Help with ported box that turned out tuned too high

[morbo]

I have just finished the cabinets for my budget 2way speaker using a TB 871 and Dayton RS225 per side. My bass alignment is meant to be overdamped, tuned to about 23-25hz in a 70-75L box. To this end purchased one of these flared ports:

http://www.partsexpress.com/pe/showd...number=269-932

The price and aesthetics were right, and the generous flare should help with vent velocity right? Well maybe I jumped the gun a bit. These are labelled as 3"ID, which they are in the center. Modelling them in BassBoxpro as a 11.5" dual flared 3" port gave me a tuning in the low 20s. However after measuring the woofer in-box with the port installed, I am getting peak port output somewhere in the 30-35hz range (33 I think). Seems like this port should be modelled as closer to a 4" ID. This is not ideal, as these speakers are meant to be used for low-level (apartment) home theater use, and after testing them in this application, I can see that there is a lot of information below the tuning frequency, and on some scenes the cones are flopping around like crazy. To their credit the RS225s seem to be near-bulletproof, but I am concerned that this will be an issue for the eventual owners of these speakers.

Does anyone have any practical ideas for how to lower the tuning frequency, bearing in mind that I've already made the huge 5 3/4" cutout for this port, and the heavy flare at the back end? My enclosures are about 16" deep internally, so I have 3-4 more inches at most behind the port to work with. So far the best I have thought of is to extend the port by gluing bristol board or somesuch to it.

[kelticwizard]

The port tubes have a 3" inch internal diameter.

I would cut the port tube in half, get 3" outside diameter PVC pipe suitable for fitting between between the halves. Then, I would get a 3" diameter elbow for putting the 3" outside diameter PVC pipe into.

If you cannot find a 3" PVC elbow, I would cut a piece of 3" PVC pipe at a 45 deg angle and joining it with PVC glue so it makes a 90 deg angle. Or better, cut two 22.5 deg angles for a smoother port. Then you can make the port as long as you need to.

By the way, what is the internal volume of your box? If you don't know, how aobut the external dimensions plus the thickness of your building material?

[morbo]

Thanks kelticwizard, that was my first thought, cut down the center and extend with a 3" piece of PVC. So I went to the store and looked at some 3" PVC.. but it is noticeably thicker walled than the port tubes I have. My concern here is that the internal diameters will be significantly different, and will result in a 'step' inside the port where my flared port meets the 3" PVC. I don't know enough about the physics involved to tell, but it seems to me this step would be a bad thing and could introduce turbulence in the vent.

[kelticwizard]

Good thinking. I don't know the physics that well either, but you don't want turbulence, that's for sure.

Don't know what to say other than PVC is soft stuff and it might not be too hard to work it so it meets the edges of the flared pipe smoothly. I haven't sculpted vinyl, but it so soft that I think you can find a suitable tool for the edges of your PVC tube to meet up smoothly with the halves of the flared vent you bought.

[kelticwizard]

Another possiblity is to cut short the PVC pipe you are making the angle with, then use epoxy glue to smooth over the junction. PC-7 and PC-11 are two excellent products for working with PVC. So is Bondo auto body filler, which might be cheaper. With your elbow made, just glue the 3" pipe onto your extensions. If you do that carefully, the two pieces should fit up smoothly-you won't need to smooth those junctions.

[morbo]

that is certainly an idea, I suppose I could file or sand down the offending edge so it is a slope rather than a step. I will have to buy some of the PVC and investigate further.

Does anyone know anything about modifying port tuning with straws or any other technique? I thought of filling the port with straws and tuning by selectively 'filling' them one by one with foam or something.... not sure that this would work though.

[kelticwizard]

Apparently they do make 3" 90 deg elbows, so you won't have to do that angle cutting.

England might be a little long to send for them, lol, but i'm just including the page to show they are not an unusual item. I am sure they are available where you are.

http://www.koicarp.net/pvc_plumbing/pvc.html

[kelticwizard]

When you fill the port with straws or some kind of stuffing material, you somewhat cut the port action. Using an electrical analogy, an open port is like an inductor, a stuffed or straw filled port is like an inductor with a resistor.

It somewhat blocks the action of the port, makes your cone move a little more than it would with an open port, but some people prefer it. There is certainly nothing wrong with trying various materials and seeing how you like it-some systems are built that way.

[bzdang]

To reduce port area. If this looks like it might fit, attach the dowel through the back.

[morbo]

So in effect you start to move closer to an aperiodic design? I think it will be a last resort due to aesthetics, but I may also try that if I have trouble getting the results I need with the the PVC.