Treatment under stairwell? Now with Pics and data!

[funkmuffin]

Hi! I think this might be my first post, but I've been reading the site for years. I have a recording/mixing room in my basement. I'm slowly over the years working to get it sounding better without spending huge amounts of $$ to do so. I'm considering another round of treatment to smooth out the low end response within the room. It's already pretty good, but I'd like to do better.

There's already a good deal of treatment in the ceiling and along the front and side walls.

Now, I'm looking at my back wall: see attachment

I'm wondering if doing some sort of treatment under the stairwell depicted in the picture would be a good idea, and if so, how would you approach it? Currently it's just wasted space, so I could fill/treat/cover it as appropriate to help with my acoustics. It's 4 feet "deep", so my thinking was that it could make a serious bass trap if done correctly.

Budget is not much of a concern, I'm more interested in the "best" thing to do there, keeping in mind that those stairs are the only way into and out of the basement, so they can't be removed.

Your thoughts?

[Soundman2020]

Hi there "funkmuffin", and welcome!

Before actually doing that, it would help to get an idea of how your room is responding currently. I'd suggest that you download REW ("Room Eq Wizard") from the Home Theater Shack website, and use that to test your room. REW is free, but you have to register at their web site to get it. It is a very powerful tool, and very useful. So I would analyze your room with that to see what problems you currently have, and then decide how to treat them.

It would also be helpful to know the actual dimensions of the room (length, width, height), and also what other treatment you have in there already. Photos would help too!

But most likely you'll need bass trapping (it is a small room, after all), and that space under the stairs looks like a prime candidate for doing that. If that were my room, I think I'd put hangers in that space, as well as some absorption on the rest of the rear wall (in the stairwell wall itself) and maybe even a "mini-superchunk" up that vertical corner on the landing.

There might also be things that you could do to your layout to improve the acoustics, such as re-positioning your speakers, desk, chair, or other things in the room, so please provide as much info as possible, so we can see what you have and what might be done to improve it.


- Stuart -

[funkmuffin]

Thanks Stuart! Good news is I already have REW and have done some sweeps, bad news is I only have done them with both speakers, and the kids are now asleep, so I'll have to run separate sweeps tomorrow. The sweeps I did showed (to my very untrained eye) a fairly flat-ish response, with a few big dips and peaks between 50 and 200hz that I'd like to smooth out.

For now, I've done a sketchup of the front half of the room so you can see the basic layout and existing treatments. The grey traps are 24" X 48" X 6" OC 703 and the three black traps are 24" X 48" X 4" OC 703 (that's a trap on the center of the front wall, not a screen). There is a drop ceiling not shown in the drawing. It is typical, but all of the space between the floor joists are filled with R30 fluffy insulation. The floor is concrete with Berber-style carpet over.

The room measures 352" long, by 132" wide, by 94.5" high. That's 29' 4" X 11' X ~7' 10.5"

Since you can't tell from the drawing, the speakers are set up such that the center of the gap between woofer and tweeter is at 48" above the floor and they are angled slightly downward. The listening position sitting upright in the chair is at ~38% (130") from the front wall. The speakers are equidistant from the side walls (36" to the center of the box) and their front baffles are ~100" from the front wall.

So, until I have sweeps, this will have to do. Sorry for the awful sketchup skills -- still learning, but I think you'll get the picture. Still frustrated by the stupid speaker stands, but just ignore the bases -- they're spaced properly for real. That was just the closest model I could download.

Thanks again!

Ryan

[funkmuffin]

OK, got some measurements done this morning. Hopefully this helps to understand what's going on in the room.

Calibrated with both speakers for 85 dB, then measured one speaker at a time. I tried to get the best views possible of the data. Please let me know if you need any other info. Thanks!

[Soundman2020]

the sweeps I did showed (to my very untrained eye) a fairly flat-ish response, with a few big dips and peaks between 50 and 200hz that I'd like to smooth out.

That's the part that matters! Below 200 Hz is where the vast majority of your problems will be, in any small room. But it's not just the frequency response curve that matters: that only tells you a small part of the story. The response in the time domain is the biggie. That's what can make or break a room, and that's what is causing the "dips and peaks" in the frequency response. The frequency curve just shows you the outward symptoms: the time domain issues are the actual underlying disease that you need to cure.

For now, I've done a sketchup of the front half of the room so you can see the basic layout and existing treatments. The grey traps are 24" X 48" X 6" OC 703 and the three black traps are 24" X 48" X 4" OC 703. There is a drop ceiling not shown in the drawing. It is typical, but all of the space between the floor joists are filled with R30 fluffy insulation. The floor is concrete with Berber-style carpet over.

OK, so you seem to have done the basic minimal treatment, but you are still going to need a lot more, I think. Your room is also big enough that diffusion might be an option.

However, it is a long, thin room, with a very low ceiling, so there are lots of challenges in there! There simply are no good ratios anywhere close to that, and there's no room for playing around, since the ceiling is just too low. You would have to cut the room down to under 19 feet in order to get close to a good ratio. You probably don't want to do that.

I'm predicting issues around 38 Hz, 55 Hz, 76 Hz, 93 Hz, and 105 Hz, to start with. Let's see how that stacks up against the actual REW results, when you have them.


- Stuart -

[funkmuffin]

Thanks Stuart!

Given the sketchups and the REW info, where would you start? I'm currently working on a local source for OC 703 panels to save on shipping which is looking promising. That will go a long way to helping get the basic treatment in place.

[Soundman2020]

Ooops! It looks like I must have posted that previous reply before I saw the REW graphs! That's what happens when I leave too many tabs open, and don't refresh before replying...

But anyway, as predicted you have large modal problems at around 40 Hz, 60 Hz, 80 Hz, 90 Hz, and 105 Hz. Pretty much spot on from the predictions! (38 Hz, 55 Hz, 76 Hz, 93 Hz, and 105 Hz). I love it when reality matches the science of acoustics...

Given the sketchups and the REW info, where would you start?

I'd need the actual SKP and MDAT files. You are only showing a very small part of each, and I need to see all of it, to be able to get a good idea of what is going on.


- Stuart -

[funkmuffin]

Thanks Stuart! I was trying to work below the forum size limit.

Here are links instead:

SKP: http://www.funkmuffin.com/pictures/Base ... 20Dims.skp

and the MDAT: http://www.funkmuffin.com/pictures/firs ... ments.mdat

[Soundman2020]

Great! Thanks! But you HAVE to do something about that mains hum problem! The 60Hz artifacts are overpowering your results in the time-domain graphs. I can't see a thing there. There are very prominent peaks at: 60 Hz, 120 Hz, 180 Hz, 300 Hz... And the one at 60 Hz especially seems to be hiding some of your modal issues...

So please fix your grounding issue, or whatever is causing that hum (it must be pretty loud!), then take new measurements and send the updated MDAT file.

But from what I can see, you do have clear modal issues at 40.9 Hz, 50.8 Hz, 93.2 Hz, 103.6 Hz and 149.9 Hz., with hints of a few others. So bass trapping is in order! You are also getting a strong early reflection at about 3ms, which I'm guessing is coming off your desk or console, as that's the only thing close enough that I can think of. The ceiling and walls are too far away to be causing that. Get a mirror and lay it flat on the desk, then sit in your chair normally. As you move the mirror around on the desk surface, I'm betting you'll find a spot where you can see the speaker reflected in the mirror...

There's another reflection at around 6ms, which is sure to be the ceiling (so there's a cloud in your future!), and a curious one at around 15 or 16 ms, which I can't figure out. There doesn't seem to be anything obvious at the right distance. There are also reflections at around 8 to 10 ms, which must be the side walls, so those panels on the first reflection points are not doing their job too well!

And (no surprises here) there are reflections at around 34 and 38 ms, which is obviously the rear wall and the side of the staircase.

But overall, it's not too bad. There are some puzzling things in there, though, such as the high frequency roll-off above 8Khz. That's hard to explain...

But anyway, please sent the updated MDAT file after you fix the hum problem, so I can see if there is anything else nasty hiding in the regions currently hidden by that...


- Stuart -

[funkmuffin]

Thanks for the insight, Stuart. Looking into the 60Hz issue... For what it's worth, the 60Hz we see in the measurement data is not audible... In other words, I don't hear any audible 60 cycle hum in the room... If it's electrical-supply related (Furman conditioner in front of everything down there....), it's not coming through the speakers when they're just on, with no audio playing.

I'm wondering if it's coming from the junky USB interface I'm using to do the measurements, since it's the only device I can get to work with REW in the virtual windows instance I'm running. We're all Macs here.

Thanks!
Ryan

Edit: Hooked up and calibrated with the Motu interface I use to record and monitor. Here's a new set of measurements:

http://www.funkmuffin.com/pictures/Feb%2028%20Motu.mdat

FYI -- The 60 Hz appears to still be apparent in the waterfall and spectrogram charts. I assure you this is not an audible hum -- I hear nothing like hum, even with my ear against the woofer with the sweep running.

So if it's not "in the air", it's the microphone/cable, which (apart from the speakers) is the only piece of the setup that was part of all measurements. I guess I'll do some tests to try and ID if it's coming from the mic...

[funkmuffin]

So if it's not "in the air", it's the microphone/cable, which (apart from the speakers) is the only piece of the setup that was part of all measurements. I guess I'll do some tests to try and ID if it's coming from the mic...

Well, found the source of the 60hz issue. It's my microphone. Swapped in several other mics, and none of them produce the 60/180hz spikes... Of course, none of them are measurement mics...

I guess I'll have to order another mic, then post up the actual test results when it arrives... At least I'll be able to get a properly calibrated mic this time.

Thanks!

[Soundman2020]

Well, found the source of the 60hz issue. It's my microphone.

I was about to suggest that that was probably the issue: ECM8000's are very prone to pic up mains hum, especially if you hold it by hand. You could try grounding the mic case itself somehow, or lifting the ground if it is already grounded.

By the way, I was wondering if you have carpet on the floor in that room?


- Stuart -

[funkmuffin]

I ordered a new mic. I suppose I could try grounding the Behringer in the meantime. It has been in it's clip on a stand from the beginning.

Yes, there is carpet on the floor. I've been considering pulling it up and installing a reflective laminate floor. Are you thinking the carpet could relate to the HF roll-off?

[Soundman2020]

Yes, there is carpet on the floor. I've been considering pulling it up and installing a reflective laminate floor. Are you thinking the carpet could relate to the HF roll-off?

Bingo! Yup, that's it. I'm pretty sure that's what your problem is. Carpet is "selective random absorption at all the wrong frequencies". Carpet absorbs highs pretty well, mids somewhat, and lows not at all, so it is the exact opposite of what a small room needs.

Lose the carpet. Jut put down some good quality laminate flooring, and see how that works. I'm betting you'll recover most of your highs.


- Stuart -