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Putting ports on the bottom questions

V63

V63

Sep 13, 2008
171
32
Alberta Canada
#1
I'm starting a do-over on a cab build but my question now, is:
If I don't what a rear ported cab, nor see the ports, and I want the cabinet to be as shallow as possible, is putting the ports on the bottom the best way to go?
Are there any problems or quirks associated with this type of porting?
How high off the floor does it need to be so as not to restrict air flow?

Thanks
 
#3
CL400Peavey said:
It's a bad idea. The floor acts like a port extention, and the floor covering will effect your sound. Port on the front. Cover with grill cloth.
Click to expand...


What about putting it on a 45 degree angle? Does this work?
I'd like to get it shallow without compromising port function.
I could squeeze them into the corners to get artificially lengthen them right? Is there math for this to know how much depth I could save?
 
#5
Downunderwonder said:
You're pushing stuff uphill with a stick. Slot port gives maximum port area for dimensional uptake. Use grill or cloth. The aesthetic in a cab with tall feet to allow any bottom port to function is looney.
Click to expand...

If I need two 4" round ports 6.86" in length what size slot port would I need?
The cab would be about 20"X26"
Could I put the slot off-side to accommodate a grill frame?
How do I adjust it to verify tuning?

Thanks
 
#7
Andy is of course right, but for a bottom port, giving it a little bit of room to breathe is necessary. Don't need much...maybe an inch...the real cab experts could fill you in on it. It would work, and I've played a cab with the port on bottom that worked quite well, but it does need breathing room.
 
#9
V63 said:
I'm starting a do-over on a cab build but my question now, is:
If I don't what a rear ported cab, nor see the ports, and I want the cabinet to be as shallow as possible, is putting the ports on the bottom the best way to go?
Are there any problems or quirks associated with this type of porting?
How high off the floor does it need to be so as not to restrict air flow?

Thanks
Click to expand...

Downfiring ports are fine if done right.

With a downfiring port, your cross-sectional area transitions from being the are of a circle (the round port) to being the surface area of a cylinder (the perimeter of the port times the height above the floor).

What we want to avoid is a bottleneck that can cause turbulence.

So, I suggest going with flared ports. The perimeter of that cylinder will now be at the outer diameter of the flare, and so will be significantly larger than the perimeter of the port itself. And, the flare smooths the airflow in and out of the port.

One inch tall feet should be sufficient if you're on a hard surface, assuming a port diameter of no more than 4 inches. On shag carpet or grass, you'd probably want to put a board underneath the cab.

The effective length of the port will be increased by the proximity of the floor, by how much I can't really say, but would guess from 1 to 2 inches for a 3" or 4" diameter port. If the downfiring port is near a cabinet edge, the increase in effective length won't be quite as much.

I've done prototype and beta-test cabs with downfiring ports with no problems when following the above guidelines, and in a couple of weeks will probably build and test another prototype with downfiring ports. Unfortunately I didn't save enough data from my previous trials to give a clearer picture of the effect on the tuning frequency; I just remember expecting it to be a few Hz lower than the modeling program predicts, and it was.
 
#11
What is being overlooked is consideration for the oscillating air mass in the vent.

This slug of air extends beyond the vent (each end).
This is the basis for the one diameter distance guideline.
The intent is to keep this air mass from colliding with the floor.

The cross sectional calculation is entirely valid, to ensure the port does not exit into a confined space.
 
#12
If the bottom port cab Duke built that I played is any indication, a flared port takes care of that. And I played it on a rug and a hard surface and it sounded killer on both ;)
 
#13
JimmyM said:
If the bottom port cab Duke built that I played is any indication, a flared port takes care of that. And I played it on a rug and a hard surface and it sounded killer on both ;)
Click to expand...

Thank you sir!

That cab has gone through at least four generations of major revision since you tested it, and I'm now back to almost the same driver complement you had but in a more compact package.
 
#15
JimmyM said:
Man, you need to let that thing run free in the wild, Duke!
Click to expand...

I hope to bring it to the Seattle GTG at the end of the month, and it may have a little brother tagging along. Seems to me the format is better suited for small gigs than for big ones.
 
#16
I have a Mac so winISD is a problem :(
The slot port idea may be the way to go. I'll have to look for a calculator online to figure it out. Has anyone worked this out already for Eminence's mid-sized 3015 cab?
[Invalid or Expired Link Removed]
 
#17
bgavin said:
What is being overlooked is consideration for the oscillating air mass in the vent.

This slug of air extends beyond the vent (each end).
This is the basis for the one diameter distance guideline.
The intent is to keep this air mass from colliding with the floor.

The cross sectional calculation is entirely valid, to ensure the port does not exit into a confined space.
Click to expand...

If there's sufficient cross-sectional area and geometrical smoothness that turbulence doesn't set in, the only significant result is an extension of the effective port length and therefore a lowering of the tuning frequency. The "slug of air" is fluid enough that it doesn't mind changing direction or shape; any resulting loss of energy is negligible.

Adhering to the one diameter spacing guideline keeps the port tuning frequency much more predictable.

Consider a slot port as a squashed round port: If the effective "diameter" is the average of its width and height, there seems to be no ill effects from ignoring the one "diameter" rule of thumb regarding distance to the internal walls of the enclosure. And the net result of the proximity to these walls is what we might expect - a lowering of the effective tuning frequency (though this isn't the only mechanism at play there).
 

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