John Sayers' Recording Studio Design Forum Archive

Soundman2020 wrote:

Quick question regarding RT60: I read on this forum that those graphs cannot be trusted from 200hz and below, is that correct?

The readings get less accurate at lower frequencies, but they are still plenty good enough to show what your room / sound system is doing. They won't be accurate enough to do a scientific research paper on the results, or write a thesis, but still entirely good for making decisions about treatment.

OK - good to know, thanks!

By the way, those ultra precise measurements leave me a bit skeptical sometimes because it would mean that if you move your head by an inch or two, the sound will not be ideal -

Exactly! Which is why there is no point in trying to design a room with millimetric precision. However, accuracy is important for the symmetric location of your speakers, and also for the positioning of the measurement mic every time you do a new test with REW. The mic must be in the exact same location in the room, in all 3 dimensions, precise to within a few mm. That is important. If not, then you can no longer compare the readings....

I understand this but in my specific case, the room is already used for composing and vocal classes so I need to remove the microphone each time I make measurements. I've put marking on the ground to help, though.

I'd really like to know what to do for those basses - I've heard of tuned traps like barrels with a whole, etc. but have never tried them. Anyone could point me to some examples?

Be careful of Helmholtz traps: lots of people talk about them, but they are very hard to tune correctly, and even harder to place correctly in the room. Panel traps, membrane traps and similar devices are your only realistic options, but they have to be big to get down to low frequencies. Many inches deep...

I see, good to know!

Am I right to think that I should first try to work on this 500Hz bump?
Apparently, I also have a serious dip between 610 and 730 Hz, anything that can be done to cure that?

For mid-range, just design the slot wall for general broad-band absorption. In other words, large percentage of open area, with the center frequency set to around 1 kHz and a gentle Q that covers at least an octave either side.

May I ask why those mid-range frequencies would need additional treatment? Aren't both the graph and the RT60 looking quite decent in this area?

I think I should look into those 500Hz, right?

It's not really worth doing anything in the mid range unless you have the low end under control already. The biggest issues, by far, are under 200 Hz. I would not aim to absorb too much specifically in the 500 Hz region either, as your decay is already low in that area, as compared to around 4k. I would do general broadband across the upper-mids, both absorption and maybe some diffusion. (However, the room is rather small for diffusion, except at high frequencies.)

OK - so let's focus on the basses first, then.

You also need to reduce the bass roll-off on your speakers a bit: Try setting that to about -3 dB.

Will do.

Which speaker did you use for that test that you posted? Left or Right? You should really do 3 tests for each measurement set: one with just the left speaker, one with just the right speaker, and one with both.

Oops - I've only used both. Will do L-R-LR next time.

2 things I've realized: closing the front window and the door at the back really changes the frequency response and the RT60 (it probably sounds evident to you but I'm amazed to see how dramatic it is).

Darn! I probably should have mentioned that right from the start! When you do measurements in a room, it is very important to have the room all closed up, with all the doors and windows closed. If not, then you are measuring the response of the combined acoustics of the room plus the other spaces around it. Sorry about not mentioning that in the beginning! Yes, it can make a very big difference.

No problem - it now sounds like common sense but I had cables running in and out of the room, thus the open door and window.

I discovered that positioning 15cm of rockwool right in the front of the desk lowers the RT60 of those 90Hz and below from 0.8 to about 0.3 seconds. I take this as good news and hope I'm right by doing so

That's fine, yes. The modal and SBIR issues are associated with the front/back axis of the room, so putting treatment on the front wall as well as the back wall is a good idea. And it works!

Should this treatment mostly be rockwool or should I plan some slat wall panels there, as well (I guess in the front only)?

I still have a very audible ringing in the highs and wonder if it's not that old heater located right under the front window (between the speakers) and/or the water pipes in the ceiling?

That's a possibility, yes. That heater radiator could indeed be doing strange things to the acoustics. But is there anything else in that room that is thin, or that has a thin air gap inside it?

Not to my knowledge, and I doubt it's the windows, as I've put rockwool in front of them but it didn't solve the issue.

I plan on completely removing the heater (or insulate it, as we don't use it anyways)

Take it out, if you can. Much better to not have it there at all. And you can use that empty space for more bass trapping!

I need to get the building owner's approval first but I plan on having it removed.

Basically, the idea is to add extra layers of rockwool (and air?) from about the middle of the wall and up to the ceiling corner traps. This way, we would keep the desired space for the sofa up to about 150cm from the ground.

That would work very well. I would suggest starting with just a simple frame filled with mineral wool, and see how that goes. If it doesn't do enough in the low end, then it might be necessary to make it into a membrane trap. Hopefully that won't be necessary, but keep it as an option.

I have an old rockwool panel (rockwool mounted on a thin layer of compressed artificial wood (not sure of the name)) that I move around to test options. Putting it angling downwards on the back wall had an effect on the 400-700Hz range but not lower than that. So I was more thinking of simply beefing up the backwall.
A question regarding this. Should I add about 15cm (3 panels) of rockwool, end of the story? Or going 5cm panel - 5cm air - 5 cm panel would be more efficient? Also, if I go for the air gap option, should the sides be closed, or is it ok to just leave the sides open?
Another alternative: I could create a self-contained slat wall built on a plywood panel? That would be plywood, a stud frame, rockwool, (thin plastic), cloth, slats. But as the plywood is hard, would that change the physical theoretical dimensions of the room?

Cloud: again, if you look at the SketchUp plot, we have the constraint of a window that opens towards the interior of the room and water pipes running at an annoying distance from the ceiling, so that leaves us with little space to play with.

Yeah, I noticed that: Not a lot of room there. Let's first see how things go with the rest of the treatment, and only think about using that area if you still aren't getting enough effect.

OK - great.

I'm wondering if I should create self-contained 15cm deep "slat panels" that I could attach to the side walls? Or is any sort of diffusion a bad idea in this zone?

Slot walls are fine on the sides, but do try to angle them enough that you don't get reflections from the speakers coming to your ears. You might need to build them as a sort of "sawtooth" design, with two or three smaller sections that are angled greatly, rather than just one section with a small angle on it. And if you tune it to high enough frequencies (for example, the mid-highs that are causing you so much trouble!!!!), make it broadband (by varying the cavity depth, and slot and gap width), with low Q (entirely filled with insulation, right up to the front panel), then that won't cause you unnecessary issues. Some diffusion is good, and the amount created by a slot wall is fairly low level, and also occurs mostly in horizontal bands (since the slats are horizontal, which is better than having it in vertical bands or 2D bands, as caused by QRDs and Skylines.

What high-mids are you talking about exactly?
I've seen John's sawtooth slat panels and this would be a project in itself but I'll keep this idea for further development.

An even better option is to dived the wall into three sections vertically, with slats for the top and bottom sections, and just absorption in the middle, where your head is closest, and where the real first reflection points are.

This sounds more realistic as of now.

Thanks for your very helpful inputs!

Last measurements: https://drive.google.com/file/d/0B6wx2y ... sp=sharing

Also, would such "acoustic barrels" serve as speaker stands and basstraps?
http://www.thomann.de/fr/hofa_basstrap.htm

According to what we've discussed for the back wall, I've created a pretty large 88x240x17cm panel. It has 10cm of Typ3 rockwool and 7cm of air gap. I put plastic on top. So, the sandwich is: plastic, rockwool 5cm+5cm, 7cm air gap -> mounted on the 5cm rockwool insulation that is already there (with cloth).

Graphs: https://drive.google.com/file/d/0B6wx2y ... sp=sharing
According to the graphs, the bass frequencies RT60 is ... worse! And I can't see any benefit in the mids and highs either, or am I missing something?
How come is that RT60 much worse than withOUT this panel? I can't see any logic here, I must say.
Not sure what to do with that huge panel, now :-/ I'll probably hang it in the lounge or something.

What do the doctors say?

So... either everybody's dead or I should upload those pics ?