My sub is talking through the vent

Discussion in 'Home Theater Projects' started by Russell R, Sep 14, 2003.

  1. Russell R

    Russell R Stunt Coordinator

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    Hello,
    I have recently built a 142.5L, 21hz tune. I ran spl's today without any other speaker playing and noticed that the lower octave has serious vent noise. Somewhere around 40hz and below. The levels are only at an average of 89db from 40hz to 16hz. What can I do to remedy this? Larger port? Multiple ports?

    Thanks,
    Russell
     
  2. Jake K

    Jake K Stunt Coordinator

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    What size ports are in there now? And what driver is it?
     
  3. Russell R

    Russell R Stunt Coordinator

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    I have a Shiva MKIII with a 4" i.d. flare. I followed the Adire EBS alignment but raised the tune from 18.1 to 21hz. After re-running the numbers with WINIsd, both tunes in a 142.5L enclosure will have high velocity through a 4" port. Why did I not notice this beforehand? Oh well, the sub is not cosmetically finished; I guess the upper end cap will come off to house multiple ports. Looking at 3 - 3" i.d. straight tubes roughly 23" long each. How does this sound? The mach with the single 4 is .19, but the mach with the 3 -3" is .04. Should be better. Any thoughts or other ideas?

    Thanks,
    Russell
     
  4. Jake K

    Jake K Stunt Coordinator

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    A 4" flared port should be more than good enough for a shiva, should it not?
     
  5. Russell R

    Russell R Stunt Coordinator

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    Well, that was my understanding before I built this. It was my fault. I should have looked closure at the velocity. Just thought that there could be nothing wrong with a CANNED recipe. Should a 4" flare be enough? Maybe I did something wrong. I calculated the internal volume of 142.5L and added end caps to this. The tune was higher, but it did not affect the velocity that much. Will try the 3-3.

    Thanks,
    Russell
     
  6. Russell R

    Russell R Stunt Coordinator

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    Yes, it is a chuffing sound through the port. Only on the lower freq's. What could cause this? If everything was perfect, according to WINisd; the calculations indicate a high velocity. What should be my next logical step?

    I played around with sinewaves today up to 90 Hz. I have some room modes that I want to address, but without adjusting my gain; there were freq's in the upper range around 120db and at the lower end of the spectrum around 87db. Room needs work.

    I will post some in-room curves soon. The sub does put out, just needs a tweak.

    Thanks,
    Russell
     
  7. Russell R

    Russell R Stunt Coordinator

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    During playback of HT and 2 channel, I do not hear the noise. If I am playing sweeps or sines then I can hear it at certain freqs. Should I change anything? It makes me nervous to think that I can hear chuffing on a 20-40 Hz tone @ ~90-95db. I placed a pillow over the port during tests and it stopped the sound. The pillow shook with the pulses.

    Russ
     
  8. Dan Hine

    Dan Hine Screenwriter

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  9. Russell R

    Russell R Stunt Coordinator

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    Ok, maybe I am making a big deal about it. You are right. Who sits around the house listening to sinewaves anyway. The only time that I hear the noise is when I am pushing the sub hard anyway to reproduce the lower freqs. PR's and sealed are in the future. I have the DIY bug.

    If I were to change out the port to multiple 3", would there be a difference? According to software, there should be slower air speed through the ports. If I had to do it all over again, would I use multiple ports; that is the question at hand.

    Will continue to tweak room and listen/enjoy the boom.

    Thanks,

    Russell
     
  10. Greg Monfort

    Greg Monfort Supporting Actor

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    (1) 6" or (2) 4" is what you need if you have program material that will drive the vent(s) hard, but that's not likely with music so the (1) 4" should be fine. HT OTOH may drive it below 21Hz, so it needs to be tuned lower.

    GM
     
  11. Joe.Kuta

    Joe.Kuta Stunt Coordinator

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    How many watts are driving the sub?

    Just curious b/c I'm trying to get a figure for what velocities in WinISD are experiencing chuffing.

    My AV15 + 4" flared port arrive in 2 days [​IMG]
     
  12. Greg Monfort

    Greg Monfort Supporting Actor

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    Hmm, different strokes..... I don't like single PRs in subs, they require too much mass to counteract, so unless the vents are really long, like enough to get down into the sub's BW.......

    OK, I didn't see this tidbit. Anyway, for HT it needs to be modded to either have a lower F3, like ~12Hz, or disabled, IMO. I mean, what's the point of having an LFE if you're not going to 'get down, grab grass, n' growl', as my HS coach used to demand? [​IMG]

    GM
     
  13. Russell R

    Russell R Stunt Coordinator

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    I have the available room/length (~50" of straight run) for the longer ports. What are the pro's and con's of going with the longer dual 4" or the single 6"? Is there a 6" flare on the market? I believe that I was running the sub very hard in the lower keys. I posted a graph on my website:
    http://users2.ev1.net/~roder/RODERPL...to_album_2.htm

    This does not show the 120db peaks around 80 Hz, but it is something to look at. Anyway, how bad is this curve without eq? I will purchase a BFD in a few days. Looking for ideas on where to start tweaking.

    This is my first sub and I will continue to build more as long as there are funds.

    Thanks,
    Russell
     
  14. ThomasW

    ThomasW Cinematographer

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    The plot if accurate, shows that the sub is a major BOOM box. This is of course a function of the room/placement.

    You should change the placement and get a Behringer before you make any changes to the design. If you can successfully cut that huge room mode (peak), the rest of the curve actually doesn't look all that bad.
     

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