Fb Enclosure tunning Vented

[webleather]

what Fb should I tune my ported box to?

I have a 12" Kicker s12L5-4, Kicker web site suggest a box with 3.25 Vt after 13.25 x 2 x 14.5 long port gives it a Fb=37, where did they get the Fb=37? and why is that best for my Sub? I could not come up with how they calculated it

Also I dont want to rip my sub apart so if tuning to Fb=37 what are my + and - limits on the Fb?

Stuart

 

[BassAwdyO]

Welcome to DIYaudio...

Kicker's reccomended enclosure size and tuning are a generic reccomendation for the average joe who knows little or nothing about enclosure design to build his own box. The performance should be OK, but nothing too exceptional.

I'm guessing you dont know too much about ported enclosure design, but here are a few basics.

Ported enclosures work on the helmhotz resonator princaple. They consist of A. an airspring and B. an air mass. As you may or may not have learned in physics a spring attached to a mass will have a certian resonant frequency. That frequency depends on A. the stiffness of the spring, and B. The effective mass.

With a ported enclosure air inside the enclosure is the airspring and the air inside of the port is the air mass. This is why you do not include the volume occupied by the port when calculating enclosure volume.

The stiffness of the airspring is directly related to the volume inside the enclosure. If you increase the airspace, the spring becomes weaker. If you decrease the airspace, the spring becomes stronger.

The effective air mass in the port depends on two things. A. the cross sectional area, and B. the length. If length is constant then a bigger cross section will result in less effective air mass, and a smaller cross section will result in greater effective air mass. With cross section constant greater length will result in greater effective airmass, and less length will result in less effective air mass.

With me so far?

So in essence the tuning of the enclosure depends on 3 things.
A. internal volume
B. port cross section
C. port length

So using their 3.25cubic feet with a port cross section of 13.25"x2" and a port length of 14.5 inches they calculated the resonant frequency to be 37hz. They were a little off because the actual tuning is ~33hz

I dont know the actual forumula off hand, but winISD has calculators that do it all for you and are pretty accurate.

Where you should actually tune your encloure depends on application and personal taste. I prefer tuning below 20hz as vented enclosures have massive group delay and high tuning sounds boomy(to my ears anyway). Where is the enclosure going? A car I'm guessing... What kind of music do you listen to?

 

[Ron E]

The effective air mass in the port depends on two things. A. the cross sectional area, and B. the length. If length is constant then a bigger cross section will result in less effective air mass, and a smaller cross section will result in greater effective air mass. With cross section constant greater length will result in greater effective airmass, and less length will result in less effective air mass.

Incorrect. More port volume is always more mass, think about it. When you alter the cross sectional area you are really changing the stiffness beacause a larger piston displaces more air. The stiffer it gets, the more mass it needs...... are you with me so far?

Nobody can tell for sure how a manufacturer came up with a recommended alignment. They could have decided that they didn't want to use more than x amount of volume, then varied the tuning frequency to get a target curve that may or may not be flat. THere may or may not be rational reasons behind it and some of their assumptions may not apply to you as a consumer.

I don't typically calculate boxes for people anymore, I might check their design, but not even that unless they provide T/S parameters. Part of the hobby is learning how the software works. Slow down, play around and come up with a design. Ask for peoples' advice, you might learn something you wouldn't have otherwise. You might soon find that you know more than the people answering your question.

 

[simon5]

He said "effective airmass", so he's not really wrong.

 

[Ron E]

"Effectively", you can put any woofer in any box with any size port and it will make sound. Why do we bother to understand these things or to precisely formulate sentences about them anyway?

"Effective" mass (due to x-sect area changes) is not a concept that has any meaning - the volume of the port plus end corrections IS the mass of the port "Mmp".

 

[BassAwdyO]

The volume of air in the port and end corrections are the only thing that matters in it's airmass? And if you increase port cross section that will decrease the stiffness of the airspring? I think that's what you're saying... I've never really thought of it like that and nothing I've read said that before, but it seems to make sense.

When you alter the cross sectional area you are really changing the stiffness beacause a larger piston displaces more air

what piston are you talking about?

 

[Ron E]

1. The volume of air in the port and end corrections are the only thing that matters in it's airmass?

2. And if you increase port cross section that will decrease the stiffness of the airspring? I think that's what you're saying... I've never really thought of it like that and nothing I've read said that before, but it seems to make sense.

3. what piston are you talking about?

1. Yep

2. Increasing the port cross sectional area increases the stiffness of the port/box system, which is why the port must be made longer to tune to the same frequency.

3. The port acts like a solid slug of air in T/S theory. The port is like a PR with infinite suspension compliance. Using a larger piston area means it will be harder to push it in - the stiffness increase in answer 2.

 

[BassAwdyO]

Thanks ron, you learn something new every day!