Port resonances killing me

[Billet]

Since you have such a constrained space and you like sealed bass boxes (me too!), have you considered an isobaric subwoofer?

[djg]

A "very constrained space". What does that mean? a 6" x 6" square space 8' long? Why do you make it so hard for people to help you?

[tvrgeek]

No. It does nothing for distortion. I have room for two in a sealed box.
Modeled about a dozen different drivers. None calculate any better. I don't understand why the build is not closer to the model.

I guess I can reduce the port to a pair of 4 inchers per driver with flairs. Makes for a strange shape box. That will push the port resonance up to 450 or so and with faired ends, maybe not too bad below 100 dB. Just don't get why I can't get the front chamber tuned high enough.

[oldloder]

FWIW, IME these drivers sound terrific in 1 to 1.25 cubes sealed.

[tvrgeek]

FWIW, IME these drivers sound terrific in 1 to 1.25 cubes sealed.

Yes they do, but have much higher distortion across the bass band as it more than doubles the excursion. I was measuring less than 1% @ 80 dB in the BP-4, sealed is almost triple that. About a third less than my old single Titanic, but does not meet my goal. I can go for a too small ported, eq it down, but again the ports resonance comes down too far.
Why it is so far off the model I do not know, I have one more trick to try today before I conclude it is not possible with these drivers.

[Chris Roemer]

With a pair of RS 10" subs in "1.0" cu.ft., if you failed to account for volume taken up by 2 cones and 2 (large) magnets, then your front chamber is prob. sig. undersized. Also, square edges (on ANY port exit) don't seem like a good idea. Why not run even just a 3/8" roundover bit around the opening?

Also, did you just average your T/S parms for the 2 drivers for your box sim (you only showed us data for one driver I guess)?

Using your parms, and your BP box dims, I do NOT get a "smooth shelf" BP in WinISD 0.50a7, I get a hump on the bottom end (like you've apparently ended up with).

[tvrgeek]

Two drivers, two 1 foot chambers, 63 Hz tuning. Flat. See below.
Yes, I compensated for the motors and cones.
Yes, I did radius what I could.
TS parameters for the two were close. Not enough to make a difference.
Increasing the front chamber makes the port even further impossible.
Increasing the rear chamber pulls the front further down, the wrong way making the port even worse.
As below, 2 5-inch ports are 23.75 inches, so bad news for resonance as it is back where I started around 200 Hz. Peak measured as high as the fundamental!

If you increase the front chamber to 2.5 foot, you get an impedance response closer to what I measure. Impedance and FR track pretty well. So I can only conclude, the model is quite wrong and these drivers are not suitable for BP-4. Picked up more plywood, so building a sealed box. Finally used my Horrible Freight gift card from the deadly jack stands I took back and got 8 more bar clamps, so a worthwhile day.

[tvrgeek]

Forgot to add. Both curves have 4th order @ 20 HP and 4th order LP @ 100. Close to in-use to make excursion realistic.

[tvrgeek]

OK, my last trick.
Added 10 g to each cone. Added a little stuffing as of course, the rear chamber is now too small. HUGE improvement. As I thought, this design needs a much bigger Vb so port sizes can be manageable. It now follows the LP slope to 400 Hz. and is in the noise the rest of the way.

Now I need to find something better than tape and nickels. Probably will need bigger chambers. Then measure the new TS parameters so I can see how bad it hurts efficiency.

offsets are not correct and levels and mic varied. Just looking at shape of curve. Will do HD measures next.

[augerpro]

Is the front box lined? Also, seems unlikely that adding 10g to a big woofer would improve the situation that much. Something else changed.

[tvrgeek]

Both chambers lined. Light stuffing in front chamber. Maroon curve.
Blue was as calculated by WinISD and originally built.
Nothing else changed. These "big" woofers are not that big and being aluminum, pretty light, so 10 g each is actually a big change. About to load the new parameters into the model.

OK, as expected, shifts everything lower, slightly less efficient. I think I am going to first, enlarge the front chamber to where the model says would be a peak and compare that to reality.

[augerpro]

How is it possible to for 10g to drop the entire >100hz by over 50dB? You have a loose dustcap or something? A badly resonant dust cap?

[tvrgeek]

It is what it is. Yes, the Dayton dust caps ring like a freaking bell. They are not defective. It is what it is. Rough cut the panels for a sealed box. Cut the holes and glue it up tomorrow.

[tvrgeek]

Wrap-up for now
Sealed system in place. Had a bit of a box resonance @ 200, but "stuffed" it away and got the Qtc down around .6 BP-4 was lower distortion by about 3 dB below the LP tuning as expected, so I want to continue to investigate but, the twins in place are way better than the single

Now I tested reversing one driver to lower second harmonic. I was advised this might increase odd. Well yes. But: It lowered even order by almost 10 dB and only raised odd by about 1 to 1.5 dB. A worthwhile trade I think. Above 50 Hz, 80 dB 1 meter, in room, it goes from 2.5% to under .5% Below 50 it is higher reaching 3.5 @ 30. The old Titanic was around 15% @ 30 Hz. Turns out the single O-Audio plate is more than large enough even driving 8 Ohms.

Waiting on my 20 Hz HP filer from e-bay. Even though the new Anthem Genesis software has a LF minimum correction, it does not kill the lowest subharmonics. Now back to the center and mains.

My conclusion is to get less than 1% @ 90 dB in room from 30 up would take a whole string of 12's. My goal of under .5% 30 up @ 80 dB even looks very hard.

Got some jazz cranked up. Typical over boosted bass but it is not boomy. Seems pretty tight but I need to do some good old violent Eastern European full orchestra. Or maybe some Copland to see.

[Adam_M]

Provide some more physical details of your build. Pictures or a drawing would do.

The frequency and magnitude of vent resonances like that are as a result of vent dimensions, vented volume dimensions, and the locations of both the vent and the driver. Depending on the physical layout of the box, it's possible to "null" the primary and most difficult resonance - the balance can be dealt with by a little stuffing in the box. For an example of what's possible, see The Subwoofer DIY Page - Projects : Enigma V2 (diysubwoofers.org)

I've spent a few hours working in Hornresp and it's always been just beyond my grasp. Do you have a write-up, or a link to a write-up of if/how this could be used to help model nulling the primary resonance in a standard ported subwoofer?