Slot port vs circular port equivalence

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Brisso57

Brisso57

Member
Joined 2005
Hi All,

I'm considering building a BR floorstander design (approx 28 litres internal volume) that has a circular port: 2" diam x 3" length.

1. Can I substitute a slot port at the base of the front baffle?
2. If yes, what adjustments need to be made?

Thanks

Doug
 
GM

GM

Member
Joined 2003
Right, the 'floor' of the cab will increase the vent's acoustic length with increasing width, ditto if the side walls are the vent's sides and at around a 9:1 width:height ratio it will begin becoming resistive in nature, i.e. moving towards aperiodic. Unfortunately, I don't remember the math from my duct designing days to figure the various end corrections and can't find my notes ATM, but I imagine a bit of Googling or maybe this site has it: http://www.subwoofer-builder.com/

GM
 
gtforme00

gtforme00

Member
Joined 2004
Thanks GM for that link!

Somehow I've never seen that site before. What you said below interests me very much about the thin slot port becoming resistive.

I am working on a small bandpass design and have wondered if using thin slot ports would be better so that the ports don't have to be so darn long.

Do you have any other references concerning slot ports?

Thanks,
David
 
Brisso57

Brisso57

Member
Joined 2005
GM said:
Right, the 'floor' of the cab will increase the vent's acoustic length with increasing width, ditto if the side walls are the vent's sides and at around a 9:1 width:height ratio it will begin becoming resistive in nature, i.e. moving towards aperiodic. Unfortunately, I don't remember the math from my duct designing days to figure the various end corrections and can't find my notes ATM, but I imagine a bit of Googling or maybe this site has it: http://www.subwoofer-builder.com/

GM
Click to expand...

Thanks GM.
I should add that I'm also planning to have these floor standers on a plinth of about 35mm height (to add stabilty) and so that should mitigate the floor gain a little.

cheers

Doug
 
P

pjpoes

Member
Joined 2005
Port volume always has to stay the same for the same tuning. This means that going from a round to slot port wont change its length if the volume is to stay the same (roughly speaking). If you are designing around slot ports that are considerably shorter, then the mouth of the slot is too small, or not equivalent to the round port you were considering.
 
Brisso57

Brisso57

Member
Joined 2005
pjpoes said:
Port volume always has to stay the same for the same tuning. This means that going from a round to slot port wont change its length if the volume is to stay the same (roughly speaking). If you are designing around slot ports that are considerably shorter, then the mouth of the slot is too small, or not equivalent to the round port you were considering.
Click to expand...

Thanks PJPOES.
I think that GM has effectively answered my real question: ie at what point does the slotted port become too "thin".
I believe I now have enough info to make the adaption with some degree of confidence.
 
gtforme00

gtforme00

Member
Joined 2004
pjpoes said:
Port volume always has to stay the same for the same tuning. This means that going from a round to slot port wont change its length if the volume is to stay the same (roughly speaking). If you are designing around slot ports that are considerably shorter, then the mouth of the slot is too small, or not equivalent to the round port you were considering.
Click to expand...

I would have to politely disagree with the above statement.

It is not solely the port volume which dictates the tuning frequency. Case in point: when a larger port diameter is used the length required to maintain the same tuning frequency is increased.

Specifically, the tuning frequency is a function of the ratio of area / length of the port as described in the following equation:

828b88b795f98db422137630067ccbff.png


where fH is the tuning frequency, v is the velocity of sound in air, A is the port area, L is the port length, and Vo is the enclosure volume.

Changing the values of A and L while keeping the product of A*L constant (constant volume) does not result in the same resonance frequency.

While it could be true that for equivalent port areas, the same overall length must be used, it is not necessarily so. The correction factors mentioned above are just that, correction factors which account for the different behaviour of slots vs circular ports.

What GM is implying above is that a slotted port with both sides and bottom being those of the enclosure will have an effective length longer than the divider used to create the slot itself would measure. There are empirical correction factors determined for different geometries of ports, and I was inquiring to locate any references GM may have on the subject.

What I have found so far comes from the archives of this very forum. Below is an image which contains correction factors for various port geometries:
An externally hosted image should be here but it was not working when we last tested it.

The associated equation in which to use these correction factors follows:
Lv = 10 * c^2 / (16*pi) * D^2 * Np / (fb^2*Vb)-k*D

Lv=length of vent
c = 344m/s or 13400 in/s
D=diameter of vent in cm or in
Vb=volume of enclosure in l or in^3
fb=tuning frequency
Np=no. of vents
k = correction factor

This information was gleaned from this thread.

My reason for inquiry was as stated, to find a way to cheat the system and squeeze a 6th order bandpass into a smaller than normal volume by using slot ports to effectively increase the length of my ports.

I have a suspicion that there will again be no free lunch, and that the flow resistance which causes the length to be effectively longer in a thin slot port will cause increased losses in the port. This may very well render inutile any benefit to the smaller enclosure to my design, but that remains to be tested.

Thank you all for your contributions.

Respectfully,
David Malphurs
 
gtforme00

gtforme00

Member
Joined 2004
I'm trying to find an English equivalent of the chart and a better description of the usage of the k factor.

The specific question in my mind is how does one use the Dv variable for a slotted port where there is no diameter? Is it the diameter of a circle of equivalent area? I am looking into this further and will post any results I find to this thread. If anyone has another source regarding this topic, contributions would be greatly appreciated.

Regards,
David
 
Ron E

Ron E

Member
Joined 2002
Rohr, einseitig bundig means:
Tube bounded (flanged) one side

Umgerechnet in Kanal means:
converted to channel

In the bottom three cases, everything before "vom" is the condition described by the picture and after "vom" is the effect, I am mainly translating the effect, here.

Kanal unterseite - vom Boden verlangert means:
Channel underside lengthened by cabinet floor.

Kanal unterseite und eine Seiten vom Boden und Eine Seite verlaengert means "lengthened by side wall and cabinet floor"

Kanal unterseite und beide seiten - vom Boden und seitenwaende verlaengert means: lengthened by floor and two side walls

One they miss is if you have a tube or channel with two flanged sides (or also a simple hole in the cabinet wall) then k=0.85. One reference for the k=0.732 and k=0.85 cases are L.L Beranek's classic "Acoustics".

I wouldn't be in a huge rush to apply these factors (other than K=0.732 and K=0.85) without a solid reference as they are likely empirical. Nothing wrong with empirical formulas, but there are sometimes a lot of uncontrolled variables that can give different results in your specific case.

Best is usually to make your port a bit too long and then correct.

I know of one guy who built a bass cabinet and reported his results to me that his tuning frequency for a shelf port built into a floor (with a brace in the middle) was the same as given by the formula with the k=0.73 case.
 
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