John Sayers' Recording Studio Design Forum Archive

There are two issues I would like mention in this post, but first I want to make clear that these considerations are being made outside of a properly designed fully insulated, room-within-a-room studio environement. My workspace is in a private home and I'm more concerned with controlling its acoustic properties than sound isolation. That said, to the meat:

1. Controlling floor resonances.

I had to have the ceiling in my workspace redone because of water damage from the roof. The old flooring in this room isn't even worth mentioning. I decided to put in a hardwood floor. I always noticed that the floor was somewhat "boomy" sounding if you put your foot down hard, so I decided to open it up and see exactly what's there. The room is 18' 4" long, 12' 5" wide and about 8' high from floor joist top to ceiling.

We started ripping out the floor and discovered the following:

The joists are 3"x 11" (actual size) placed 19" apart (centers) spanning the narrow side of the room. They're "hung" on the edges of the outer joists and are very good shape, though they appear to have been re-cyled from another location. There's about 4" clearance below the joists till you hit the gyprock that is the ceiling of the floor below; there's a few inches of fiberglass pink insulation between the gyprock and the joists. The plywood above the joists is only 5/8" and is being removed; for some reason they used floating wood strips between the joists to screw the plywood sheets to each other and straight nails into the floor joists that became loose and noisy over the years... a real shoddy job... nuff said!
I need to know if the following makes any sense:

Considering that:

- there are 11x 3"x11" studs under the floor
- a new 3/4 to 1" plywood subfloor layer will be used to structurally reinforce it
- a standard thickness full hardwood floor will be installed on top of that
- and that there's no acoustic absorption at all in that floor currently

Does it make sense to go through some effort to dampen floor resonances further?

Options (I can think of):
- install 6" to 10" thick rock wool between the floor joists to improve the absorption.
- add additional joists between existing floor joists to even out the support, or add perpendicular pieces to offer support in the other direction (between the floor joists). How many?
- use some kind of math formula to make the placement of cross-pieces acoustically significant.
- stagger the vertical depth of the rock wool with some acoustic plan to even out absorption at various frequencies

2. Use large wide broadband absorbers to alter the reflection angles of the room from front to back.

We all know the pitfalls of parallel walls in a room, but what if we built wall-length sound absorbers to try to control those reflections. Does making and installing something like this make sense? (dimensions/thickness are arbitrary; just for the inquiry) :
Thanks for any feedback!

Angels wrote:Does it make sense to go through some effort to dampen floor resonances further?

Yeah, I think that's a good start for the room. Effectively you have a big drum head there.

Angels wrote:Options (I can think of):
- install 6" to 10" thick rock wool between the floor joists to improve the absorption.
- add additional joists between existing floor joists to even out the support, or add perpendicular pieces to offer support in the other direction (between the floor joists). How many?
- use some kind of math formula to make the placement of cross-pieces acoustically significant.
- stagger the vertical depth of the rock wool with some acoustic plan to even out absorption at various frequencies

I'm going to say that for maximum value you'll need to fill the entire cavity (without squashing it in) with around 50kg/m3 rock wool or 30kg/m3 fibre glass.

Adding any timber to the structure may be necessary, but you'll need to consult someone suitably qualified to let you know if that has to happen. Here in Oz there would definitely be 'perpendicular pieces' between the joists to stop warping and add integrity to the structure. That may or may not be applicable where you are.

I'm not sure the maths comes in to it too much with placing any cross braces in, but I believe the larger the individual spaces the better as it will lower the resonant frequency of each space.

I don't believe staggering the rockwool will help as once the floor is sealed it will still act like a tuned drum head and have it's own resonant frequency, but filling the cavity with rock wool will absorb an amount of that freq.

Angels wrote:2. Use large wide broadband absorbers to alter the reflection angles of the room from front to back.

We all know the pitfalls of parallel walls in a room, but what if we built wall-length sound absorbers to try to control those reflections. Does making and installing something like this make sense? (dimensions/thickness are arbitrary; just for the inquiry) :

Yep, that's on the right path but you'd be going floor to ceiling at the rear of the room as well. You'd also be super chunking the corners at the back and putting in thick absorbtion on the back wall. The front wall would also look somewhat different depending on how you were mounting your monitors.

all the best,

Steve

Thanks for the corrobation and suggestions, Steve. Coincidentally, my name is Steve too...

Re - the floor: Roxul products are easy to get in my part of the world, so I'll be going with that. Rockboard 60 has the right density but maxes out at 4" thickness, so I'm thinking 2 layers (8" deep) between every joist. Since it's 48" long, adding 2 - 2.5" perpendicular cross pieces separating the joists into 3 areas allows me a perfect fit of 3 batts between 2 joists covering the 12' 5" of the width. But I will lose material having to shave down the 24" side.

As for the room-length panels, I'm not looking for a dead acoustic space, just a controlled one. I track a lot of stuff live in-room (computers are in a different room). After I finish the floor, I'll do the testing and math and see if I can integrate this idea into something functional.

Again, thanks for your thoughts.