Question on shelf ports

[OldogNewTrick]

With round ports, changing port diameter results in a corresponding change of length to maintain the same volume/tuning.

Does this hold equally true for rectangular shelf ports ?
And for a triangular corner port ?

Anybody have an algorithm to describe the relationship between aspect ratio (height x width) to length in order to maintain a constant resonace frequency ?
or have any links to other resources expanding on this ?


Much appreciated if anyone could point me in the right direction !

[Mr. Foxen]

Port area is the important bit against length, not so much shape, but you have end correction, which I think is basically the the back of the cab and the end of the shelf acting like a right angle end piece of the port.

[bgavin]

For slot ports, I add 1/2 the slot height to the actual length.
This results in a measured tuning slightly lower than the calculated result.

The actual tuning is done by empirical methods.
Measure the impedance sweep and adjust the slot length to fit.

[OldogNewTrick]

Originally Posted by bgavin

For slot ports, I add 1/2 the slot height to the actual length.
This results in a measured tuning slightly lower than the calculated result.

Bruce, thank you. If I understand your approach correctly, it is independent of slot aperture and considers only slot height, correct ?

Greenboy suggested at some time to divide the WinISD calculated shelf length by 1,5 to arrive at the actual shelf length.
Since the such derived shorter shelf length increases Vb, you need a process of iteration to arrive at an approximately correct value.

That is the extend of my knowledge so far.

[Mr. Foxen]

Width is just as important as height, since it is port area that is important, it determines how much air can flow through it. The length along with that determines the way the air resonates as it does so.

[OldogNewTrick]

Originally Posted by Mr. Foxen

Width is just as important as height, since it is port area that is important, it determines how much air can flow through it. The length along with that determines the way the air resonates as it does so.

If I understand you correctly, you are suggeting a linear relationship, based on volume alone, behaving identically to a round, non-flared port.

Pity that WinISD does not calculate the shelf length correctly, then I could simply forget about the why's and how's and just do it....

[Mr. Foxen]

You can try it in WinISD, since it shows port area. Might need Pro, but since it is free to, no reason to use the older version. Behavior won't be identical, because there are factors like turbulence, and the effect of the rest of the cab (hence the end correction), but the important thing is port area, no a single dimensional measurement. It isn't about volume so much, area determines airflow, (correspondingly airspeed) and then factor in length for tuning purposes, so keeping volume constant, but messing area vs. length will not give you the same tuning.

[bgavin]

The problem with a slot port is the extra effective length created by the sides and bottom of the slot.

This is compounded by the rule of thumb that requires 1 diameter unobstructed space behind the port. I have a design going into RTA tonight that violates this rule. Tinkering will be involved.

The Length + 0.5 Height is pretty generic.
I find this tunes the cab slightly lower than desired.
If the slot panel is glued into place, this becomes a Dremel exercise trying to shorten it.
Prototype your cab with a replaceable slot panel. It is less grief.

[Mr. Foxen]

Round ports have a significant convenience advantage in that you can make them telescopic with the right size tubes.

[OldogNewTrick]

Thanks for all your input, guys.

I've been excersizing my google skills and came up with lots of information, but no straight answers.

Been reading the wikipedia article on Helmholtz resonators, which is the basis for all port theory. Found some convinient software tools to convert round to rectangular ports. Gotten some background on a Mr. Onken, who seems to be the Godfather-once-removed to all double-sided slot port bass cabs (Aguilar GS 210/410, Eden XLT). Played with the port converter software and WinISD Pro just to empirically confirm that round ports and rectangular ports contain different volume for the same tuning frequency.

With all that physics making my head spin, I think I am good for some good CH3-CH2-OH chemistry to balance my head out...

[bgavin]

OP, Here is what you are searching for. See Post #9.

[OldogNewTrick]

Originally Posted by bgavin

OP, Here is what you are searching for. See Post #9.

Bruce, great, man, thanks


happy dance





really appreciate your tenacity, need now to work this through..

[bgavin]

I measured the 2.227 K for the full width slot port.
It is dead on.

[OldogNewTrick]

A question, Bruce, is that K value of 2.227 also true for multiple shelf ports ?

Like the fEarful port with a center divider - do you just ignore the devider and quietly deduct its thickness from the combined port width or are that 2 seperate ports ?

What about an Onkken ported box with lets say 3 equal shelf ports on each side, so 6 in total....

You8r help and everbody else's is greatly appreciated !

[bgavin]

The divider isn't really a factor unless it is a complete divider. In that case, treat the slot port as normal, but subtract width equal to the divider thickness.

[dhomer]

Instead of starting a new thread, I thought I'd dig this one back up to pose a WinISD based question. WinISD Pro has an end correction input on the port calculation. Plugging in that 2.227 value results in a much longer port, not shorter as the correction would need to be. Does this value have to be negative, or is the program incapable of making that calculation? Would I have to get out the scientific calculator and do it by hand??