Sound transmission/reflection of glass (windows)

[ThrillHo]

I've been acoustically treating my room with rock wool panels and then the question struck me: how does sound react to the windows in my room? I speculate that low freqs would pass through but high freq would reflect. These are basic single panel windows facing the outside. I know it depends on the window, but does anyone have a general idea of what happens?

-Ian

[jchas]

The typical window glass (and i'm assuming you're talking about windows to the outdoors) is about 1/8" thick and follows a pretty steady line of sound transmission loss from 0db at 125htz to 30db at 8khz. A second sheet of glass (like a storm window) will essentially double the amount of transmission loss because you are doubling the amount of mass (160lb/cu.ft.).
The greater the gap between the 2 panes of glass will also increase transmission loss. An unfortunate side effect of double-pane glass is that if the panes are the same thickness, the window will act as a resonance chamber and pass certain frequencies almost unattenuated.
Glass has aproximately twice the mass as plexiglass and works twice as well.

i just finshed reading Sound Studio Construction On A Budget by F. Alton Everest which explains it all in excrutiating detail.

[Tibbon]

Originally Posted by jchas

The typical window glass (and i'm assuming you're talking about windows to the outdoors) is about 1/8" thick and follows a pretty steady line of sound transmission loss from 0db at 125htz to 30db at 8khz. A second sheet of glass (like a storm window) will essentially double the amount of transmission loss because you are doubling the amount of mass (160lb/cu.ft.).
The greater the gap between the 2 panes of glass will also increase transmission loss. An unfortunate side effect of double-pane glass is that if the panes are the same thickness, the window will act as a resonance chamber and pass certain frequencies almost unattenuated.
Glass has aproximately twice the mass as plexiglass and works twice as well.

i just finshed reading Sound Studio Construction On A Budget by F. Alton Everest which explains it all in excrutiating detail.

Right on... I personally feel that if you're really trying to block sound to use three panes of glass, the middle one being thicker to combat the resonance issue that you mentioned.

I would put it so that the outer two are angled.... so that the panes would look like this from the side..

/ | \ ( hope you can understand the drawing), or the opposite with them tilted slightly downward instead. Either way, it will help prevent a little bit of the immediate slapback from the glass. Just depends on the rest of the room.

Also, remember to caulk or seal them someway well. Crap, just read that you're talking about your outdoor windows. Hmm. Best of luck on those. I had a mix room that had that problem at one point. Curtains helped (although opening them changed the sound of the room a bit).

If you're really needing to kill some sound, but want to keep the light... block up the windows using that glass brick stuff if you're really needing to kill the sound. They are heavy, and the mass will help a lot.

[ThrillHo]

[quote=jchas;1257462]The typical window glass (and i'm assuming you're talking about windows to the outdoors) is about 1/8" thick and follows a pretty steady line of sound transmission loss from 0db at 125htz to 30db at 8khz. quote]

I just wanted to clarify, by transmission loss do you mean the amount the transmitted wave attenuates? And whatever doesn't transmit gets reflected? I'm not concerned with soundproofing, more phase cancellation.

Oh, and the room happens to be my bedroom. I thought it be a good topic because I know their are many more like me with windows to the open world next to their workstations.


-Ian

[ThrillHo]

Originally Posted by jchas

The typical window glass (and i'm assuming you're talking about windows to the outdoors) is about 1/8" thick and follows a pretty steady line of sound transmission loss from 0db at 125htz to 30db at 8khz.

I just wanted to clarify, by transmission loss do you mean the amount the transmitted wave attenuates? And whatever doesn't transmit gets reflected? I'm not concerned with soundproofing, more phase cancellation.

Oh, and the room happens to be my bedroom. I thought it be a good topic because I know their are many more like me with windows to the open world next to their workstations.


-Ian

[jchas]

Originally Posted by ThrillHo

I just wanted to clarify, by transmission loss do you mean the amount the transmitted wave attenuates? And whatever doesn't transmit gets reflected? I'm not concerned with soundproofing, more phase cancellation.

Oh, and the room happens to be my bedroom. I thought it be a good topic because I know their are many more like me with windows to the open world next to their workstations.


-Ian

Transmission loss would be the amount of attenuation between you and the outside world. Important if you don't like the sound of the outside world in your recordings - equally important if the outside world isn't crazy about your music

Glass, being about as hard, shiny, and dense a surface you can find - will make a perfect reflector. As it has no surface imperfections there will be no diffusion and next to no absorption. If you are already treating your walls, and aren't singing directly towards the window, the reflections off the window won't be as important as the nearly sqare box that makes for most bedrooms.
If it still bothers you - simply hang a piece of contoured foam over the window when recording. It won't help for isolation, but will help make the reflections unsymetrical.

[ThrillHo]

Thanks for the responses. When you say glass is a perfect reflector, is that frequency dependent at all? Don't bass frequencies below 125Hz pass seemlessly through unattenuated (and unreflected?)? Pardon my confusion.

[Ethan Winer]

Originally Posted by Tibbon

if you're really trying to block sound to use three panes of glass, the middle one being thicker to combat the resonance issue that you mentioned.

Intuitively that might seem more effective, but it's actually worse than using only the outer two panes. Technically this is called a "triple leaf" and it's best avoided. The attached drawing shows the principle with STC numbers. It's meant for wall construction, but the same thing applies for window glass too.

--Ethan

[Ethan Winer]

Ian,

Originally Posted by ThrillHo

I'm not concerned with soundproofing, more phase cancellation.

Glass reflects more or less the same as any other rigid wall material, but a little more at higher frequencies. People often obsess over glass, but it's not that damaging unless it's at a reflection point. In that case you need to cover it with absorption, but you'd need that with sheet rock too.

--Ethan