FR125 MLTL slotted port question

[valleyman]

Hi all

I received my new FR125s today and want to set about building a pair of GMs MLTLs for them. Specs as follows:

45 Hz ML-TL, driver at ~ear height (~+/- 2 dB):

L = 39.56"
CSA = 43.68"^2
driver down ~3.3"
vent = 3" dia. x 3" near/at the bottom
stuffing density = ~0.3 lbs/ft^3 from top down ~30"


I want to substitute the round port for a slotted port at the base of the speaker so that the bottom of the speaker would become the bottom of the port.
The port should be 3" diam x 3" long. Port area is therefore 7in^2 so 7" would be a convenient internal width for the cabinet with a 1" high port. This leaves the box 6.25" deep. All fairly simple so far I hope

I read in a post that slotted vents whose dimensions exceed a ratio of 9:1 start to become resistive. I'm not sure where the figure comes from, but if it's true I'm ok.

My problem is that I also read that using the edge of the cabinet as part of the port messes things up. Information about this seems scarce, and I have seen designs that do use the edge of the box as part of the port (Godzillas slotted fe127 box as an example).
I can see how using the edge of the box would cause problems, as air would only be able to flow in and out of the vent from one side rather than from all directions as would usually be the case.

Is this something I need to be worried about or should it be fine?


Thanks

[Scottmoose]

It's true. You should be fine with a slot vent.

[Godzilla]

http://www.zillaaudio.com/fostex-127e-slotted-box.htm

When i built this cabinet i was thinking of a few things. There was a lot of discussion about a reference project using the 127e at the time. I wanted my speakers to be smaller (dreaded WAF). I also wanted to maintain decent bass response. Keeping the baffle wide as well as placing the speaker close to the rear wall seemed to improve bass and overall sound. I nearly put a round port on the bottom front but liked how the wood looked and did not want to hide it behind a grill. I nearly put the port on the rear but decided this may not work well if putting the speakers close to the rear wall. When i calculated the slot the side seemed to be the logical choice based on its size. I always guess when figuring the slot. Really all i do is convert the round port into square using WinISD. Then i figure where it will fit best in the cabinet. Among other projects, I have been thinking of doing a Hammer Dynamics 'clone' (using the 12lta )with a slotted port at the bottom. For me, the slotted cabinet works well. Having the slot on the side allowed additional tuning possibilities in my living room, where the speakers still sit, small and out of the way. Wifey never complains about them. And they light up the room with sound. Eva Cassidy comes alive! But Moody Blues just don't sound as full as they should. These speakers work better in smaller rooms and with less demanding music. No tweeter on the back either.

Building this cabinet was so easy.

[valleyman]

Ok, thanks guys. I'm guessing then that the slotted port will be fine and I'll go for it. Worst cast scenario, I can chop off the bottom inch and put a round port in.

A quick question about bracing... I'm guessing that braces having their thin edge along the length of the line would interfere least with the quarter wave action, am I right?

[Scottmoose]

Do long as you don't go overboard window braces shouldn't cause problems. A vertical brace is a good plan though.

[valleyman]

Great, building and report will hopefully come next week. Thanks

[GM]

Greets!

I don't know if the >9:1 began as slot diffraction theory or actual measurements, but it's true.

Right, the floor and walls of the cab increase the acoustic length of the port, so the vent's top plate will be somewhat shorter to maintain a given Fb.

Anyway, if the attached is accurate, then our standard vent has a 0.732 end correction, so a 3" diameter x 3" long tube has an acoustic length (Le) = radius*0.732 = 1.5*0.732 = 1.098 + 3 = 4.098". For the shelf vent it's 1.5*2.227 = 3.3405 + 3 = 6.3405", so the equivalent requires a top plate length = 3 - (6.3405 - 4.098) = 3 - 2.2425 = 0.7575".

In short: 4.098 - 3.3405 = 0.7575", so a baffle thickness cutout is close enough IMO.

A common formula derived from corner loading yields a considerably different result though:

Le = 3 + 2*(8/(3pi)*((7.068/pi)^0.5)) = 3 + ~2*(0.8488*radius) = 3 + ~2*(0.8488*1.5) = ~5.5464"

~5.5464 - 4.098 = ~1.45" long

Not sure if this is the correct way to compare them though, so trying them both (or some length in-between seems the prudent thing to do if you don't do any measurements. I always did impedance traces and at this point in time only remember they were somewhat shorter than standard vents.

GM

[GM]

Forgot the 'attached':