# Shelf port length question

Discussion in 'Amps and Cabs [BG]' started by JGrooveguy, Oct 28, 2018.

1. ### JGrooveguy

Feb 22, 2018
I am using WinISD to calulate a rectangular/shelf vent for a cab...

Is the "vent length" the length of the shelf only (sketch A)? Or is part of the corner also considered part of the "vent volume" therefore making the shelf and the calculated vent length slightly different (sketch B)?
See sketch I made that hopefully helps explain my question.
Grey question mark area is where I am unsure if this is "cab volume", "vent volume", or a combination of the two (as in sketch B).

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2. ### Old Garage-Bander

That’s a good question.
What if you thought of this in terms of round/tube ports?
Where does port volume end and cab volume begin?

3. ### JGrooveguy

Feb 22, 2018
I did try to consider vs a round vent, if it was a round tube (or even rectangular still), but away from the edges of the cab, then it would be more simple. Its the fact that the vent ends in the corner of the cab that is causing my confusion. If a round port was placed tightly in a corner and fairly close to the back im sure it might alter the acoustic impedance as well...

4. ### JimChjones

Aug 6, 2017
SE England
AIUI its all about vent end correction values. I suggest the first thing to do is to try a whole range of feasible values and see if it makes enough difference to worry!

6. ### Raf Seibert

Dec 16, 2013
Texas
You might calculate it both ways and see what the difference is. I wouldn’t be surprised if the difference wasn’t too significant.

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7. ### Downunderwonder

Aug 26, 2009
New Zealand
It depends on how close into the corner your shelf ends. There is a correction to be made as you approach the shelf depth distance but I don't know details.

8. ### Downunderwonder

Aug 26, 2009
New Zealand
Hopefully your sketch is miles off in shape. You can't have the shelf end closer than its own depth, even within 120% could be no good. You might have to make it a more squat shaped cab.

9. ### DukeLeJeunerational romantic mystic cynical idealistSupporting MemberCommercial User

Nov 24, 2008
Princeton, Texas
Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
So now we're "esteemed"? I kinda like that!!

Unfortunately I don't use shelf ports so this will just be "armchair quarterbacking"...

Referring to the drawings in the original post, my guess is that the effective port length would be greater than "L" as depicted in either A or B. I think the effective port length would extend "around the bend" in this case, and up the back of the cabinet a little bit. The reason is, we still have all 4 boundaries that define the port all the way up to the top of the shelf, and then we have 3 of the 4 boundaries that define the port above that, so the port-like effects probably do not stop abruptly at the top of the shelf.

Also as our Wonder from Down Under noticed, the cross-sectional area is "pinched" at the back of the cab by the port shelf. I believe this would tend to lower the tuning frequency, and/or could cause turbulence ("chuffing") at high air velocities. But if the drawing is to scale, it would take enormous cone motion to get the air velocity high enough to cause chuffing, so imo that would be unlikely.

If I had to guesstimate the effective length of the port based on the drawing, it would be about 1.5 times the length "L" shown in drawing "A".

This armchair quarterbacking analysis is free, and worth every penny.

If by any chance our venerable equine master guru, @agedhorse, happens to post in this thread, I agree with him instead of with me.

Duke

Last edited: Oct 28, 2018
10. ### Bill Whitehurst

Sep 25, 2010
Fort Worth, Texas
I can tell you that the distance from the end of the port shelf to the back of the cab should always be more than the distance from the top to the shelf port.
I think I read 1 1/2 times is the minimum.
Since the top and sides form 3 out of 4 sides of the port, it will have some effect on tuning.
How much probably depends on how close and maybe more factors I haven't thought of.

I expect agedhorse or any of the more knowlegable folks can add detail and experience to that.

agedhorse likes this.
11. ### agedhorseSupporting MemberCommercial User

Feb 12, 2006
Davis, CA (USA)
Development Engineer-Mesa, Product Support-Genz Benz
Good question, here's my attempt at an "esteemed" answer...

What you have with a port is a resonant air mass contained within the volume of the port being subject to vibration by the driver. If the port gets very long, 2 things happen to the equations. The first thing is that the long length adds some loss to the function of the port due to the "friction" of the air moving along the inside surface of the port. This is generally not a big deal until the air velocity increases.

The second is is that when the port gets so deep that it's within say <1x the height of the port, that insignificant transition opening between the back side of the port and the inside of the cabinet becomes another significant resistive loss.

As this distance becomes small the equations begin to transition from ported to a lossy sealed enclosure.

Rule of thump is to keep the port depth at least 1.5x the port height and generally you can safely ignore it.

12. ### StumboWherever you go, there you are.Supporting MemberCommercial User

Feb 11, 2008
Song Surgeon slow downer. https://tinyurl.com/y5dcuqjg
Good one!

Old Garage-Bander and interp like this.
13. ### Kirk Bryant

Awesome sauce! So darn tasty my feeble brain barely tasted (understood)
About 10% of everything that was just said above. Thanks for chiming in old pony! I should’ve thought of you too.
Mea culpa.

14. ### Downunderwonder

Aug 26, 2009
New Zealand
I think the advice from Agedhorse got a bit fast and loose.

If I join the dots I get that you need fully 1.5x the port height worth of distance from the end of the shelf to the back wall. In your sketch it looks like about 0.7x the height.

To do that you need to increase the cabinet depth and reduce its height to maintain cab volume and the port area.

15. ### HaphAsSard

Dec 1, 2013
Italia
Then again,
If the above - a stricture between the back opening of the port and the internal surface(s) of the cabinet not causing problems thanks to a wide vent cross-section to start with - can be translated into shelf port situations (i.e. if the corners don't create another set of variables, which I don't know), I guess these more forgiving circumstances can happen in an overall wide box; more compact builds tend to require the port to either shrink proportionally, and thus chuffage needing to be a consideration, or become impractically long.
The extreme scenario is what @fdeck calls the "small box crisis", when the port width and length that the cab needs can't practically fit into it.

agedhorse likes this.
16. ### JGrooveguy

Feb 22, 2018
Thanks agedhorse and all, this helps me make make my decisions. I'll check WinISD for the length and whether I can tune it nicely with more than 1.5x the shelf height left at the back or not.
This is actually for a retrofit where I want to turn a sealed 1x10 (w/ 5" mid crossed @ 500Hz) into a vented cab to try one of the new Eminence CA-1059's. The mid driver and crossover are already in a separate compartment (forming the basis of the rectangular port I would make), but I now I'm leaning towards just taking this all out and put a round port where the mid driver used to be (and gain cab volume while saving weight). I actually dislike the tone with the mid driver engaged for my playing style so it wouldn't be missed.

Last edited: Oct 29, 2018
17. ### JGrooveguy

Feb 22, 2018
I feel like properly calculating the vents/tuning as per my sketch would be more like a double bass reflex. Aka two vents in series (but usually with significantly more volume in between).
The gap between cab and slot would be the first/internal port, with a vent length of 3/4" (thickness of the plywood). The actual slot length "L" as per sketch "A" would be the second port. The grey area would be the second cab volume, but the volume and port size ratios are way off from a usual double reflex designs you see in hifi speakers...
I could be totally off, just a thought I had. Not going with this method anyway, unless I would building off a proven design (and probably for hifi speakers).

18. ### agedhorseSupporting MemberCommercial User

Feb 12, 2006
Davis, CA (USA)
Development Engineer-Mesa, Product Support-Genz Benz
An unintentional typo at its best

How did I get fast and loose?

To fix the problem with the existing box, the area would need to get smaller and the port depth shorter.

Another problem with slot ports is that if the height gets too small, the same non-linearities occur in the port equations. Generally, if a slot port gets smaller than an inch and a quarter to an inch and a half, this too can be an issue. It's why slot ports aren't generally a good choice for smaller boxes... they no longer act as ideal ports.

Jan 27, 2014
Tampa, FL
Fantastic!

20. ### Downunderwonder

Aug 26, 2009
New Zealand
Not my best choice of words but I was having trouble joining the dots between OP problem not being fully explained and your response because you left out the detail of the distance to the back wall. Sorry about that.

Another option although way more work for a retrofit project would be folding the port around the back wall, also with unpredictable losses in the bend.