What's the rule of thumb on distance of port to top plate of sonosub ?

Discussion in 'Home Theater Projects' started by Mike Dr, Mar 21, 2002.

  1. Mike Dr

    Mike Dr Stunt Coordinator

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    I'm putting finishing touches on my tempest sonosub. my design will have the port firing up and so i am going to put a plate about 4" over the port to prevent things falling into it and I like the look of it too.. what I'm wondering is how much space to allocate between the port and this top plate ... I knwo for inside dimensions they tell you to allow at least as much room on each side of port as the diameter of it, but I dont know if the same goes for outside..

    Also, how much room should there be between the tempest itself and the bottom plate?

    thanks
     
  2. Mike Strassburg

    Mike Strassburg Second Unit

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    Dan W recommended a minimum 4" space for the bottom of my Tempest so I'd assume the top would need the same.
     
  3. Harry Lincoln

    Harry Lincoln Stunt Coordinator

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    I saw a post here about a week ago, from Greg Monfort I think, he recommended the area of the opening should be equal to the sd (surface area of the driver). Run a search on his name and you should be able to find the thread.

    Harry.
     
  4. Harry Lincoln

    Harry Lincoln Stunt Coordinator

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    Just re-read your post, thought you were talking about the driver at first.

    Harry.
     
  5. Jeff Meininger

    Jeff Meininger Second Unit

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    I'm dragging this thread back up again because I think it deserves more discussion... especially when we're dealing with straight (non-flared) ports.
    I'm planning to build an AV-15 sonosub, and I'm curious about effect of the distance from the port opening to the baseplate. I didn't worry about it with my EBS shiva, but that was a 4" flared port. Now I'm talking about a straight 6" or 8" port with only 5" of breathing room.
    I understand how this works for the driver; it is a large diameter diaphragm that doesn't move back and forth much more than 46 mm point to point. 5 inches seems like plenty of breathing room. Judging by the feeling of putting your fingers at the exit of a 6" straight port with an AV15 pumping at tuning frequency, though, I'd characterize the "slug of air" in the port as a small-diameter diaphragm that travels a much greater distance back and forth than the driver does... maybe even all of 5 inches.
    So many people have built downported sonosubs that I realize it MUST be okay to do this. If you think of the port as generating pressure, you'd understand that as soon as the air is outside of the confines of the port, the pressure expands in whatever direction it can.
    On the other hand, if you think of the port as containing an almost cohesive slug of air, you start to worry. And a simple test seems to indicate that we are indeed dealing with a slug of air. If you move your finger barely out of the straight port's exit hole, you stop feeling the whooshing air. The air exits the port in a straight line, and seems to want to continue in a straight line.
    What do you folks think? Anyone know the physics behind this?
     
  6. Mike Dr

    Mike Dr Stunt Coordinator

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    Personally, I dont think you'll have a problem since I dont think in this case the 6" port will ever be utilized fully anyway in such a way that would require a lot of room in front of it, but then this is only backed up by my common sense .. and that could be wrong [​IMG]
    out of curiousity, why not put a flare on it?
     
  7. Jeremy Stockwell

    Jeremy Stockwell Supporting Actor

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    For what it's worth, my DIY Tempest Sonosub (290L, 17.5Hz) has a 6" straight port with a little more than 4" of clearance between the bottom cap and the bottom plate (both the driver and port are down-firing - 24" diameter sonotube).

    Loud content well below the tuning point (17.5Hz) does cause audible chuffing, but it's not too bad (though it is the only limitation of my design at my preferred listening levels - 10-15dB below reference).

    A flared 6" port would have been nice, but I had some 6" PVC around that I couldn't justify not using at the time.

    JKS
     
  8. Seth_L

    Seth_L Screenwriter

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    In the case of the driver the opening should have the same SD as the port or the driver begins to be mass loaded by the air. I can only presume the same thing applies for the port.
     
  9. Mike Keith

    Mike Keith Second Unit

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  10. Mike Keith

    Mike Keith Second Unit

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    I've found that prototyping is always best, install the port and prop some blocks or legs under the tube and play some loud 15-20Hz test tones, then place the platform that you intend to use above the port and see what happens, this way you can make changes accordingly. I did this and found that if your port diameter is large enough it’s hard to make any audible change from obstructing the flow at 3-6" above the port. I went the other direction and had a Valve shaped spire (or Finial) turned out of Oak and will place it on top of the port to redirect the airflow out and around to the sides. I have drawings that I would post, however this site is very picture unfriendly so I’ll have to post a link in the future when it is completed.
     
  11. Greg Monfort

    Greg Monfort Supporting Actor

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    JM,

    A vent's action is very close to a piston of equivalent diameter (there are

    some losses due to the air's slight compressibility), but its 'excursion'

    (for lack of a better description) is limited to ~0.613x its radius is free

    space or ~0.8488x (8/3pi) its radius when terminated on a baffle.

    From this it doesn't take a rocket scientist to figure out that with a small

    pneumatic piston of limited excursion it must be oscillating pretty fast

    (vent mach) to energize enough molecules in the time the frequency dictates

    (1/f) to create a ~22.48ft diameter note (~0.0625sec, or 16Hz in this

    example) and why you feel a rush of air coming out of the vent. After all,

    even at the standard criteria of 5% mach 'speed limit' for low audible

    distortion we're talking ~38mph, so it takes some distance to dissipate.

    As MK noted though, deflecting this seeming 'excursion' doesn't audibly

    affect vent output and if you suspend a piece of foam/polyfil sheet/or equal

    above it beyond its 'excursion' limit will damp it down somewhat without

    affecting the pipe's fundamental and lower harmonics. If you position it

    within the 'excursion window' then you'll lower the Q of the vent, reducing

    group delay (improve transient response) at the expense of peak vent output,

    with max damping being when it's positioned at the terminus. This is

    essentially what a flare does, so I recommend this tuning tweak when too

    small a vent is used and there's no flare.

    GM
     
  12. Jeff Meininger

    Jeff Meininger Second Unit

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    Greg: then the cool trick would be figuring out exactly how much "air excursion" your port produces for a particular alignment, and then just make sure the plate is far enough away.

    Where did you get your maximum port excursion numbers? Wouldn't a smaller diameter port have to be excursing farther than a large diameter port for a given output level?
     
  13. Greg Monfort

    Greg Monfort Supporting Actor

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    My understanding from studying pipe/duct theory (where I got the 'excursion' numbers) and my own simple measurements is that the 'excursion' is a constant based on the pipe's effective diameter, IOW the SPL coming out is varied only by the amount of pressure exciting it at the 'closed' end, and of course any losses within the pipe due to friction, bends (if any), and cross sectional area (compression).
    This implies that for a smaller diameter vent to achieve the same SPL as a larger one it must resonate faster (higher vent mach). Since the various cab design programs reflect this I'm going to go out on a limb here and say I'm on the right track. [​IMG]
    From this it becomes apparent that the larger the radius of a straight pipe or the more you flare it, the greater the 'excursion', so calculating it with the above formulas should suffice regardless of output level.
    GM
     
  14. Jeff Meininger

    Jeff Meininger Second Unit

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    Producing bass requires moving air. For a given output level at a given frequency, a specific amount of air has to be moved. In the case of a sub driver, you can achieve a given level of output with a large diameter cone excursing a little bit, or a small diameter cone excursing quite a bit. Why would ports behave differently?

    We are in agreement that the air coming out of a smaller diameter port moves faster. And I think we're in agreement that the same amount of air has to be moved by a port no matter the diameter to produce a given output level (which is why the small diameter port's air must move faster). So if you took a tall skinny glass and filled it with 1 cup of water, and took a short fat glass and filled it with 1 cup of water, wouldn't this be a good model of the air exiting the port? The water level in the tall skinny glass would be higher, indicating longer excursion.

    Right?
     
  15. Greg Monfort

    Greg Monfort Supporting Actor

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    >Producing bass requires moving air. For a given output level at a given frequency, a specific amount of air has to be moved. In the case of a sub driver, you can achieve a given level of output with a large diameter cone excursing a little bit, or a small diameter cone excursing quite a bit. Why would ports behave differently?
    ====
    Because a driver is in theory a pistonic constant acceleration device and a vent is a 1/4WL resonator.
    ====
    >We are in agreement that the air coming out of a smaller diameter port moves faster. And I think we're in agreement that the same amount of air has to be moved by a port no matter the diameter to produce a given output level (which is why the small diameter port's air must move faster).
    ====
    Correctomundo.
    ====
    > So if you took a tall skinny glass and filled it with 1 cup of water, and took a short fat glass and filled it with 1 cup of water, wouldn't this be a good model of the air exiting the port? The water level in the tall skinny glass would be higher, indicating longer excursion.
    >Right?
    ====
    Not by my understanding of the physics involved. If anything, it would show that a larger radius pipe has more excursion since it has more unfilled cross sectional area. [​IMG] I can't think of a suitable analogy for what's going on to describe it any better than I already have, so I guess I'm out of explanations. [​IMG] Anyway, since we agree on the other points, doesn't it stand to reason that if they're true, then a fixed 'excursion' is a de facto criteria?
    GM
     

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