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How to get rid of an acoustic null in the room.


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I have two corner traps 24" x 24" x 33" x 7' permanent mounting. 1 is in the back corner. 1 is in the right corner. The left corner has a sliding glass door on one wall so I couldn't install one there. Though I have some lernd corner traps coming so that corner will be taken care of as well.


The fiberglass boxes - wooden frames 1" x 10" 's. The boxes are 4'x6' x 10" deep. They are mobile. The construction is R38 Fiberglass insulation - Chicken wire - 2" polyester - Black burlap. Kraft paper is still on the fiberglass. I have the paper towards the wall and not the speakers.

 

It's pretty unconventional to treat the wall behind the monitors. You seem to have an overall lack of low end and an unsually high room mode (125 Hz) I'm guessing that's why. :idk:

 

A 10" deep "trap" would do quite a number around 125 Hz. If it's not too much trouble, at least try removing the traps behind the speakers and re-analyze the room. Speakers with their backs to a wall generate considerably more low end than free standing speakers. Normally, you don't want that but you're lacking low end across the board and absorption behind the speakers isn't going to do anything to the high end, thus my suggestion.

 

Yes all these treatments are broadband.

 

They're intended to be, I realize. How thick are your panels and how far from the panel to the wall joint?

 

The monitors are Event PS 6. Powered monitors and no back wall compensation that I know of.

 

Looks like Event doesn't make those anymore, and I couldn't find the specs on the Event website. With 6" woofers, I guess you're not going to get a lot of low bass out of them so maybe that's part of the problem.

 

However, the 125 Hz mode is a problem you have to fix. One more thought: any chance your speakers could be out of phase with each other? Can you turn the speakers on one at a time and run the RTA? That's pretty easy to do, and it reveals a multitude of problems.

 

The speakers are not really in a hard corner. My drawing wasn't perfect to scale. The Attack wall which is 3 - 8 ft walls have a roughly 175 deg angle between each of them. So corners aren't that sharp.

 

OK, good. :)

 

I can move the boxes out and check. I admit I'm skeptical of that. I done measurements in the room in the past without treatments and found the curve to be very sawtoothed. Though it wouldn't hurt to check again. Are you thinking the treatments are reducing the overall bass response.

 

I'm sure of that. Having speakers near a back wall, on the floor, or in the corner always increases the bass response.

 

Question where to put more traps if I do decide to do so. I have all the corners taken care of already except 1. So Along the ceiling-wall intersection maybe? or out in space? Near or away from the speakers?

 

I'm not a big believer in using thin fiberglass panels as bass traps, mostly because I've measured their response. I have no way of knowing how well yours work.

 

I know how well MY bass traps work, because I RTA'd the room before and after. Sounds like that would be a big pain for you to do at this point. I don't like panel traps unless they have a vibrating membrane inside at a minimum. You tend to need a lot of panel traps to do much and I just don't have room for them. My treatment of choice for bass trapping is Auralex MegaLENRDs, having tried various other products. I'm not a fan of standard Auralex LENRDs, they just don't give much absorption below 120Hz even if you cover all your corners with them.

 

However, I've found that by moving the listening position closer I get more bass response.

 

That makes sense in a room with untreated or inadequately treated modes.

 

My question however is do you think I'm better using the curve with the dip at 125 which is a really flat curve except the severe dip. Or am I better off with curve where I moved the speakers away from the wall, and now have a less severe dip at 80 but with a spectrum that is not bad but not as flat.

 

That 125 Hz dip is big enough to be a serious problem. If you mix with that response, you're going to make muddy mixes, probably. It's a big deficit to overcome mentally.

 

You're much better with smaller dips located at various places than one big dip.

 

The more I read your posts, the more it sounds like the wall behind your speakers is causing that big dip - and the chief suspect has to be those unusual boxes you put behind the speakers. I've never seen that done.

 

Generally, in a well designed studio either the main speakers are mounted into the walls so as to eliminate the back, bassy reflection from the speakers, or nearfields are used, either sitting on the console or on stands away from the back wall.

 

My main monitors are, by necessity, fairly close to the back wall. I'm aware that this increases the bass response, but my 824s have "space" compensation, meaning a switch for variable bass rolloff to accommodate the speakers being set in a free field, with a wall behind them, or in a corner.

 

I really would try RTAing one speaker at a time, and maybe remove the trap behind that speaker to see what difference it makes. If you get good results with one speaker but not with both, suspect a phase problem. If you get more jagged results with the rear traps removed, that would still be better than that huge dip at 125 Hz.

 

Terry D.

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It's pretty unconventional to treat the wall behind the monitors. You seem to have an overall lack of low end and an unsually high room mode (125 Hz) I'm guessing that's why.
:idk:

A 10" deep "trap" would do quite a number around 125 Hz. If it's not too much trouble, at least try removing the traps behind the speakers and re-analyze the room. Speakers with their backs to a wall generate considerably more low end than free standing speakers. Normally, you don't want that but you're lacking low end across the board and absorption behind the speakers isn't going to do anything to the high end, thus my suggestion.




They're intended to be, I realize. How thick are your panels and how far from the panel to the wall joint?




Looks like Event doesn't make those anymore, and I couldn't find the specs on the Event website. With 6" woofers, I guess you're not going to get a lot of low bass out of them so maybe that's part of the problem.


However, the 125 Hz mode is a problem you have to fix. One more thought: any chance your speakers could be out of phase with each other? Can you turn the speakers on one at a time and run the RTA? That's pretty easy to do, and it reveals a multitude of problems.




OK, good.
:)



I'm sure of that. Having speakers near a back wall, on the floor, or in the corner always increases the bass response.




I'm not a big believer in using thin fiberglass panels as bass traps, mostly because I've measured their response. I have no way of knowing how well yours work.


I know how well MY bass traps work, because I RTA'd the room before and after. Sounds like that would be a big pain for you to do at this point. I don't like panel traps unless they have a vibrating membrane inside at a minimum. You tend to need a lot of panel traps to do much and I just don't have room for them. My treatment of choice for bass trapping is Auralex MegaLENRDs, having tried various other products. I'm not a fan of standard Auralex LENRDs, they just don't give much absorption below 120Hz even if you cover all your corners with them.




That makes sense in a room with untreated or inadequately treated modes.




That 125 Hz dip is big enough to be a serious problem. If you mix with that response, you're going to make muddy mixes, probably. It's a big deficit to overcome mentally.


You're much better with smaller dips located at various places than one big dip.


The more I read your posts, the more it sounds like the wall behind your speakers is causing that big dip - and the chief suspect has to be those unusual boxes you put behind the speakers. I've never seen that done.


Generally, in a well designed studio either the main speakers are mounted into the walls so as to eliminate the back, bassy reflection from the speakers, or nearfields are used, either sitting on the console or on stands away from the back wall.


My main monitors are, by necessity, fairly close to the back wall. I'm aware that this increases the bass response, but my 824s have "space" compensation, meaning a switch for variable bass rolloff to accommodate the speakers being set in a free field, with a wall behind them, or in a corner.


I really would try RTAing one speaker at a time, and maybe remove the trap behind that speaker to see what difference it makes. If you get good results with one speaker but not with both, suspect a phase problem. If you get more jagged results with the rear traps removed, that would still be better than that huge dip at 125 Hz.


Terry D.

 

Thanks for taking the time for the detail response.

 

A couple points.

 

My corner traps are not panel traps. They are densely packed fiberglass.

 

The dip 125 is definitely the due to the distance away from the wall. Not so much the treatment.

 

Already proved that by moving the speakers it immediately changed the response.

 

Now the treatment maybe causing the overall low response in the lowend so that makes sense.

 

I seriously think a majority of my problem is the size of my room. In order to keep the listening position close the 38% position that forces me to keep my monitors close to the wall which was initially giving me issues with early reflections. Made the room sound echoie. Thats I why put the 10" deep absorbers behind them.

 

Regardless, I am gonna try your experiments. Haven't had a chance with Easter and all.

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The dip 125 is definitely the due to the distance away from the wall. Not so much the treatment.

 

Never seen a big dip like that caused by the speakers being in front of a wall.

 

Regardless, I am gonna try your experiments. Haven't had a chance with Easter and all.

 

Good luck, and let us know what you find. :thu:

 

Terry D.

 

P.S. You need more/better bass trapping, but not behind the speakers! :)

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Would you say that this Behringer unit (deq2496) is a good way to measure the room? Or is there some comparable priced PC software (or better yet - freeware) that would work as well if not better.

 

I'm sure that Behringer RTA is giving good enough numbers.

 

I believe there is freeware RTA software (might need Ethan to chime in here), but I don't know particulars as, due to my job, I have access to the hyper expensive stuff. :idk:

 

Terry D.

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Would you say that this Behringer unit (deq2496) is a good way to measure the room? Or is there some comparable priced PC software (or better yet - freeware) that would work as well if not better.

 

The behringer unit is working just fine.

 

Whether I like it or not. :(

 

The thing I like about it as opposed to just using the RTA within wavlab for instance, is its like having a 3rd party observer looking at my system from the outside. The measurement is not influenced or jaded by my speakers, soundcards, mixer, preamps, etc. It just measures the result of my total system from a bystander viewpoint.

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The results are in.

 

This measurement is with the speakers at 27" from the wall and listen spot at 38% the original listening position, without the treatment behind the speakers.

 

Well the dip is now a lot worse, and the rest of the response seems similar but has a couple of narrow peaks that weren't there.

 

Overall, I'd say no way to this.

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The second result is similar.

 

This is with the listening spot move closer by about 10" and the speakers moved off the wall to 3-1/2 feet, still without treament.

Now the dip is deeper but also the peak I was concerned about in this position is bigger.

 

So I think I need my treatment.

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Exploring this last curve.

 

I have some corner traps coming, so I will have to test the room with more traps once they arrive. However, the 5 dB boost between 180 - 315 is concerning me. I wonder if this has to do with a LBIL peak as Ethan was talking about. This spot doesn't correspond with a 1/4 wavelength, but it does correspond with a 5/4 peak on his calculator. Just wondering if a 5/4 peak has the ability to alter the response in the way it has done here. Also the boundary that corresponds to this 5/4 peak is treated with 703 panels.

 

Another thought. When I moved closer to the speakers, my listening position also gets closer to my computer monitors. I have dual monitors. They are about 1'10" from my listening spot. That is pretty close to a 1/4 wavelength of 180. Even though they are not a boundary, could they have a similar affect.

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Well here I pulled the speaker out to 52" from the walls. But This makes it so the monitors are now only about 18" from my head. Seems a little closterphobic.

However, here is the curve.

 

Much better but is emphasizing the mid range a little more than before that said I pulled up an old mix it did sound muddier and I was having issue with my DI box on the bass seeming to midrangy. Thats not the case with this configuration. Now the DI seems boomy. Still I'm not sure its wise to have the speakers so close. Will this throw off my stereo imaging? It did balance not only the listening spot but the whole room.

 

I wonder with more traps in the room if I will be able to back the monitors off a little bit and still keep a flatter response. I will see.

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Well here I pulled the speaker out to 52" from the walls. But This makes it so the monitors are now only about 18" from my head. Seems a little closterphobic.

However, here is the curve.


Much better but is emphasizing the mid range a little more than before that said I pulled up an old mix it did sound muddier and I was having issue with my DI box on the bass seeming to midrangy. Thats not the case with this configuration. Now the DI seems boomy. Still I'm not sure its wise to have the speakers so close. Will this throw off my stereo imaging? It did balance not only the listening spot but the whole room.


I wonder with more traps in the room if I will be able to back the monitors off a little bit and still keep a flatter response. I will see.

 

 

That curve looks very good, but you do need to have your listening position in the equilateral triangle spot where all three legs of the triangle are the same (speaker to speaker, left speaker to your head, right speaker to your head).

 

Did you ever try RTA'ing each speaker separately?

 

And you are asking a lot from those little 6" speakers.

 

Terry D.

 

P.S. What are your existing traps in the back of the room like?

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Thanks for your help. Actually, I ended up taking your advice by removing the panels behind the speakers. I played around with the speaker position some more got the speakers back off me (36" away). Once I did that I got small dip in the low then I removed the panels and it flattened out some. So I then took the panels and put one in my untreated corner, and the others along the walls not behind the speaker That seemed to do the trick. So then my 703 panels were covered by the traps, so I took those off the wall. My new curve had some mid range bumps. Small but there, so I put 3 of those on the wall behind the speakers. And also my spot placed a speaker right under my plywood cold air return. So place one above that speaker.

 

 

Yes I'm not too fond of those Event ps6's anymore they've been notorisouly lacking lowend since I got them.. But its what I got.

 

Anyway here is the latest and greatest curve not too shabby.

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Thanks for your help. Actually, I ended up taking your advice by removing the panels behind the speakers. I played around with the speaker position some more got the speakers back off me (36" away). Once I did that I got small dip in the low then I removed the panels and it flattened out some. So I then took the panels and put one in my untreated corner, and the others along the walls not behind the speaker That seemed to do the trick. So then my 703 panels were covered by the traps, so I took those off the wall. My new curve had some mid range bumps. Small but there, so I put 3 of those on the wall behind the speakers. And also my spot placed a speaker right under my plywood cold air return. So place one above that speaker.



Yes I'm not too fond of those Event ps6's anymore they've been notorisouly lacking lowend since I got them.. But its what I got.


Anyway here is the latest and greatest curve not too shabby.

 

That's better than not too shabby. That's damn good for a room. :thu:

 

OK, one final piece of advice: the reason you're getting such large changes by moving your speakers and/or listening position is because your room's not sufficiently bass trapped. What you've done is fine, you've found the "sweet spot" but be aware that moving your listening position even a bit is going to produce a very different result.

 

If you get your bass modes better under control, you'll find the sweet spot gets larger and there will be much less variation with listening position.

 

My preferred methods for bass trapping is to place it on the back wall only, and my preferred devices are Auralex MegaLENRDs for the corners plus a tuned resonator for any severe problem frequencies. Ethan's Real Traps work too, but you need quite a few of them. The trade off, however, is that his traps aren't completely "dead" meaning they don't suck up all the high frequency in the room. So I use some of each.

 

I think Ethan makes some larger traps than the ones I have now. I suspect those might work better for bass trapping, but I've not tried them. The MegaLENRDs really did the trick for me, and they didn't take up any room space I was using.

 

Good luck, and congrats on what you've done! You'll find your mixes translate a lot better on other playback systems. When I finally got my control room acoustics under control, I no longer needed the several speaker systems I was using, and I never have to "run out to the car" anymore to check my mixes. They also sound very nearly the same in the mastering lab as they do in my room.

 

Terry D.

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TAnyway here is the latest and greatest curve not too shabby.

 

Just to be clear, a third octave measurement like that hides the true extent of the peaks and nulls. Especially the nulls. The best way to measure LF response in a room is at 1 Hz intervals, or 1/24th octave or higher. The graph below shows the very same measurement displayed as 1/3 and 1/12 octave. Guess which one is correct? :D

 

--Ethan

 

art_mon3.gif

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