Skinny line array port issues

[jvphotog]

Hi all. I have a 39” x 11.5” x 4” speaker with (6) DMA 90s in it- theyre 3.5” full range drivers. The cabinet is 1/2” mdf so it should be slightly less than .7ft3. The port calcultor I used says a 2.75” port at 1” should put me around 90hz. However DATS says it’s 65hz. I pulled out the port and was left with a 3” hole that’s effectively a 1/2” deep port and DATS measured that at 70hz. Why are my measurements so far off from the calculators? My only guess is that the inside of the cabinet is 3” so possibly my port is acting more like a slot port..? Any help is appreciated!

[billfitzmaurice]

For 90Hz tuning in 0.7 cu ft I get a 4" diameter 3" long port. Smaller will have too much velocity. I wouldn't tune it that high, it gives a 4dB hump at 100Hz. 80Hz gives a 2dB hump, which is tolerable. Because of the shallow depth I'd put it on the top or bottom. To prevent quarter wavelength resonance the end opposite the port should be generously damped. Or you could take advantage of that, leaving one end open, making it a transmission line with a 90Hz Fp, although it would be better if tapered.

[jvphotog]

The ID of the cab is 3”, so nothing bigger than a ~2.75” port will fit! They’re going to be rear surround speakers, crossed at 90-100hz, that’s why I tuned it so high. I was going for max output above 90hz and Dsp can kill that hump. I could install two ports?

Edit: I’m 3D printing the port, so it doesn’t have to be round… I could make a slot port in the hole I already have

[jvphotog]

As an experiment I printed out a 2" port at 1" length thinking that maybe the opening of the port was too close to the inside walls. .7ft3 cab with a 2" port at 1" should be around 70hz. Dats measured this new port at 57hz! What the heck is going on?

[Chris Roemer]

You're measuring the "valley" (between the 2 peaks), right?
Show us a screen shot of your DATS plot, please.

A PAIR of 2"id ports that are 3" long should work for EITHER the 4ohm or 8ohm vers. of that driver (you didn't specify - which are you using?).
Should give you a tuning in the low 30s.
With THAT tuning, the DMA-4 should "do" 70Hz (F3).
The -8 should do closer to 60.

[jvphotog]

This is with the last 2" ID port @ 1" long.

Edit: Theyre DMA90-4s

[jvphotog]

I cut out the circle on the back (for the port) and decided to try a slot port. I put a 3" x 4" port at 1.5" and again it was way below what the calculators said. I took out the pieces of wood so now I just have a 3" x 6" hole and DATs is measuring 76hz.

[jvphotog]

This is the speaker in question with the now giant hole cut in the back!

[billfitzmaurice]

And now it's a transmission line. You may be getting pipe harmonics above the quarter wave frequency. If that's the case you'd fill the line with polyfill to damp them.