What about the mention of using hardwood for the frame?
The type of wood doesn't really matter. What matters is the mass (or density). and the ability to support the weight of the window. From that point of view, more dense woods are arguably better.
It was written by Paul Gilby Co-Founder of Sound On Sound but I guess it's out of date info.
Well, he might be great at editing an excellent magazine, but perhaps not so great at acoustics. As you say, the information might be out of date: acoustics is an ever-advancing science, and new research can bring changes to the way people design rooms. For example, that's why nobody uses the original true LEDE concept any more: research in psycho-acoustics shows why it wasn't a pleasant place to work. Extensions of LEDE, such as NER, CID, and RFZ have been shown to be much better.
I haven't read anything about angling the glass for sound reflection, more so to reduce the resonance between the two panes and glare/light reflection.
Light glare is the only valid reason for angling glass.... or maybe there's another valid reason: aesthetics (the owner thinks it looks cool).
No, it does not reduce resonance between the panes. The volume of air inside the sealed cavity is the issue: that is what resonates, not the shape. You can have any shape space you want for a sealed cavity. The original Helmholtz resonator was spherical, with a round pipe or neck sticking out at one point, but you can make it cylindrical, rectangular, a cube, a pyramid, a dodecahedron, or any other shape, and as long as the volume of air trapped inside is the same, all of those would resonate at the same frequency. In studios, we build them mostly rectangular these days, because is is easy, but here's how they used to look originally:
Image not preserved: A_selection_of_Helmholtz_resonators_from_1870,_Hunterian_Museum,_Glasgow.jpg
The actual equation is:
f= (c / 2 * PI) * SQRT ( S / V * L )
where f is the frequency,
c = the speed of sound in air,
S = the surface area of the hole,
V = the volume of air in the resonator's body
L = the length of the neck or port
As you can see, there's no variable in there to adjust for the shape of the container.... the only thing that matters, is the volume of air inside.
OK, so a two-leaf window is not a Helmholtz resonator, but the same holds true for any resonance in a cavity: the shape doesn't matter. It's the volume of air inside that matters. In fact, for this case the equation is far simpler:
f = 60 / SQRT (M*D)
Where:
M = the surface density of the panel,
D = depth of the resonant cavity.
And that's where the confusion comes from! People see the "D" and assume "Well then, I'll change "D" across the cavity! I'll make "D" greater at some points and less at others, so that I get different frequencies!" Cool idea, but it doesn't work quite like that. You have to understand the derivation of the equation to arrive at the conclusion that "D" has to be constant if you want to use this simplified version of the equation, because that "D" actually is only valid for that case... once again, what matter is the VOLUME of the cavity, and if the width and height are held constant (as assumed by the simplified equation), then you can just use the depth. If you do decide to change the depth, with one side being deeper than the other (by angling the glass, for example.....) then the resultant resonant frequency is given by the AVERAGE depth: So add up the greatest depth and the smallest depth, divide by 2, and that's your "D". (in other words, D = (d1 + d2) / 2) Thus, by angling the glass the only thing you achieve is that you change the resonant frequency! That's all. You do NOT stop the resonance, or make it go away: you just move it to a different frequency. And since the only way to angle glass in a fixed depth window frame is to push it INWARDS at the top of bottom, the volume of trapped air always becomes SMALLER than it was, so the resonant frequency always goes UP. You do get a slight reduction in Q as well, which is also not good, because the resonant region is now broader... So your window isolates less well than it would have...
So no, angling glass does not make the resonance disappear: it just changes it.
Some people also mention the huge scary monster of "standing waves", saying that angling the glass gets rid of those nasty creatures inside the cavity.... also not true! For the same reason that you cannot get rid of standing waves in your room by angling the walls, you also cannot get rid of standing waves inside the cavity of the window by angling the glass: from that point of view, the cavity is identical to your room, and the glass panes are identical to your walls. You can angle the walls (or glass) all you want, and all that you will accomplish is moving them to a different set of frequencies. You cannot eliminate standing waves: they will always form at some frequency or other, and all that you change by angling the glass is to choose a different frequency set, with a different "spread" of frequencies around the spectrum. If you do manage to angle your glass by a very large amount (more than about 12° or so), then you can actually "get rid" of some axial modes... but they then become tangential modes instead, so there's no real change in total....
There's no free lunch here. You can't get something for nothing.
" It's also common practice to angle the piece of glass on the studio side downwards slightly, which stops the view through the window from being obscured by reflections from studio lights and helps to prevent the build-up of standing waves between the sheets of glass. "
Exactly. That's the one reason why you might want to validly angle your glass. But in that case, you have to model your entire studio in 3D first, including the locations of all lights and all viewing positions, to ensure that the angle you choose really does stop glare! You might remove glare from the overhead lights themselves, but then create a situation where all you see in the glass is a reflection of the rear end of your console....
:)
Here It shows rubber or cork under the glass pane(not floating the frame tho) and angling the glass
Why would you want to put rubber or cork under it, if the reason for that is not to float the glass? Makes no sense.... The ONLY valid reason you would use a springy, rubbery mounting is to decouple the glass, so that vibrations in the glass are not transmitted to the frame, and vibrations in the frame are not transmitted to the glass.. If you have no interest in preventing vibrations from getting through, then why do it? What other purpose would there be?
I almost never have any studio windows taller than 36" or 90 cm. WAY too much glass and unnecessary for line-of-sight and communication.
I disagree. Sometimes it is necessary to have large windows, and it's not just about line-of-sight communication: it's about the overall aesthetic of the studio. Glass gives the sensation of open, airy, bright spaces, which is important too. Especially for small studios, where the rooms are not very big anyway. You don't want it looking tiny, cramped, dingy, and claustrophobic, so use more glass between the rooms to help avoid that. As long as it does not detract from the acoustics of the room, and as long as the budget can handle it, there's no reason to limit the size of your windows.
Take a look around all of John Sayer's studios (click on the logo at the top right corner of this page): notice how many of them use large expanses of glass, and how good it looks.
I see huge expanses of glass in a control room because they are sacrificing the acoustical accuracy of the space for visual impact.
Not true! As with most generalizations, this one isn't true either.... (Yeah, I realize the irony of make a generalization about generalizations....). If you know what you are doing, and design the rooms accordingly, there is no need to "sacrifice the acoustical accuracy of the space". I doubt that anybody would say that the many, many studios designed by John Sayers somehow "sacrifice acoustical accuracy". That simply isn't true. It is entirely possible to have a studio that uses a lot of glass, and is also very accurate, acoustically.
According to the opinion you posted above, this room must be terribly disgusting:
Image not preserved: glass-soffit-5.1-diante-do-trono.jpg
In reality, it is "Diante do Trono", one of the top studios in Brazil (and all of South America, for that matter), and was designed by WSDG, one of the leading acoustic consulting companies in the world. But look at all that glass! Based on the quote above, that cannot be any good at all, and this studio must be terrible....
:)
I think you can see why it is that many of us do not agree with that claim about it being necessary to "sacrificing acoustics" to use glass: As Dan pointed out, glass is just another building material that can be used any place it is needed in a studio, as long as the necessary precautions are taken. I guess I can understand why a studio designer who does not know how to use glass, and is struggling to make a name for themselves, might want to belittle the design criteria used by those who do know how to use it, perhaps in an attempt to conjure up a competitive advantage, or something... It's unfortunate, though, as glass is an excellent material for studios... if you can afford it, and understand how to use it.
Now I getting conflicting info from You and Greg about using two panes of glass of different thickness?
I don't think we are in conflict: We are both saying that the glass in each leaf needs to be at least the same density as the leaf it forms part of, preferably greater. I added to that, saying that if you are concerned about panel resonance from two identical panes of glass, then make them different thicknesses: in other words, one of them will have to be THICKER than the minimum thickness needed to get the same density as the leaf.
If you are concerned about that, then here's another equation for you to play with: it give you the free-field resonant frequencies of any panel, based only on the dimensions and the mass:
The resonant frequencies are: Fr = 0.45 * Vl * t * [(r/w)^2+(r/h)^2]
where:
Vl = the longitudinal velocity of sound in the partition (m/s),
t = the thickness of the panel (m),
w = the width of the panel (m)
h = the height of the panel (m))
r = the harmonic number (1 for the fundamental frequency, 2 for the first harmonic 3 for the second harmonic, etc).
Vl for glass is somewhere in the region 4000 - 6000 m/s, depending on the type of glass. You can figure it more accurately using Hook's law:
c = (K / ρ)1/2
where:
K = Bulk Modulus of Elasticity (Pa)
ρ = density (kg/m3)
You'd need to look up the modulus of elasticity for your specific glass in a table, or ask the manufacturer.
- Stuart -