It's a good idea... if you like black mold and lung cancer.
VB's exist for a reason.

You asked a lot of the right questions, and then one really bad one... in the right place for no one to set you striaght.

Reflections are what make a speaker and room sound really good. Leave first reflections alone; there are far better places for room acoustic treatments.

Like all good things, that assumes a degree of moderation. One can overtreat rooms very easily, and BassThatHz has given you a great example of what the spectrogram of such a room looks like. Bass has done a lot of work on his room and system, and made some very good contributions here, but he's successfully implemented a very unique set of tastes, preferences that are not shared by the vast majority of listeners.

Your room, your call. My advice is:
- get a basic acoustic measurement system, so you can create the spectrogram of your room
- decide on a goal for the room decay rate.
- bring your data back here for specific suggestions (ignore anyone who suggests treating sidewall reflections)
- apply room treatments and remeasure, to see if your goals were met.

One common goal is 0.3 sec for 40-60dB of decay, across the spectrum from ~10Hz to 10KHz. BtH's room is roughly at that level below ~200Hz, but his room is dead above there, with a hole in the 800-1600Hz octave. Having seen pictures of his HT, I can project what's causing this response, but the key piece is that this was his goal, his room is right where he wants it to be. That's his choice, and one great thing about DIY; it lets you go where you want.

I made a different choice, since my room sounded pretty good, and asked why I didn't need to follow BtH's approach. The fact is, with reasonable goals, many residential rooms have reasonably good acoustics. Turns out, conversation drives us to furnish our homes to decay rates close to the 0.3 sec. level. If your room sounds funny (and most basement HTs do), there's likely work needed. If the room sounds good, what's to fix?

Have fun,
Frank, a minority opinion, perhaps, but a necessary one if you want the whole picture.

Thanks for your feedback, I know i have much to learn. What i've done so far is worked to find where my first reflections are. And once i learn some more, i'll figure out what if anything to do about them. Since my room is not square, i plotted each speaker (in wall) across the first boundary angle wall and again off the side wall. I'm not sure if this approach is correct. In any case, i've attached a copy of the sketchup file should anyone want to take a look.

Here is a link to the sketchup file: https://www.dropbox.com/s/3vaaxi6dep...Model.skp?dl=0

I also measured my new subs, they are in front of the front wall at the corners where the front wall meets the angle wall: https://www.dropbox.com/s/vyog71lbwm...si18.mdat?dl=0

Attached Thumbnails

   

I'm using diffusion panels (see my article referenced above) at the reflection points, and it works very well. The sound is uncolored but richer (if you follow that - I mean that the treble/bass balance isn't altered but the sound is made more spacious without sacrificing clarity or intelligibility). Behind the listening chair (about 4 ft back) is also a great location for these.

Those panels you built are great. I wish I could find a more commercial looking implementation picture to show the wife to get buy in. I personally think they would look really neat floor to cieling on each wall of my cove if I painted them to match the wall. If I somehow got buy in, would I put them on all of those walls and ceiling or would I need to mix in absorption?

Also, It looks like my subs have some ringing in the 28hz range so I may have to find a bass trap.

And this is the spectrogram that results from the first measurement (they all seem similar, save for level). I did a second that shows even stronger resonances, but the pattern's the same.


You've got a 28Hz resonance and an 18Hz resonance. These correspond with distances through the relation distance = 565 / frequency (which is just speed/2 = wavelength x frequency, with spped/2 because the fundamental will be a half wave resonance) of about 20 and 32 feet. Some folks might like that 18Hz resonance (I have one at 15Hz due to room coupling), but the 28Hz resonance has real legs and needs treatment. I'm seeing 40dB decay in ~1.5 sec.

There is one thing you should confirm, using a full range speaker. From this data, it looks like your room is pretty good above 100Hz against a 0.3 sec decay time goal. If confirmed, that means you're not in the market for the vast majority of room treatments, because they'll absorb in the wrong frequency range.

Also, did you move the mic at all? Mic position has to vary to get an accurate picture, because modal variations can occur over very short distances. My standard is a diamond pattern around the main listening position, front/rear and side/side. You'd be amazed at the variation over that simple pattern, selected because I fidget and move my chair, but I stay within that area most of the time

Note I haven't suggested any room treatments yet because I think you're still optimizing things. If this resonance pattern persists, it'll be a challenge to hit those resonances because you've got a hole between them, at 24Hz, that decays in about 0.5 sec., in addition to not wanting much absorption above 100Hz.

I want confirmation this pattern's real before starting to discuss your options.

Have fun,
Frank

Thanks Frank! They were all from the same spot. I was trying to see how much SPL I could get at MLP. Also, these sub's are brand new and need to be broken in. Truth be told, the wife left for a bit so I snuck down and fired up rew and some bass demo disks. These things are incredible. I read the "jerry" rew guide last night and am becoming familiar with the waterfall, decay and impulse graphs. I've a long way to go. I will try to break them in a bit, gain match them to my other speakers, move them a bit out from the wall and try a diamond pattern as you suggest. I'll also follow the guide and do more than the sub's so we get a better picture. I'm also going to look at measurements from my old sealed sub's that these replaced and see if the measure similar since I can start to understand the rew plots better.

Cheers!

Here is a little better file I did a
in January without sub's with my in-walls running full range.

https://www.dropbox.com/s/i73sf4x4pa...ains.mdat?dl=0

We should talk a little about acoustic decay rate measurement. The secret to getting accurate data is starting loud enough. If you want to see 40dB of decay, you measure at a level ~50dB above the room's noise floor. Unless you have a soundproofed room, 70dB isn't enough, you need 90-110dB signal to see a real 40-50dB decay in a typical residential space. Otherwise, you get that this last data set shows - tremendous false resonaces. Compare a spectrogram from this data with the loud sub runs and you'll see what I mean, once you line up the frequency axes (this run started at 30Hz, the subs at 10Hz)

The other thing I like to do is set countour lines at 10dB spacing, so finding the 40dB decay is as simple as crossing 4 contour lines. I default to scaling based on maximum value, too.

Again the goal is to evaluate the room once you're happy with it, to see what residual issues may be present. My room's construction and furnishings did what was needed, so I haven't changed a thing. You, too, may be surprised with what you get, at least above 100Hz.

Have fun,
Frank

Got it, I will find the time to do a full set of measurements at a good enough volume and will start over trying to analyze the data.

Thank you!

Don't mean to hijack the thread but can you post a link to that REW guide you referenced? I have already added some diy absorbers using 0c703 but want to measure what affect it has had and if I've got any resonances, and if adding any more panels would be necessary. I've got omnimic, and I'm assuming I can use that to measure things appropriately (applying some principles of the REW guide). Thanks!

NP. https://www.avsforum.com/forum/91-aud...ostid=22823228

Keep in mind that the stock umik maxes out at 105dbZ. You have to mod it to get the full +120dbZ out of it. (0db gain, not 18db).

With a gain of 12, it will do 120dbZ ceiling to ~40dbZ noisefloor. Which is what I'd recommend for most people.

Since I run my subs so damn hot it gives the appearance that I have weak treble.
I should make one without any subs on, and played at 95db and see what decay I get, especially in the 1-2khz range.

0.3 to 0.4s is a decent decay target to aim for.
My 0.2s decay is a bit on the dry/neutral side for sure...

Does that also go for the umm6? I have the stock Dayton version. I read about the cal company after I bought it. Doh.

Quickly took some measurements tonight. I set my inwall l and r to go full range, center crossed at 80hz and had double bass turned on the onkyo. So I basically had 4 bass sources including my two sub's. It seems that was not a good idea. My rew measurements look much worse. I just moved the mic around on the table sitting behind the couch. Had it pointing strict up on its little tripod. It's just a smidge behind the mlp. I'm going to have to retry all this with the mains crossed over to the sub's with the double bass setting off(it disables anyway when you choose anything other than full range).

Here is the mdat anyway.

https://www.dropbox.com/s/cncjudnc0e...Audy.mdat?dl=0

They say "payback's a bitch," but so is data interpretation. Two things I missed when reviewing your spectrograms
- the range is 91-51dB, or 40dB, too short to show actual 40dB decay
- I would swear you'd included contour lines, but that's not the case. Contour lines at 10dB spacing I find very helpful in interpretation.

I suspect if you do a 95dB sweep sans subs, you'll see more like a 0.3 sec decay for 40-50dB, based on extrapolation of the data you provided. And I have no arguement with the room curve you've dialed in. Dry would be a good description, I suspect!

HAve fun,
Frank

PS Thanks also forthe rational reply. Not every forum poster would have spoken to the data, rather than the poster.

The idea behind x.1 for bass is threefold:
- send the most power hungry program to a special driver(s) designed to handle the load
- separate the modal (bass) frequency generation from the specular frequency generation
- allow the program developer (who does the final x.1 mix) added capability for LFE - low freqeuncy effects.

That second one is what you're playing with here.

The audible spectrum within a room is divided into three frequency regions which are clearly evident on your unsmoothed sweeps.
- the modal region, where room boundaries define resonances, and drivers set up a sound field of frequencies with wavelengths comparable to room dimensions. Sound quality in this region is dependent on driver placement relative to room boundaries, not relative to the listener.
- the specular region, where room boundaries can be seen as mirrors, reflecting sonic energy in a manner consistent with Snell's law (angle of incidence = angle of reflection). Speaker placement is determined relative to the listener, with specific speaker locations (LCR and surrounds for 5.1) required for proper playback
- a transition region, inbetween.

Modal - looking at measurement #1 , note that below ~100Hz, your unsmoothed curves are nonetheless comparatively smooth. This smooth trend continues through the data; you're not seeing the strong peaks/dips characteristic of bass mode issues. There are resonances - tones that persist - but not cancellations, like those sharp dips seen higher in frequency. It looks like you've chanced upon a pretty good place to put those subs!

Specular - this is why we smooth FR data. The hash you see above ~400Hz is real, but ubiquitous; it's what we're used to hearing. That hash is due to wave interference; every sharp dip is a frequecy cancellation due to a second wave at the same frequency that's out of phase with the first. The key is that these cancellation modes get closer together as frequency increases, until they become inaudible. High modal density is characteristic of the specular region, as well as the ability to make predictions based on straight line propagation. Dolby speaker placement guidelines assume this behaviour. Note that speaker location relative to room boundaries can have effect, but you're in-wall so much of that goes away.

Transition - modal density is increasing, but modes are still audible. Speaker output interacts strongly with room boundaries, not to set up standing waves, but rather peaks and dips. Allison effect, floor bounce, rear wall dip all occur in this range. Here's a link to DIY speakers, but I want you to look at the excerpts from Toole's book at the top.
- first pair show a freestanding speaker placed as designed, freestanding, and mounted in-wall, like yours. What's they don't tell you is that the big hump in the bass range is a design variable (baffle step compensation) that can be changed to make in-wall placement as flat as freestanding.
- the second pair show the effect of surface mounting vs. flush mounting a speaker on the wall. Note the dip at 200Hz due to an out-of-phase wall reflection, is in the transition region
- the third pair show the effect of bookcase mounting, with one possible remedy. Note the dips are again in the transition region.

So, let's see what you get with proper 5.1 bass management! Set everything to small and try again. Overall, this isn't that bad; the slight downward tilt at higher frequencies is a very common room curve target. The only thing I'd want to change so far is the 400-450Hz bump from L and R speakers, but not center. This could be boundary reinforcement - not all waves are out of phase - and it's consistent, appearing in app measurements with L or R.

I also did a spectrogram of #1 , and you will want to treat this room once it's fully furnished. I spoke earlier about strong resonances below 30Hz, but I also see a rise in decay time exiting the transition region. By ~400Hz, it's taking 0.6 sec for 40dB decay. You'd like half that. The trick is you're running close to 0.3 sec. in the transition region, 100-200Hz. You'd want bass-dead treatment to hit the 400Hz+ range, and bass-specific treatments below 100Hz, to avoid messing with the one frequency range that's already close to a reasonable target decay time.

Thankfully, there are ways to do that. The thickness of acoustic panels has a strong effect on LF absorption, as does the presence of air space between the panel and wall. Once the final requirements are known, you pick the thickenss and spacing that meets your goals. We can then discuss placement, to perhaps to address the 400Hz peaking if it's a boundary interaction.

Bass issues are not as easily addressed. Porous absorbers like those used in panels work better in areas of high air velocity (back to wave motion, pressure vs. velocity and their respective nodes). Bass modes have velocity nodes at the walls; the wave is all pressure, so wall-mounted bass traps need to be pressure sensitive, not velocity. I suspect you'lll find commercial product capable of absorbing below 50Hz, and there are several shown in Everest. you may not DIY, but you can always get a contractor to do the work, based on your plans.

Good stuff!
Frank