Room acoustics improvement project

[Joe_b]

I love this wonky thread,
now if you will excuse me, I would like to get even wonkier.

Something about these room measurements is really hard to read. The ca. 10dB drop from 400Hz to 10KHz is a real problem, but is likely to happen in certain rooms. The 6-7dB drop from 400Hz to 180Hz just doesn’t make sense. The unfiltered measurement just looks like a bad measurement. No way your speakers/room sound like that. For instance, look at the drop from 80dB at about 140Hz to 55dB at about 160Hz. Thats 25 dB over 1/8th of an octave. These things just don’t happen in the real world. Can be a truck drove by, dog barking or floor rumble coming up the mic stand. I don’t know this particular piece of software, but in SMAART it is called that the measurement has poor Coherence.

Measuring each speaker at point blank so yo can evaluate the room vs. just the speaker can help in reading the room measurements later.

Taking a room measurement with just one speaker so phase interaction between the pair don’t contaminate the measurement is also a good intermediate step.

Another question, what sort of filtering takes place? is this equivalent to A and C weightings?

[dtr]

Thanks for your input. Doing this for the first time so might very well be making mistakes. What is your expertise in this field?

@Joe_b

The unfiltered measurement just looks like a bad measurement. No way your speakers/room sound like that. For instance, look at the drop from 80dB at about 140Hz to 55dB at about 160Hz. Thats 25 dB over 1/8th of an octave.

I haven’t given details about my room. It’s quite an uncommon arrangement (tight space under sloping roofs), with problematic asymmetries (2 chimneys running though it, one on the front and one on the back wall). When I walk through the room I can hear the huge dips in low frequencies very clearly. Readings match my hearing. Also, see here other people’s examples of before/after treatment, 30dB dips present:

Ideal rew graph for mixing and mastering - Gearslutz

Some surfaces have been covered with porous materials that likely act on the high freq response by the previous inhabitant.

I’ve also done a lot of 1 speaker measurements to find the optimal (or least detrimental) positioning for them in relation to the construction asymmetries. Haven 't done point blank measurements though.

I ll post more details about the room, 1 speaker measurements and software settings next week. Need to deliver a project on monday first.

[Joe_b]

“Expertise” would be a gross overstatement.
I 'm a live-sound guy who’s had to deal with acoustics practically for over 20 years and who happened to once take a 3 day training in Smaart. I also had a hand in building a few rooms and trying to make the whole process just a little less ghetto.

To be fair, I always used smoothing when taking measurements because I was usually measuring for time alignment of speaker clusters and not EQ and this guy posts non-smoothed graphs. Also, I was always measuring either much bigger rooms or outdoors, so variation in results is to be expected.

The guy who posted that example of a good looking/sounding graph has some really good advice here: http://ethanwiner.com/acoustics.html. If you have the time and patients, it will tell you way more than anything I have to say

Back to the ghetto…I think your 2 biggest problems are the big bump around 400Hz which is probably local may be treatable with a single pannel at the right spot.

The crazy dips in the low end are reflections phasing out direct sound and just require better overall bass trapping. If you have some dead space above the ceiling in the attic it can be used as a bass trap by leaving an opening in the ceiling and dumping a lot of absorptive material up there. If such a space exists untreated, it may cause the bump you have at 40-50 Hz. as well as some of the other anomalies in the low end.

I am also assuming the drop at 60Hz is just your monitors naturally falling off but the point blank measurement should tell you if that is true or not.

As for the shape of your room, the sloped ceiling is probably a pro since no parallel surfaces and much less problem with room-modes. Asymmetry makes stereo imaging problematic, that is one area where headphones are are quick and reliable fix. Also, when dealing with non-acoustic sources as most people around here do, stereo is mostly panned mono sources which are not as delicate as stereo-pair acoustic recordings, where the realism of the image just breaks down if you are getting the wrong cues from your monitors/room.

[lesstalkmoredisco]

@shroom

Also if you want it to be effective at low frequencies you’re going to need to build frames and have a free hanging membrane inside them (Lead loaded vinyl)

Like this ->

Next up in the DIY Rockwool acoustics train is me! Just bought a couple packs of the densest variant I could find in my country (100kg/m3).

Q @shroom the link/image that you referred to (“Like this ->”) is not working anymore. Could you share or explain? I’ve got 8cm thick rockwool; which I’ll cover with fabric (which type is the next search). How many cm’s of loaded vinyl / free hanging membrane do I want? Does I go behind or in front of the Rockwool 233? Or sandwich the membrane between two layers of rockwool?

Thanks for helping if you can!

[lesstalkmoredisco]

And second question: what kind of fabric to use to cover them?

Some research online says: “tightly woven but breathable”. Which is super vague. Cotton?
Some other online forums suggested that type of black fabric that’s apparently used in gardens/grass against weeds or something. Is that a good idea?

Any other suggestions by those who’ve already done this research?

[loopdude]

I’d use painting fleece and then whatever cloth you want/like

[Capoten]

I like the traps where I used linnen, the most, due to apperence, ability to make a tight fitt och not dusting.

I also did not coat the wool with glue to secure the fibers, but wraped the trapp in polyester-wool, befor fitting the linnen. This also helped with the fitt and akt as a fiber filter.

[lesstalkmoredisco]

These top ones seem a bit difficult for my current handy-person level (don’t really have equipment also for very straight cutting), but the deep frames below them seem super doable

[Capoten]

I have not used loaded vinyl, but I have 4 floor to celing wardrobes with additional absobents on top. These will also work as base-trapps.

This is how I built the corner-broadband trapps.

[lesstalkmoredisco]

@loopdude

painting fleece

This kind of stuff?

[lesstalkmoredisco]

Why did you cut the rock wool in all those strips instead of putting them in as flat pieces? Just to get the depth to your ideal measure, or some other reason?

[shroom]

@lesstalkmoredisco

How many cm’s of loaded vinyl / free hanging membrane do I want? Does I go behind or in front of the Rockwool 233?

It’s been a few years and I’m far from an expert but usually in front, the thicker the vinyl the more it absorbs but I think a single piece is fine.

https://www.soundonsound.com/sound-advice/q-what-kind-bass-trap-do-need

[lesstalkmoredisco]

@Capoten

Is this the one you put on top op the closet? I realize now that the structural integrity of it is helped by the metal angle things on the inside. Might give them a try if I can find them in my local hardware store.

[Capoten]

Yes, in order to get enough depth and better use of the wool. It does not show in the pictures but those corner-trapps also was wraped in polyester-wool, but I used jute fabric as final wrap with I regret due to dusting.

[loopdude]

Yes, malervlies. It acts as a filter.

[Capoten]

No, the ones on to of the cabinets were made with thinn studs similary to the corner ones, but with a thinn 4mm board below for stabillity.
Yes, I am in an apartment, so I dont want to fix anyting to the walls or celing, That is why the corner-trapps are free-standing, the side trapps are mouted to the shelf and the clouds are resting on a mini truss. This was also usefull for getting an air gap behind the trapps. (About 1/3 of the thicknes of the absorbent.)
The framed trappa was quite easy to build, due to the wood being pre-cut shelves from a hardware store. Also the cloud trapps was filled all the way, with does not show in the pictures.

[lesstalkmoredisco]

@Capoten

This was also usefull for getting an air gap behind the trapps. (About 1/3 of the thicknes of the absorbent.)

Yeah I’m going to aim for this too, behind wathever I end up building. Didn’t know what exact ratio to go for, but I guess I’ll take your 1/3rd:)

That’s it for now for me today, love you all, so helpful in a moment where I was starting to feel overwhelmed in how to proceed. Cheeers

[lesstalkmoredisco]

Update! With a clearer plan in mind things moved quickly this eve, tried a first frame! Actually turned out really good. I keep being impressed with how forgiving wood is, as long as you pre-drill your screw holes and use 90 degree angles everywhere.

Next step will be the two types of fabric, and wood for the next one. I had to saw some old wood planks to size now; I guess when you use timber that’s already the right breadth (as wide as one or two rockwool slices) then it should be done really quick.

Now the next decision will be if I put three slices of Rockwool in there, or two with an air gap in between. (I remember reading some things about space between rockwool parts but don’t remember the details)

[loopdude]

Looks nice
Please keep us updated

[lesstalkmoredisco]

Will do. Fabric will take some thinking. I realized that coarse burlap is pretty flammable I think, which isn’t the best combo with burning candles in my studio. I might order some cheap bed-linnen online as fabric. Seems more accessible then finding a fabric shop since only food shops are open currently with Covid measures…