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RETIRED theater builder
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It is a stretch to say it is built because there is a lot more work to be done, The fabric, the molding, the ceiling, screen wall panels. But we reached a calculated point that Morph planned into the project time line to take a listen to the room before be bury the acoustic treatments behind fabric, to see if there was anything we should do.


First a walk around the room and the current acoustical treatments


Thick carpet and fabric chairs


The front screen wall is entirely covered with 1 inch of Linacoustic. The side walls are covered up to the first row with Linacoustic.








After the first row, above the chair rail is poly batting.






Back corners are bass traps and the back wall is Linacoustic






We haven't added any scrim reflectors to the panels yet but it is certainly possible at this point to liven the room. The fronts of the columns are not in position, they will be stained wood with back-lit fabric Shoji screens. The interior of the columns will be treated.


prototype:




the cherry fronts




Now for the measurements


The quiet room




The RT60 measurement (reverb)




Dennis says .35 to .4, we are at .23 so on the dead side.


the ETC graphs




we see a large reflection which we thought was the ceiling under the soffit

Morph held a wad of fiberglass under the soffit and that peak went down 4-5 db so we know what/where it is. Not sure what we can do about it, there isn't a lot of room to do much. The screen is tight to the underside of the soffit.




Bass Decay:




So Morph and I want all the opinions and expert advice on this as we move forward.
 

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Quote:
Originally Posted by BIGmouthinDC /forum/post/22047239


we see a large reflection which we thought was the ceiling under the soffit

Morph held a wad of fiberglass under the soffit and that peak went down 4-5 db so we know what/where it is. Not sure what we can do about it, there isn't a lot of room to do much. The screen is tight to the underside of the soffit.

I'm posting mostly to subscribe to the thread, because I want to see how this develops (and I'm taking copious notes
).


However, with regard to the reflection from the ceiling, is there enough room to put some treatments above the LCR behind the screen to attenuate the energy before it gets to the ceiling? You know, treat the source before it becomes a problem.


Also, any idea what's causing the peak in the quiet room response at 60 and what looks like 120? Could it be electrical noise at 60Hz and at the first harmonic rather than actual acoustic noise?
 

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Discussion Starter #3
JPA, good ideas and Morph did a brief experiment with putting a Linacoustic "visor" over the center channel and it did produce about the same result as a wad of fiberglass on the ceiling.


The 60 and 120 is a puzzle and may just be an artifact of my microphone and laptop. I got the same result this AM when I hooked it up at home, so at this point you have to question those blips. We were testing in the proximity of 4 large transformers for the chairs maybe some interference? Should have unplugged them.
 

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Being that the peaks are at multiples of 60 Hz, it does look a little suspicious. Like maybe the power supply to your laptop isn't filtering out the 60 Hz sine wave from the outlet. Although, looking at the graph again, that second peak may not be quite at 120 Hz.


If you're troubleshooting those peaks, you might try turning off as much electrical stuff in the room as possible, like lights and the chair transformers (which can both produce audible and electrical noise), and maybe even let the laptop run off of batter power long enough to make the measurements.


It may very well be a noise in the room, and not worth the effort to locate. I was just curious



Glad to hear my other random thought was a success!
 

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Like J_P_A, I mostly want to get subscribed to this thread - I'm exceedingly interested. Thanks to you guys for taking the time.


That said, there might be an improvement to be made in the way your ETC graph is put together. The spike at 0 is, I believe, the initial direct sound from the speaker(s?). There is something to do with loop-back (I think) in REW that allows the time to start when the sound begins to be emitted, and the first spike happens later in time - it helps to look at it this way to place everything in the same reference for time and space. Also, I'd suggest producing ETC graphs in this way for each of the seven (?) main channels individually. We may find that the surround and rear channels are less useful in tuning the overall response, but I think there will be useful information to be gained - not to mention it will be helpful to label each graph very precisely in terms of driven channel and listening (mic) position. And if we're being thorough, one should properly know the response of the speaker outside the room (both on and off axis) - that will help, I think to define the nature (frequency content) of specular reflections.


I say all this as though I know what to do with all this extra data when I see it. That's far from the truth, but it's all part of the full picture. Also, can we get room dimensions and speaker distances to the listening position(s)?


Someone please correct me if I'm wrong - I don't want to give these guys extra work to do without reason.


Regarding the 60Hz noise - was the sound card calibration run under the same conditions as the measurements? (in terms of battery operation, power supply, etc)


Fred
 

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Well, I was really hoping that someone more experienced than myself (as I have no - ZERO - practical experience) would offer some ideas.


In the absence of that, let me give you some suggestions for what to try. Feel free to ignore me, or maybe my wrong-ness will at least spark some conversation.


My feeling is that one should start with the bass. The porous absorption that will be used (unless you are open to more advanced tools, that I can't really help with) will continue to suck high frequency energy out of the room. It would seem backwards to me to go about balancing reflected energy with direct energy and then go add a bunch of porous absorption messing it all up - so start with the bass so that you know how much of the reflected sound energy you've lost and can go about carefully adding it back (by either removing linacoustic and poly batting, adding thin reflective panels, or adding diffusors or some sort). Plus, if you've got an option to move subs, it's best to do it now, not later. Is that possible?


I don't recall exact built details, but I see in Morph1c's first post in the build thread he's indicated one sub behind the screen. It would seem that moving it side-to-side behind the screen would be easy. What about moving it out into the room or adding another? I'm looking at the decay times and seeing the ringing at 28Hz, which I would assume corresponds to a response peak (but we don't see a graph of the frequency response - can we get white or pink noise through the sub and see the response?) There's a similar hang-over around 50Hz. If these are troubling when listening to music, they would be the first thing I would look into. If they are a problem, I'd suggest (in order of decreasing effectiveness): adding more subs, moving the sub(s), a little EQ cut, a tuned bass absorber (or two), more bass traps. Of course, if the bass response is acceptable, we can move on to specular energy (above approximately 500Hz).


The psycho-acoustics I have read about say that you should not have high gain reflections at the mic within about 10-20 ms of the direct sound. Second, as you've noted, the room is somewhat dead. Given that, the goal should be redirecting or diffusing specular energy (in the horizontal plane as much as possible) without absorbing it. As you noted, the first high-gain reflection comes very soon after the direct energy (only 2 or 3 ms). If that's coming off the ceiling, then at least you've found it. Allow suggest a tilted panel that would reflect the sound to the back wall, over the heads of the listeners. If you have a spare sheet of plywood, maybe you can rig up a way for it to hang down in front of the screen at an angle? Here's a visual reference:



To go along with the "quiet time" following the direct sound from the speaker, at least in a two-channel environment, listeners often like a "kicker" to clearly define the end of that period. (I couldn't tell you why, or if it will translate well to home theater, but that's the only frame of reference that ever gets discussed and cited around here) So, if you want a kicker, where do you generate the high-gain reflection that will produce that psycho-acoustic effect? A little math - sound travels approximately 1.13 ft in each ms. For the reflected energy to arrive at least 10ms after the direct energy, the path it travels must be at least (10ms divided by 1.13ft/ms equals 8.8ft) 9 feet longer than the direct sound. In order for the reflected sound to be of adequate gain, it will need to come from a large area. The large central area of the rear wall is probably adequate size. If it's 4.5 feet from the rear rows ears, I'd say it's a good candidate. All that is to say: try it again with no linacoustuc on the rear wall and see if that helps.


And naturally, as a general statement, it's tough for anyone to know with any confidence what changes in a graph are going to be audible with real content. At every step, listen to some real sounds for reference. Maybe pick the first three minutes of The Dark Knight and watch it over and over - it's got good bass, surround effects and dialog so that you can compare.


Now that I've vomited that into the thread, I hope it's useful.


Fred
 

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The HVAC Noise



The Quiet Room



Going back to the 60Hz and 120Hz noise, is there any reason that we can't compare these two graphs? I'm sure the mic locations weren't the same, but clearly there is much less noise in the new graph. The 60Hz and 120Hz spikes weren't in the measurement from the HVAC duct noise. Any explanation? If the testing methodologies were the same, I don't have any...
 

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Discussion Starter #9
Fred I'm thinking of potential variables. From the original measurement to the recent, we wired up the recessed lights in the light tray and plugged in four massive transformers for the chairs. We did try turning off the lights, didn't try unplugging the chairs. I'm still processing your other ideas on what to do with the room
 

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Quote:
Originally Posted by BIGmouthinDC /forum/post/22047239


It is a stretch to say it is built because there is a lot more work to be done

Screen and PJ are hung, isn't this where are work stops?
 

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Quote:
Originally Posted by BIGmouthinDC /forum/post/22049456


Fred I'm thinking of potential variables. From the original measurement to the recent, we wired up the recessed lights in the light tray and plugged in four massive transformers for the chairs. We did try turning off the lights, didn't try unplugging the chairs. I'm still processing your other ideas on what to do with the room

Would the chairs transformers even make noise when the chairs aren't moving? - seems like no movement implies no current being drawn...


Before obsessing over thinking this room is too dead, I've heard that different criteria might need to be applied for systems with surrounds. Specifically, I heard that it might be desirable to have a slightly dead room. Does anyone know anything about it?
 

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Discussion Starter #13

Quote:
Originally Posted by Morph1c /forum/post/22049516


Would the chairs transformers even make noise when the chairs aren't moving?

I'm wondering if my system is picking up some EM interferance not actual noise
 

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Quote:
Originally Posted by BIGmouthinDC /forum/post/22050177


I'm wondering if my system is picking up some EM interferance not actual noise

Dennis posted the answer. It is irrelevant. No sense in worrying about issues that are irrelevant. I would also look into acquiring a better sound card than using the one in the laptop as well as a multiplexer.
 

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Actually I pulled the OmniMic out this morning and the 60hz problem is now worse and in the audible zone. My first unit had an internal ground issue that caused the 60. I think this one may have developed the same issue.
 

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Cheap, unreliable equipment. Cheap, unreliable results. Equally to the point, you'll never get accurate results when you're attempting to measure beyond the ability of the equipment. Equally to the point, you should not be measuring room response until you've measured the speakers. BTW, a $2000 microphone becomes a cheap microphone if it is dropped, the speaker stand falls over or you just toss it in your overnight bag.
 

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Given your proximity to the rear surrounds, you might experiment with a 6mil or so plastic on the front of the bass traps to make them reflective at higher frequencies. Same could be true on the back wall at surround height. Not sure that will solve any of your measurements, but would liven up the surround presentation.
 

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Another random rambling, but I wonder if you can make the rough pass with the equipment you have. Get the low hanging fruit, and get comfortable with taking the measurements and how room changes affect the results. Then, could you find somewhere to rent a high end mic for a day? I know a lot of photography shops rent those high end lenses that us normal folks can't afford, seems like an audio/recording shop might rent a high end calibrated mic as well.
 
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