gabo
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Posts: 8
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Post by gabo on Jan 19, 2023 21:39:03 GMT
I am in the process of working with a guy who has a new studio control room. The room is 25'x15' and has a vaulted ceiling that is 9' tall on the sides and goes to 15' in the center and is vaulted lengthwise in the room. I was thinking of building it similar to Ethan's room, where he built 21 of the absorbers alternating between the Deep Bass, Mid Bass, and High Bass absorbers. However, due to the way we're going to build the front wall, I have quite a bit of space in the corners. The front is going to be sort of rounded around the corners with flush mounted monitors installed there. Maybe it's best to just make that completely full of rockwool, which would provide about 4' depth in the corners and around to about a foot on each side out to about 9' into the room. The alternative would be to build it with the alternating absorbers down the side walls to the corner and then again along the front of the room. For the back wall I am considering the alternating traps, but do have a couple of questions. First analyzing the traps, the Deep Bass traps are 4" deep, with insulation, air gaps, and 1/4" plywood on the front. This looks like a limp mass trap having a formula for the center frequency of f=192/sqrt(mass x depth) where 1/4" plywood has a mass of 0.7, so 192/sqrt(0.7x4) = 115hz Which for this room doesn't seem low enough. I could replace the 1/4" plywood with 2lb/sqft MLV and get it down to 143/sqrt(2x4) = 68hz. Of course the cost of the MLV is quite high, but maybe I use that for the Deep Bass and then use the 1/4" for the alternating panel. The formulas I'm using are from this article, www.acousticsciences.com/artblog/limp-mass-membrane-bass-traps/Thanks for any discussion. GB |
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Post by Hexspa on Jan 19, 2023 22:53:37 GMT
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gabo
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Post by gabo on Jan 20, 2023 0:28:30 GMT
Thanks, yes I have both of those publications, they are excellent.
I have measured the room and can tune them to some room modes. However, in Ethan's article, he actually didn't do that. He recommended these three panels, one for Low Bass, one for Mid Bass, and one for High bass and alternated them on the walls. I believe his objective was to trap most of the low frequencies by using 3 traps tuned at different frequencies in alternating fashion to effectively create a broadband absorbing wall.
However, when I look at his trap builds, I get the 115hz I posted in my previous note for the Low Bass traps. With the size and volume of my room, I have a schroeder frequency that is around 104hz +- about 10hz depending on what specs you use for the desired amount of decay.
So it seems to me that my "bass" trapping needs to be at frequencies below the lowest trap he made, 115hz. Does that make sense?
I have measured the room, empty with only drywall on all the walls, and have peaks at 26, 53, 71, and 104 and nulls at 39, 64, and 97 hz.
So it seems to me if I wanted to do something similar, I would be better served to build my 3 traps (low, mid, high) tuned to maybe 53, 71, 104?
Or maybe I would be better off just using broadband absorption, I already plan on doing that on the front corners (4' thick) and wall (8" or more), and ceiling (8" clouds with about 80% coverage). There is so much info that says that won't work on lower frequencies, but then when you talk to people who've done it, it seems to work.
Just trying to understand if alternating wall sections, tuned to specific frequencies, is better than about 8" thick broadband. On paper it seems to be, in practice I'm not sure.
GB
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Post by Hexspa on Jan 21, 2023 7:42:21 GMT
A few things: Ethan has depreciated those absorbers, I remember him saying. Last I heard, he's mostly with the standard frictional absorber with or without FRK and air gap. From what I understand, nobody here can really help you with resonant absorbers including me.
Second, small rooms do not have a Schroeder frequency because that requires a diffuse field which small rooms do not have. Small rooms have a 'transition' frequency which is much higher than you'd get from calculating a SF. That said, the small rooms certainly have axial modes and you calculate them thus:
calculate modal harmonics (axial mode):
F = n1130/2D (imperial) 1125 ft/s
F = n345/2D (metric) speed of sound 343m/s @ 20deg Celcius
F is frequency
n is harmonic (1-4)
D is dimension (distance between walls)
first harmonic 16ft length
F = 1x1130/32
35.3125Hz C#1
Something to keep in mind is that you will need broadband absorption regardless if you use resonant absorbers. Last I remember of Ethan's advice was get all your broadband in then see where you stand. In practice, sub bass will ring longer and that is ok - particularly below 63Hz so long as it's a tapered lengthening; you don't want a single long ring. Besides, this is for a control room and not a live room. You can reduce sub frequency ringing with multiple subwoofers - I've seen decay times reduce with three subs. I can share my notes with you but, again, this isn't my area of expertise. The point is that you might be able to deal with the sub bass range electromechanically via subwoofers or you might need a combination of techniques.
Everyone has their own approach. I recommend getting everything in the room, getting in your broadband then seeing where you're at with LF resonances. Have your predictions ready via the formula I gave you and determine the right tool for the problems, if any. Obviously, you will need targets - there are many available. You will only see how far you are once you take your first shot. I've never heard an acoustician say that everything on paper translates perfectly into the real world.
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gabo
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Posts: 8
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Post by gabo on Jan 21, 2023 13:24:10 GMT
A few things: Ethan has depreciated those absorbers, I remember him saying. Last I heard, he's mostly with the standard frictional absorber with or without FRK and air gap. From what I understand, nobody here can really help you with resonant absorbers including me. Second, small rooms do not have a Schroeder frequency because that requires a diffuse field which small rooms do not have. Small rooms have a 'transition' frequency which is much higher than you'd get from calculating a SF. That said, the small rooms certainly have axial modes and you calculate them thus: calculate modal harmonics (axial mode): F = n1130/2D (imperial) 1125 ft/s F = n345/2D (metric) speed of sound 343m/s @ 20deg Celcius F is frequency n is harmonic (1-4) D is dimension (distance between walls) first harmonic 16ft length F = 1x1130/32 35.3125Hz C#1 Something to keep in mind is that you will need broadband absorption regardless if you use resonant absorbers. Last I remember of Ethan's advice was get all your broadband in then see where you stand. In practice, sub bass will ring longer and that is ok - particularly below 63Hz so long as it's a tapered lengthening; you don't want a single long ring. Besides, this is for a control room and not a live room. You can reduce sub frequency ringing with multiple subwoofers - I've seen decay times reduce with three subs. I can share my notes with you but, again, this isn't my area of expertise. The point is that you might be able to deal with the sub bass range electromechanically via subwoofers or you might need a combination of techniques. Everyone has their own approach. I recommend getting everything in the room, getting in your broadband then seeing where you're at with LF resonances. Have your predictions ready via the formula I gave you and determine the right tool for the problems, if any. Obviously, you will need targets - there are many available. You will only see how far you are once you take your first shot. I've never heard an acoustician say that everything on paper translates perfectly into the real world. Thanks, yea that's good advice and pretty much what I had come to as well. We'll just do the broadband, especially in the front and down the front part of the walls, and the ceiling, then shoot it again with REW and see where we are. I'm not sure about the Schroeder frequency in this room, I wouldn't really consider it small at 25x15x12, but I do agree that even in any room Schroeder isn't a hard point and is more of a transition region that actually spans multiple times whatever the Schroeder calc comes out to be. But those are all frequencies that are easy to treat anyway, so those won't be the big issues. I may do the ceiling first, since that is vaulted it has a corner at the top, and it's easier to do the ceiling when you don't have anything in the room. My plan is to do the ceiling with pretty much full coverage DIY rockwool panels that are 8" thick and hung down with an air gap behind them. Thanks for the thoughts and comments, very helpful when you're working alone and don't have anyone to bounce info off of!! gb |
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Post by Hexspa on Jan 23, 2023 6:30:17 GMT
No problem. Yeah, acoustically anything under 10,000ft3 is considered small. Your space's 4500ft3 is nice but it still gets the regular small room approach. I've been in a tiny reverb chamber that was definitely diffuse so I guess the lines can blur but I still think you're looking at the transition approach rather than true schroeder. But, hey, I don't have a degree in this so your guess might be as good as mine.
Also, like I might have mentioned, 6" is the optimal thickness for rigid panels. Beyond that, you might get some reflectiveness which means you're starting to waste material. Ethan's findings show that more coverage beats less but with greater thickness. In other words, if you can stretch out your material to make more 6" panels to cover more area, you have reason to think you'll get a better result. Be sure to measure and find out which dimensions are ringing so you can put your insulation there unless you plan to treat that with pressure units.
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