AVS Forum banner

Status
Not open for further replies.
1 - 9 of 9 Posts

·
Registered
Joined
·
656 Posts
Discussion Starter · #1 ·
I'm planning on starting my construction in a couple weeks to finish my basement and turn a section of it into a home theater room. In looking at some books on basement finishing pertaining to walls, they all say to attach a top-plate directly to the floor joists above, and then toenail the studs into the top plate (or construct the wall on the ground and raise it up, but still attaching directly to the floor joists). My concern with this is whether this would transmit sound rather freely into the rooms above. I have limited vertical space to work with, and was planning on doing a suspended ceiling already. Is there any other recommended way to put up your walls without having to attach them directly to the floor joists (keep in mind, I only have 7.5 feet to worth with from floor to joists)? Or does this method really not transmit as much sound above as I think it will as long as I double-drywall on top of the studs - hoping the sound will have been filtered out enough by the time it reaches the studs that there won't be much left to transmit into the joists and the rooms above??
 

·
Registered
Joined
·
249 Posts
Attaching the studs to the ceiling joists will transmit sound, low frequencies in particular.
 

·
Registered
Joined
·
84 Posts
Looks like this could be the start of a controversy.


Attaching the wall studs to the ceiling joists will transmit sound. But how? And is it a problem?


First, you probably are only talking about the interior walls, but let's state out loud that you only have to concern yourself with interior walls. Walls at the perimeter of the basement will presumably connect to the floor joists near the ends of the joists, which would be unable to vibrate due to being connected to the outer walls of the house. Thinking of a vibrating string, these places will be nodes -- no vibration. These exterior walls would be easy to connect to the foundation (I guess) about half way up, drastically stopping their motion anyway.


As for the inner walls, when you connect them and they vibrate, this vibration will be coupled to the floor joists. But does that matter? These walls will vibrate laterally, and their tops, when coupled to the floor joists, will also be vibration nodes as the floor joists will not be able to move laterally, so the only vibtration that will couple to the floor will be from...let me take a second and think this through...


Let's say the wall is seven feet high and you blast it with enough bass to move it back and forth about a half inch total(it would probably be less). Since the wall's length is not flexible, its height will vary as it vibrates back and forth. A wall moving about a half inch might vary in height by, say, about .020" (a guess). That will transmit vertical motion to the floor joists, but what starts out as a half inch becomes twenty thousandths of an inch. That is a reduction factor of 25 times the force, which is about 28 dB (did I get that right? Force is like voltage, not like power, so one half = a 6 dB reduction). So whatever amount this wall is moved will be transmitted to the floor joists, attenuated by some number in that neighborhood.


Anything you can do to keep that wall from vibrating will attenuate that particular coupled sound even more, but even if my calculations are wrong, the attenuation is 14 dB, and look at the next paragraph.


You probably will have more of a problem with the fact that the floor joists, and any drywall you put on them, will be acted on directly by the moving air, just as that wall was. Because the sound's effect literally has to go around a corner for that wall to move the floor joists, and the sound goes directly to the floor joists above the room, worry first about rigidifying the floor joists, second about adding mass to them to make them harder to vibrate, third about how thick the door is at the top of the basement stairs, fourth how well that door seals, fifth whether you have any vents from the basement crawl space to the outdoors, and maybe after that worry about the inside walls being coupled to the ceiling.



Okay, time for a joke based on fact. You could greatly attenuate the effect of the sound on the actual floor joists by running a 4x4 at the center of the room from the ceiling joists to the basement floor. Assuming the basement floor is concrete, that is.


I'm from Southern California, where a basement is either a crawl space (90% of them) or a room up to six feet by eight feet, crammed with heaters and stuff (9 1/2% of them), so forgive me if I don't seem to have imagined your space properly.
 

·
Registered
Joined
·
40 Posts
Quote:
Originally posted by Jeraden
I'm planning on starting my construction in a couple weeks to finish my basement and turn a section of it into a home theater room. In looking at some books on basement finishing pertaining to walls, they all say to attach a top-plate directly to the floor joists above, and then toenail the studs into the top plate (or construct the wall on the ground and raise it up, but still attaching directly to the floor joists).
The best solution is to decouple the ceiling from the floor joists. Do a search for 'floating ceiling', and you will see a bunch of threads where this is discussed in detail. The main problem will be when you attach your ceiling drywall to the floor joists, allowing a wonderful medium for transmission of bass frequencies into the floor above. Mass (double-drywall) is only part of the solution. You also need to decouple.


Chris
 

·
Registered
Joined
·
306 Posts
Actually the suspended ceiling may be much more of a problem than attaching walls to the floor joists.


A suspended ceiling has cracks around the ceiling tiles. The tiles probably don't absorb much of the sound, especially low frequences. The ceiling tiles may rattle.


To reduce ceiling tile rattle one may put silicon caulk around the metal channel which will hold the tiles. He let that dry before installing the tiles. That helped the tiles float a bit more than resting directly on the metal. Above the tiles put as much fiberglass insulation as you can. You may also want to add more weight on the tiles by gluing MDF or sheetrock to the back of the tiles. Note if you add weight you will need to use many more support wires. Most suspended ceiling framework is fairly flimsy.
 

·
Registered
Joined
·
3,681 Posts
My very basic understanding is that you need to build "a room within a room". In other words, the theater's ceiling is only attached to the theater's walls.


SM
 

·
Registered
Joined
·
1,329 Posts
What Chris meant was to actually have ceiling joists that attach to the side walls and are independant of the ceiling/floor above, then cover with sheet rock etc. He did not mean a suspended ceiling made of tiles.
 

·
Registered
Joined
·
7,655 Posts
HomethtrLA


What you have to remember is that those floor joists are connected to a series of x by y 'drum heads' made up of the above sub-floor. Even though you are reducing the vertical movement, you are not considering the fact that you are also, following your logic, moving the walls on the first floor which are attached to the ceiling and the walls of the second floor.


While the movement will continue to be decreased, it is still there, vibrating every piece of plywood, drywall, etc. it is connected to physically. All this is not even considering the lateral movement of the walls below which will flex the floor joists horizontally.


Try this out. Go downstairs with someone upstairs. Take your fist and pound straight up on the joist. Then hit it on the side. As the person upstairs which one was louder. The one on the side will also probably sound less 'focused' and coming from all over in the room upstairs.


Jeraden,


Building a room within a room will significantly reduce structure born noise transmission. During construction, the walls will be very flimsy. Not to worry. When you get the false joists up, tied to the walls, and all of the drywall hung, it will firm up nicely. You can help the rigidity but putting some diagonal braces on the outside of the inner walls which will never be seen.


Have fun.
 

·
Registered
Joined
·
348 Posts
just screw the drywall to the joists above you and buld a sofit for any duct or beam that is lower than the joists
 
1 - 9 of 9 Posts
Status
Not open for further replies.
Top