a little bass extension, shorten your port and reduce your port velocity a little. Make your enclosure a little bigger and that can help too. Massive wattage can help too. I built my now roomie a sealed sub to be more music oriented, plus he was living in an apartment at the time, and what I built could shake all three floors anyway! I like the way Carl's sub sound's on Beethoven's Eroitca 1st movement of the Bass Viols plucked. Solid, no "boom", the over and under-tones are there. It's no chest-crusher on movies, but I think it's enough for most small/medium-small rooms with the room gain they'll give.

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                  • #10
                    Re: Optimum vent velocity?

                    Use a 3" precision port. Peak particle velocity with that amp and driver, in a box under 2.5ft^3 tuned to ~20hz should not exceed ~30m/s. Push the box size smaller and the port longer, the port output (and velocity) will be further damped. From 30hz up velocity will not exceed 10m/s at full output so music will be unaffected.

                    The only time you'll ever get any port noise out of this system is during movie special effects, which, will be associated with so much wide-band ruckus, and, likely structure/furniture involvement that the port chuffing is more or less masked.
                    Pro/Fi Cinema Speaker project: "From the Ashes"

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                    • #11
                      Re: Optimum vent velocity?


                      www.bodziosoftware.com.au/PORTS.pdf


                      http://www.subwoofer-builder.com/flare-testing.htm
                      “I cried because I had no shoes until I met a man who had no feet”

                      If we all did the things we are capable of doing, we would literally ASTOUND ourselves - Thomas A. Edison

                      Some people collect stamps, Imelda Marcos collected shoes. I collect speakers.:D

                      Comment


                      • #12
                        Re: Optimum vent velocity?

                        Dustin,
                        Have you tried the "flare it" utility here:
                        http://www.subwoofer-builder.com/flare-it.htm

                        There is an article about testing various port velocities for audibility on the site that is well worth reading.

                        -Bob
                        -Bob

                        The PEDS 2.1 mini system
                        My A7 Project - another small desktop speaker
                        The B3 Hybrid Dipole - thread incomplete and outdated

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                        • #13
                          Re: Optimum vent velocity?

                          Unibox has a brown line at 17m/s and a red line at 26m/s so I imagine those would be ideal.

                          If you have a 4" port that's too long, try (2) 3" ports, you may even need 3 or 4 ports to get a good compromise on vent air speed and length of the ports.

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