tuning question...

[outspoken 10+ year member]

so i know this may get me flamed, but, in an l shaped ported box, longer port = lower tuning, shorter port = higher tuning? i would like to modify my box until i can get a new one built. flame me if you must, i probably deserve it anyway //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif

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[P.rick 10+ year member]

"longer port = lower tuning, shorter port = higher tuning?"

yes sir

 

[outspoken 10+ year member]

o.k., that's what i thought,now how do i figure out how much i need to reduce the port to get it to the freq. i want? it's at 32 now need it to be at more like 38...

 

[simpsonbuck 10+ year member]

http://www.bcae1.com/spboxnew2.htm

 

[mlstrass 5,000+ posts]

also realize that changing the port length will change the box volume...

 

[9seconlives 10+ year member]

Is it true that every 1/2" in port length (shorter or longer) eitherway is the equivelant to 1hz increments of tuning?

 

[dragnix 5,000+ posts]

Is it true that every 1/2" in port length (shorter or longer) eitherway is the equivelant to 1hz increments of tuning?

never heard of this and i don't think it would be right anyway. Tuning also has to do with box size and port area.

 

[Rashaddd 5,000+ posts]

Is it true that every 1/2" in port length (shorter or longer) eitherway is the equivelant to 1hz increments of tuning?

Absolutely not.

 

[9seconlives 10+ year member]

never heard of this and i don't think it would be right anyway. Tuning also has to do with box size and port area.

Understood BUT why then by making the port shorter or longer does it change the frequency to which the box is tuned? just curious and not trying to pick a fight with a senior member but it "almost" made sense to me when i heard it the first time. i understand that the size of the box and port are a major factor but i'm use to building by DD's specs most of the time and if i have a 2"-3"x16"port tuned at 40htz then how come if i make it an inch or 2 longer ir will drop it down to like 36 or so? cause you're tuning the box not the sub....maybe it's too late and my meds are kicking my a$$ but than you guys for clearing that up for me. //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

here's what i mean though...doesn't matter if you have a 1510 or a 3512 the area of the port is different but the length and tuning frequency is the same..i also understand that thes #'s are not set in stone..

http://www.ddaudio.com/dd/caraudio/enclosureInfo/boxSize_hiEff.asp

SORRY TO JUMP OFF TOPIC!!//content.invisioncic.com/y282845/emoticons/crazy.gif.c13912c32de98515d3142759a824dae7.gif

 

[mcsoul 10+ year member]

"longer port = lower tuning, shorter port = higher tuning?"
yes sir

[quote name=Juha Hartikainen

Designing your Box, help file from winISD]

If your mach rating is too high, the only way to lower it is to either increase the number of or the diameter of the ports. Of course, here comes that dreaded "trade-off" word again! If you make your port(s) bigger in diameter, they will be able to pass more air with less effort. However, in doing this, you have lowered the "loading" of the speaker(s) in the box. This, of course, changes the -3dB point of the box. The only way to get back to your desired -3dB point is to "load" the speaker(s) again. How? Well, you gotta make the new, larger port(s) longer! This forces the speaker(s) in the box to work harder to move the air into and out of the box, and brings your -3dB back to where it should be. THE END OF YOUR PORT(S) MUST BE AT LEAST 3 INCHES AWAY FROM ANY INTERIOR WALL OF YOUR ENCLOSURE.

OP, keep in mind the a the depth of the port gets longer for the same tuning when

the port area (port length x width) are increased. The depth needed will also increase for every port when you add additional ports.

 

[Rashaddd 5,000+ posts]

Understood BUT why then by making the port shorter or longer does it change the frequency to which the box is tuned? just curious and not trying to pick a fight with a senior member but it "almost" made sense to me when i heard it the first time. i understand that the size of the box and port are a major factor but i'm use to building by DD's specs most of the time and if i have a 2"-3"x16"port tuned at 40htz then how come if i make it an inch or 2 longer ir will drop it down to like 36 or so? cause you're tuning the box not the sub....maybe it's too late and my meds are kicking my a$$ but than you guys for clearing that up for me. //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gifhere's what i mean though...doesn't matter if you have a 1510 or a 3512 the area of the port is different but the length and tuning frequency is the same..i also understand that thes #'s are not set in stone..

http://www.ddaudio.com/dd/caraudio/enclosureInfo/boxSize_hiEff.asp

SORRY TO JUMP OFF TOPIC!!//content.invisioncic.com/y282845/emoticons/crazy.gif.c13912c32de98515d3142759a824dae7.gif

This only works if you have the exact same proportion of box size to port area regardless of the particular box. For example...if you always use exactly 12sq in of port area per cube, then yes, you will always have the exact same change by lengthening or decreasing port length by a specific amount. DD recommends what I believe 16sq in per cube and that's what all their recommended boxes are calculated at, so all of their boxes have the same ratio of port area to air space, which means all of their boxes will be affected by a 1" decrease or increase in port length in the exact same way.

Certainly not a good idea to use this as a rule of thumb, and it will never be 1" = 1hz for any number times you do it, because the rate of change in frequency will also change over the range of port lengths due to the fact that the port is inside the box and by changing it's length you also change the boxes air space and therefore tuning is affected twice. The 'L's in a slot ported box will also make variations in the rate of change of tuning frequency based on change in port length because of the extra wood that is being used and the wood length to actual port length being inconsistent.