Symmetrically placed subs not measuring the same?

[Sonnie Parker]

I can't remember this ever happening before, but I moved some subs around and I have two of my SB16's place in the front corners, symmetrically in my symmetrical room, yet there is an 8dB swing at 28Hz between the two. I don't get it... they should measure the same, or so I thought... and I don't ever remember other subs I've had up front not measuring the same.

Anyone got any brilliant ideas as to what the cause could be?

 

[JStewart]

REWs room simulator should work pretty well for your symmetrical room. Be curious to see what it calculates for that placement.
Is it possible you missed clearing something in the subs built in dsp?

 

[Sonnie Parker]

REWs room simulator should work pretty well for your symmetrical room. Be curious to see what it calculates for that placement.
Is it possible you missed clearing something in the subs built in dsp?

I added the measurements to the first post.

I checked everything on both subs. I did have one subs about 3/4" farther from the back wall than the other, so I aligned that, but it made zero difference in the response. I checked all the settings in the processor as well... nothing. I'm no doubt scratching my head.

I have not dug into the room sim yet.

 

[Wayne A. Pflughaupt]

Wow. Puzzler for sure.

I'm sure it's a pain to move them, but swapping locations and re-measuring might be revealing...

Regards,
Wayne

 

[Sonnie Parker]

Wow. Puzzler for sure.

I'm sure it's a pain to move them, but swapping locations and re-measuring might be revealing...

Regards,
Wayne

That did cross my mind, but need help. I have a cracked/broke/bruised rib (3rd time now same spot) and while it does not hurt as bad as it did a few weeks ago, it still hurts enough. I've already picked them up and set them on top of my 18's that I'm not using right now, and that was bad enough. Anymore moving of them will require help. Wanna make a road trip to Bama?

 

[AustinJerry]

I would expect them to measure roughly the same as well. To rule out that there is not an actual difference in the performance of the subs, I would run a near-field measurement of each. If the near field measurements are the same, then I would certainly try Wayne's suggestion. Even though the room may be symmetrical, room effects can do strange things.

 

[Chris_Adair]

Is one of your room dimensions 3.06m ? that equates to 1/4 wavelength which would be a room mode where one sub is cancelling the other at 28Hz, switch the sub phases and you may find the swing the opposite way round.
No easy solution, but improving the deep bass absorption would help, though 28 Hz is hard to absorb - you'd need a tuned membrane absorber.
I'll try it in my room in next few weeks. It's interesting.

 

[Harbormaster]

Try swapping the phase (180) on one sub

 

[DanDan]

Are the L and R walls made of the same material? One exterior the other partition? Are the doors, windows?

 

[Sonnie Parker]

I tried various phase adjustments up to 180 degrees with no changes of any significance.

Walls on both sides are 6 inch staggered stud with 5/8" sheetrock over 3/4" plywood.. with solid wood door near the back side of one wall, which I suppose could make a small difference. I can see some minor differences, which sorta kinda is in the grand scheme of the response, but still an 8dB swing seems like a lot.

Room is 19.5ft wide x 23.5ft deep x 8.5ft high.

I'll do a near-field measurement on each... didn't think about that... good idea to eliminate the subs themselves being the culprit.

I thought it might be something to do with these huge ports I have in the riser that are stuffed with a pillow (use to be part of the enclosure for my riser 18's). One may not be stuffed the same and could be acting as a trap of sorts.. as that's a huge cavity underneath. However, I think there was a time where I experimented with those being stuffed and not, and there wasn't any noteworthy effect.

 

[damic]

Is the furniture placed in room absolutely symmetric as well?

 

[pinox67]

For a perfect symmetry in measures all shall be symmetrical: room, subs type and setting, placement and any furniture. Also a door or a window not in symmetric positions can cause differences.

However you should also measure the sum of two subs to see what you ear in a normal listening situation. Depending on room resonance modes and sub/listening positions there are always interference between the subs, constructive or destructive, so the overall frequency response is different from that of singular subs, even if they have the same response. In REW you can do simulations to understand better the behavior in all working conditions.

 

[DanDan]

Seems we should note the same frequency has a boost on one side and a dip in the other. Try opening that door to see what happens.
A near field measurement in the outdoors will tell if they both are working properly.

 

[Sonnie Parker]

Is the furniture placed in room absolutely symmetric as well?

Yes... the only difference being a small cabinet with a turntable on top on one side hear the back. I'll move that out later today after worship service and see what happens.

For a perfect symmetry in measures all shall be symmetrical: room, subs type and setting, placement and any furniture. Also a door or a window not in symmetric positions can cause differences.

However you should also measure the sum of two subs to see what you ear in a normal listening situation. Depending on room resonance modes and sub/listening positions there are always interference between the subs, constructive or destructive, so the overall frequency response is different from that of singular subs, even if they have the same response. In REW you can do simulations to understand better the behavior in all working conditions.

Absolutely... the summed response looks excellent. Dirac will sum them all for correction anyway. I simply like to see what the response is and if there are any huge peaks I will knock those down to keep the levels correct, although Dirac does use normalization on the levels... still... huge peaks can affect that normalization level.

Seems we should note the same frequency has a boost on one side and a dip in the other. Try opening that door to see what happens.
A near field measurement in the outdoors will tell if they both are working properly.

Too much trouble outside, but we can still get a good idea with a measurement 2 inches from center with each sub. I'll see what opening the door does as well... no doubt it will effect both subs, but might be interesting to see where it affects them. It's a pressure sealed door with another door behind it. These are older picks... need to update them.

 

[DanDan]

I am interested to see if measuring close to the subs works. IMO Modes feed pressure back onto the speaker cone, but who knows. Also the door, that's big, light, near a corner on one side.
Can you easily swap the subs?

 

[pinox67]

From pictures, I see that the door is in a side and open or closed, it is not in a symmetric position respect to subs emission, in the front wall. This is sufficient to cause some differences in their response.

If you cover the door and its symmetric position in other side with a heavy curtain, the difference should be reduced a lot.

 

[Sonnie Parker]

Can you easily swap the subs?

Not with a cracked rib... need help. I could take the rubber feet off and slide it though.

From pictures, I see that the door is in a side and open or closed, it is not in a symmetric position respect to subs emission, in the front wall. This is sufficient to cause some differences in their response.

If you cover the door and its symmetric position in other side with a heavy curtain, the difference should be reduced a lot.

It stays shut, so it's basically a very hard surface that continues down the side wall. The sheetrock is also covered with about 1/8" of textured mud, which makes it a harder surface like the solid wood door.

 

[Sonnie Parker]

Here are a few measurements... No Smoothing