Port noise from AV12 w/ 3 3" flarred ports???

Discussion in 'Home Theater Projects' started by Stephen Hopkins, Oct 26, 2003.

  1. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    I just finished an 3 ft^32 AV12/Rythmik based sub for a friend (pics in this thread http://www.hometheaterforum.com/htfo...hreadid=165330 ). We decided to go with a 3 port multiple tuning point design similar to that used in the SVS Box subs. We used 3 3" x 11.5" long flarred ports, 2 facing out the back and one out the bottom (the side until we get some of the big PE sub spikes/feet). Even with all 3 ports open and a 28hz rumble filter in place we can still hear some chuffing type port noise on heavy bass hits in music at high volumes.

    In contrast, my own EBS alignment AV12 w/ PE 272w amp has 2" downfiring non-flarred ports (leftover PVC i had at the time, it was originaly a Shiva) and I hear no port noise at similar volume. I have a pair of the same 3" x 11.5" ports to replace the 2" ports I have now just incase.

    Why would he be getting the port noise w/ 3 of the 3" flarred ports and i'm not getting any w/ 2 non-flarred 2" ports?

    Thanks [​IMG]
     
  2. ThomasW

    ThomasW Cinematographer

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    How close are the inside port openings from any of the interior surfaces or objects (braces/etc)?

    There should be a distance equal to the port diameter between the edges of any port and any interior surface.
     
  3. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    There's nothing Parallell within 3" of any port opening, but there are things perpindicular to the port openings that are closer. Imagine the 3rd port running up and down parallell to the amplifier and perpidicular but beside the other 2 ports. I'll see if I can post in inside picture sometime that may clear things up a bit.
     
  4. Rob Formica

    Rob Formica Stunt Coordinator

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    Just brainstorming... have you considered the possibility that you are hearing an air leak rather than port noise? It may not be the case... but even a small crack can be quite annoying. It's worth the time to maybe block the ports temporarily and check carefully around the sub for any air escaping or noise.

    Just a thought...
    Rob
     
  5. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    That was my first thought. We've tried this and best we can tell the port noise goes away when all 3 ports are blocked.
     
  6. Brian Bunge

    Brian Bunge Producer

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    Thomas' comments are all that I can think of at the moment.

     
  7. Rob Formica

    Rob Formica Stunt Coordinator

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    Another thing you could try (if you haven't), is to see if one port or all three are the culprits... Block one or two at the time... and see if this will illuminate the noise.

    If the port running perpendicular is really the cause, blocking it should reveal it?...

    Still thinking out loud...
    Rob
     
  8. Mike.Hjorten

    Mike.Hjorten Auditioning

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    Have you modeled your box at all? If you plug in a 3 ft3, and AV12 and 3 X 11.5in ports you get a tune of 31 hz. That gives you an airspeed in your ports of about 150 ft/s at the resonant frequency. That seems to be a very high tune for an AV12 that has an Fs of 21 Hz.

    I just completed a similar sized AV12 sonosub that ended up 100 L (a little more than 3 ft3) with three 3” ports and had to make the ports 34 inches long to get a tune of 20hz or so. I notice next to no port noise unless your right next to it and playing a 20 hz sine wave.
     
  9. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    The reason we have the 3 x 3" x 11.5" ports is for the exact reason you mentioned, the high tuning point... more accuratly, for multiple tuning points. W/ 2 ports blocked (1 open) we get a tune of 21.32 hz and an F3 of 21.15hz. With 1 port blocked (2 ports open) we get a tune of 30.15 hz and a F3 of 25.7hz and a 3.75db boost at 34hz. With no ports blocked (3 ports open) we get a tune of 36.92hz and an F3 of 30hz w/ a 6.5db boost at 41hz. The Vent Match for 1 port was the only one in the red (.22), w/ 2 ports it was .04 and w/ 3 ports it was .01.

    The reason i mentioned plugging all 3 ports was to eliminate the possibility of the noise being due to an air leak comming from a less than perfect seal being made with the driver, amp, or one of the ports. This doesn't seem to be the case.
     
  10. ThomasW

    ThomasW Cinematographer

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    Given the 'something' for everyone' multi-port config it, doesn't surprise me that there's weird turbulence inside the box where the ports intersect.

    This design idea is interesting, but probably better to have tried it in a larger box where proper spacing was available.
     
  11. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    True, i think the problem may lie in all three ports not being the same orientation. That said, it's only audible at the listening position at ear bleeding levels that wouldn't be tolerable for more than a minute or two at a time.

    Thanks for everyone's input [​IMG]
     
  12. Rory Buszka

    Rory Buszka Supporting Actor

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    How high do you elevate the sub off the ground? If it is low enough to the ground, the down-firing port could be loading to the space between the subwoofer and the floor. I think if you raise the sub 5" off the ground on some stout legs and with a baseplate (plinth) then you might be able to give the bottom port more breathing room.
     
  13. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    Right now the 3rd port isn't downfiring, it's side firing. We're waiting on the feet to get here from PE, but in our testing there was no audible difference in the port side-firing or downfiring w/ 2" legs and no base plate.
     

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