Dual Maelstrom Subs, Port & Alignment Questions.. (1 Viewer)

[Mike Keith]

OK, I've tried the tuning by watching the cone movement thing and it seems to be more difficult than I first thought. I can’t tell the difference in cone movement between a 6" X 10.72" port (20Hz) and a 6" X 30" port, so how could I possibly fine tune a inch or so?

I was using a 20 Hz Test tone for this preliminary test but haven’t tried the Sweep yet, however I cant imagine having any better luck when I cant tell any cone movement differences between such a great length differential.

What I did find out though is that a 6" Port will simply not work with a Maelstrom sub no matter the length of the port. I get severe cuffing with the shorter port and far less with the longer port, but it's still audible, so I’m back to a 8” port.

Also I tried a digital multi-meter (Fluke 77) at the speaker terminals to find the lowest impedance and in a word, "Impossible" it can't be done practically, maybe a analog meter would work but no way any digital meter will update fast enough to see the readout while a test tone is playing.

I wish WinISD would let you change the port length just to see what was going on when I tried the different lengths.


Questions;

My Win ISD modeling says the port velocity to be at 115 feet per sec with a 6” X 10.72 port in a 355L enclosure, my question is how fast is too fast as far as port velocity is concerned (obviously 115f/s is far too fast)?

What’s the max group delay for a sub at 80 Hz and down (I’m at 32.3 ms at Fb according to the modeling)?

Besides Transfer Function Magnitude, Port Velocity, and Group Delay is there any thing else I should pay special attention to?

 

[Kyle Richardson]

Try the Avia test tone sweep. Its handy because it displays the frequency on the TV as it sweeps.

 

[MichaelAngelo]

"An 8" port tuned to 20Hz in 340L is only ~22.5" long, and its "organ-pipe" resonance is at 245Hz, "


Jack,
How did you arrive at that freq? Is there a formula to determine "organ pipe" resonance?

Thanks for any help,
Mike

n/m:: found it

 

[Mike Keith]

Kyle, do you know the volume displacement of the Maelstrom so I can figure this into the net volume, I’d like to get as close as possible to the actual length of the port.

Additionally Does Win ISD account for the volume of the port, if not the modeling idea is a complete waste, as the net volume changes the port length, so every change (decrease) in volume lengthens the port which in turn decreases the volume, and again lengthens the port.

 

[Mike Keith]

I've made a jig that allows me to change out ports on the fly so to speak. My first model used a several 6” diameter port's and they all had port noise ranging from severe to medium, the longer the port the less severe the port noise. Then I tried an 8” diameter port and to my surprise, I still got some audible port noise although it's probably not audible in a room while playing music. So I went back and checked my WinISD modeling and found that I'm right at 19m/s with the 8” port and the 6” ports had anywhere from 35m/s (shortest port 8”long) to 29m/s for the longest port (30”).

So based on this I believe that 17m/s is a good max level, also I was playing a pure 20Hz test tone at very loud levels to get this port noise with the 8” port at 19m/s velocity, so this probably wouldn’t be a problem with any music material.


Conclusions:
The lower the tuning frequency the longer the port and the slower the air velocity.
Also the box size seems to be proportional to the air velocity, the larger the box the higher the velocity on any size port and although the FR looks more extended on the larger box the group delay goes up and the port velocity becomes a real problem unless you tune it ridiculously low (15Hz). In my case with the 355L enclosure I can't get the velocity down below 19.7 m/s if I tune it to 20Hz with a single 8”X 21” port which is about as low as practical for the Maelstrom sub. Now when I cut the enclosure volume to about 280L all things in the port velocity area look much better, also group delay goes from 33.5 ms to 28.6 ms which is much better. In other words my enclosure is too big for a 20hz tuning.

So I will cut off about a foot of the 24” X 48” dia Sonotube making it 283L, then if there is no port noise I'm back to trying to find some test tones or Sweep Tones that go down below 20Hz, I have 3 different test discs and none of them have any test tones below 20Hz. I know that WinISD has a signal generator but no way I can strech a 100' RCA connector across my house, and I don't want to drag this huge beast into my computer area.

Any one else have similar experiences to back this up or refute any of my findings?



I would appreciate any feedback.