Soundproofing a large shop front window

[Pandaskater]

Hi all,

I have an old shop front i’m (ever so) slowly converting into something more studio like, the space is used for rehearsing, some home recording..my partner and i are both musicians, it's filled with guitars and keyboards and various associated bits.….time has come to deal with the very large front window.

So..

The window itself is approx 3.6m x 2.2m, not sure how thick (looks maybe 5mm) and is laminated.
The building itself is double brick, those old solid ones.
The floor is concrete and no noticeable vibration or noise is transmitted through it, so at least that's good!
The plan is to build a large wall/panel to block it off, soundproofing it as much as practical/reasonable. The wall will be single frame structure, standard absorption/leaf style, the leaf will be two layers with Green Glue (or equivalent) between.

Looking from the outside of the shop in it would go:

Window - air gap(approx 100mm) - absorption/leaf wall.

Traffic noise measured inside the room 1 meter back from the window is around 60dB (C weighted) on my little Radio Shack meter , and the aim is to cut this noise as much as possible.

My question:

The best type of “stuff” to use as the abortion layer in this scenario?

Currently have some 75mm thick Rockwool Fibertex 450 panels for the absorption layer, which i believe is 80kg/m3, quite dense, my thinking being that as well as acting as absorption/dampening inside at the resonant space created between window and wall, it will also help with bass attenuation, both coming in (from the occasional truck) and going out (from the occasional bass guitar) as denser is better, right?

Or would it be better to use something lighter/fluffier? Because...i'm not sure why this would be the case, but....

I’m aware of issues to do with flanking noise and decoupling and all the other “challenges”, just this particular little detail has me head scratching at the moment, so any informed opinion (or definitive answer even! If such a thing exists..) greatly appreciated!

Thanks

Carl

[Soundman2020]

Hi Carl, and welcome!!

not sure how thick (looks maybe 5mm) and is laminated.

If it is laminated, then it is likely thicker than 5mm. It would be good to determine the real thickness...

Also, is that an operable window, or fixed pane? Either way, it will need to be sealed in place permanently.

The plan is to build a large wall/panel to block it off, soundproofing it as much as practical/reasonable.

In othre words, you plan to complete the missing side of your outer-leaf by putting a single-leaf wall across the room, in front of the window. Then you will later build the actual inner-leaf studio at some point int he future?

The best type of “stuff” to use as the abortion layer in this scenario?

If you use mineral wool insulation, the the density should be about 50 kg/m3. If you use fiberglass insulation, the density should be about 30 kg/m3.

my thinking being that as well as acting as absorption/dampening inside at the resonant space created between window and wall, it will also help with bass attenuation,

Contrary to popular belief, higher density is WORSE for low frequency absorption (but better for high frequency). If you want better low frequency absorption, you need lighter insulation (but that will be at the expense of high frequency absorption...).

as denser is better, right?

Nope!

Or would it be better to use something lighter/fluffier? Because...i'm not sure why this would be the case, but....

The answer is "gas flow resistivity". That's the actual parameter that matters. It's a measure of the acoustic impedance of the material, and is expressed in units of MKS rayls: You want something around 10,000 rayls. Actually, anywhere from about 7,000 to about 15,000 would be fine. There's a very approximate but not very linear relationship between GFR and density, and since most manufacturers don't measure the GFR, but do measure the density, it's easier to talk about densities. The problem is that the relationship is different for each type of insulation. Hence mineral wool = 50kg/m3, fiberglass = 30 kg/m3. In both cases, that should be around 10k MKS rayls, give or take a few k.

I’m aware of issues to do with flanking noise and decoupling and all the other “challenges”, just this particular little detail has me head scratching at the moment, so any informed opinion (or definitive answer even! If such a thing exists..) greatly appreciated!

You are aware that you'll end up creating a 3-leaf system on that side of the room, when you do eventually build the actual inner-leaf, right? You'll need to compensate for that...


- Stuart -

[Pandaskater]

Hi Stuart, thank you for the response, i haven't time for a considered reply at present; the food for thought is appreciated.

The answer is "gas flow resistivity". That's the actual parameter that matters. It's a measure of the acoustic impedance of the material, and is expressed in units of MKS rayls: You want something around 10,000 rayls. Actually, anywhere from about 7,000 to about 15,000 would be fine. There's a very approximate but not very linear relationship between GFR and density, and since most manufacturers don't measure the GFR, but do measure the density, it's easier to talk about densities. The problem is that the relationship is different for each type of insulation. Hence mineral wool = 50kg/m3, fiberglass = 30 kg/m3. In both cases, that should be around 10k MKS rayls, give or take a few k.

Great info, i'll do some hunting around (this forum, what a fantastic resource!)

Other points noted and due head-scratching in progress!

Thanks again.

Carl

[Soundman2020]

(this forum, what a fantastic resource!)

...and it gets better every day!


- Stuart -