Glass critical frequency
Different thicknesses of glass in recording studio’s are used in a mass/air/mass configuration but it is not because they may rattle at the same frequency. The thickness of the material, any material, will have a critical frequency. This is an acoustic hole that allows sound/vibration to pass thru as if no barrier existed.
The graphic above shows the critical frequency of 4mm, 8mm which are the ones we are interested in at the moment. The 4mm glass has a critical frequency at around 3200Hz. The 8mm glass has a critical frequency of around 1600Hz.
You may not have noticed but the doubling of the glass (from 4mm to 8mm referred to as mass) divided the critical frequency in half. If you had a piece of this same glass and it was 16mm you could expect the CF to move down to around 800Hz.
So they both have holes acoustically speaking. But if you place the 4mm piece of glass in one wall and the 8 mm in the opposite wall you have a better ability to plug this acoustic hole since the sound that passes through the 32ooHz hole will be barred from passage through the next glass pane that has the 1600Hz critical frequency.
Adding mass directly on top of existing mass, well, they may have been thinking the same thing there as well. That and refraction. With refraction as a tool the thinking is this. Think of a clear body of water. Now push a stick directly into the water until you hit the bottom. When you look at the stick it seems to have “bent” downward. This is the visual companion to refraction.
Vibrations move faster through solids, the more mass it has the faster the vibration will travel.
So using dis-similier materials the thinking is that you alter the path the sound takes which takes energy from the sound wave thereby creating a better isolation assembly. But the effect is so small that it is not worth the effort.
Using more of the same mass in a wall or overhead assembly is the answer. The overall TL of the added mass moves up with the addition of another panel which means a better isolation assembly.
Back to the windows.
What happens if you use, say, thermal glass in a double wall assembly? If you have 2 pieces of double insulated glass you effectively have FOUR pieces of glass with the same critical frequency…It IS a free pass acoustical tunnel directly in and out of the rooms, from either side to the other.
When the control room OR the tracking room is tested, if they ever are, there will be a spike at the critical frequency of the glass in these double insulated panels. People will scratch thier head for weeks trying to figure out why there is so much 3200Hz frequency getting into the room and why every time it is eq’ed out…it just comes right back in.
And that is why you do not use double insulated glass;) Well that and multiple air spaces which are known to reduce the ability of an assembly to isolate.