Room tuning/treatment help - what should I do and where?

[theconqueror]

Recently refinished a basement room and it has some weird angles to it. Before I get too carried away with anything I'm looking for some guidance on how I should treat this room.

A little bit about it - this is in the basement, so it's studded out with normal 2x4 with 1/2" sheetrock and fiberglass insulation and then behind that is poured concrete. The weird little jut outward with the door in the top right of the diagram is where the boiler is and that area has 5/8" sheetrock and rockwool insulation. The ceiling height in here is 6' 8". The floor is ceramic tile. We have some carpets down right now, but they have no padding underneath them. We have no curtains for that window just yet either because I wasn't sure if I was going to need to make some heavy duty ones to try absorb sound. I'm also planning on building a media console for the TV and have it wide enough to sit the speakers on it as well, that's why everything looks ghetto rigged. The speakers and TV are in their approximate location, but I can still move them. The main listening position will be the little couch/love seat

Receiver I have is a Denon AVR-S660H. Speakers are JBL 530s. I have the matching center channel as well, but haven't hooked it up yet as I'm not entirely sure it's necessary? Subwoofer is a borrowed Definitive Technology SC4000 until I figure out what I'm after.

I took some measurements with and without audyssey enabled. I had a hard time getting it to run all the channels at once, so they're labelled individually. If there are any other measurements or info I could provide, let me know.

mdat file attached but here is a quick preview

 

[dsnyder0cnn]

Frequency response of the two channels (without correction), taken independently, looks totally workable, especially given the low ceiling height. You're about 5 dB down between 100 and 300 Hz before summing the two channels, but that's not a huge deal.

The challenge in an asymmetric room like this is that the phase relationship at low frequencies is not matched between the left and right speakers. The wavelaunch from the right speaker travels into that open area on the back-left side of the room while oblique radiation from the left speaker is trapped by the back-right corner. The result is that the output from left and right will not sum nicely below the room's transition frequency without some spending some money on bass traps.

To help illustrate this, here's the excess group delay (approximate) for the left and right channels:

You can see that the right channel looks pretty good, besides a couple of very high-Q (likely inaudible) spots between 100 and 300 Hz. However, the left channel will be fighting with the right channel at 54 Hz and again at 95 Hz and 115 Hz. I've used Acourate to do a magnitude sum of the separate impulse responses to show the effect of these mis-aligned phase relationships:

Frequencies where you see that the Green curve (L+R) is significantly below either the L or R curves indicates ranges where the speakers are fighting with each other due to their phase relationship. Here, the 54 Hz, 95 Hz, and 115 Hz spots are easy to see, even though my normalizing of the curves may be a little off.

To solve this, I'd suggest investing in significant bass trapping for the front-left and back-right corners. Something like this, stacked floor to ceiling should make a big difference: http://realtraps.com/p_megatraps.htm

The RT60 for your room is around 500ms, which is pretty good. I don't think you'll need to invest much in treatment besides bass traps. I would want to add bass traps and re-measure before investing in anything else anyway since that will things a lot. Good luck!

 

[theconqueror]

Awesome, thank you so much for that reply. I'm going to chew on that info for awhile.
I certainly have room for a bass trap in the front left corner, but the back right is a little tough because there is a door to a closet there. It's a coat closet and packed with coats, so maybe if I open the door it'll work as a pseudo bass trap.

I was thinking about putting up regular absorption panels to try and cut the RT60 down, but I'll make bass traps a priority now

 

[welldun]

Since the room is already built, I would think it would make sense to just get all your furniture and speakers situated where you will have them and then take measurements from that point on. This will give you the actual response for the room with that setup (which could change if you move furniture around and/or the location of the speakers).
Without the measurements from the actual listening position, along with proper distance settings in the AVR, it's all a guessing game.
If you're calibrating the system for one main listening position and you will be using a subwoofer, then more than likely you won't be using the speakers set to large (full bandwidth). This means that a crossover set in the AVR will pull some of the energy out of those speakers below that crossover point. If you can then find a good location for the subwoofer where it gives you a nice frequency response, at the MLP, then integrating the sub and mains at the proper crossover point could result in an overall decent response.

 

[theconqueror]

Yeah, everything is situated in its "final home" I just haven't permanently put it there in case some weird acoustic issues popped up. I have the flexibility to move things and get better sound before I lock everything down. If someone on here tells me I can fix the 54hz null by moving the mains another one foot away from the wall I dont have to destroy and rebuild a custom made media console.

And yes, that .mdat file I posted has the sub measurement in there too

 

[welldun]

Ok. My point about finding a good location for the subwoofer, is that if you have a good frequency response from the sub all the way past say 100Hz, then you can just use the sub to deal with the null at the 54Hz mark (along with any other issue that the mains might be causing below 100Hz). This would then allow you to run the mains as Small and set the appropriate crossover at the AVR (say 100Hz in this example), so that all speakers work well within their frequency range. If you can do that with the sub. Then the next thing would be finding the desired crossover point and integrating the sub with the mains at that point. Assuming that you will use some sort of auto calibration to set the speaker and sub distances, you can then use the REW Alignment tool to find the best distance setting for the sub (if the AVR got it wrong) so that the sub and mains integrate best.
Based on the graphed, it looks like both speaker and the subs have the same peak between 30Hz and 50Hz, this could be a room mode aka standing wave due to where the MLP is from where you took the measurement.
If that's the case, and you have a way of EQ'ing that peak down (either by moving the MLP or using DSP) the your overall frequency response would be smoother in that range.

Based on the graph, the sub itself doesn't seem to add much low end extension, so if you decided to go without it, you would want to do the same (EQ by moving MLP or use DSP) and you would probably get similar results.

** Note: You might want to also set the mic at ear height when taking those measurements. The same would apply to the Audessy mic.I know that some will frown upon setting the mic on anything other than a proper mic stand, but in your case, you can just as well set stack some books, boxes, or pillows and set the mini mic stand on that. the goal is to have a clear 360 degree path to the mic.
Also, based on the pics that you supplied, the couch being so close to that rear wall could be adding (exaggerating) to the low end response. try taking the same measurement with the mic positioned a foot or two further away from that back wall to see how the measured response looks then.

 

[theconqueror]

Got it, got it. That makes total and complete sense. Audyssey (I can never spell this right) set my mains as "large" and I think that sets them to 40hz crossover. I've been messing with stuff so much I'll have to double check that. I have a new sub on order which should take it down to 20hz (SVS PB-1000 Pro), so we'll see how that changes things.

When taking the measurements I propped the mic up on the little cardboard audyssey stand so it was at ear height.

So homework will be to:
- mess with the crossover frequency to see if that changes the peaks
- move the couch out some and see if that does anything
- I'll also try opening the closet door to see if that does anything too

 

[welldun]

If you still have the borrowed sub, run a sweep of it with the couch/measurement mic further from the wall. compare that result to the previous one to see how moving away from the wall affects the frequency response (FR). If the original mic location was at a room mode (peak), then moving the mic away from that location will change that peak on the new measurement. If that resulting FR is smoother throughout the range, then you have potentially found your new MLP. If you can live with the MLP in this new location, you can now measure the mains (set to small) with the crossover set somewhere between 80Hz and the 100Hz (for example), If the FR for the mains looks good too, then you can move on to setting the proper subwoofer distance so that it crosses over nicely at the desired crossover point.

If after finding the new MLP location and running audyssey it ends up setting your mains to Large, just go into the settings and set them to small, and set the proper crossover. Then Enjoy!

 

[theconqueror]

I got my new sub and new center channel and took the same measurements as before plus one where I opened the closet door that is in the back right of the listening position, and then where I moved the MLP forward a bit. The sub doesnt see a dip at 54 hz at all and it's not as pronounced with the center channel. Opening the door does change the group delay. It changes the peak at 54hz to a null. I took a look at those bass traps you posted, dsnyder0cnn and they won't fit in the corners I have. I'm looking into building some membrane or diaphragm type, but finding a design that has bulletproof consensus seems difficult.

Using amroc I found that 54hz is a room mode/standing wave for a 10' 7" distance, which is the width of the wall behind the MLP. In the test where I moved the MLP, I moved it out forward enough to get past that wall, but not much changed on the graph.

The receiver currently has the mains crossover set to 40hz, I wonder if I just bumped that up to 60hz or 80hz and allowed the subwoofer to handle those frequencies like welldun was saying, if that would be the best option. The sweep I have of the sub doesnt show off everything it can do and it has DSP built in. The last measurement of it I did had audyssey enabled and shows it's quite flat up to 80hz.

 

[welldun]

looking at all of the measurements, 2 things stand out.
1st, looks like with the exception of the sub measurement with audyssey active, the rest of the sweeps were done at a relatively low volume.
2nd, rather than a null at 54Hz, I'm thinking that for the mains, the 35-50Hz range is actually a peak that makes 54Hz look like a null.

With that said, If you are committed to using a subwoofer, then why not let it do it's job to the fullest? I understand the allure of wanting to get your mains to play as deep as possible, but when you have a competent subwoofer that can do the job better and more efficiently, why not let it do just that?
Based on the measurement, it looks like the sub does a fine job up to at least 80Hz (not sure if you have the crossover set to 80 or lower for that measurement), so if it was me, I'd set the crossover to 80 and set the mains to small, and them just allow audyssey to do it's calibration. After that, you can use REW to make sure that the AVR set the subwoofer distance properly so the crossover point is smooth. If it didn't set the correct distance for the sub you can use the alignment tool in REW to find the correct distance.

 

[theconqueror]

Yep, I already set the mains for 80hz from your previous suggestion. I have to double check that I did it right though, the menus get confusing. If it is set correctly, I should see the mains slowly build up to their peak SPL at 80hz, like they're doing for 40hz in all of the graphs I've already posted (Audyssey set them to 40hz), right?

1- Does a low volume cause any inaccuracies? I left the volume set at where I had been listening to it the night before.
2- If you take a look at the right speaker the drop off isn't nearly as pronounced.

Is there any benefit in testing the system as a whole rather than testing each speaker one at a time?