Port resonances killing me

[tvrgeek]

All my previous subs have been sealed, so never had an issue. Testing my new BP-4 build. Port is 4 x 11 x 22 inches. Keeps velocities down under 7 @ 120 dB for no swooshing.
But, I am getting a 200 Hz-ish resonance big time. I can move it a little, make it better or worse, just not get rid of it yet. Is one big port not clever? Should I switch to a bunch of round 4's or mix 3 and 4's? I noticed a lot more midrange comes out the port than I expected. Not important as way above crossover. It does give the second order LP response to about where my port problem jumps up. Without that it would be clean to 500 or so. I could lower the total area if big ports just don't work.

Odd things. Just can't get the BW as wide as sim. HP too low though after compensating the EQ, @ 80 dB, 1 foot, I succeeded in getting all distortion below .5% from 20 to 120. Single Titanic sealed ran as high as 5.6%

[Kornbread]

Slot port?

[Wolf]

Lay/glue some thin craft felt on the walls. That should do it.
Wolf

[billfitzmaurice]

You're keeping port velocity below 7 what? Meters per second? 18 to 20 is OK. Not knowing the full details of your build I'm assuming the port area could be greatly reduced, along with the length, which would push the port resonant frequency much higher. Midrange out of the port usually indicates insufficient damping.

[Brian Steele]

All my previous subs have been sealed, so never had an issue. Testing my new BP-4 build. Port is 4 x 11 x 22 inches. Keeps velocities down under 7 @ 120 dB for no swooshing.
But, I am getting a 200 Hz-ish resonance big time.

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)

[Steve Lee]

Another source of info on port tuning --> http://www.troelsgravesen.dk/vent_tuning.htm

[tvrgeek]

You're keeping port velocity below 7 what? Meters per second? 18 to 20 is OK. Not knowing the full details of your build I'm assuming the port area could be greatly reduced, along with the length, which would push the port resonant frequency much higher. Midrange out of the port usually indicates insufficient damping.

The only sub I built ported before, I had port velocities in the 15 M/s range and it was audible. I know the rule of thumb is below 17 but can cheat with big flairs. Guess I'll cut the side of the box off and try some smaller pipe.

[tvrgeek]

Making a WinISD model match my impedance and response, I can simulate it with the rear chamber 1 cu ft, but front 2.5 tuned to 50. Both chambers are 1 cu ft. I tried chucking a block of 4 x 4 in the font chamber and it had little effect. 2 4 inch round ports with 350W in, gives me 16M/s. Maybe I woudl never peak that high and it would be masked.

Here are the simulations of target ( blue) and what it takes to match my measurements ( yellow) Other than the above band output reality correlates except for a huge hump @ 200 only 5 dB down.
This is two RSS265HE drivers, BP-4.

[tvrgeek]

Port reduced, entire box lined with "boom-mat" an underlayment I use in my MG and TR. Will see when dry.

[Chris Roemer]

Please share your measured T/S parms.

[billfitzmaurice]

Boom-mat isn't what you should be using. It's a good material for car doors, to reduce vibration, but it doesn't have the damping qualities of polyester batts, Type 700 fiberglass or open cell foam.

[Rory Buszka]

All my previous subs have been sealed, so never had an issue. Testing my new BP-4 build. Port is 4 x 11 x 22 inches. Keeps velocities down under 7 @ 120 dB for no swooshing.
But, I am getting a 200 Hz-ish resonance big time. I can move it a little, make it better or worse, just not get rid of it yet. Is one big port not clever? Should I switch to a bunch of round 4's or mix 3 and 4's? I noticed a lot more midrange comes out the port than I expected. Not important as way above crossover. It does give the second order LP response to about where my port problem jumps up. Without that it would be clean to 500 or so. I could lower the total area if big ports just don't work.

Odd things. Just can't get the BW as wide as sim. HP too low though after compensating the EQ, @ 80 dB, 1 foot, I succeeded in getting all distortion below .5% from 20 to 120. Single Titanic sealed ran as high as 5.6%

Acoustic filters suffer from these types of additional non-ideal behaviors: vent turbulence, organ pipe resonances, section resonances, and out-of-band leakage. These behaviors can also be excited by distortion products, so filtering of distortion products and out-of-band material is not as effective as some marketing materials have claimed; bandpass enclosures normally don't do this well, but some ultra-cheap systems have tried to use the acoustic bandpass as the only low pass filter.

The best way to eliminate these non-ideal behaviors is to not excite the system at these frequencies at all, using high-order upstream filters. Distortion products from the transducer are still able to excite these resonances, so minimizing the cumulative vent area while maximizing the diameter or characteristic dimension of each individual vent will keep the port length to a minimum and avoid turbulence noise. Then the vent area should be reduced further so that a threshold of vent velocity producing wind noise is not exceeded (~20 m/s max vent velocity works pretty well for avoiding audible noise).

All these things can be optimized using a design tool like the current version of WinISD and then checked with a program like Hornresp that will model both the reflex and quarter-wave behavior of the port.

[tvrgeek]

"Boom Mat" I have is an older brand name. Not "dynamat" kind of stuff. it is a 1/2 inch fiber poly sound absorbing mat. Looks like trade names have changes as the current Boom Mat is just a mass loading sheet.

Brought vent down to 2 3/4 inches by 11 1/4. At 80 dB 1 foot; chuffs like a M&&*%^& . I was correct at 4 inches. I did get the port resonance down by 5 dB and the lining with some low density fill in the ported chamber tamed some of the higher frequencies, but not much. Still can't get the bandpass anything near right. Big spike in distortion at the tuning frequency ( 63 Hz)

I think I'll build what I know works. A nice sealed box. No vent garbage. Won't have the acoustic filtering and much higher excursion from HP down to tuning, but won't have problems either. Using two drivers will lower my excursion and so lower my distortion inherently. The RSS is stiffer and a little lower distortion than the Titanic in the first place. I'll play with the old Titanic woofer in BP and see if I can figure it out, but right now, time to "shoot the engineers and begin production" Biggest lesson is WinISD is no where near correct in BP-4 simulation. I sort of wonder if the small Vb is acting more like a port.

Drivers are a little different as one is years old and the other brand new, but not that far off.
Measured of the old one follows. Win ISD with both chambers 1 cubic foot, port tuned for 63, gives a very appropriate bandpass. I can't get close to it in reality. If I half the port, I can approach the double hump response at considerably lower efficiency, but even then, can't the high end out far enough. 80 Hz is as far as I could get the impedance peak. Should be 95. Could be these are just the wrong drivers for this design. EBP being 55. I think they were intended as woofers, not subs.

With a 20 Hz 4th order HP, and equivalent 100 Hz LP, there is actually very little difference in simulated response BP-4 to sealed. I was hoping for "magic" acoustic filtering, but not the case. Seems the port only filters in the octave above the pass band and then the dimensions are such, the port is as free a path as a DR.

[augerpro]

How are you terminating the port?

[tvrgeek]

Slot port. Square edges. No, I don't have the physical room to use nice 5 inch round ducts with nice big flairs. Very constrained space. Even so, it does not explain why I can't get the front chamber tuned high enough. I even took the end cover off so the port was nothing but the hole, and all it did was make it look like a single sealed chamber with a Q around .95, just as one would expect with only the rear chamber.

It could be that if I made two independent systems, then the relative area of the port to the front chamber dimension would be greater and it would behave like a tuned chamber, not like just a tube.