Port Questions

[hjmiller]

Thanks to all for the advice provided in the last two weeks regarding my ported sub project. I have pretty much finished the cabinet design for my sub. It will be prism shaped to fit tight into a corner. Someone provided images of their design that was used for inspiration. The sub will be about 170 liters, tuned to roughly 20Hz. I'm using an eD 13O2 driver and an eD LT350 amp.

OK my questions. Based upon the size of my box I was planning on using two 4" ports with flares roughly 25" long or so. The inside height of the sub is 30". My first question is in regards to port velocity. I am below 17 m/s until under 20Hz. My peak is probably right around 22 m/s. Is this a major risk for port noise? If I go to a 6" port I'll have to add an elbow which has a negative impact on noise as well. I know some are more conservative than others in regards to maximum allowed velocity, but with the dual flares I figure I can probably push the 17 rule at least a bit.

Inside I have 4 to 5 inches of clearance above the port, both the driver and ports will be down firing and roughly 4" off the floor. Do these clearance seem adequate?

[Mike P.]

I am below 17 m/s until under 20Hz. My peak is probably right around 22 m/s. Is this a major risk for port noise?

No.

Inside I have 4 to 5 inches of clearance above the port, both the driver and ports will be down firing and roughly 4" off the floor. Do these clearance seem adequate?

Inside yes, outside, not sure. If the sub is prism shaped to fit tight into a corner, then the sound wave can only radiate from the front of the sub. Here is a diagram from Collo to make sure the area is big enough. Post #10.


http://www.hometheatershack.com/foru...html#post65272

[hjmiller]

By" tight to the corner", I will still probably still have 3 to 6 inches of clearance from either wall. Again the ports and driver are planned for down-firing orientation, with 4" legs to keep it off the floor.

One other question regarding the ports is related to the orientation of the ports relative to the driver. Is there any advantage as to the orientation (facing towards, away, or perpendicular) of the port relative to the driver. I was thinking about when I line or stuff the sub, how careful would I have to be to leave a free air path through the center from the driver to the end of the port, and how that distance may or may no affect the sound.

[Mike P.]

I was thinking about when I line or stuff the sub...........

It's one or the other. Lining a ported sub with egg crate foam dampens the sound wave to control boominess. Stuffing a ported sub with polyfill has the affect of making the cabinet seem bigger to the sub. Adire Audio used to recommend this for some of their designs. A ported sub would only need to be lined if the cabinet is big enough.

[hjmiller]

Never really got a complete answer on my port sizing questions. Looking at most of the commercial sub designs it seems that for a 12" driver you rarely see any more than 2 - 4" ports. Modeling of my design, using the 17 m/s rule as a limit I show I need either 3 - 4" ports or a single 6" port.

Are the DIYer's in general just more meticulous about port noise than the commerical designs, or is there something else specific about their (the commercial designs) design constraints (smaller boxes typically, maybe less Xmax on the drivers they use?, maybe more liberal use of bass boost to get more output from the driver and less dependance on the port at lower frequencies).

[collo]

Many commercial subs appear under-ported because they're tuned up around 30hz. Ports are much better behaved at 30hz than they are at 20hz.

As you move to more expensive subs, the tuning is lower, and the issue is addressed via more ports ala SVS, passive radiators ala Deftech, or sealed with equalisation ala Velodyne.

The amount of portage we diy'ers use depends on the application. For music your peak velocity is at a higher frequency than it is for home theater operation. If you run HT in the typical household, you have kids, wives and small animals that don't like the floor to move, so you're in the same boat as for music. If this is you, go for the pair of 4inchers.

For real HT though, you just know that when things get subsonic, you don't actually want to hear anything. If you can fit it, I would go for a six.

Have a play around with flare-it to see how diameter and flares affect things.

Here's your dual 4inchers





Three fours....





And a single 6incher

Each case modelled with 30mm flares. Even though a six has less area than 3 fours, it can run at a higher velocity....

[Ricci]

Collo,
Thanks for that program link. It is nice to have! I have a question for you...What do you think about the Polk power port system? I assume that you have heard of it since you are the port authority? I'm using it in a design that is being built for me. I didn't know if you had played around with it?

[hjmiller]

Thanks for the feedback Collo. I am probably just going to stick with the dual 4" ports as that fit the best in my design. Realistically I will rarely be pushing this anywhere near the limits but want to do it right.

My next question is in regards to tuning. Should I go with a tuning of somewhere near 20 Hz which gives me a very flat response, or tune down to 18 Hz or so which will give me some slope from 30 down to the lower frequencies. I know that with room response I'll get better some additional output at the low end and in theory if your sub is modeled perfectly flat you may get some extra response on the low end (boomy right?). I'll attach the FR for both alignments and let me know what you think. Construction wise going down to 18 or lower requires an elbow which isn't the end of the world. I have also included a picture with one port shown how they would look at 20 Hz and one port shown how it both would look at 18 Hz. Any feedback on my plan or bracing is welcome as well.

[collo]

I would go for the higher tune. You've got good extension to 20hz and leaving out the bend saves on cost, build time and possible turbulence.

Your bracing looks fine and designing for the corner of the room is something I'm sure others will follow with interest. Nice drawing BTW.


Regarding the Polk port, it's useful where you dont have the full port diameter clearance between the port exit and an adjacent wall. It would be possible to adjust the curves of the 2 pieces so that velocity is unchanged until reaching a flare at the end....

[hjmiller]

The higher tune was my original thought but then I started doubting myself. Think I will stick with that plan. I was a little worried about room gain but my room is large and open so designing flat as possible is probably the right way to go.

Someday when I get the itch I could always get a driver more suited for the deeper extension and create a real beast. For now this will be probably more than I can really handle without dirty looks from my little girls and wife.

Thanks for the compliment on the drawing. I'm a mechanical engineer so I have access to nice CAD tools and the know-how to use them. It is really nice because I can get extremely accurate volume measurements from the tool.

I have the itch to just have something to listen to in the system and am thinking of just throwing together a simple sealed box this weekend if I can find time. I have a plan in my head for making a roughly 15W X 16H X 22D box out of a 1/2 sheet of MDF if the big boxes have that. Might be worth just going through that exercise before trying the bigger ported box, plus it would cool my urge to rush anything.

Thanks as always for all the feedback.