Acoustical Treatments Master Thread - Page 197

Pepar,

Are you using frictional absorption for the bass or something else, and how much? Those decay times look quite low, is there anything about the sound quality which you might refer to as stuffy or dry?

General question for anyone, can floor resonances of the overhead floor cause significant 10-30hz ringing (I've been thinking for a long time this might be my issue ) and if so can absorption be a practical solution other than redesigning the floor?

Twenty-nine lineal feet of 17x17x24 superchunked OC703. Check my website linked in my sig.


Yes, a bit. And that's why I am going to remove some of the 2" Linacoustic on my front wall, reduce the size of the front left and right first reflection point absorbers (2" OC SelectSound Black), mount diffusors above and below the newly sized absorbers and install some diffusors in the rear of the room. Not sure I'll net much because I am installing about another twenty-four lineal feet of superchunk in the rear. I really want to knock down the ringing below 80Hz.

So i figured i'd run a waterfall of the soundcard... This is what i got:



I do not know what i was expecting, but this doesn't look right to me. What do you think? What's it supposed to look like? Since it's the loopback, shouldn't it basically drop off right away instead of decaying that slowly?

EDIT:

Well, i ran the same test on a desktop computer, and its soundcard gives me this waterfall:



I guess that's normal for soundcards then? Still puzzles me a bit; I would expect the decay to be immediate...?

I don't think that "stuff" below 50Hz on the purple graph should be there. Is that a laptop's integrated sound?

Here is a waterfall of REW's measurement of my USB sound "card" from the process of generating a CAL file. It is more of a professional device, so the nearly ruler flat response doesn't surprise me. What does surprise me is how very non-linear the two are that you measured. Perhaps a waterfall of a looped back sound card is not a valid measurement; mine has a similar falloff like yours, but then has some sort of ongoing broadband noise that ... goes off a cliff like a gate closed just short of 250ms.

Yup, it's the laptop's integrated sound. I guess i should not be surprised that it has those problems in the low end.


Interesting, thanks for posting that. Looks like it might be normal for the soundcard waterfall to have that "horizontal cylinder" look then. And i'm wondering as well if the loopback test is not a valid measurement to graph a waterfall.

The red graph i posted is for the on-board audio of a cheap desktop i had sitting around. It's a Microcenter special, the sound chip is a RealTek. I'm not terribly surprised that it's not ruler flat. The "saddle" type curve seems to be inverse to the FR graph, interestingly enough. I do not know if REW is using the calibration of the soundcard in that waterfall to overcompensate perhaps.

My normal desktop has an Audigy card, and that card's FR is ruler flat as well. The problem was when i hooked it up to my AVR to test, i got a mid-frequency "hum", which i was unable to eliminate, so i couldn't use it to do further measurements

A laptop on battery power is the way to go. But the integrated sound on laptops is usually not duplex, i.e. cannot output on one channel while inputting (the test mic's signal) on the other channel. So nearly everybody uses a USB device.

Thanks for the info Pepar. Just for kicks I turned off the audio to produce a waterfall of the room/house itself. First graph is room with fridge running upstairs, next is with fridge turned off. As you can see the fridge being situation on the same floor span as the HT ceiling creates quite a spike at 20hz and it's harmonic at 40hz, both would appear to be excitation of floor resonanes at sub-harmonics of the fridge motor. I have no idea what's causing the 34 and 43hz spikes.

In professional movie location sound recording, one of the first things they teach is "unplug the refrigerator."


- Terry

That would be lesson #2, lesson #1 is alternative methods of keeping beer chilled.

Craft services.

Actually, alcohol on the set is generally a no-no. Otherwise you may never finish shooting the movie! Gotta hold out until the wrap party.

And the shaky-cam was born.

Actually, the real "shaky cam" was developed by director Sam Raimi.

Enough with the OT already! I'm the worst offender!!

Nice graphs and interesting audio insight from that darn fridge . .
Remember I mentioned a little while back about the "noise" level of the house in relation to RT20/30/60 discussion? You've got at least 65-70dB background noise! Would the other noise be from say the furnace?

Hopefully the fridge noise is temporary, It's been moved while some reno's are being done in the kitchen, if not some isolation devices maybe in order (the wife's going to love that one). I have no other mechanical systems operating in the home except another fridge in a room next to the theater which completely slipped my mind and I suspect is the culprit of the 34/43hz noise.
I've suspected for a long time floor resonances from the span may have been causing problems with decay times down low and evidenced by the excitation of the 20hz peak induced by the fridge I think I may have been correct. I originally designed the ceiling/floor to flex and dissipate energy at 20hz, I wonder went wrong .

Further to my recent room waterfall, here's the latest with both refrigerators off. Also, with the room at the most quiet I can make it (except for desktop running adjacent to HT to run program) here's a comparison frequency response of the room with and without the projector running.

For the theater designers, what ambient noise levels do you target for a theater with the projector running.

Hughman - About all you can do is build a hushbox or buy a quieter projector.

Is that LF rumbling from the projector??

Before we bother talking about decay times, we've got to get the FR looking right... it looks like you've got the "C-Weighted Mic" radio button in the MIC tab of the settings window, which should be unchecked for running a loopback measurement, or for that matter any measurement where you're not using a C-Weighted mic...

I think I'll have to leave it as is or by a more quiet projector. I have a shelf system which enables me to raise or lower the PJ to adjust for bulb aging (high power screen). I'm presently running the PJ on high lamp mode.

I surmise at this point the Sub 20hz noise is likely an artifact of the rather large lake I live on and the ice which is in a constant state of expansion/contraction.

I'm 99% certain i had it unchecked for the loopback test. But thanks for that thought, i will check it again tonight or tomorrow night when i get a chance.

A few points about reading REW graphs for everyone:
1: When posting a waterfall, make sure the frequency axis is in LOG mode. There is a known bug with the Freq axis in LIN mode that makes the plot unreliable.
2: Post an FR along with the waterfall with the same graph limits to make it easy to match up.
3: When posting your FR, before making the jpg, make sure to check the soundcard cal and mic cal buttons under the plot to display these traces on the plot. This allow people to see where in the very low or very high end your data becomes unreliable due to the effect of the cal files on your noise floor.
4: Don't sweat persistent noise very low, unless you absolutely have to and are absolutely sure your setup has a noise floor capable of these measurements.

Somewhat afraid to post here as I think youll have a field day with this but Id love to get your thoughts. I realize the room will never be acoustically perfect.

Ok---been lurking here for years. Now getting serious and actually close to finishing my theater. Heres the catch....its in an outbuilding/barn/playhouse behind our house. Its a neat building with a large playroom/gymnasium. The theater will be in the loft that overlooks the play area. The theater is 20 feet wide by 30 ft deep. The first 20 ft of the loft will be theater, with a bar gathering area in the back 10 ft. Jason from AVS did my prewire when I was very ignorant. Now nearly two years later and I know a lot more (but still learning) and Im doing most of the finish work myself.

Heres the dilemma: the loft is open to the gym below. Roughly 18 of the 30 ft of wall are open to below. Now I know the audiophiles are going to freak, but it is a cool thing to see. Hopefully these pics I post show up.

I dont care about limiting sound in the sense that its a separate building so I dont care about sound getting into the gym. We wont be playing hoops and watching movies simultaneously. I am very concerned when watching movies about echo bouncing back from the gym. So--Id like to be able to close off the opening during movie watching (I dont think it will matter much while watching sports as I likely wont be cranking the sound). Ive got some thoughts but would like to see what the forum thinks.

Thanks in advance....heres the pics to help explain:









Tony

I'd just remove all first reflection point absorption so your room isn't as dead assuming your waterfall is accurate.

Hi, Looking to improve my Room Acoustics as they are probally needed.. All I have availble is a SPL Meter along with a Test Tones CD. Will I be able to test my room with that or do I need a Fancy Gizmo to do the readings??

Here is a Pic of my Room it is in the Basement with the Right and Rear wall being Cynderblocks with Drywall over... On the Front and Left Wall When I built it I put in 2" Insulation between the Studs to the Ceiling, the Styrofoam kind of Insulation..

Would Someone be able to tell me what to Add or do I need to make Measurments first??



By the_nephilim at 2010-02-16



Also here is my Webpage with some Before and after shots. Also I havea 7' Drop ceiling and did NOT put any insulation inbetween the Joists above the ceiling Should I??


http://home.comcast.net/~glafsr/wsb/...ome.html-.html

Can't open the link...well, I can but it just takes me to the photobucket home page.

Frank

Thanks, Mike. The FRP absorbers are needed. There are other things I can do to increase reverb time if I want to.

Jeff

Fixed....pics now in post. Blame the new guy!

Tony

You don't need the FRP treatment. They don't do anything beneficial, probably more harm than good being only 2" thick. They basically take your speaker's off-axis response and strip away the higher frequencies and send you back stuff between 200-500hz. I find treating the FRPs hurt dialogue in my experience. I would at least suggest taking the side and ceiling treatment down and seeing how it sounds. You can always put it back up.

I had the benefit of using my theater without the absorbers for a while and then adding them one or, in the case of the front left and right, two at a time. I can assure you that they are doing a lot in the way of improving imaging of LCR. I understand your comment about them being only 2", but the bass traps handle what would be absorbed by increasing to 4".

I'm talking about 200-500hz reflections coming from the FRPs. Bass traps in corners will not magically remove those.


There's a lot of information here if u have a little bit of time.

http://www.harman.com/EN-US/OurCompa...ions/13686.pdf