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[Mike Holtzinger]

WinISD says Vent diameter 4.02x4.02in. Vent length .24in. Vent mach .13. I like the idea of triangular ports, but Win ISD apparently doesn't have a triangular port option AFAIK.

[David Morison]

WinISD says Vent diameter 4.02x4.02in. Vent length .24in. Vent mach .13. I like the idea of triangular ports, but Win ISD apparently doesn't have a triangular port option AFAIK.

Can you confirm what Vb & Fb you're modelling, and the number of drivers and ports please?
I'll try replicating that when I'm back at home and see what might be up.

One simple option may be to increase the number of ports until you get the port length up to at least the thickness of the baffle - this will have the secondary benefit of reducing port airspeed (Mach) further.

As a first approximation, you can change the shape of a vent as long as you keep its area and length the same, so if you find a size where 4 vents of XX area work, building them as triangles may work.
There are a couple of catches though, for example when a port is built in along the wall of a cabinet it will behave as if it's longer than it really is to some extent. This is due to 2 (or more, depending on shape) of the port walls being longer than the nominal length of the port. Some trial and error may be required to finalise that, by starting with a longer vent and cutting it down until it's right.

HTH,
David.

[Mike Holtzinger]

 Thanks David,
 According to Win ISD, Vb is 21.4 cu Ft and Fb is 35.31.

[David Morison]

Thanks David,
 According to Win ISD, Vb is 21.4 cu Ft and Fb is 35.31.

Okay, that's laaaaarge.

As a rough rule of thumb, typical 218s would be from about 10ft³ at the compact end of the scale up to 14-16ft³ for a more generously sized unit.

So first question, are you sure you want to handle a box that big?

Now, a consequence of a large box is that it allows the driver to move pretty freely, which means high excursion at even modest power levels, so a box this big will start to exhibit distortion at lower power levels than a smaller box. In this case, you reach the nominal Xmax at an input of about 40V for the pair, which equates to 400W into 4Ω or 500W into the minimum (in-band) impedance of 3.17Ω.  (The 640W mentioned in the pic refers to that voltage being applied at the minimum impedance overall, which is at about 200Hz so wouldn't be as relevant for subwoofer use.)


Reducing Vb &/or increasing Fb will help with that, however at the expense of LF extension.

So, assuming you stay with the big box: Vent sizes.

There are different rules of thumb regarding just how high an airspeed can be tolerated, the highest I've seen quoted for PA use is Mach 0.1 (ie approx 34ms^-1), with lower estimates for really keeping clear of a risk of hearing the air movement in the vent down to half that.
If you accept that higher level, then 4 ports each being 4" x 4" will get you just under 34ms^-1 even without an HPF. Each one would need an effective length (after allowing for any "extra" length from building in against the cab walls) of 3.14".


If you want to hit the lower target you'd need twice the area, so 8 4" square vents, each needing to be 9.58" long now:


Alternatively, 4 square vents each 5.5" per side would give similar area and require less length (a large vent is predicted to be more efficient than smaller ones, so you won't eat into the cabinet volume as much this way):


Hope this helps,
David.

[Mike Holtzinger]

Thanks again David. First off, it is Large, but it's the roughly same size as the JBL 4719. So I know what I'm in for as far as moving them. How about a single large port in the middle like the SR4719x has? Seems it would be easy to build and it would add some rigidity (if that's a word) to the box.

[David Morison]

Thanks again David. First off, it is Large, but it's the roughly same size as the JBL 4719. So I know what I'm in for as far as moving them. How about a single large port in the middle like the SR4719x has? Seems it would be easy to build and it would add some rigidity (if that's a word) to the box.

Re SR4719X, it's about 21.4ft³ external, so will be more like 15-16ft³ internal after accounting for the cabinet walls, baffle being set back to give grille clearance, the drivers themselves, the ports, handles and any bracing. You're looking at about 50% bigger on the inside, which would be about 14% extra on each linear dimension.

Re single mid mounted vent, yes that can be an effective option. WinISD predicts that if you go for a single rectangular port with the same area as the 4x5.5" square ports (eg 22"x5.5") then the required length drops to 3.1".

Cheers,
David.

[Mike Holtzinger]

So if I recess my baffle(s) I can get the volume down to 16.9 cu ft. Will this affect the mid vent dims?

[David Morison]

So if I recess my baffle(s) I can get the volume down to 16.9 cu ft. Will this affect the mid vent dims?

Yup, you'd need to recalculate those for any change in Vb or Fb.
If you get that net internal volume and want to keep the same Fb, then my previous example vent of 22" x 5.5" would need to be 6.35" long. That would take up 0.5ft³ in the box, don't know if you've already factored that in in your 16.9ft³.

Cheers,
D.

[Mike Holtzinger]

Thank you David for making me use my brain. I believe Win ISD is working for me now. Maybe you could do me a favor and confirm my results.
 With 2 2241 drivers in a 17.255 ft3 box, for a tuning freq. of 30 hz, a single vent 5x18" should be 6.89" deep and will give me a vent mach of .11.
 

[Richard Turner]

https://www.jblpro.com/pub/manuals/enclgde.pdf

vent quantity and length are given on these plans, if you cannot calculate the panel cut list you are in over your head, dont forget to add the lip ahead of the baffle to protect the driver/attach grille to.

These plans work well, I made single and dual 15" cabinets off these plans many years ago and they were quite serviceable.

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