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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Drinking Straws posted by Rich S. on November 3, 1999 at 12:29:10:
Use of drinking straws in a port or vent primarily provides a resistive element to the vent. The normal operating principle of the air in the vent is to act like a pure inductance, which resonates with the air volume compliance inside the cabinet which is acting like a capacitance.Lining the walls of the enclosure adds resistive damping to the capacitance, a fully stuffed vented box has a LOT of resistive damping of the box's air volume compliance. If an enclosure is heavily lined or fully stuffed with absorbing material, then I would not add any resistive damping to the vent, as this would undoubtedly seriously overdamp the system. If there is booming bass with such a system, then I would look to speaker positioning, etc., rather than trying to put a band-aid on the speaker.
Adding straws to a vent adds a lot of resistive damping to the vent 'inductance'. This resistive damping can also be achieved by using a piece of polyester batting of a suitable density (hi-loft type) and stretching it across the vent opening, and adding layers until the proper amount is reached.
There is one thing that straws do to a vent that the batting will not do, and this is the primary reason that a high end manufaturer would use straws, as a few pieces of batting are much cheaper to implement: They break up the single continuous volume of the vent, and prevent the usual organ pipe resonance of the vent from occuring. Without the full diameter to resonate, the vent resonance is broken up into a multitude of two different higher frequency resonances, which most plastic straws will not readily support, the resonance of the straw inner air volume, and the resonance of the spaces between the straws, at an even higher frequency, and even more dispersed due to the triangular shape.
Since the penalty for using straws is to heavily damp the vents action resistively, and to slightly lower the tuning, this gain of losing the vent organ pipe resonance does not come without a penalty.
Most vented systems are supposed to be 3 dB more efficient than a sealed box, and if the vent was perfect with no turbulence, this would be the case for a given box volume and similar -3 dB cutoff frequency. However, the actual gain is usually only about 2 dB, due to vent turbulence, and other losses. Add in straws, and this will drop to 1 dB or less, in some cases, totally throwing away any gain in sensitivity at all.
BTW, if you are going to use straws in a vent, they should have a dab of superglue here and there to keep them from moving around, and of course, you should only do this if you are sure you want to keep them in the vent! Another thing is DO NOT make them longer than the existing vent, as they are already going to lower the tuning a bit, cut them to be the length of the unflared portion, or if the vent is not flared, a little shorter than the actual vent length, and make sure none are poked into the inside more than the others.
Another way you can eliminate the vent resonance WITHOUT adding such a large amount of resistive element, is to use a sheet of mylar or similar thin plastic, roll it up into an "S" shape, or a folded over "C" shape, and stuff that into the vent, length considerations same as the straws, and glue it into place using a dab or two of super glue. The sheet should be thin enough to fold/curve over, and yet want to spring out enough to almost hold itself in place due to friction alone. This will reduce any vent resonances to minimal effects, and yet not add a lot of resistive element. The main thing for this type of vent tweak is to make sure that the shape formed by the rolled up plastic sheet is consistent and the same from the front of the vent to the rear of the vent. We still want to maintain turbulence free air motion within the vent.
It helps to think of the vent air volume as a slug of air, and the best thing is for it to move back and forth in the vent as a solid unitized slug. Anything that might cause this slug of air to be dispersed is going to raise the resistive element, usually in a non-linear manner with level, and cause turbulence. At the point where the air goes totally chaotic, the vent is said to lock-up, in essence the air slug can not move effectively any more, only broken segments are moveing side to side instead of in and out (this description is not strictly correct, but is used for purposes of illustrating the situation more clearly).
I hope this clears up the straws in the vent myths and wives tales a bit, and allows some intelligent experimentation.
Jon Risch
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Follow Ups
- How Drinking Straws in a Port Work & How to do Better - Jon Risch 21:25:44 11/03/99 (4)
- Nice post, but - BillH 08:28:57 11/05/99 (1)
- Re: Nice post, but - Jon Risch 16:01:57 11/06/99 (0)
- Re: How Drinking Straws in a Port Work & How to do Better - Doc B. 08:43:36 11/04/99 (0)
- I may be very wrong but - Mart 00:17:23 11/04/99 (0)
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