Bass null in my new treated room, help! w/pics

[Retinal]

Hey guys,

Today I put the monitors, Adam P33A, in my new control room, and even tho' I do like how the room sounds, very tight and focused, I noticed a bass null right in the mixing spot.

Now, since that I have all Digidesign systems I can't use Roomeq and post a graph of the test, I used a Phonics PAA3 which didn't show the null pretty much at all (with the pink noise, pointless to do the sweep since it didn't freeze the spectrum like REW), but I was listening to my reference cds and I *know* there was a null, huge null, like the whole bass and sub bass spectrum.

Now, the room is treated with:
Floor to ceiling superchunks in the corners behind the monitors
6 inches bass traps positioned in this way:

Two floor to ceiling in back corners
Two behind the monitors, next to the chunks,
Two on the front and rear wall/ceiling corners,
Two on the ceiling,
Two on both side walls (wall/ceiling corner)
One on top of the couch behind the mixing spot

Traps are made of Fiberglass/mineralwool sandwiches 70/80 Kg for the fiberglass and 40/50 Kg for the mineralwool.

Plus, 3 inches panels on the first reflection points on the sides walls
And some spare lil panels as shown in the pictures (white ones)
Plus two pieces of foam on the side walls (see pics)

I thought that'd be enough treatment for a room that measures 4,20 meters x 2,90 and celing is 3.30

But like i said even tho' I do like how it sounds, this bass null was pretty noticeable, at least when I was at the 38% spot.

I moved the monitors closer to the wall behind them and things were a lot better, but the best improvement was to actually to switch the monitors.

P33A (for those who don't know) have a woofer that acts as a full range with the other joining it only below 150Hz.

Adam manual says that you should have the full range woofer in the inner side, but when I switched them L/R (so to have the low freq woofer in the inner side instead of the full range) it was way better!

Lot more bass presence, more focus on the whole spectrum.

Now, switching the monitors and putting them closer to the wall (front wall for me) made things a lot better, but still didn't sound right. If i move backward with the chair I can hear the bass frequencies way better, but I can't mix 6'/7' far from the monitors, the crossing point that should be behind my head would be 4' far in front of me :/ doesn't sound right.

But in the way they are positioned right now (which is the best sounding one so far) they are something like only 2' far from the front wall, and it seems too close.

Space between the tweeters is 3.37"

How do I solve this null?

Sorry for the long post, here's the pictures



Back wall


Back and side







Ceiling


Thank you in advance!

[SAC]

Assuming the null is a result of pressurized room modal behavior, the short answer is that you move your listening position.

[DanDan]

I suggest that you identify the mode/s in question. This will inform you on which axis you and your speakers should move. Play some sine waves in the room. Manually slowly sweep from say 30Hz upwards. When you hit a mode there room will let you know all about it. Write down the mode frequency. Move about the room and identify the peaks and nulls. Write them down. The first length mode at around 41Hz will have enormous peaks at the front and back walls, and an uncannily almost silent null at room middle. Move on , three or four modes should be enough. You could virtually look at all this here. hunecke.de | Room Eigenmodes Calculator
It should now be clear which mode are causing the null which you find problematical. Adjust your speaker and listening positions to optimise. This is an intelligent compromise exercise. I suggest that you use REW or FM for this rather than an RTA. Both work fine through Digidesign Hardware.
Optimise one speaker position at a time, use averaging, but check the combination. NB that height is up for grabs. Try your ADAM's vertical. This is much better, trust me, I have secret tests....
This vertical orientation will also enable you to place the woofer up or down, possibly avoiding or stimulating a peak or null. Try it on top, it might help your 50-70 Hz issues. Adjust height and if necessary get those Mo Pads or similar to angle the speakers downwards.
Use height and distance from the front and side walls to tune your dips. Get the most even compromise. This helps a lot, search.
Wall_Bounce_Calculator_2D.xls
If you lose level in finding the most even Bass, boost below 150Hz on your ADAM's. My S3A's are about +2 LF, -2 HF.
You will never get really even bass in such a small room. However, you can certainly make the best of it. When mixing, I spend a lot of time in between and quite close to the speakers. Big headphones, tweeters off axis. I frequently move to the back third of the room really hear the bass.
A workaround but an informed one. By frequently referencing familiar favourite mixes you can make this situation deliver very reliably.
DD

[Retinal]

Thank you Dan!

Much appreciated.

I will do these tests, but I cannot for the life of me be able to make REW work with Digi hw since it's not possible to select digi drivers as default (i'm on Windows XP, not Mac), and the drivers are output only for anything that's not PT :/

I need to find a way to get a graph with REW I will try to find a workaround.

Another thing, I did try to put the monitors vertically but it was worse, but I didn't try to move them once they were vertical, so I will definitely try that again.

I will be back with the results!

Thanks again


Crazy idea: putting monitors on what is now the side walls? I assumed that would be way worse and not even an option but I will ask just to make sure..

[SAC]

Placing MTM/D'Appolito configured speakers horizontally places the polar lobing anomalies in the horizontal plane, exacerbating your problem with nulls.

Setting them upright (the orientation for which the configuration was designed to correct for a 15 degree upward polar tilt with a 3rd order LR crossover) keeps the polar lobing anomalies in the vertical plane and generally out of the listener occupied space.

In other words, they are best used vertically.

The remaining issues are due to their interaction with the room.

[Wes Lachot]

Originally Posted by Retinal

Hey guys,
Now, switching the monitors and putting them closer to the wall (front wall for me) made things a lot better, but still didn't sound right. If i move backward with the chair I can hear the bass frequencies way better, but I can't mix 6'/7' far from the monitors, the crossing point that should be behind my head would be 4' far in front of me :/ doesn't sound right.

But in the way they are positioned right now (which is the best sounding one so far) they are something like only 2' far from the front wall, and it seems too close.

I would not automatically assume that 2 ft. from the front wall is too close for your monitors. What you are most likely experiencing is a speaker/boundary interference null, as opposed to a modal null. This would make sense, because you would expect better sub and low frequency response by moving the speakers closer to the wall if this is the case, and that is the result you decribed.

Moving the speakers from 4 ft. to 2 ft. from the front wall will move the front wall null up one octave, from 70 Hz to 140 Hz. You stated in your original post that "there was a null, huge null, like the whole bass and sub bass spectrum." I doubt that was really the case--it's not really possible for a single null to encompass the entire bass and sub-bass range. But a 70 Hz null will surely give one that impression, positioned as it is in the very meatiest part of the bass range. A null like that will cut the bottom right out of a kick drum, for instance. On the other hand, a null at 140 Hz, though far from ideal, will not do as much damage to the overall bass response, and furthermore, a 140 Hz null is easier to solve with garden variety trapping.

If you really want to have your speakers 4 ft. from the front wall, then you're going to have to build traps on that wall that are very effective at 70 Hz. The traps shown in your pictures are not sufficient for this purpose. Given the trapping as shown, you are probably much better off with your speakers 2 ft. from the front wall, or possibly even a bit closer.

-Wes

[PaulP]

Originally Posted by Wes Lachot

I would not automatically assume that 2 ft. from the front wall is too close for your monitors. What you are most likely experiencing is a speaker/boundary interference null, as opposed to a modal null...

Wes has also written an excellent paper that goes into this in a bit more
detail : Bass Waves in the Control Room.

Paul P

[DanDan]

Wintel, sigh. You could use a sine generator, perhaps in your DAW, plus your SPL meter, to manually do freq response graphs. You are really only interested in LF, so this is doable. This could be done in combination with my earlier 'find the modes' sine quest.
You could focus on your worst dips, the most significant one/s bearing in mind what Wes said. The ADAM's are three way, one of those two drivers outputs below 150. They are thus not a classic design, and are intended for horizontal use.
However, the laws of physics do not change, and I really have those tests. Go vertical.
Furthermore rotate the tweeters to get an even better horizontal HF dispersion.
+1 to Wes, try closer to the wall, almost touch it if necessary.
You do have a lot of treatment. I wonder if those panels behind your speakers might be better used at the back, beefing up the bass traps back there.
IMHO they look too shallow to do much about SBIR.
Question, your ceiling, is that that a dropped tile ceiling? If so, is your height, 3.3, to the tiles or to the ceiling above them.

DD

[SAC]

Thanks, Victor. I was not intimate with the speaker internals.

But they will still exhibit horizontal polar lobing in the overlapping bandpass of the crossover region between the woofers.

And a vertical orientation between spaced sources places the polar lobes in the vertical plane instead of in the horizontal plane. This feature alone alone will suggest that their dispersion is optimal when vertically oriented.

There is good reason for the preponderance of vertically stacked drivers in speakers.

Also Wes is absolutely correct that no polar or modal null is going to have the extensive bandwidth indicated!

[Retinal]

Thank you guys,

all this is really helping me to understand.

So everybody agrees, I will go vertical

One question Dan, should I try with the low freq woofer on top, i got it right?
And to answer your question about the ceiling, yes is a dropped tiles ceiling and 3.3 mt is from the floor to them.
I do have the Auralex mopads, so Dan, you'd try to put them higher and then tilt them with the pads? Like tweeters higher than my ears, right?
Yes I can do the sine+spl meter test manually, even tho' I'm thinking about borrowing a laptop from a friend with a non Digi hw and install REW, I get the feeling that that would save me a lot of time..
I can built other traps for the back of the room, i still have material left.

Thanks Wes,
No I got no problems keeping the monitors closer to the wall, I just thought that wasn't correct, I will try even closer.
Yes that was exactly what I was hearing, bass drum being completely empty, just hearing the attack pretty much :/

I will try vertical tomorrow and try to move closer to the wall as well and report,

Thank you so much!

[DanDan]

Ret, so can you give a good guess as to the 'real' ceiling height?
The vertical/horizontal thing is a question of balance of compromises. Horizontal will get that LF woofer closer to the side wall, or further to avoid or provoke width modes. A very powerful tool, particularly since your issue is a bass one. Note, my speakers and tests are S3A. My vertical suggestion is not the same as SAC's, I am referring to HF horizontal dispersion, and a real world test, not theory. I am saying that vertical delivers much better results in those aspects. I can't say more or they will find me......
I am not questioning his reasons for favouring vertical at all, but I would also assume ADAM have addressed such design issues. For this same reason I would also not be too absolute about my vertical. Abbey Road seem to use them horizontal! Your problem is a bass one, so the power of being able to shift that woofer in or out may trump these other issues.
By all means try the Woofer on top. That's a suggestion, not a recommendation, as I don't know the height or the frequency area of your nulls. 38% of the real height may be a great height for the woofer, but it depends. As I said that 2D Wall bounce can also be used vertically. Speaking of which, I have just been playing with that re my own room. Give it a go, it is quite an eye opener. Try different listen angles to get the speakers closer to the side walls.
By simple sines or REW, do identify they null/s, guess or measure the ceiling height, and let us know. A calculated recommendation versus whatever you end up with by testing and listening would be an interesting comparison
DD

[Retinal]

Ok good, thank you Dan.
I can measure the ceiling tomorrow and give you the real high, then I will try putting the monitors at 38% (i had no idea the 38% rule applied also in that way, but of course it makes sense now that I think about it)

Like I said putting the LF woofer in the inner side improved the bass response quite a bit, while I only tested the monitors vertically when they were still far from the front wall. I will give it a try first with the LF woofer on top and then at the bottom as well

Testing and reporting is the way to go, I got it, thank you all guys for following me on this one

[Retinal]

Here we go,

thanks to all of you guys, Dan in particular I solved the problem pretty well and the room now sound amazing!

The problem was indeed the rear wall which I had underestimated.

This is what I've done: put the monitors vertically, with the LF driver on top,
got even closer to the front wall and things started to get better but what made a huge difference was to replace the already existing trap on the rear wall with a way thicker one and build another one (same thickness) and put it under it, where the couch was going to go, at this point it will go in front of it or something, I don't care I rather the room to sound good!
Here's some pictures and also the test with the analyzer:

Those two are the new traps, 4 panels, two fiberglass e two rockwool, the fiberglass is
3 inches thick each panel, and the rockwool is 1.6, overall the traps are slightly more than 7 inches thick





Monitors are now vertically placed and closer to the front wall, I will try (once I have the desk that I'm waiting) to put them slightly higher and tilt them with the mopad and see how that goes







And that's the result of the test in the mixing spot, please note that i keep the tweeters low by 2db (back of the monitors panel).





not bad, Like I said I will try to put the monitors a lil bit higher and tilt them to see if that small peak around 200 or something gets better but I'm already happy as is it!

Thank you guys for your help!!

[Retinal]

Lol i didn't even notice it was on the side until you pointed out

[DanDan]

Thanks for your kind comments retinal. Sometimes I wonder why I post here, but your result is certainly a thumbs up.
DD

[Retinal]

Each and every one of your advices turned out to be good, the way the room sounds improved so much, no trace of the bass null now, and even if I will still try to tweak the room more and make it better I think that if mixes will not come out right from in there it won't be because of the room or the monitors