Cabinet size vs. port displacement

When calculating port parameters, is the volume of the port subtracted from the calculated size, or is it usually compensated for in the design? For instance, in a particular 3.5 cu.ft. design I'm working on, the optimum port size to avoid turbulance works out to .75 cu.ft. port displacement volume in this case. Does the 3.5 cu.ft. size usually take that into account, or should the cabinet size be increased to 4.25 cu.ft. for the same performance? The programs I'm using are not really clear on this point.

It seems like it should already be compensated for, or else you'd constantly have to recalculate values to get a final refinement. It's just that .75 cu.ft. is a pretty big chunk of displacement out of 3.5 cu.ft., so I'm a little concerned...

Thanks for any info - John.

Alas, port volume must be added to box size, or the calculations redone based upon the "net" volume, unless the program specifically states that it takes it into consideration. Same with driver displacement.

The air in the port is moving, not compressing, and thus does not contribute to box volume. The port is the relief; the enclosed volume "sees" only its interior opening. If you put the ports on the outside of the box, you wouldn't add them to the box volume, either.

WinISD help, for example, states:

"The bad news here is that rectangular ports must be built into the box, and therefore require much more skill and equipment to accomplish. As with flex tubing, rectangular ports consume a lot of internal space, and so the entire design must be re-done to allow for the area displaced by the ports."

Well, it's the volume displaced by the ports, not the area, of course....

Gurus may smack me here, but that's my reading. We now have contradictory views to resolve. I use "net" volume and successive approximation. :p

My last vented system was 8.99 cubic feet gross physical volume. The driver, ports, and bracing took up 1.29 cubic feet so the net physical volume was 7.70 cubic feet. The dual 6" diameter ports each had 13.25" ducts tuning the volume to 31.35 Hz. That means the actual effective volume of the system was 7.98 cubic feet (counting system losses). Lining the inside with good old 2" thick Owens Corning fiberglass dropped the tuning to 29.86 Hz which equates to an actual effective volume of 8.79 cubic feet - just 0.2 cubic feet shy of gross and also happens to be the volume JBL says is displaced by the 2242/2245. The ducted ports are quite large for this system so I had to use 2" instead of 1" fiberglass to compensate.

You may notice that the default loss value in WinISD is something like 7. Better JBL systems measure around 10 to 12. These particular 8.99 cubic foot boxes I built measured just over 14, so "typically" the effective volume with a loss of 7 would have ended up with a higher effective volume resulting in an even lower tuning frequency. Enclosures that measure total losses around 3 are considered junk. In other words, how well you build the box will also have an effect. Keep in mind that Vb can vary by up to 20% without detracting from performance whereas Fb can vary by up to only 10% without detracting from performance.

Well, 3.5 cu.ft. is the max gross internal volume, with the speaker volume taken into account. I'll run a few different versions to see how much the theoretical difference is. I haven't used WinISD yet - 'guess I'll download a copy of that to see what it says.

Thanks for the help - John

Somewhere on JBLPRO site there is a list of boxes for popular drivers, along with a few good rules of thumb. one of these R-O-T states that (on larger boxes) you can forget about deducting bracing, driver, and (within reasonable size limits) port volume deductions if you insulate the box on all sides with 2" of fiberglass. It "adds virtual volume to the box", as they say. Just keep it well clear of the ports (staple it back and/or cover it with gause to hold it away), and don't stretch it tightly over the backs of the woofers.


If still unsure, make your port a little on the long side-you can always shorten it. If you know what to expect from your port dimensions and length, you can measure the Fb of the finished product and mathematically figure out what size your box is actually behaving as. If you want to really get cranky about it, you can add (or subtract) loosely stuffed or mounted fiberglass in the cabinet to further tweak the effective net volume of the box. you can easily add 20% "virtual" box volume this way to make the T/S parameters get right in a box that is too small. In theory, you can add 40%. In theory.


But the more important criteria is Fb, once you get pretty close to the ideal volume.

Look what I just found in a dark corner of C:http://audioheritage.org/vbulletin/i...es/biggrin.gif

Well, dammit! I can't paste a .pdf to this, so I am suddenly compelled to go to JBL's site and seek out the link-
Try: http://www.jblpro.com/pub/manuals/enclgde.pdf

Yes, a link suffices nicely here....

On this theme - I have decided to stick my next port on the _outside_ (top) of the box.
It's for a 2245 subwoofer and it only has to reproduce up to 66Hz so I can make the port quite deep before pipe resonance becomes a problem.
This means I increase the area and avoid port compression.
But the port volume is about 80 litres! (3 cu. feet) - so I plan to stick it up the top like a stovepipe!
It will be tucked up as part of the horn baffle so won't look weird.
Theory says it should work - Any one see any problems with this?

Dave.

Quote:

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Originally Posted by johnaec

Well, 3.5 cu.ft. is the max gross internal volume, with the speaker volume taken into account. I'll run a few different versions to see how much the theoretical difference is. I haven't used WinISD yet - 'guess I'll download a copy of that to see what it says.

Thanks for the help - John

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John;

In general, vented enclosure volumes up to approx 11 cu.ft. ( 311 L ) will have a QL of 7. Larger boxes than that, the QL is typically 5. Changing QL changes the F3 and amplitude response. Whatever volume the port takes up inside the box has to be added to the original calculated box volume.

What loudspeaker program are you using BTW ?

Dougie

Quote:

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Originally Posted by Giskard

You may notice that the default loss value in WinISD is something like 7. Better JBL systems measure around 10 to 12. These particular 8.99 cubic foot boxes I built measured just over 14, so "typically" the effective volume with a loss of 7 would have ended up with a higher effective volume resulting in an even lower tuning frequency. Enclosures that measure total losses around 3 are considered junk. In other words, how well you build the box will also have an effect. Keep in mind that Vb can vary by up to 20% without detracting from performance whereas Fb can vary by up to only 10% without detracting from performance.

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Quote:

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Originally Posted by Rudy Kleimann

Look what I just found in a dark corner of C:http://audioheritage.org/vbulletin/i...es/biggrin.gif

Well, dammit! I can't paste a .pdf to this, so I am suddenly compelled to go to JBL's site and seek out the link-
Try: http://www.jblpro.com/pub/manuals/enclgde.pdf

---

Yeah, we've posted that document at least two dozen times. We weren't jerking off around here. We were pretty busy making sure the best possible information was available to the forum.

Holy thread resurrection, Batman! :blink:

John

Properly bracing the box increases QL and pays dividends but adding 2x4's will reduce cabinet volume. I think the tradeoff of a larger box with proper bracing is better than a smaller box which flexes.

Another wacky idea is to brace the box externally. If they can be hidden in a wall installation or will be road warrior speakers 1x2 braces could be run along the outside.
I saw a road speaker that was built this way and was carpeted over the braces. The braces were spaced in way that allowed two pair to be stacked and this did not require the box panels to be enlarged to compensate.