Advice needed - measurements after installing basstraps -

[ri.noceros]

Hi,

I need an advice from you experienced guys.

After installing porous absorber bass traps all over the room the overall freq-response is a bit better, but i got a (new) huge(!!) peak at 110.8.

Measurement-sheets and images of the room with basstraps are attached.
How do you read the results? Are`t they a bit poor after all that afford?

Installed basstraps -

# 1 Layer (=5cm) of Rockwool Fire-Batts 110kg/m³ (cornertraps 100x60cm)
# 2 Layers (=10cm) of Rockwool Firebatts 110kg/m³ (Minitraps 60x50cm @ all 8 room corners)
# 2 Layers (=10cm) of Rockwool Acoustic Batts 40kg/m³ (large traps 190x120)
# 1 Layer (=5cm) of Rockwool Acoustic Batts 40kg/m³ f(100x60cm) Mid-High absorber at early reflection points (position checked with mirror-test)
# 1 Layer (=5cm) of Rockwool Firebatts 110kg/m³ cloud 120x100cm over the mixing position

Any suggestion how to tune my basstraps (distance to the wall, attach sealed plywood o one side etc.etc.). I need to understand how I can get rid of that 110.8Hz peak?

Thx for any advice!!!!

Richard

[Soundman2020]

# 1 Layer (=5cm) of Rockwool Fire-Batts 110kg/m³

That is WAY too dense for bass traps! It might work OK for high frequency absorption, but not for bass trapping. Its more than twice as dense as it should be, and at only 5cm it isn't anywhere near thick enough, either.

60x50cm @ all 8 room corners)

Those are not very big, and there are twelve corners in a room, not just eight. Which corners do you treat, and which not? A diagram or photos showing what you did would help.

# 1 Layer (=5cm) of Rockwool Acoustic Batts 40kg/m³ f(100x60cm) Mid-High absorber at early reflection points (position checked with mirror-test)

That's too thin. You should go for at least double that, ore even more if you can afford the depth.

# 1 Layer (=5cm) of Rockwool Firebatts 110kg/m³ cloud 120x100cm over the mixing position

Once again, that's too thin and too dense.

You also didn't mention what angle you placed all these panels at, or what separation you left behind them.

I need to understand how I can get rid of that 110.8Hz peak?

That isn't your only problem by any means, but it is most likely related to a room mode However, seeing that you didn't post your room dimensions or diagram, it is impossible to say where it is coming from or how to treat it.

It would also help if you were to describe how you took those measurements, and what we are actually looking at in each case. The impulse response graphics don't make much sense at all, and it seems you have some serious issues going on in there, but once again without any of the information on how you got those or what they actually are, it's impossible to even understand what is going on in your room.

But to start with, I'd suggest that you replace those extremely dense, thin and small bass traps with something a bit more substantial of lower density, and also post a detailed diagram and photos of how you treated that room, as well as details of what kind of room it is, what it is meant for, construction details, and dimensions.

Finally, there is an announcement at the top of the forum about what to do to assure getting as many responses as possible. The announcement leads to this post (click here). Actually, several people, who are experts on this forum, will most likely not reply if you don't do what is written in that post. Many others who are very helpful, will most likely not reply out of respect for the moderators' wishes.


- Stuart -

[ri.noceros]

Stuart, thank you for taking your time. I appreciate your critics and hope I will learn here how to make things better.

It is about my home studio in an old house with thick walls in ST.Petersburg/Russia. My goal is to realize my recordings at home and following the recommendations of my friend (a mastering engineer with great reputation) who said - guys - don´t invest in expensive equipment before you didn`t treat your room in a way that you can hear what these great devices are actually doing I try to acoustically treat my room and try to get the best I can at least for the mixing position.
The flat is located in a rather quiet yard and making noise during daytime is no real issue here in Russia, so no problems with that.

Add corner traps -
I used 110kg/m³ Rockwool, because I read an article of Ethan Winer, who offers tons of valuable informations and he recommends 705-FRK that is four inches thick and spaced 16 inches away from the wall. That shall be quite effective to frequencies below 125 Hz.
705-FRK has 96kg/m³ and 110kg/m³ was the closest to that I could find here in Russia.
As he wrote "Using 705 fiberglass that is two inches thick does a good job, but using four inches works even better" I choose double layered Mini-elements (corner-traps) for 8 corners of the room.
They are completely senseless for low-freq?

Add side traps
The space behind the traps at the early reflection points on the side walls (beside the mixing position) is 20cm/moutned with no angle.

Add cloud
Space from the ceiling 77cm to 105 cm. Small angle, see photo attached.

Add corner traps at wall/ceiling position.
Angle 45-60°. Space behind max. 30-35cm.

Additionally large traps 190x120x10 filled with 40kg/m³ Rockwool (to close the 1m gap where the windows are)

++++

# Measurements were done with Room EQ Wizard, using a sweep-tone.
# The microphone was placed at the mixing position (38% from the front wall centered)
#The distance from monitors to the side is 117,5 (first early reflections from sidewalls return at around 5.43 ms)
#Nearfield-Monitors and mixing position form a triangle with equal sides (146cm).
# The dimension of the room is L502xW382xH306cm (ratio: 1 : 1.25 : 1.65 )
# Volume: 58m³

Add impuls response - Same measurement (sweep tone)

When I play the main problematic freq, the volume is different on different positions.
An image is attached which scales the volume from 1-10 (10=highest volume in the room)

So far I got recommedations to treat the front wall (before mixing position) with a large, thick trap (around 15-20 cm), as well as the back-wall and place the traps if possible where the problematic-freq is loudest in the room.
Another plan is to add two layers of 40kg/m³ on top of the cloud.

I also tried to shift the mixing position a bit backwards, but as one freq got lower another peak appeared, so I´d like to stick with placing better (or more) traps in order to get this solved.

A photo of the room is attached.

Hope this makes already a bit more sense.

[Soundman2020]

who said - guys - don´t invest in expensive equipment before you didn`t treat your room in a way that you can hear what these great devices are actually doing

Very good advise!!!!

They are completely senseless for low-freq?

Not completely senseless, no. Just not very efficient at low frequencies. Contrary to intuition, more density does NOT help low frequency absorption, and actually hinders it. Higher density helps at HIGHER frequencies, but lower density is better for absorbing lower frequencies.

Take a look at this report that Andre linked to a few days ago. OK, that is about how absorption acts inside an MSM wall, not directly as treatment, but the same principle applies. Higher density helps more with highs than it does with lows.

Better still, take a look at the published figures for four inch thick 701, 703 and 705, and you'll see what I mean.

--Type ---- Density --- 125Hz -- 250Hz -- 500Hz -- 1kHz -- 2kHz -- 4kHz -- NRC
701, 4" -- 24 kg/m3 --- 0.73 --- 1.29 ---- 1.22 --- 1.06 --- 1.00 --- 0.97 --- 1.15
703, 4" -- 48 kg/m3 --- 0.84 --- 1.24 ---- 1.24 --- 1.08 --- 1.00 --- 0.97 --- 1.15
705, 4" -- 96 kg/m3 --- 0.75 --- 1.19 ---- 1.17 --- 1.05 --- 0.97 --- 0.98 --- 1.10

705 is twice as dense as 703, which in turn is twice as dense as 701. Yet the coefficient of absorption for both 701 and 705 at 125 Hz is much LOWER than it is for 703: So even though 703 is much less dense than 705, it absorbs low frequencies BETTER than 705.

Now look at absorption at 250Hz: once again, 705 is worse than both 703 and 701! Here, the real low density 701 has better absorption than either 703 (twice the density) and 705 (four times the density). Even at 500 Hz both 701 and 703 STILL beat 705 for absorption. It is only at higher frequencies that things start to even out, and all three products are roughly the same. The overall NRC tells the story: both 701 and 703 are better than 705, in this aspect.

Also, it isn't just about density: it is about gas flow resistance too. All materials have different flow resistance characteristics, so mineral wool needs to be more dense than fiberglass for the same thickness and absorption.

It's complex, and I don't claim to understand all of the math behind it, but I do listen to the experts and try to understand the basics, and the conclusions are pretty clear: Lower density is better than higher density for bass traps, within a certain range. And higher density is better than lower density for dealing with high frequencies.

Mounting also has a huge effect. You say your traps are "four inches thick and spaced 16 inches away from the wall". The data I gave above is for mounting the panel directly on the wall, so here is the exact same data but for mounting it over a 16" air gap:

--Type ---- Density --- 125Hz -- 250Hz -- 500Hz -- 1kHz -- 2kHz -- 4kHz -- NRC
701, 4" -- 24 kg/m3 --- 0.87 --- 1.14 ---- 1.24 --- 1.17 --- 1.18 --- 1.28 --- 1.20
703, 4" -- 48 kg/m3 --- 0.65 --- 1.01 ---- 1.20 --- 1.14 --- 1.10 --- 1.16 --- 1.10
705, 4" -- 96 kg/m3 --- 0.59 --- 0.91 ---- 1.15 --- 1.11 --- 1.11 --- 1.19 --- 1.10

Now the differences are even MORE noticeable: low density 701 (only 24 kg/m3) is MUCH better across the entire range, from 125 Hz up to 4 kHz.

Now, having said all that, you also said that what you are using isn't even 705. You said "705-FRK has 96kg/m³ and 110kg/m³ was the closest to that I could find here in Russia." So your's is even more dense, and will therefore have even lower absorption, but yours isn't even Owens Corning brand, so in fact you don't have any idea what the real characteristics are. You should contact the manufacturer, and ask for the acoustical properties of the exact product that you are using, and for all his other products, to see what you have right now and which one you should replace it with in order to get the best performance possible.



- Stuart -

[Soundman2020]

More...

Based on your dimensions, that 110 Hz peak is probably relate to your 1,0,0 axial mode, which is theoretically at 112.5 Hz. And your peak at 142 Hz is probably your 0,1,0 axial modem which is theoretically at 148 Hz.

The peaks at 36, 60, 66, and 94 are meaningless in terms of modes: the room doesn't support any modes that low. More likely to be spurious data, or reflections off something in the room, than actual modal issues. Or even speaker artifacts, or issues with the measurement mic. But not modal issues, and not worth worrying about yet.

Anyway, based on the above, it seems like you don't have enough absorption on your ceiling and rear wall. I would increase the size of the cloud, hang it higher up (closer to the ceiling), and/or put a reflective back on it with a large air gap (to help break up any vertical stuff).

Also, those poly-cylindrical diffusers on the side walls are probably not doing anything to help, and might even be a part of the problem. I'd take those out, I think, and replace them with absorption.

Another big potential problem that I see is at the front of the room: there are two alcoves (windows? closets? bookshelves?) that you have just covered with absorption panels. Those are most likely doing strange things to the room, especially if they are deep. I'd seal those off with thick drywall plugs, then reposition the panels diagonally across the corners, and put some more absorption on the front wall, directly behind the speakers.

So my plan would be:

1) Replace the wool in some of your absorbers (the high density ones) with something of much lower density, and much thicker (at least 4"). The 40 kg/m3 stuff that you used in a couple of them should work fine, but that 110 kg stuff is way to dense. It isn't useless, but it also isn't very efficient.

2) Make the cloud much bigger, use thicker wool, raise it up closer to the ceiling, and put a solid back on it (plywood over an air gap).

3) Plug those alcoves at the front and put thicker absorption diagonally across those front corners, plus behind the speakers (on the front wall)

4) Put more absorption on the rear wall, thick and large.

5) Remove the polys and put large absorption panels where they wall.

6) Measure again, and see what happens / what still needs doing.


Other people may not agree, and might have better ideas, but that's what I'd go for.


- Stuart -

[Ethan Winer]

I just want to clarify a few things. First, differences in the ranges I see listed here, such as 0.73 versus 0.84, are very small, and can be due to run-to-run test variations or other factors. If the tests were done in different labs then all bets are off. A difference of two to one is significant, but anything less than 10 percent can be safely ignored. At least at low frequencies like 125 and even 250 Hz.

Second, what happens at 125 Hz may not be the same as what happens at 50 Hz. Official test data typically goes no lower than 125 Hz, and only a few labs in the world are even certified to report as low as 80 Hz. When I tested different materials for my Density Report a few years ago, 705 measured the best density for very low frequencies. However, that was just one test, and it'd be great if someone else did the same test and published their results. Much more about the flaws of standard lab tests for bass traps from this article in Sound & Vision magazine:

Alternative Test Methods for Acoustic Treatment Products

--Ethan