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Building new slot ported box

 

Bronze Member
Username: Boopers

Lima, Ohio United states

Post Number: 59
Registered: Jul-04
I'm thinking of building a new slot ported box for my 12" speaker. I was thinkin of dropping the height like 3 or 4 inches and adding that to the width of the box. Only real problem I have is how will that affect the slot port, how would I adjust it to keep the same freq, or should I just tune it to a different freq. Not sure what freq it's at now so maybe I should just set it at what would sound best regardless?
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 1134
Registered: May-04
Keep the surface area of the slot port the same and you'll be ok.
 

Silver Member
Username: Jmloughrey

Farmington, CT

Post Number: 129
Registered: Jul-04
surface area, what a wonderful thing...
 

Bronze Member
Username: Fryguy

Barrington Passage, Nova Scotia Canada

Post Number: 54
Registered: Jun-04
I think you mean the port volume.
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 1137
Registered: May-04
I meant the surface area and exactly that. With the surface area and length the same, the port volume will also be the same, but port volume isn't what tunes an enclosure, the friction of the port's surface area is what tunes it. Example, a 5" port tuned to 35 hz will have more port volume than a 4" port tuned to 35 hz, yet the tuning frequency is the same, while the length and surface area are different.
 

Anonymous
 
You tell 'em Jonathan!!!
 

Bronze Member
Username: Fryguy

Barrington Passage, Nova Scotia Canada

Post Number: 62
Registered: Jun-04
width x height = surface area
width x height x depth = volume

If it was the surface area that determines the tuning of a port why is it that if you blow into a tube (bottle) with a different volume (Internal size) that the tone is different? Also if it was the surface area then you could add ribs to the inside of the port to give more surface area. Just a thought.
 

Bronze Member
Username: Fryguy

Barrington Passage, Nova Scotia Canada

Post Number: 64
Registered: Jun-04
The larger the port the longer it needs to be to have the same tuning. too small of a port and the air velocity it too high and you will get alot of port noise. so you have to determine a few things first. what frequancy do you need it tuned to? whats the port opening area are you limited to?
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 1144
Registered: May-04
My point was that if he expects to keep the same tuning frequency and port length, the surface area of the slot port has to be the same as the round one. The volume has nothing to do with it. He could double the surface area and shorten the length of the port for the same port volume, yet his tuning frequency would be higher. Which is why he can't match the port volume, he has to match surface area if he expects to keep the same length. http://www.jlaudio.com/tutorials/ports/index.htmlfrequency This explains my point quite well.
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 1145
Registered: May-04
http://www.jlaudio.com/tutorials/ports/index.html
bad link above.
 

Bronze Member
Username: Fryguy

Barrington Passage, Nova Scotia Canada

Post Number: 65
Registered: Jun-04
We are talking about the same thing. i just mis-read what you said. I thought yau had meant that the tuning was just about the port opening size. But i now get that you mean that if he/she is to get the same port frequancy as before he/she needs to have the same size. We are both right i just took the explination too far! sory for any confusion i may have caused.
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 1146
Registered: May-04
By the way, tuning frequency doesn't work like blowing in a bottle. The purpose of the air in the port is to control the cone excursion within the tuned frequency range, by means of resistance to motion. Blow air in a port, I doubt you'll hear much of anything, you're blowing a constant feed of air, subwoofers on the other hand pulsate at certain frequencies. This creates resonance. When air is moved at the tuning frequency, resonance occurs inside that air in the port, which is what increases SPL at that frequency. If you run WinISD Pro, you'll see that the cone excursion is at it's least at the tuning frequency. When you tune a port, you are tuning the air's resistance to motion, and the resonance of the compliance of air within. This is why smaller ports need less length to tune to the same freq., they offer more resistance to motion. This is also why surface area of the port is the primary factor in determining tuning of an enclosure, the cross sectional area of the port determines how much resistance can be offered, only when you know this can you choose the correct length. Last thing, adding ribs inside the port is going to decrease the surface area.
 

Bronze Member
Username: Fryguy

Barrington Passage, Nova Scotia Canada

Post Number: 66
Registered: Jun-04
It was an example of the principle of how it works, is not far from how blowing across a bottle reacts. Oh and the ribs thing was a joke :-D chill. I didn't want or mean to start any flames.
 

Bronze Member
Username: Boopers

Lima, Ohio United states

Post Number: 61
Registered: Jul-04
lol, you all start getting all worked up. I can't use WinISD because my speaker isn't listed and I don't know all the specs to enter into it. also I don't know what the port is tuned to now, but if I give you the measurements would you be able to tell me? or is there just a good freq I should use regardless?
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 1150
Registered: May-04
I wasn't really worked up, just proving my point. Sometimes I tend to be a bit extreme, though, lol. What kind of sub do you have? and yes, if you give measurements of the port and the box it's in, how thick the wood is, and the type of sub, we can figure the tuning freq. and how big the slot needs to be. There may be a better frequency to go with depending on what you want, but we'll consider that after you give us the dimensions.
 

New member
Username: Specboatteam20

Post Number: 1
Registered: Aug-04
Plus, that whole blowing in a bottle has nothing to do with a port, it is the resonant freq based on the volume of the bottle.
 

Silver Member
Username: Fryguy

Barrington Passage, Nova Scotia Canada

Post Number: 103
Registered: Jun-04
duh...but its the moving air that makes it work.
 

Bronze Member
Username: Koz1031

Monticello, In United states

Post Number: 46
Registered: Jul-04
I've ran into the same problem as jermie, I can't use my WinISD because I haven't got all the figures. I bought used subs, no sheet, and the site doesn't give them either, Can you help me out here Jonathon or Glass or anybody
 

Bronze Member
Username: Koz1031

Monticello, In United states

Post Number: 47
Registered: Jul-04
I've ran into the same problem as jermie, I can't use my WinISD because I haven't got all the figures. I bought used subs, no sheet, and the site doesn't give them either, Can you help me out here Jonathon or Glass or anybody
 

Gold Member
Username: Jonathan_f

GA USA

Post Number: 1288
Registered: May-04
What kind of subs do you have? You can usually email the manufacturer for optimum boxes or thiele-small parameters, or both.
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