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  • Front Port vs. Rear Port

    Porting a subwoofer box. Will run up to 120 Hz. Any major difference between porting from the front vs. porting from the back?

    Assuming the cabinets are kept roughly 6" off the wall.

    Would appreciate any comments. Thanks!

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    • #3
      Re: Front Port vs. Rear Port

      yes and no, part of it will depend on overall room placement. IMOE, crossing at 120hz tends to make the sub easy to localize, so I am generally more apt to fire the port and driver away from the listening area.
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      • #4
        Re: Front Port vs. Rear Port

        Originally posted by herb View Post
        Any major difference between porting from the front vs. porting from the back?
        One has the port on the front, one has it on the back.
        Assuming the cabinets are kept roughly 6" off the wall.
        With a 6" distance to the wall you should consider having the driver and port both facing the wall. The driver would gain some acoustic loading while above bandwidth harmonic content and port noise would be filtered.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          Re: Front Port vs. Rear Port

          Originally posted by billfitzmaurice View Post
          One has the port on the front, one has it on the back.



          With a 6" distance to the wall you should consider having the driver and port both facing the wall. The driver would gain some acoustic loading while above bandwidth harmonic content and port noise would be filtered.
          I don't think I will go that route. Thanks though.

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          • #6
            Re: Front Port vs. Rear Port

            Originally posted by herb View Post
            I don't think I will go that route. Thanks though.
            Kids LOVE to shove stuff inside ports, IF they can see them.

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            • #7
              Re: Front Port vs. Rear Port

              I think in most cases having the vent on the same side as the driver will give tighter bass because there is less chance for phase differences between the driver and vent.

              Sometimes a rear vent may give you the room gain of the vent without the room gain of the driver. If your aiming for more low end and the tuning frequency is low enough, the wall or corner will reinforce the vent without making the lower midrange sound muddy. If vent freq. is too high this won't work as well.

              Like just about all things audio it just depends on the variables. Is the port only gain worth the trade off of possible phase issues? The room loading, sub position, crossover freq, etc, all play into what you hear from your seat.

              Depending on the size of the cabinet and other factors it's possible to have a strong flat response from 25hz to 70hz and 90hz to 120hz, with a severe peak or dip between 70hz and 90hz because of phase summation, cancellation from having the vent not on the same side as the driver.

              From personal experiance with car audio where there is much more cabin gain, vented cabinets always sound tighter with the vent on the same side as the driver

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              • #8
                Re: Front Port vs. Rear Port

                Originally posted by Chris Roemer View Post
                Kids LOVE to shove stuff inside ports, IF they can see them.
                I don't disagree!
                Cellphones, wedding rings, toys and food can go there especially if it is a big port! So go ahead put it in the front;).

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                • #9
                  Re: Front Port vs. Rear Port

                  Brian in car audio having the vent on the same plane can actually be a detriment. If the enclosure is designed as to take advantage of the phase anomaly, the vent may end up 90 or 180 degrees from the driver plane. In an SUV its common to fire the subs up and the port to the rear, 90 degrees out. I've built subs with the sub firing forward and the port to the side, a chambered transmission line. In that instance firing the port and driver on the same plane actually would have reduced output as the port was out of phase with the driver causing cancellation. The sub I am currently working on for a friend will fire forward with the port to the rear, a slot that will rebound into the corner (part for looks, part for kids, part for reinforcement)
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                  • #10
                    Re: Front Port vs. Rear Port

                    Originally posted by brian View Post
                    I think in most cases having the vent on the same side as the driver will give tighter bass because there is less chance for phase differences between the driver and vent.
                    That would require a driver to port outlet distance of five feet and more, so it's seldom a concern.
                    Sometimes a rear vent may give you the room gain of the vent without the room gain of the driver.
                    The same situation applies.
                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

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                    • #11
                      Re: Front Port vs. Rear Port

                      Brian in car audio having the vent on the same plane can actually be a detriment. If the enclosure is designed as to take advantage of the phase anomaly, the vent may end up 90 or 180 degrees from the driver
                      Ok, I'm always open to new info. So how would you calculate this? Is this just trial and error, or does this require testing equipment?

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                      • #12
                        Re: Front Port vs. Rear Port

                        Originally posted by brian View Post
                        Ok, I'm always open to new info. So how would you calculate this? Is this just trial and error, or does this require testing equipment?
                        Deal is, for a 30Hz tone, with a wavelength of (1125 fps/30 cps =) 37 feet, you'd have to have a vent that was 18' away (or 9' @ 60Hz) for its "pulse" to reach the woofer's cone 180* out of phase.

                        Firing a port out a box at a 90* angle to the baffle doesn't make the sound "out of phase" by 90*. It's inconsequential what side of a sub the port exits w/respect to the driver at "subwoofer" freqs, regarding phase.

                        Chris

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                        • #13
                          Re: Front Port vs. Rear Port

                          As long as you keep the port from directly behind the woofer you shouldn't have any negative effects from where you place it. Also try to keep it so the port is not too long that it's opening is close to any enclosure wall.
                          Thanks ,
                          JB

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                          • #14
                            Re: Front Port vs. Rear Port

                            http://www.trueaudio.com/st_ventq.htm
                            "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche

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                            • #15
                              Re: Front Port vs. Rear Port

                              Is placing a port directly behind a woofer an issue?
                              "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche

                              http://www.diy-ny.com/

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