I know these will introduce some resistance to flow if aspect is high enough and lots of wall surface is present for a given CSA. I have this scenario: a 43 liters (net) enclosure that I want to tune to 20Hz to produce a quasi 1st ordrer roll-off. If building with Ultraflex vents, 6 of them, 0.5" wide by 4" high, for a total of 12in^2 CSA and 12" long, the predicted lumped tuning is ~40Hz. How much reduction in Fb can I expect from the resistive effect?
IG
IG
Expect 0 Hz reduction...
This is not a joke, just a guess based on my current understanding of vented loudspeakers. Generally, a resistance only damps a resonance, but does not affect the resonant frequency.
Mmm... True that a resistance only damps. I have it in my head that one effect of the high-aspect vents reduces the resulting Fb - not R in this case I guess. Anyway, thinking about it is pretty clear that 40Hz won't be taken down anywhere close to 20Hz in this case.
IG
Are you talking about like a Jensen Ultraflex cabinet, or something totally different I haven't heard of? If the former, the effective length of the vents is extended quite a lot for reasons not having to do with the vent shape.. On the inside, the column of air working for bass reflex is extended as it sort of bends into the open space after the ends of the slots, and on the outside it acts a little longer as it reaches out onto the wall (if you have it in a corner). Those kinds of effects are really hard to predict, though. You pretty much have to build with more cross section than you require, test Fb, and add pieces to reduce the cross section as needed.
Hey dumptruck,
I'm pretty certain that it's the Jensen Ultraflex scheme I have in my head, probably better known as Onken to most people. With these enclosures, very large Vb (~360L for Onken) it's not really hard to tune low with larger and shorter vents, but when dealing with a planned 43L of net reflex volume, that's another story and pretty much PR-territory. I'd have to look at pretty high aspect vents to get me anywhere close to my target and building and measuring is the only real way to know. Worse case scenario, the vents "do nothing" and Fb is 40Hz and it produces a maximally flat reflex for my driver with f3=40Hz. Best case is it ends-up tuning super-low and I can reduce length until best result is achieved.
FWIW, the bottom slot-vent on my TD15M box is three 6.5"x1.5" sections, all 10.25" long. I built the box with 5.0cu.f of gross volume to achieve at least 4.5" of net volume. The lumped vent geometry predicts Fb=35Hz for 5.0cu.f and Fb=37Hz for 4.5cu.f. Actual measured Fb is 32Hz. I guess there is a wee bit of the Ultraflex-effect going on, but this was not the goal and vent aspect is not that high. BTW, I'm getting my phaseplugs real soon, they're sitting at my Vermont PO box, waiting for me. 🙂
IG
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These effects you're talking about are not from the aspect, they are from the way the ports are terminated. It's that "end correction" factor some simulators use. If you're simulating the same way as you simulated for the TD15M (that would end correction of 0.75 or 0.85 by my sim for the numbers you posted), you can probably expect to end up a little lower in Fb, just as you did for the TD15.
I don't know what your construction is like, but I've seen people plan it so that two or more dividers between slots can be left unattached until last and varied in width a lot, so you can use them to raise and lower Fb as needed (hopefully not needing to make them so wide that your total port cross section gets too small and air velocity becomes a problem).
I don't know what your construction is like, but I've seen people plan it so that two or more dividers between slots can be left unattached until last and varied in width a lot, so you can use them to raise and lower Fb as needed (hopefully not needing to make them so wide that your total port cross section gets too small and air velocity becomes a problem).
Yes, there could indeed be ways to leave a provision for adjusting the slot vent height. Airspeed is OK for the vents I have layed out on my plan, 5% Mach is hit around 90W and mid-band output is close to 110dB by then and Xmax probably exceeded. While my ideal target would be 20Hz, I could live with 25Hz or so based on what the sim shows me. It's all conjecture until I make some sawdust anyway; who knows, I might prefer the 40Hz ~max-flat alignement, but I'd really like to at least give the over-damped reflex a try.
BTW, this would be for the Peerless 830869 and BMS 4540ND on the 6"x6" JBL PT waveguide, another E-Wave of sorts. I might have to get hip with PCD this time! 🙂
IG
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