Control Room Window: How to determine proper glass thikness?

[Giancarlo]

Hello Forum

I'm in the construction process of my new home recording studio. And I am looking for information about choosing the proper glass for the CR Window.

If it gets really loud in the studio there are approx 100 dB inside. The studio is located in a commercial building with other little business around like a carpenter, a saddler, some storage and offices. Therefore we chose to build the studio walls as a double leaf stud wall construction to reach a STC of minium 60. The CR window should reach a similar STC and espacially isolate the low frequencies.

Wall details (inside to outside):
15 mm Gypsum board / 5.5 mm of self adhesive sound deadening sheets* / 60x80 mm stud / 70 mm air cavity / 60 x 80 mm stud with 60 mm woodfibre insulation / 12.5 mm Gypsum board

*made of plastisized bitumen (see http://www.perakustik.ch) with a weight of 10 kg per sq.m. This equals three layers of 15 mm gypsum board.

The CR window will have a size of 2500 x 800 mm (WxH). The window frame is made of 25 mm beech wood, the setting blocks will be 30 mm thick. I will isolate the cavities as suggested by John. Detais see attached picture.

I've bee reading everything I found about proper glass thikness in this and other forums and I talked to different gaziers in town. I let them quote glasses 10 mm and 8 mm thick as suggested by John in his construction manual. One glazier that has been involved already in recording and radio studio construction said that with a wall with an STC of 60 10/8 mm of glass woudn't be enough. He suggests a combination of 20 mm LSG glass (2x 10 mm with an acoustic foil inbetween) plus 19 mm float glass. This is about double the thickness of what John is suggesting to start with. And I was wondering if it isn't a bit exagerated?

The glass pane combination my glazier suggests would cost approx AUD 1400. I have a budget of AUD 800. The 20 mm LSG pane costs 500 AUD and the 19 mm float glass AUD 900 (interestigwise the LSG costs less). He sais, that the 20 mm LSG has a STC of 42 dB, and for example a 10 mm floating glass an STC of 34 dB.

My questions:
- I'm trying to find a good solution within my budget. I don't need a super-window but it should reach an STC of let's say between 55 and 60.
- Its the solution my glazier suggests the way to go or is it exagerated?
- I was thinking of a combination of a 20 mm LSG plus a 18 mm LSG. This would cost about AUD 900. Would that be a good solution?
- John (and the SAE) suggest to place the thicker glass pane on the CR side. My glazier sais to put it on the life room side. What is correct and why?
- Some say a float glass offers the better low frequency insulation than an LSG of the same thikness because of its mass. Is that correct?
- How important is it to have the glass panes at an angle? Some say it is better not to because one looses air space inbetween the panes which is important for insulating the low frequencies.
- If I construct the window like on my drawing (and like on Johns drawing) there will be a part at the top of the window where the window will actually be outside of the wall structure. There the sound isn't stopped by the wall structure but just by the window frame plus the insulation. Is that still the correct way to do it or would it be better to have a smaller angle between the glass panes so that that window would be completeely within the wall structure?

Thanks to anyone who will have some advice for me.

regards
Giancarlo

[AVare]

Therefore we chose to build the studio walls as a double leaf stud wall construction to reach a STC of minium 60. The CR window should reach a similar STC and espacially isolate the low frequencies.

- I'm trying to find a good solution within my budget. I don't need a super-window but it should reach an STC of let's say between 55 and 60.

- Its the solution my glazier suggests the way to go or is it exagerated?.- - I was thinking of a combination of a 20 mm LSG plus a 18 mm LSG. This would cost about AUD 900. Would that be a good solution?.

What you want is for the mass to be equivalent to the leafs of the wall. Glass is about 3x denser thatn drywall, so you want one third the thickness of the leafs. Any more and you are wasting money because go through the wall.

- - John (and the SAE) suggest to place the thicker glass pane on the CR side. My glazier sais to put it on the life room side. What is correct and why?.

The thickness pattern should be the same as the leafs. Thicker glass in the leaf with the thicker drywall.

- - Some say a float glass offers the better low frequency insulation than an LSG of the same thikness because of its mass. Is that correct?.

Laminated is better than float for the same mass.

- - How important is it to have the glass panes at an angle? Some say it is better not to because one looses air space inbetween the panes which is important for insulating the low frequencies..

Angling of panes IS NOT DONE FOR ACOUSTIC REASONS. For any window depth, angling reduces the isolation.

- - If I construct the window like on my drawing (and like on Johns drawing) there will be a part at the top of the window where the window will actually be outside of the wall structure. There the sound isn't stopped by the wall structure but just by the window frame plus the insulation. Is that still the correct way to do it or would it be better to have a smaller angle between the glass panes so that that window would be completeely within the wall structure?.

Build the frame to the same density as the wall/glass density.

Transparently,
Andre

[Giancarlo]

Andre,

Thank you for your suggestions and your quick reply. I have some more questions about your quotes:

What you want is for the mass to be equivalent to the leafs of the wall. Glass is about 3x denser thatn drywall, so you want one third the thickness of the leafs. Any more and you are wasting money because go through the wall.

Isn't this an observation on average sound proofing performace of different materials?
Could it be, that in a specific frequency bandwith a drywall reacts differently from glass with the same mass?
And therefore could it still be beneficial using thicker glass panes for a better performance with low frequencies?

Laminated is better than float for the same mass.

Here for me the question is, what is "the same mass"?
A LSG is two layers of glass bonded by a polymer interlayer. So in a certain way the two glass layers are decoupled. The conclusion could be, that a LSG of 20 mm doesn't have the same mass as a float of 20 mm because the two 10 mm layers of the LSG don't react to sound waves the same way as single 20 mm float pane does. Is this correct?

Angling of panes IS NOT DONE FOR ACOUSTIC REASONS. For any window depth, angling reduces the isolation.

What would then be the reasons for angling of panes?
I Thought angling panes would help to eliminate internal standing waves and to reduce direct reflections from the window into either the CR or the LR?

Thanks for your time
Regards
Giancarlo

[Soundman2020]

Isn't this an observation on average sound proofing performace of different materials?

No, it's an observation on the equations that govern the principle of MSM isolation.

Could it be, that in a specific frequency bandwith a drywall reacts differently from glass with the same mass?

There will be differences in things like the coincidence dip, but that won't affect low frequency isolation much. Coincidence frequencies are way high in the spectrum. What matters most is mass and air gap.

And therefore could it still be beneficial using thicker glass panes for a better performance with low frequencies?

No. Once you get to the same level of isolation as your wall, then it is pointless adding more mass to the window, since the wall is then the limiting factor. You could make your window STC-100, but if your wall is only STC-60, then the entire structure is STC-60.

Here for me the question is, what is "the same mass"?

The "same mass" means the same mass! Difficult to confuse.... It means that if you compare two pieces of 10mm glass, one laminated and one not, but both weighing 100 kg, then the one that is laminated will show higher isolation.

The conclusion could be, that a LSG of 20 mm doesn't have the same mass as a float of 20 mm because the two 10 mm layers of the LSG don't react to sound waves the same way as single 20 mm float pane does. Is this correct?

Lamination hardly affects the mass at all. The mass of the laminate material is insignificant as compared to the mass of the glass. You seem to be confusing mass and damping.

What would then be the reasons for angling of panes?

Lighting. If you have vertical windows the you can get annoying reflections of lights shining back in your eyes, if you don't plan your lighting carefully.

I Thought angling panes would help to eliminate internal standing waves

Yes, but you lose a lot more isolation from the reduced air gap than you do from internal resonance. Correct installation of cavity edge damping and use of different thickness glass helps reduce resonance issues.

and to reduce direct reflections from the window into either the CR or the LR?

You will still get reflections, no matter how much you angle the windows! The only difference is that they will go in different directions, but they will still be there.

The CR window should reach a similar STC and espacially isolate the low frequencies.

Like Andre said, if you want the same isolation on your window as you have in your wall, then you need the same mass and the same air gap in your windows as you have in your wall. You cannot improve the isolation of your wall by making the window work better at low frequencies than the rest of the wall. As Andre said, if your window isolates better than your wall then you wasted money on the window, because the wall is now the limiting factor. Whichever part of the entire structure is worst is what defines the overall isolation. So if you have a fantastic wall and fantastic window but you forgot to seal the cable duct between the two rooms, then the cable duct is the limiting factor and everything else was wasted. The same applies if you have incredible walls but didn't isolate the air conditioning ducts. The same applies if you put electrical outlets in the walls and didn't build up the mass around the boxes. The same applies if you drilled a hole trough your door to put in the handle. Etc. There are MANY places where you can destroy the isolation provided by a really good wall, and whichever one you forgot to do right is what is going to define the overall isolation of your room. Even a tiny crack under the wall or a break in one of the seals where the wall meets the ceiling is going to destroy your isolation. The wall is a tuned system, and all the parts work together: Anything you do that changes the tuning in any way, is going to change the isolation.

- Stuart -

[Giancarlo]

Hy Stuart

Again I get good advice from you. Thanks.
(Viva Chile for the soccer world championship. First team in Group H! My compatriots just did a incredibly stupid job today (Italy)...)

Lamination hardly affects the mass at all. The mass of the laminate material is insignificant as compared to the mass of the glass. You seem to be confusing mass and damping.

I certanly am confused by the contradictory information I'm told from different people. It all started with my glazier telling me, that the two layers of glass in a LSG with leaf inbetween don't act the same as a single compact float glass mass, when they are hit by soundwaves. Thus at first one glass layer would resonate with the soundwaves then the energy would go through the foil to the second layer. This would be the reason why, in terms of sound insulation of lower frequecies, may glazier says that a floatglass is better than a LSG.

I then asked a building physicist and he confirmed, that for example in a 20 mm LSG the two 10 mm glass layers would react to soundwaves like to decoupled single 10 mm panes, not like a 20 mm float glass...???

I trust the knowledge of you people on the forum far more and your answers make decisions about choosing the glass easier for me

Cheers
Giancarlo

[AVare]

You are not trusting our knowledge. Look at test data. Pdf page 63 in the Saflex Acoustical Guide. Test results including varying the interlayer thickness.

Andre

[Giancarlo]

Andre, of course I trust your knowledge. Maybe it's a translation problem wth my English, sorry about that.
What you and Stuart are recomending to me sounds by far more reasonable than what my gazier and the other guy said. Therefore I wasn't to bother anymore their comments and planned to go on as you suggested. Nevertheless I apreciate a lot the saflex PDF you send me the link for. Very valuable information. Thanks a lot.

In the meanwhile I got detailed STC data for the LSG glass I was thinking to use: STC = 42 for a 10 mm / 0.76 mm / 10 mm glass
Hz / STL
50 = 32.9
63 = 28.1
80 = 31.3
100 = 29.9
125 = 35.2
160 = 34.5
200 = 34.4
250 = 34.9
315 = 35.6
400 = 37.3
500 = 37.8
630 = 38.9
800 = 39.8
1000 = 40.9
1250 = 42
1600 = 45.1
2000 = 49
2500 = 52.5
3150 = 55.87
4000 = 59
5000 = 61.4