Should I build a False floor.

Posted: Thu Mar 13, 2003 11:08 am

Scott,

As to your question:

Quote:

Would you estimate that it's within 3dB?

I can't give you a number, since complete systems of materials can't be quantified by simply adding the STC values of the various components, and there are an almost infinite (billions and billions?) number of possible combinations of materials. No one I'm aware of has ever attempted to measure them all.

Anyhow, one of the worst types of noise pollution that you'll have to deal with from the overhead foot-traffic is the ultra-low frequency "drum effect" caused by the entire joist system flexing as kids jump around, or adults do what ever it is they might be doing up there. Let's be real, you're never going to totally sound-proof the place from these type of vibrations, no matter what you do.

So what do I mean by "almost as isolated"?. Let's think about it for a minute and use common sense. When you simply screw layers of rock directly to the joists, there is a lot of physical connection between the ceiling and the floor system above--the entire length of every joist. Surely the "drum effect" will transfer to the space below in a big way.

With scenario one (resilient channel), we reduce the points of contact to every 16" in each direction, a very serious reduction in percentage terms, which is why in the STC charts, you'll often see an improvement of 12dB or more, just from the addition of resilient channel to the equation.

With scenario two (hanging shock mounts), you've got roughly the same number of contact points, but now they're tiny points, and the shock absorbers will allow the floor above to flex without transferring that "drum effect" to the room below nearly as much by means of mechanical transfer, (often referred to as structure-born vibration). However, the less efficient method of air born transfer will still take place, and you're not going to stop that short of pouring a slab overhead.

The essence of your question is the difference between shock absorbing hangers and a whole new joist system. I think that in the real world the answer is, not very much. Note that both scenarios two and three have the added benefit of a much deeper airspace between the layers of rock, which will extend the sound-proofing down to the low octaves, where it's really needed. (Remember that STC values are based on weighted curves that allow for a lot of slackness on the bass region.)

In your case, it sounds like you need isolation more than added ceiling height below. Eight or nine ft. of ceiling height is plenty for a small studio, given that's what's left over after dealing with overhead isolation. So all things considered, I'd go with scenario two. But get it done professionally. It'll still be really cheap, looking at the big picture.

Now that'll cost you a nickel, my going rate on this forum. But I might have to raise my rates.

--Wes

Posted: Thu Mar 13, 2003 2:30 pm

Thanks Wes. That was well worth the nickel!
Scott

Posted: Fri Aug 22, 2003 12:02 pm

Wes,
Wouldn't it be even more effective if I use "Scenerio 1":
_______________________________________
"Build new, load-bearing walls (inside the existing walls) to carry new ceiling joists for the isolated ceiling. Leave at least 1" airspace between the gypsum and the new joists. Then hang a couple of layers of gypsum from the new joists."
_______________________________________
but instead of screwing the gypsum directly to the existing floor joists above the new ceiling joists, I suspend the gypsum on a spring loaded ceiling grid (basically a combination of "scenario 1" and "scenario 2")?

My only concern with this design (and "scenario 1" in general) is that it seems to violate the principle of having a "mass-spring-mass" system, with an uninterrupted air space (spring) between two "leaves" of mass. The top "leaf" of mass is the existing floor on TOP of the currently existing floor joists, right? Arn't we adding a 3rd "leaf" inside the air space by attaching gypsum to the bottom of the currently extisting joists?

Thanks,
Scott

Posted: Fri Aug 22, 2003 1:44 pm

Kris,
Thanks for your advice. I might not have been clear on one point - I am not suggesting putting the spring grid on the new set of joists, but my proposal was to put the spring grid on the existing joists, then build the new ceiling/joists below that. One big advantage I was hoping to get by doing that was to drastically reduce the level of the "drum effect" from foot traffic overhead. Is this still not worth it in your opinion?

Regarding your comment about diminishing returns - yeah, I know what you mean, that's a good point. However, is it actually correct to say that two barriers each with STC of 47dB will only provide 50dB of attenuation when used together? I know that when adding two sound sources the logarithmic math works that way, but is the effectiveness of two separate sound barriers determined by pure subtraction like that?

Thanks,
Scott

Posted: Fri Aug 22, 2003 1:44 pm

Scott,

Quote:

Scott wrote:
My only concern with this design (and "scenario 1" in general) is that it seems to violate the principle of having a "mass-spring-mass" system, with an uninterrupted air space (spring) between two "leaves" of mass. The top "leaf" of mass is the existing floor on TOP of the currently existing floor joists, right? Arn't we adding a 3rd "leaf" inside the air space by attaching gypsum to the bottom of the currently extisting joists?

The two leaf rule is a good one, but like all rules it is not appropriate 100% of the time. The problem with the floor above as a leaf in the system is that it may not be up to snuff, and so I'll occasionally simply ignore such a leaf and start from scratch.

Another example is the sudio I'm building right now for Brad, who hangs out on this group (lunatic). His building had external siding (3/4" cedar channel rustic) and Celotex sheathing. Since the cedar/ sheathing combination has an STC approaching zero, I chose to ignore the outer stuff altogether and built a two leaf sytem inside the building. I promised him he wouldn't hear those Harleys that cruise up this road, so I'd better be right...

--Wes

Posted: Mon Aug 25, 2003 6:44 am

Thanks Kris.
If I go with the "scenario 1" described above, should I:
1) NOT attach RC on either the new or existing joists, because they are already de-coupled (will rigid attachment of the gypsum provide better low freq transmission loss)?
2) Attach resilient channel only on the new joists (that are supported by the new load bearing walls) before the 2 layers of gypsum, but no RC on the existing joists?
3) Attach resilient channel only on the existing joists before the 2 layers of gypsum, but no RC on the new joists (that are supported by the new load bearing walls)?
4) Attach RC to both the existing joists and the new joists?

Scott

Posted: Mon Aug 25, 2003 11:17 am

Scott,

I would go with door number one. But I'm interested to see what Knightfly has to say.

--Wes

[ August 25, 2003, 12:18 PM: Message edited by: Wes Lachot ]

Posted: Mon Aug 25, 2003 12:09 pm

Personally,

If i were constructing this - for myself - i would go the route of door # 4. This is going to be maximum isolation.

However - probably the greatest actual bang for the buck is door #3. I don't believe the gain from 3 to 4 is going to be that great (i would spend the money even for the small gain - but that is just me) But the gain from 1 or 2 to 3 is going to be quite significant.

The BIG problem here is impact noise.

It's the biggest problem we face in hotel construction....... and the drywall applied directly to the bottom of the existing floor joist is not going to help with that AT ALL.

Turn this sideways and look at it - it's a wall assembly - we know how much the rc helps with that - and even more when you're dealing with impact noise.

So i am going to reccomend the rc - and stop the noise where it's at it's greatest - which is the underside of the existing floor assembly.

Then deal with what sound does make it through that with the decoupled ceiling joist/insulation/gyp mass below.

Quote:

will rigid attachment of the gypsum provide better low freq transmission loss

NO - your best isolation is done through the increased gyp (2 layers) mass and the decoupling action that the RC provides from the structure. This doesn't change because of the ceiling below.

Happy Hunting

Rod

(by the way - i am also interested in what Steve has to say about this)

[ August 25, 2003, 01:20 PM: Message edited by: Rod Gervais ]

Posted: Mon Aug 25, 2003 3:19 pm

Rod,

>It's the biggest problem we face in hotel construction....... and the drywall applied directly to the bottom of the existing floor joist is not going to help with that AT ALL.

Very good point. I retract my previous statement. Scott, you should have kept the box Sherryl Merryl was holding.

--Wes

Posted: Mon Aug 25, 2003 5:21 pm

Quote:

Originally posted by Wes Lachot:
Very good point. I retract my previous statement. Scott, you should have kept the box Sherryl Merryl was holding.

Wes,

Thanks, after i posted that (and it was more intuitive for this situation than any thing else....LOL) i went and did some "looking around".

Interestingly, I found at Acoustics 101 (an Auralex.com site) exactly this issue.... here's what they had to say:

CEILINGS
The method for controlling structure-borne sound that’s passing through ceilings is much the same, but generally will include adding additional layers of SheetBlok & drywall over the existing ceiling, preferably hung on resilient channel. If you’re lucky enough to have vertical height to spare, drop down 3 1/2" and frame another ceiling — resting it only on top of your new walls; insulate it with mineral fiber and cover it with 2 layers of 5/8" drywall mounted on resilient channel. If you have an unfinished existing ceiling, insulate it with mineral fiber, cover the joists with 2 layers of 5/8" drywall mounted on resilient channel (you can use 1/2" drywall if you want, but 5/8 has been verified to be better if space, time, funds & motivation permit) and then drop down 3 1/2" and frame your new ceiling. Some people have asked about insulating their existing unfinished ceiling then layering it with drywall. One of the million dollar room guys says this is not the wise thing to do because the layers of drywall segment the dead air space between the floor above and the new lowered ceiling. Another of the million dollar guys says the opposite. I must admit that at our old studios we followed the first guy’s advice and now regret it. So you do the smart thing: go ahead & put layers over your existing ceiling joists before you drop down and frame a new ceiling. Most of us, however, aren’t so lucky and won’t have the height to spare anyway.

:c:

Happy Hunting

Rod

[ August 25, 2003, 06:22 PM: Message edited by: Rod Gervais ]

Posted: Tue Aug 26, 2003 11:40 am

Rod and all

I find these very interesting: http://www.kineticsnoise.com/architectural/af.html

I guess they're not for sale in Europe.

Cheers, Han

Posted: Tue Aug 26, 2003 11:54 am

Han,

Kinetics is a great company with good products....... it's too bad you can't buy from them in Europe.

One of the components of my current project is a 21 story hotel with the swimming pool/spa/exercise room located on the top floor.

We're doing floated concrete slabs for sound isolation from the rooms below and are using Kinetic products in our design.....

this link will show you that product:

http://www.kineticsnoise.com/architectural/lsm.html

As far as i am concerned - this company is one of the players in the field. :tu:

Rod

Posted: Tue Aug 26, 2003 6:15 pm

Quote:

One of the million dollar room guys says this is not the wise thing to do because the layers of drywall segment the dead air space between the floor above and the new lowered ceiling. Another of the million dollar guys says the opposite. I must admit that at our old studios we followed the first guy’s advice and now regret it. So you do the smart thing: go ahead & put layers over your existing ceiling joists before you drop down and frame a new ceiling.

Jeez, what the hell? Is there no such thing as sound proofology? This ain't rocket science. It would be pretty simple for some masters student- hell an undergrad senior project- to figure this out. Guess in the whole scheme of things, it's really not too important. Maybe someone already has and we just don't know where too look.

Posted: Tue Aug 26, 2003 6:41 pm

[quote]Originally posted by ap:

Quote:

Jeez, what the hell? Is there no such thing as sound proofology? This ain't rocket science. It would be pretty simple for some masters student- hell an undergrad senior project- to figure this out. Guess in the whole scheme of things, it's really not too important. Maybe someone already has and we just don't know where too look.

AP,

reality is that there is (often times) more to this than strictly science.......

you go to a hotel - new - and leave frustrated because you could listen to the people in the next room.

You curse the owners - the architects and the bums who built it.

The wall design has an STC rating of 54...... but is actually more like 38.

The tested assembly has no doors in it - no outlets in it - has had someone construct it who has nothing but time on his hands - and is paid to painstakingly make it perfect regardless of how long it takes. Perfect science - perfectly executed - and totally unrealistic for normal construction - because we need doors and outlets - and because we would have to charge 5 grand a night for a room if we built it like they did in the lab. And i could find different things wrong with the same wall assembly in 30 different properties.

Same thing with this side of construction..... find me 30 acoustic engineers and i will find you certain things that they have 30 different points of view on - most of it based on personal experiences.

Reality is that i would not expect to find a handfull of engineers that would picture this from my perspective.

I know Wes personally - and think that he is an extremely qualified professional whom i hold in the highest regard.

Yet the though of impact transmission didn't cross his mind.

This does not mean that i am better in the area of acoustics - because i most certainly am not... nor does it mean that he is somehow not competent... because he is more competent than I.

It simply means that the 2 of us have had different experiences which make our minds see different scenerios.

I'll bet a dozen donuts to your dollar that the 1st million dollar room man had built all his rooms from scratch - and nothing above him - and had always had great results without the RC - just as i have in the cases where the only concern i have is air borne noise transmission. In that case i would never EVER reccomend to anyone that they throw away their money on that application.

The 2nd million dollar room man however had built in conditions where he had operations overhead - and learned the hard way that what serves for air borne sound doen not necessarily work for impact transmissions.

2 different paths - both right based on their experiences - one path to the most perfect room - and the 3rd person learns from both.

the math might be pure - but the math has no experience - it doesn't lie - but it doesn't tell the truth either it only answers the questions asked - and it doesn't know whether they are the right questions or not - there are very many different "truths" when it comes to this -

Happy Hunting

Rod

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