Recently I built some 15" coaxial surrounds (Vortex 15) for my home theater and I'm having an issue with what I believe to be standing waves in the box. The boxes are 48.5x17x12, ported and tuned to about 35Hz.
I ran an impedance sweep on the finished design and this is how it looks with no stuffing.
This is approximately what I'd expect. Resonance due to a standing wave of the longest dimension right around 140Hz. So I messed with putting a block of safe n sound insulation in the middle of the box, as well as trying a pillow in the middle of the box, and while they kill the resonance at 140Hz, a resonance shows up at 100Hz. See below.
It doesn't look that bad on the impedance sweep, but it is audible. Here's a nearfield sweep in REW. I plugged the port for this to make sure I wasn't getting any port weirdness.
I've built other box sizes with these same drivers/crossovers before, and didn't have issues, so I'm ruling out any issues with the drivers or crossovers. I built four of these new boxes and all of them exhibit the same behavior. All of them have drivers and crossovers pulled right out of the old boxes into these new ones.
I'm kind of at a loss of what to try next. I've done testing with the port plugged to make sure the port isn't the issue, and this issue is independent of the port. I've tried adding clamps and boards to the outside of the box to add more bracing to make sure it's not a panel resonance, and I haven't been able to eliminate it that way either. I'm hoping people on this forum will have some suggestions.
My game plan after work is to try putting a pillow right behind the driver, in addition to a pillow halfway down the box, and maybe some loose polyfill in the bottom of the box. But maybe you guys have some other ideas that could help.
Thanks for any and all help.
I ran an impedance sweep on the finished design and this is how it looks with no stuffing.
This is approximately what I'd expect. Resonance due to a standing wave of the longest dimension right around 140Hz. So I messed with putting a block of safe n sound insulation in the middle of the box, as well as trying a pillow in the middle of the box, and while they kill the resonance at 140Hz, a resonance shows up at 100Hz. See below.
It doesn't look that bad on the impedance sweep, but it is audible. Here's a nearfield sweep in REW. I plugged the port for this to make sure I wasn't getting any port weirdness.
I've built other box sizes with these same drivers/crossovers before, and didn't have issues, so I'm ruling out any issues with the drivers or crossovers. I built four of these new boxes and all of them exhibit the same behavior. All of them have drivers and crossovers pulled right out of the old boxes into these new ones.
I'm kind of at a loss of what to try next. I've done testing with the port plugged to make sure the port isn't the issue, and this issue is independent of the port. I've tried adding clamps and boards to the outside of the box to add more bracing to make sure it's not a panel resonance, and I haven't been able to eliminate it that way either. I'm hoping people on this forum will have some suggestions.
My game plan after work is to try putting a pillow right behind the driver, in addition to a pillow halfway down the box, and maybe some loose polyfill in the bottom of the box. But maybe you guys have some other ideas that could help.
Thanks for any and all help.
Hm. I don't think so? But I don't actually know. Any type of stuffing I put in that spot makes the resonance happen at 100Hz. I might try moving the stuffing above the port and see if it changes anything. Or try both.
Here's what the box looks like. Right now the big block of stuffing is under the port by about 5-6in.
Here's what the box looks like. Right now the big block of stuffing is under the port by about 5-6in.
Given your box shape it is an ML-TL not a BR and you likely are fighting the quarter-wave resonance (lower than 70 Hz given the vent) and not taking advantage of it.
dave
dave
Hi, driver is probably top of the front panel? This would maximally excite the mode, while driver exactly at the middle would not, almost at all. I also guess the port is at the other end? A fix might be to put the port (inlet) at the top section as well, and fill the bottom of the box to dampen the resonance.
Resonance moving down in frequency would indicate longer path length, perhaps the safe'n sound in the middle makes like a wall, sound goes around it instead of attenuating.
So perhaps easiest is to put damping on the walls instead. Perhaps fill in the whole bottom half of the box.
edit. well images appeared before sending this 😀
Resonance moving down in frequency would indicate longer path length, perhaps the safe'n sound in the middle makes like a wall, sound goes around it instead of attenuating.
So perhaps easiest is to put damping on the walls instead. Perhaps fill in the whole bottom half of the box.
edit. well images appeared before sending this 😀
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Is that just due to the tall skinny nature of my box? It was intended to be a BR box tuned to 35Hz and the impedance sweep shows that behavior.
Hmm, assuming the vent is the hole below the driver, then there's the driver/floor 'bounce' and a secondary vent/floor one, which creates a mid-bass notch in the response.
The impedance sweep with the resonance at 100Hz has the bottom 1/3 of the box filled with polyfill, plus a pillow at just below the halfway point, also filled with polyfill. Then some mattress topper stapled on all walls. Though I think the mattress topper probably isn't thick enough to really do anything...
Yes, the shape. When any one dimension starts to be significantly larger than teh other 2 the box transforms from a BR to an ML-TL and needs to be modeled as such. HornResp is the commonly available tool for that.
dave
Interesting. Would this still show up with the box sealed? That's where I'm most confused. I figured in that case it's mostly just an air volume.
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OK, HR sealed sim using generic 18.3 Hz/15" with minor full length damping shows notches at 100, 200 Hz, ~120 Hz impedance peak, so wondering if measurements are including floor/whatever resonances.........
My measurements are with the mic about half an inch from the cone. So I don't think floor or room interactions would show. Do you mind posting your HR sim? I was going to run it when I get home.
Several things could be at play. Filling the box extensively with damping material lowers the speed of sound in the filled section, lowering the fundamental resonance. A vibrating piece of damping might cause trouble too. But I fail to see why the rise in the impedance plot is accompanied by the drop in SPL. Just like there’s some kind of acoustic short-circuit at play.
you definitely want some damping directly behind the driver. Is there any chance that you took some of these measurements with the driver not secured to the box? I've found that a driver that is just set it the hole will cause a resonance around the 100-150hz area. I've had the most sucess with material behind the driver and at the very top and bottom, with nothing on the sides or back of baffle. The stuffing you put in the middle of the box is messing things up. You're losing efficiency around the second peak which is down over 10ohms and shifted from 57 to 65hz which shows to me that you are now interering with the whole system instead of just attacking the resonance.
I didn't have much time to play with this, since I have to head out soon. But I pulled all the stuffing out of the box as a clean slate. This is what it looks like.
Here it is with safe n sound right behind the basket, and 1/3 way down the box.
Here's safe n sound only behind the driver.
It seems like what's happening is any amount of stuffing is simply lowering the resonant peak's frequency, and not actually removing it. Life would have been so much simpler if I didn't build such enormous boxes!
Here it is with safe n sound right behind the basket, and 1/3 way down the box.
Here's safe n sound only behind the driver.
It seems like what's happening is any amount of stuffing is simply lowering the resonant peak's frequency, and not actually removing it. Life would have been so much simpler if I didn't build such enormous boxes!
I wonder if a 'shelf brace' or two would help breakup the resonance ?
Like a solid shelf that goes across half or two thirds of the cabinet.
Thus diverting or breaking the wave as it tries to flow through the cabinet.
Like a solid shelf that goes across half or two thirds of the cabinet.
Thus diverting or breaking the wave as it tries to flow through the cabinet.
Yeah, most of mine and for others were in the ~10-30 ft^3 range and thanks to both an EE from W.E. in its/his early cinema sound design times + a DIY ~TL design addict, learned TL theory PDQ to make offset driver column/tower speakers' pipe action damp the vent, making damping a minimal requirement to the point where some folks pulled all of even my recommended minimal amount out (Altecs' top, one side, back in the driver area only).
Another I used early on was Bozak's (and later some Altec's) suspended full width/depth, acoustical 'blanket' (thick roll insulation), though FWIW/YMMV, etc., instead of parallel to the baffle, my female(s) SQ 'squad' claimed that suspended diagonally was slightly better overall.