Anatomy of a Polk RT3

[TLS Guy]

Just wish I lived closer to you guys; I'd come over and learn something.
Rick

Rick, everyone wishes they lived closer to Walberswick House, on beautiful Benedict Lake!

 

[Guiria]

A 2" flared vent would be 12.5" long. If you don't have room for that then use non flared cardboard tube. 1.75 diameter needs to be 8.4" long and a 1.5" tube needs to be 6" long. Use the largest diameter you can fit, as that lowers port velocity. Vent velocities are 15, 21 and 30 m/sec respectively. The lower the better.

Isn't 21 and 30 m/sec getting to a point that creates chuffing?

Perhaps a slot vent would work? what would be the dimensions of a slot vent or are slot vents only used for lower frequencies?

Also with a 2" flared vent could I mount it vertically? The vent would be outputting omni-directional frequencies anyways.

Just a day or two more of questions and I will be cutting wood.

 

[TLS Guy]

Isn't 21 and 30 m/sec getting to a point that creates chuffing?

Perhaps a slot vent would work? what would be the dimensions of a slot vent or are slot vents only used for lower frequencies?

Also with a 2" flared vent could I mount it vertically? The vent would be outputting omni-directional frequencies anyways.

Just a day or two more of questions and I will be cutting wood.

For a speaker that size 30 just about gets in. However if you want to build a slot vent, then a 4 X 1 inch vent 16 3/4 inches long flared both ends will do the job, with a vent velocity of 12 m/sec.

You seem to be headed for a speaker significantly above the vast majority of commercial offerings.

Yes, you could mount one of the tube vents vertically, and that is probably what I would do

 

[Guiria]

Yes, you could mount one of the tube vents vertically, and that is probably what I would do

partsexpress and madisound carry the same 2" flared both ends port. It's total length is 11", you previously recommended 12.5". How does the shorter length change the tuning. If the change is unacceptable then I will be going for the slot vent.

 

[TLS Guy]

partsexpress and madisound carry the same 2" flared both ends port. It's total length is 11", you previously recommended 12.5". How does the shorter length change the tuning. If the change is unacceptable then I will be going for the slot vent.

I think if you go to the plastic plumbing section at a home improvement store you will find a connector that fits. You need to go there to pick up the Oatey system for gluing on the flares anyway. I think you will find you can cut the port and extend it with a coupler, or better just use the flares and get some plastic pipe and cut it the right length.

 

[Guiria]

What about an elbow in the port length? Does that create port turbulence or anything?

 

[Guiria]

I've got my design drawn up, few hours of grid paper and pencil, couple of drafts and I like the current configuration.

I'm going with the slot vent route and plan on running the slot horizontal then vertical up the back wall to get my 16 3/4" length. I wanted to mount the rockwool on the front facing side of the vertical wall directly behind the driver. The kicker is with 4" of rockwool that leaves me with 1.5" of space between the rockwool and back of the driver. That's if I don't recess the driver. (I plan to recess the tweeter). Is 1.5" too close? Will the rockwool absorb the frequencies I am trying to generate with the slot vent (74 Hz)?
I can modify the box dimension if the rockwool will be too close and get 2.5" of space between them without changing my overall net volume. Basically make the box one inch shorter and one inch deeper. Any more than that and I'll run out of space on the front baffle for mounting.

Given my design and assembly experience I am going for the simpler route of mdf all around including bracing. I will be using cross bracing against all 6 faces. Xover will be mounted underneath the cabinet in its own space.

I haven't narrowed down a finish yet.

There is a lot that goes into a DIY speaker. I am really enjoying it and I haven't even started building.

I think the key will be to follow Treebeard's advice and "Don't be hasty".

I'm off to research rockwool...

 

[sploo]

Will the rockwool absorb the frequencies I am trying to generate with the slot vent (74 Hz)?

I'd think that very unlikely. 74Hz is pretty low, and you'd need quite a lot of rockwool to really hit that.

You may find the following useful:

http://www.ethanwiner.com/acoustics.html
http://www.ethanwiner.com/density/density.html

 

[TLS Guy]

I've got my design drawn up, few hours of grid paper and pencil, couple of drafts and I like the current configuration.

I'm going with the slot vent route and plan on running the slot horizontal then vertical up the back wall to get my 16 3/4" length. I wanted to mount the rockwool on the front facing side of the vertical wall directly behind the driver. The kicker is with 4" of rockwool that leaves me with 1.5" of space between the rockwool and back of the driver. That's if I don't recess the driver. (I plan to recess the tweeter). Is 1.5" too close? Will the rockwool absorb the frequencies I am trying to generate with the slot vent (74 Hz)?
I can modify the box dimension if the rockwool will be too close and get 2.5" of space between them without changing my overall net volume. Basically make the box one inch shorter and one inch deeper. Any more than that and I'll run out of space on the front baffle for mounting.

Given my design and assembly experience I am going for the simpler route of mdf all around including bracing. I will be using cross bracing against all 6 faces. Xover will be mounted underneath the cabinet in its own space.

I haven't narrowed down a finish yet.

There is a lot that goes into a DIY speaker. I am really enjoying it and I haven't even started building.

I think the key will be to follow Treebeard's advice and "Don't be hasty".

I'm off to research rockwool...

I think you need to be careful about the amount of absorbent material you use. You can indeed kill the resonance of a ported enclosure. I would recommend 2" thick panels and cover half the area of each panel. The general starting point for a ported enclosure is to cover no more than half the surface area. This is what is meant by "fill light" in the modeling.

Fill the crossover compartment with Polyfill.

 

[Rickster71]

Rick, everyone wishes they lived closer to Walberswick House, on beautiful Benedict Lake!

It would be too hard for me to concentrate on speaker building, with that kind of scenery.
Rick

 

[Guiria]

Just posting the actual build link thread to tie the two threads together.

http://forums.audioholics.com/forums/showthread.php?t=47566