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post #1111 of 1177 Old 07-06-2014, 01:21 PM - Thread Starter
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Grills on order: (2x) 4"x24" bar grill and (1x) 8"x18" bar grill (all three brown, plus one balancing damper for installing inline in the return duct, just in case. I figured I need a coupler anyway, since the 25' duct I started with wasn't quite long enough, so why buy a coupler when the damper is only a couple bucks more. The damper will be easily accessible under the stairs, a few feet from the air handler. Price is actually very competitive with what's available on the shelf at Lowe's, plus this is actually the right stuff...
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post #1112 of 1177 Old 07-06-2014, 01:35 PM
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Originally Posted by HopefulFred View Post
Grills on order: (2x) 4"x24" bar grill and (1x) 8"x18" bar grill (all three brown, plus one balancing damper for installing inline in the return duct, just in case. I figured I need a coupler anyway, since the 25' duct I started with wasn't quite long enough, so why buy a coupler when the damper is only a couple bucks more. The damper will be easily accessible under the stairs, a few feet from the air handler. Price is actually very competitive with what's available on the shelf at Lowe's, plus this is actually the right stuff...
Great to hear......one step closer!

BTW, I screwed up on my info JFYI. That 85 square inches for returns should be for each grille.......there are four return grilles, not two.
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post #1113 of 1177 Old 07-06-2014, 01:40 PM - Thread Starter
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8x18 is about the largest I could fit, in any case. I'm not really doing any real math here, just putting it together in a gettin'-it-done kind of way.
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post #1114 of 1177 Old 07-06-2014, 01:56 PM
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Originally Posted by HopefulFred View Post
8x18 is about the largest I could fit, in any case. I'm not really doing any real math here, just putting it together in a gettin'-it-done kind of way.
Well, if room volume was directly related to cross section of supporting ductwork, you're well with in required air exchange.....but it's not. I'll bet you are fine........
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post #1115 of 1177 Old 07-06-2014, 05:33 PM
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With 2 supplies and 1 return, the return should be approximately equal to the total area of the supplies, right? Or is there some adjustment to be made since I'm combining the two into one? (I wouldn't think so, just checking.) So I'm expecting to need somewhere around 200 square inches for return.

Shopping at HVACQuick.com, I'm looking at (2) Dayus DABL - Bar Linear Grill for an opening of 24" by 4" or do I need to go larger to get that much actual grill area? (pn in my basket is DABL-00-A-24x4-Brown-STD)
Area of the diffuser has nothing to do with the CFM. Very simply, you calculate the CFM of supply you will need for the room based on the calculated heat load. Your return CFM should approximate the total supply number otherwise you can get into a positive / negative pressure situation in the room where the door will either be very hard to open or very hard to close depending on the room's pressure and because the room is so tightly sealed. The best approach to get equivalent CFM is to install an in-line baffle on the return side at the end nearest the Air Handling Unit. All the diffuser size "area" tells you is what the face pressure will be and nothing more. 4x24 is the minimum for a 6" flex at max CFM, but you could easily go bigger and lower the face pressure even further....although sometimes this can be at the expense of slightly less efficient air mixing if the CFM is spread out over too great of an area (i.e. too big of a diffuser).

Hopefully this makes sense as I just got finished with a 10 hour car ride, complete with screaming baby *sigh*....
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post #1116 of 1177 Old 07-06-2014, 05:40 PM - Thread Starter
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Thanks, TMcG - that does make sense. I was just trying to back into a determination for face velocity (<250 fpm) at the intake grill, working from the assumption that the pressure would be equalized. In any case, I feel pretty comfortable that the grills I have ordered will be appropriate, and I have the balancing damper to go along with them - I think I'll be good!
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post #1117 of 1177 Old 07-07-2014, 06:15 AM
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I feel your pain on screaming baby
Where you headed ?

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post #1118 of 1177 Old 07-07-2014, 08:07 AM
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As usual, I'm late to the party. Best I can tell you should be good on the grills as well. You've got .7 s.f. of grill for each 6" supply. At 75 cfm, that gives you a face velocity less than 110 fpm.

With balanced flow, the return is drawing 150 cfm from the room. An 8"x18" grill gives you 1 s.f. of grill, so that puts you at 150 fpm on the return.

Both look good to me (I'm sure you did these calculations yourself, anyway). I think you've got a little extra margin considering you will likely have less than rated flow form those supplies.
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post #1119 of 1177 Old 07-07-2014, 08:33 AM - Thread Starter
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I did not do that math - so thanks. If i do math, it's usually in public so someone can check my work.
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post #1120 of 1177 Old 07-07-2014, 02:30 PM
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Originally Posted by J_P_A View Post
As usual, I'm late to the party. Best I can tell you should be good on the grills as well. You've got .7 s.f. of grill for each 6" supply. At 75 cfm, that gives you a face velocity less than 110 fpm.

With balanced flow, the return is drawing 150 cfm from the room. An 8"x18" grill gives you 1 s.f. of grill, so that puts you at 150 fpm on the return.

Both look good to me (I'm sure you did these calculations yourself, anyway). I think you've got a little extra margin considering you will likely have less than rated flow form those supplies.
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I did not do that math - so thanks. If i do math, it's usually in public so someone can check my work.
CFM = velocity x surface (in square feet)

Therefore

Velocity = CFM / Surface (in square feet)

A 6" flex duct has a max velocity of 75 CFM and about 0.2 square of surface area (28.27 square inches). So velocity (face pressure) is calculated at 75 / 0.2 = 350 fpm.

If you used a 4" x 12" bar diffuser with 75 CFM, that equates to 0.33 square feet and calculates to 225 fpm which is under your 250 fpm target. However, it gives you very little overhead to deal with any air turbulence.

The 4" x 24" diffuser is a nice size that gives you plenty of overhead at around 107 fpm without being obnoxiously huge.

EDIT: I forgot to mention that the face velocity calculation is just that....a calculation....and that the only way to get the real numbers is with an anemometer as no supplies ever run at perfect maximum and no AHU or duct layout is ever the same. The cost difference between what you ordered and the absolute minimum is negligible compared to the benefit of having the diffuser slightly oversized to be well under the 250 fpm max target.

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post #1121 of 1177 Old 07-16-2014, 02:38 PM - Thread Starter
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Mfusick reminded me that I haven't been updating my thread here (a week passes very quickly, doesn't it?). I don't have much to show, but here it comes.

Some HVAC work progresses slowly. I've received the 8" balancing damper that I will install in the return line near the air handler. So all that is on hand and ready to install, but no progress, no pic. The supply grill and boot that we talked about last week was for the left side of the theater. The right side is different, in that the soffit encloses plumbing (4" drain, headed to the sanitary sewer).



Since the whole purpose of the soffit is to enclose that pipe and limit its noises, I need a separate register boot for the duct in this area. I built this one, but have not completed the flanges and whatnot that will be required to secure it into the soffit. It looks like it will be a nice tight fit, once in place. It's 40" long, about 7.5" tall and 8" deep, more or less. I've glued it together and caulked any suspect seams. I will line it with 1" duct liner (and plastic?) before I install it. Also still need to cut the 6" hole in one end to attach the duct to.



I am almost done carrying sand. It's been a long process, as I only move sand in my VW and I don't have excess space to store any while it dries. Sometimes I empty a bag or two and then end up waiting a day or two for it to fully dry before I add any more. It looks like probably two more bags will completely fill the stage. I also just realized I haven't posted any "in progress" shots of the stage. You'll see from this picture that my construction technique is non-standard. I built using spare 2x4s instead of 2x12 or larger. It just made sense from a practical standpoint, and since I still haven't completed a design for visual features of the stage I figured I could just add on any steps or curves or whatever I might end up with later.

Mostly, the stage is just a basket of 2x4s, stacked on edge 4 high. The studs were 10 feet (IIRC), so they don't reach the full width of the stage in a single run. I stagerd the seams about two feet from each end, and reinforced the joint with a vertical 2x4 on the inside. To make up for the weaker structure of 2x4s, I reinforced the sides in three additional places with "H" shaped braces. I assembled the braces on the floor, and then inserted them in the stage, screwing them into both the front and rear faces. This should (and I believe does) keep the faces from bowing out under the strain of the sand. I also ran a pair of sistered 2x4s the length of the stage (left to right) at the top, as a joist, to provide a structure to screw the decking to. The stage is 30" front to back and 14 inches tall (4 2x4s). I've lost count, but I think it has a little over 20 cubic feet of sand in it at this point. Let's call it 22cuft finished.

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post #1122 of 1177 Old 07-16-2014, 02:52 PM
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Awesome work, my friend! What plastic are you considering putting in your register boot?

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post #1123 of 1177 Old 07-16-2014, 03:01 PM - Thread Starter
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If I were, which now that I think about - I don't see the point - it would be the same black 6 mil plastic I used to line the stage and riser for sand. I was concerned about keeping the boot air tight, but I don't think the plastic will be needed, now that the boot is glued and caulked. I will probably use it on the other side, where the soffit end needs to be sealed, but the gap is larger than a bead of caulk.

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post #1124 of 1177 Old 07-16-2014, 03:55 PM - Thread Starter
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Speaking of not seeing the point: this is a little discouraging. REW is hard to use, and the results can be confounding. J_P_A has been trying to figure out his subwoofer solution, and I suggested that he put a sub in his in-progress theater and take some measurements to see how the modal interactions would play out. Analysis of that would be useful in deciding placement and bandwidth requirements. Sounds pretty straightforward... Since my room is at a similar state of assembly, I figured I should take my own advice and see what shakes out. Here it comes (the first round at least).

I have one assembled sealed subwoofer. I placed it in the rear corner of the room, hoping to strongly couple the driver's response to the room's response. In theory, all the modes should be energized with this placement, and the frequency response and waterfall should show them most clearly. This seemed like step 1 in determining what low frequency room treatments will be useful. (actually, this is step 2 - step 1 would be to verify the free-space performance of the sub. I skipped this step.)



The measurement mic should also be placed in a corner, for the same rationale. Here's my ECM8000 on the floor in the other rear corner of the theater.



The mic cable runs out through the doorway, to the USB soundcard I have (Art USB dual pre) which connects the mic and amp to REW. In this next picture, you can see the hardware outside the theater, all connected. I put it all outside so that I didn't have to listen to the loud sweeps, and so that I could use the wire I ran for the subwoofer. I also ran mic cables, so that I could connect it all from outside - and avoid running the mic cable through the doorway, but those cables aren't terminated yet. You'll also see in this picture that there is no door! I have a single 5/8 sheet of drywall covering the doorway (mostly). Obviously this is poor soundproofing. I'm not sure how much this impacts the validity of this test. Perhaps if/when J_P_A duplicates this test in his theater, we can see the influence of my "door." You can also see here that I'm powering the sub with my Sherbourn multi-channel amp - 7x150. 150W is not a lot for a subwoofer, but it's still enough to rattle the house.



That's not the only soundproofing issue. Obviously, the wall partition itself is incomplete, with no drywall on the outside. Also, the HVAC return is just a giant opening in the wall. I covered it with some OSB scrap - who knows to what avail. This is directly above the subwoofer.



I'll leave the measurements and results for the next posts to simplify this for readers.

Last edited by HopefulFred; 07-16-2014 at 04:16 PM.
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post #1125 of 1177 Old 07-16-2014, 04:16 PM - Thread Starter
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Given the set up, I was hoping to see a frequency response that mirrors what was predicted by a room mode calculator. I like to use this one - http://www.hunecke.de/en/calculators...igenmodes.html - because I like the 3D image it uses to remind me of the orientation of the standing wave in the room. Here's the measurements I put in:



The measurements of the room are pretty close, with two or three notable caveats. First, the entryway to the theater is an additional 2 feet or so of room length, limited to the center of the rear wall. Second, there is no option to indicate the varying room height that results from the presence of the riser, stage, or soffits. I used the full height, making no effort to find an average shorter height. Third, the calculator rounds to the nearest decimeter.

Here are the individual eigenmodes predicted by my dimensions.



As I said, in theory there should be peak in the frequency response that corresponds to each eigenmode, because the driver and mic have been positioned in tri-corners. So I was hoping to see peaks at 26 and 46 for the first length and width modes. Then the region from 50 to 80 was probably going to be pretty messy, as there are a number of modes in there, some of the tangential and oblique. Then in theory, there should be another strong peak near 80, at the third length mode.

If someone would like to offer a different expectation, I'd love to hear their interpretation of this input data.
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post #1126 of 1177 Old 07-16-2014, 05:24 PM - Thread Starter
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Before I post the actual measurements I gathered, let me say that I spent a lot of time trying to get it to work properly. This is only the third time or so that I've taken any sweeps. The first time, I measured my Cheap Thrills, both inside and outside, mostly just to verify that I put them together properly. I also used REW's RTA function for a while, just trying to learn how to use it. When I connected everything today I had several problems. There are too many little things that get changed for me to recap what I did or didn't do and the full set of warnings and failures I had. I think that by the time I took these measurements, the only weird thing was that I had to set the sweep level to -25dB (instead of the expected -12, I think) in order to avoid clipping the input (or some such). I also had a number of measurements where I got a warning that "the impulse peak is not where it should be." I threw all those out. For the first session of sweeps, I was trying to work through these problems and found that the results were hugely varying. Even without adjusting anything, two sequential sweeps would generate plots that shared basically nothing. I put it aside and came back to reconnect it, and everything became predictable (aside from the low level: that is, during the level check, it would warn the level was low, but it would leave almost no headroom).

Do I have a lot of confidence in these plots? No, not really. But the repeatability got so high, I couldn't think there is something wrong with them. Also, I have set the limits and scales to what I presume is non-standard dimensions. I didn't do that to be difficult, but since I just want to be able to see them as clearly as possible and at the highest resolution - this is what I've got. I have the .mdat if anyone is interested. The SPL here is meaningless; I did not calibrate the level. I ran the sweeps only from 20 to 200Hz, and I used the 512k resolution. The sweeps lasted close to ten seconds. I tried some sweeps starting lower and running higher, also some at even higher resolution. I wasn't getting much output below 20Hz, and the higher resolution didn't change anything, so here's what I got:



The waterfall is also non-standard in its limits, but with no treatment and basically nothing in the room the decay was very long. I wanted to be sure I could see the noise floor in the measurements, so I ran the time out to 600msec.



I have a few thoughts about this, which I will save for later. I hope that I can get some objective feedback first, then I'll weigh in myself.
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post #1127 of 1177 Old 07-16-2014, 07:44 PM - Thread Starter
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I'll just lay out a few observations and see what means anything later.

Parts express says that sub should have an f3 in the mid 30s. I think that matches what I have here pretty well, so I'll assume there's nothing too weird going on with the driver or enclosure.

It seems to me that most of the variation in the frequency response is constructive interference, so what we're seeing is additive peaks, not cancellation nulls. I say that mostly based on the idea that the mic and sub are too close to the walls and floor to have any cancellation in this frequency range. (Wrong?)

There's some peaking between 20 and 30 - maybe this is the first length mode. Maybe it's two smaller peaks because of the extra length of the room in the entrance.

The stronger peak in the mid 30s doesn't correspond to anything on the chart - so either I'm wrong about it being a peak and there's some noise or something, or the lobby area (the space separated by the single sheet of drywall) is acoustically coupling in this range and we're seeing that energy come back in at that frequency.

The fairly strong peak in the upper fifties doesn't seem to correspond to anything, but maybe it's the combination of the second length (2,0,0) and first diagonal (1,1,0) modes. They should have been lower, but the riser and soffits shift them higher in frequency.

The modes in the 70-100Hz range are tightly packed enough to produce a smooth enough response as is.

In general, I think the level was too low. The difference between the signal and noise floor makes it harder to see the decay.

Later, I'll try with the mic at a midpoint or something, and see if I can compare again. That'll be more complicated, because the mic will be at a node for some of the eigenmodes.
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post #1128 of 1177 Old 07-16-2014, 08:06 PM
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I'm guessing your mic was near the floor? Could you measure about halfway up the wall? You have a lot better handle on this than I do, but my first thought with that peak in the mid 30's is I wonder if the mic being close to floor is seeing an acoustic shortening of the room due to the riser and stage height? I really have no idea how you would go about estimating the acoustic length, or if being close to the floor will make any difference or not. That is, does the acoustic length change as the height changes?

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post #1129 of 1177 Old 07-16-2014, 09:07 PM - Thread Starter
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I'm guessing your mic was near the floor?
Yes, the mic was lying directly on the riser, as in the photo in post 1124. In fact, it's still lying on the riser right now. I haven't touched it at all since I set it down.
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Could you measure about halfway up the wall?
I can, and I will. But it may not be soon or easy - I need to buy a mic stand.
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I wonder if the mic being close to floor is seeing an acoustic shortening of the room due to the riser and stage height? I really have no idea how you would go about estimating the acoustic length, or if being close to the floor will make any difference or not. That is, does the acoustic length change as the height changes?
I think you're thinking of it the wrong way. The mic either sees the high pressure of the modal response, or it doesn't. The modal response exists because of the real dimensions, and to some degree the storage of energy in the boundary. If we position the mic at a place (near a boundary) where the pressure of a standing wave must be high (pressure is high, velocity is low), then the mic should record the highest pressure possible - the highest of any place in the room. If we position the mic at a node of the standing wave, the pressure should be the lowest of any place in the room - theoretically zero at that frequency.

There is a separate question of what frequency of wave constructively interferes in a resonant way between any two surfaces (or combination of several surfaces). This is the reason for this test. In theory, we can just plug the numbers into the calculator and know what frequency the resonances will be centered on, but it's always more complex than that. I'm not fully educated on the intricacies here, but this is a common area of misconception I think, so let me lay out what I know and think I know. (This may seem condescending, but I hope you don't take it that way. I know you are well-educated in a lot of this, but others are not. Maybe others will find this appropriately informative.)

If we start with the idea of a simple impulse wave, we see the wave reflects off a rigid boundary.


If instead of a single pulse wave, an endless series of full waveforms (both positive and negative halves) propagate through the medium toward a boundary, there will be some frequency where the reflected waveform exactly superimposes with the incident waveform, and perfect constructive interference is achieved. If this phonomenon is present at both ends of a confined medium, we have a standing wave. In this case, no energy is lost to the perfect boundary, and the wave reflects or resonates forever, even without further input from a source driving it.

The length of the space between the boundaries and the speed of the wave reflecting between them are the only two factors which determine the frequency of maximum constructive interference - the resonant frequency. But again, this is the theoretical perfect scenario. Real boundaries do not perform this way at most frequencies - especially low frequencies.

Instead, with real boundaries we see some portion of the wave energy will be transferred from the particles of the incident medium (air) to the particles of the boundary medium. The energy now contained within the motion of the boundary medium can do one of two things: either remain in the medium and be gradually dissipated as heat (frictional losses, in effect) or it can be transmitted back out of the boundary medium into whatever is adjacent (air). With most home construction partitions, the energy of low frequency sound will pass into the boundary and out the other side. Some of the energy will be immediately reflected at incidence. Some of the energy will be transmitted into boundary, but not immediately through to the air space on the other side. It's this energy that is interesting.

Imagine a boundary confined at the edged, but free to flex in the middle. It's fundamentally like a drum head in that way. Each small section of drywall is like this, and as a whole, the wall exhibits this behavior as well. When the high pressure of sound impedes it, it flexes. That flexing is the sound energy being transmitted out of the air, and into the wall structure. Once that happens the energy deflects the wall stretching it like a rubber band storing energy within it elastically. The pressure lessens, and the wall begins to return to its original shape. The wall, as drum heads and basically any other deformed object, will rebound past its original shape and oscillate until the energy dissipates. When the wall flexes back into the room with the sound source, it has become an acoustic source of its own, emitting sound back into the room. However, the sound that comes back into the room is delayed by whatever time it took the wall structure to flex back and forth. This will vary with mass and construction technique - it's difficult to predict, even in lab conditions as I understand it. So now the wall must be added into the model of how the standing wave is formed. Instead of the speed of sound and the distance between the walls being the only variables, we've added in an extra bit of time (not just the time that the wave needs to move at its own natural speed in air). This extra bit of time has the effect of lowering the resonant frequency in the standing waves. Two boundaries that would establish a standing wave at 35Hz if perfect might establish a standing wave of 32Hz if one or both is flexible.

There are other more complex phenomena present in the complete analysis of the modal interactions and decay. At some point, we'll need to consider the bandwidth of the resonances, and the way that through overlapping bands one resonance can stimulate another. The acoustic coupling of adjacent spaces (as I suggested for the 30Hz region in my test) is another one, but I don't think I have the knowledge to deal with that.

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post #1130 of 1177 Old 07-17-2014, 05:19 AM - Thread Starter
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Here's a surprise. I placed my order with HVACQuick on the afternoon of July 6. The grills arrived at work yesterday. That's super-quick, especially given that the website and order confirmation email say standard production is 3-4 weeks.

The blue is the protective film, the grills are brown, as expected.

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post #1131 of 1177 Old 07-17-2014, 06:34 AM
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I think we're on the same page with regard to room resonances…. Well, you're actually probably a little further in the book than I am, but I agree with what you said above. My question has more to do with how the wave is deformed at the front of the riser and the faces of the soffit. I was considering the first length mode of the room and trying to think of reasons for the room to be acoustically "short" enough to create a mode at ~35 hz. That is, do the features of your room make it appear to be roughly 15 ft long?

Where I may be operating under a mistaken premise is that as you move away from the interference, the modal distribution will change. While the corner will have all modes energized, do we know that the corner sees the same modes due to the riser? I feel like I'm doing a poor job of explaining this, which probably means it's a flawed argument. None the less, here is a quick diagram with a couple points that I have questions about.



Obviously this is exaggerated, but the question is, what is the length mode in the bottom half compared to the length mode in the top half? Is the modal distribution in each of the three corners the same?

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post #1132 of 1177 Old 07-17-2014, 07:14 AM
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No I don't think they are the same.
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post #1133 of 1177 Old 07-17-2014, 07:24 AM
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If you look at a lot of professional designed theaters some common design traits emerge like the classic Erksine stepped ceilings, some even stepped wall(s). It's not aesthetic or an accident either; it's an intentional design element for audio reasons.
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post #1134 of 1177 Old 07-17-2014, 07:38 AM
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Cool Fred.. great progess.. I had alot of tests I wanted to run before the riser construction this time, mostly to decipher the mystries of the "broadband riser trap" but after 6 hrs of searching for my mic (which I havent seen since I moved) I gave up.
A random thought from the monday morning quarterbacks seat.. Your sub and mic are both on the floor as you and JPA have discussed.. isn't this effectively "energising" your riser resonanace.. looking at it from simple panel resonance stand point, 2 layers of 5/8 with an 8" depth sould resonate at ~ 32, ~27 with a 10" depth, The panel dampening and internal absorbtion lower the Q effectively making your riser a broadband trap.. so total WAG but the peak you see at 35hz may be more related to you riser resonance than the room interaction.. if the case most of everything your seeing below say 50hz can be thrown out with baby....

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post #1135 of 1177 Old 07-17-2014, 08:18 AM - Thread Starter
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That may be it, JPA. The length from the top of the riser to the bottom of the opposite soffit is about 14' (7'4" height from riser to soffit, and 12' across the room - Pythagorean from there).

Maybe I can place the mic at the midpoint of that span and remeasure.
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post #1136 of 1177 Old 07-17-2014, 08:23 AM - Thread Starter
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KNKKNK - riser resonances crossed my mind, but the sub is on sand at that position, so I hope not. Only the 5-6' down the middle of the riser are open (with insulation).
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post #1137 of 1177 Old 07-17-2014, 08:24 AM
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I wish I knew more about FEA and FBA algorithms. I'd love to write a program to solve the wave equations for these rooms and look at how each incremental change affects the modal distribution.

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post #1138 of 1177 Old 07-17-2014, 08:37 AM - Thread Starter
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You've got books, right? Get on it!
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post #1139 of 1177 Old 07-17-2014, 08:52 AM
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Quote:
Originally Posted by J_P_A View Post
I wish I knew more about FEA and FBA algorithms. I'd love to write a program to solve the wave equations for these rooms and look at how each incremental change affects the modal distribution.
There is a web page from Europe that does this, I've seen it posted once in the DIY audio forum. It's all in meters but seems to work.
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post #1140 of 1177 Old 07-17-2014, 09:00 AM
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Originally Posted by HopefulFred View Post
You've got books, right? Get on it!
Yeah, it's the time that's a problem. As soon as I find some 36 hour days I'll get right on it

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Originally Posted by Mfusick View Post
There is a web page from Europe that does this, I've seen it posted once in the DIY audio forum. It's all in meters but seems to work.
Can you find a link? That seems like it would be a pretty heavy duty algorithm just to have as an app on a web page.

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