Studio wall knocking test

Today, I have a question regarding testing my gypsum wall. I went with a two wood frames system. Each frame carries 3 layers of drywall board. There is a doorway, which will be sealed later, so the construction is not yet air-proof. The floor is decoupled as is the ceiling. Only the wall flanks were not decoupled, as they are massive concrete and should therefore carry hardly any vibration. When I built the wooden frame I kept knocking on studs while touching the studs on the other side, to make sure no vibration was transmitted. It worked at that point. Now, with one layer of boards attached on both sides and the insulation inside, I do get a mild vibration, when knocking on the other side with low frequncies on some spots. My question: Could that be related to the insulation material touching each leaf? I made sure, I didn't pack it too tight, but the whole wall is filled with it. This is, how it looks like: Would it be better to remove the middle layer of isolation? Do I have to fixate the other layers then, that they will stay in place? Like this: Maybe, the other wall is just being excited through the air, as it is low frequencies only, that are being transmitted ... I just don't want to find out, I did some mistakes with the insulation, before I add more layers on the other side. right now, it would still be pretty easy to remove one layer of drywall ... Thanks!

As long as it is not over packed the insulation is OK. If it is over packed it creates a bridge. In most all cases there is not a need to entirely pack the wall assembly with insulation. Personally I would err on the side of caution and not place so much insulation in the assembly. But this statement "The floor is decoupled as is the ceiling." could be the reason for some of your troubles as well. A decoupled floor is fine but it requires mass...not light weight building materials. So if you have that type of floor then you have more issues coming your way in the not so distant future since you could have developed a diaphragm, like the skin of a drum. The decoupled ceiling was a good idea depending on how you actually did this but the floor. Let us inspect this edited statement: "I went with a two wood frames system....Only the wall flanks were not decoupled, as they are massive concrete and should therefore carry hardly any vibration. If you have an exterior concrete structure and you build a double wall assembly inside of this you have effectively created a multiple air space assembly...the common three leaf known to reduce the TL at low frequency. So this and the other things mentioned have got to be giving you issues. But, if there is sufficient distance between the exterior structure and these interior framed wall assemblies, you may be alright...like three feet or more from the existing structure, more always being better.

Thanks for the answer! By decoupled floor and ceiling I do not mean a floating floor. I just cut throug the tile floor with a flex between both wall frames, so vibration of one wood frame would not be transferred to the other one. So, there actually is quite a lot of rigid mass carrying both frames. Also note: it is not a room in room construction, just a wall to separate one room to two rooms. I did the knocking test very often without the gypsum boards attached, so I am quite shure these are the only possible errors: - flanking walls (it is actually not concrete, but a massive brick wall!) - insulation packed to tight - just transmission via air, as the boards create lower sounds than the wood frame itself did. In this case, I wonder, if the problem will be gone, as soon as the walls will be sealed air-tight and the doors installed. I just wondered, if this has been experienced by others, too, during construction. And if those problems later disappeared. If not, I wil finish this side and open the other side later.

To give a better picture - this is the room I am talking about with pictures: viewtopic.php?f=1&t=17825

Only the wall flanks were not decoupled, as they are massive concrete and should therefore carry hardly any vibration.

Wrong. Solid concrete conducts sound rather well, actually. If you tap the wall gently with a hammer in one room, you will hear it in the wall in the other room. No doubt about that. So your rooms are still coupled through the walls. We did warn you about that in your original thread.

I just cut throug the tile floor with a flex between both wall frames,

How did you do that? :shock: :!: I just do not understand what you are saying here! You originally said that this is a build in a cellar, and that the floor is "consists of heavy Tiles on concrete." Are you saying that you cut through your entire floor, all around the edges, to separate it from the walls? If you did that, cutting around the edge of the floor and separating it from the foundation walls, then you now have a major, serious, really bad problem! You really should STOP what you are doing right now, and pay a qualified structural engineer to come in, look at what you have done, and tell you how to fix it. You CANNOT simply go and cut the floor out from the foundations like that! The floor might well be part of the structural system of the house, and even if it is not, then you now have a floor without any foundations under it! That slab will very likely crack sooner or later, since it is not supported correctly any more.

So, there actually is quite a lot of rigid mass carrying both frames.

Whoa!!!! Sop you are saying that, not only did you cut your floor away from the foundations, you then also built a massive wall on top of it WITH NO FOUNDATIONS UNDERNEATH????? :shock: :!:

Also note: it is not a room in room construction, just a wall to separate one room to two rooms.

Actually, you were warned about this very problem too, back in your original thread. I quote myself:

You CANNOT isolate two rooms unless you decouple everything: Just building a wall between them will NOT achieve good isolation, unless you also cut all the flanking paths. And the only way to do that, is to build the room correctly, as four decoupled walls with a decoupled ceiling on top

Brian also warned you about it:

Perhaps it will be easier to think of each side of the wall as a diaphram, picking up all the vibrations in the room. If there is a mechanical connection between the two sides, this vibration will conduct and be reproduced on the other side. ... there should be no flanking i.e. transmission path between the two rooms via the wall acting as a bridge.

Brian also warned you many times about other problems with your plans, and gave you suggestions on how to fix those, but you didn't seem to want to listen to him. And I also tried one more time to get you to re-think this:

Stop. If you build that wall, or anything else, BEFORE you understand what you are doing, then you would be absolutely wasting your time, your money, and a lot of hard work, all for nothing.

I even tried to give you the method that you needed to get this done right:

first of all, just stop building. Don't waste any more time, money, effort, materials etc. Then get Rod's book, read through it a couple of times, ask questions here (or ask Rod himself), until you understand the concepts. Then design your entire room properly, taking into account ALL of the factors, concepts, potential problems, etc., Then post the complete design here, and only when everyone agrees that you have a good design, only then should you be picking up a hammer to start actually building.

And I even warned you about the exact situation you have gotten yourself into:

It will take a lot more time, for sure: weeks, perhaps months. But that is MUCH better than roaring ahead at full steam, doing things that will not produce the results you are hoping for. The solution then will be even worse: Rip it all down and start again, doing it right the second time.

But it looks like you went ahead and built something that you were told would not work. You never did any of what I suggested: you never asked any more questions here, and you never did post the complete design here, for us to check. Instead, you just went ahead and built it your way. And now you have discovered that we were right. So the only solution now is to take it all down and re-build it properly. Sorry. - Stuart -

Woah! Stuart, please relax a little. None of your assumptions meet, what I did! When you read through the original thread, you'll see I wanted to go to a professional acoustic builder and pay him to plan the wall correctly. I did that! What he said was exacly that: cut through the floor between both wall frames. And I didn't even do that myself! The problem was, that there had been a floor radiator installed, so I had professionals come and replace it by conventional radiators, to make the cut possible. They also did the cut (down through the Estrich), so there are certainly no structural risks taken here. I also separated the wood frame and gypsum plates of the hung ceiling. The only possible flanking that could be left now is the side walls and and the concrete ceiling above the hung ceiling. Of course I specifically asked my acoustic consultant, what to do about that: Nothing, was his reply, that wouldn't cost a fortune, but considering the mass connected underground, it would be ok and meet my expectations. I could even verify that myself, as I knocked many times against the wooden frames and saw, if transmission was happening. There wasn't! So, the transmission could only happen due to the gypsum boards being attached or the insulation inside. My acoustic guid is Michel Schreiber from ACM-Acoustic. The guy built many studios and is off next week for a concert hall. So, definitively not a fake one ... I was just trying to see, if I could find out the mistake on my own. If I won't, I will call him again ... I DID follow every advise given here, up to the point, that I payed someone to help me. So, please don't tell me, I didn't. It's simply not fair and wrong. Constructive criticism still welcome! :-)

And yet, here you are back again, asking why it is working. You are obviously very concerned because the wall is clearly not doing what you expected it would, and you have not even finished yet.... There's another thread here on the forum that I suggest you read, from a guy who found himself in a very similar situation to where you are. You might find it interesting: viewtopic.php?f=2&t=17363

I DID follow every advise given here, ... So, please don't tell me, I didn't. It's simply not fair and wrong.

well, I just went back and looked over your thread again, and I still don't see the place where you posted your final plan before you started building. I guess I must have missed that, and it must be there since you say that you did follow all the advise you got here. So please could you point out where you posted that, as I'd like to look over it again, and see how you proposed to cut the floor and decouple the walls. - Stuart -

FriFlo wrote:

To give a better picture - this is the room I am talking about with pictures: viewtopic.php?f=1&t=17825

Actually that is a sketch up. A picture is more detailed in what was actually constructed. It seems like, reading what Stuart has said, you have passed this off previously and he and some others tried to slow you down and you did whatever it was you wanted. So I have to ask, why do people even come here if they already know the answers? The reason is that most do not know the answers, myself included, but given the proper amount of input and information we have worked out some great recording rooms. Sure sometimes Mr. Sayers has to jump in and re-direct us, but that is what happens here. No bs, we want you to succeed but if you choose bs over attention to detail, especially if you dismiss the instructions that the moderators give you...then you are just shooting yourself and your budget and your acoustic environment in the foot, so to speak. If I can say this without hurting your feelings, I think you owe Stuart and possibly whichever Brian is being referred to an apology. It reads like they tried to help you, you did what some hack said who knew little, and now you started a new thread in order to get support for the things...they already told you NOT to do. When will they learn right?

Is now everybody trying to repeater that? I already said! That is not the case. 1) I did not rush. I wanted to go on, but I didn't because of the advice. I read the book and learned other stuff. The other thread is almost 2 month ago. How on earth do you call that rush? 2) I did never ignore any advice given. It is just, that materials are different in other countries and I was not always sure, if I was understood correctly in my purposes by everybody. So, in the end - to be sure - I hired a professional acoustic guy, because I was told to do that! How could I possibly follow the advice more than by doing that? Everything I did, was following his instructions. I even consulted him about all the doubts I learned about here. So, if you want to repeat your lectures, please care to read, what I had to say and stop saying things, that I already said weren't true! I don't even know, if my wall works or not, as it is not done yet. However, I have doubts because of the knocking test. There are still modifications possible, because I might remove from one side.

By the way: what Brian said, was that I needed to build 4 separate walls and ceilings to decouple everything: that was not possible. I would loose to much space, money and time. That is why I was trying to find another solution and the acoustic consultant gave me that.

This is, what i did following the instructions by the pro acoustic guid. So far, I couldn't reach him by phone. 1) I completely removed the new wood frame and removed the old drywall from the tracking room side. 2) let the builders cut through the floor between the old frame and where the new one is supposed to be. This gap should decouple vibrations of the floor between both rooms, as the acoustic professional suggested. The gap was filled by a special acoustic foam (he also suggested): Würth pure logic. This was done, that the gap would not close again over time. The floor decoupling was tested by hammering on one side and listening on the other side. It worked. 3) I closed the air gaps between hung ceiling and actual ceiling on both sides by fitting in wood pieces and foaming with the same würth stuff mentioned before. 4) I rebuild the new frame, fastening it to floor, ceiling and walls with big screws and duebel. I also did that with the old frame, whereas it was not attached to the floor before. 5) I cut the frame of the hung ceiling (and obviously the drywall there) between the old and the new frame, after screwing it to both frames. Now it is decoupled between both rooms. 6) I added two cable hoses and wooden boxes for light switches. Both do not create bridges between the walls. The cable Hoses are plastic foil held in shape by a light wire spring. 7) I tested by knocking with my fist and a hammer on both ceiling and frame on multiple locations and had some one els check, if any vibrations were transmitted to the other frame or ceiling. It worked at that point. This is, how everything looked then: 8 I added two layers of gypsum board. Each time, I chaulked the edges and mudded afterwards. Then I did the knocking test again, and now the vibrations upon knocking with my fist were transmitted to the old wall and vice versa.

Soundman2020 wrote:

There's another thread here on the forum that I suggest you read, from a guy who found himself in a very similar situation to where you are. You might find it interesting: viewtopic.php?f=2&t=17363

I actually cannot see, where this is much similar to my needs. This guy is looking for a room in room construction to seal off sound to the neighbors. I am looking for ways to improve the isolation between two rooms inside of my studio. No problems with any neighbors. To do that, I cannot build a shell around both rooms, as that would cost way to much money, room and time. It simply is not possible for me ...

I finally reached my acoustic professional. He said, I need not to worry. Pounding against the wall with my fist would create an air compression between the sealed walls, that could not escape fast enough to the sides, therefor excite the other wall. However, with reasonably loud monitoring, there would never be as much vibration happening, as with knocking wit my fist, so I am fine. Also, the weakest link in that wall would always be the doors between both rooms, hence, my wall quality is good enough - maybe even to good, considering the transmission via doorway.