Cement board causing resonance in floor?

Hello all, I have lurked here for a while, trying to absorb enough to successfully build, and I'm under construction as I write... and, I've hit a "snag", heheh. Here's the background, sorry if it's too much/too long: The rooms that I'm modifying are in a two-story, wood frame apartment building. I rent a two-bedroom unit on the upper floor. The ambient noise on the outside of the building is rather high, as my unit is located appx. 250 feet (76m) from a busy freeway, and 125' (38m) from a busy commercial street. The building was built in the early 70's. NO insulation was used, anywhere at all. The exterior walls are stucco on the outside, the interior walls and ceilings are plaster (3/8" applied over 1/2" gypsum lath). All walls are 2x4 framing, and the floor between the upper and lower apartments is 2x10. The sub-floor is 1x6 planks nailed to the joists, covered by 3/8" particle board stapled to the planks. The floor covering is mostly carpet/pad. The roof is 2x8 framing. I record my voice only: narration, commercials, etc. I considered, and rejected, a small isolation booth, and decided to modify a bedroom (13'-5" x 11'-5" x 8'-3", or 4.0m x 3.5m x 2.5m) to use as a recording space. My project, long in the planning stage, was pushed into action when the nice, quiet tenant living below me moved out, and a young family with two screaming, running, stomping, children, heheh, ages 1 and 2, moved in. As a renter, I can't do many of the modifications that would produce the best results. For example, I can get away with making SOME construction noise, but stripping the plaster down to the framing would not be possible, nor would any modifications that can't be made from within my apartment. My goals are to provide enough isolation to record successfully in the presence the "average" ambient exterior noise and the "average" noise generated by the family below me. I'd also like to be able to record during the night, when "mom and dad" are sleeping in their bedroom, located directly below the room I'll be using for my "studio". Given the restrictions on what I can do, I don't expect to be able to record through door slams, heavy pounding feet, aircraft, heavy truck or bus traffic, etc. I won't go into the full details of construction plan here, because I am looking specifically for some quick answers to the previously mentioned "snag" that has come up...but it's pretty straight forward: for isolation, add insulation and mass to the walls and floors, change/add some doors, and modify/plug a couple of windows. Then treat the interior walls as needed for the square box that it its, heheh. I'll provide more details, a floor plan, and some photos in separate post at a later date. I decided that blowing cellulose insulation into the floor and wall cavities would be my first step. I started with the floor of the living room, not because I would be recording there, but because it would help with TV noise and kid noise coming up from below. It was also a MUCH easier space to conceal my construction activities from my landlord. The results were better than what I expected: Before my old neighbor moved, I could hear his TV regularly through the un-insulated floor. With the insulation in place, I could hear the toddler and infant "exercising" their voices, screaming at the top of their lungs, only faintly, and only with my ear literally to the floor. In the living room, the process to blow in the insulation was to cut out a section of the particle board down the center of the room, cut out a piece of the underlying diagonal plank above each joist bay, blow the insulation in, replace the piece of diagonal plank, and then replace the particle board. That was a month ago, and last week, I started the process of insulating the bedroom floor and walls. Because this is to be the room I will be recording in, I decided to take an extra step that I had not taken in the living room floor phase: replacing the particle board top-layer of sub-floor with something "better". I thought "I should add some MASS to this floor assembly". OOPS. (Now, I know from what I learned here that a "decent" subfloor assembly would involve removing the existing planks, replacing them with plywood, and adding a second layer of particle board or MDF above it, with Green Glue between the layers. Even better might include starting with a limp-mass layer first, then the damped sheet material as described on top of that. Cost, time, construction noise all made this prohibitive.) Because I wanted to add the maximum mass but little or no HEIGHT, I chose Hardibacker brand 1/2" cement board to replace the existing particle board. It weighs appx. 2 lb. per sq. foot, about the the same as 3/4" MDF. After blowing in the insulation, I pulled up part of the existing particle board and installed the cement board. I even took the added step of filling in the gaps between the diagonal planks with acoustic caulk. Today, when I was finished with about 2/3 of the cement board, my neighbor below and the kids just happened to spend time some time in the room directly under the room I had just spent days insulating and sub-flooring...and YIKES! Even with the insulation in place, and the heavier top layer in place in 2/3 of the room, I could hear them MORE loudly than before I began the "improvement"...OUCH. Now, I am trying to allow, in this self-perceived increase in noise transmission between the spaces, that the carpet is rolled back 2/3 of the way in that bedroom. But, I just can't imagine that the carpet and pad will cut much noise...I had expected that the nearly 10" of insulation and much heavier subfloor would have made a much bigger improvement in isolation between the rooms. At last, here is my question: What I am GUESSING might be happening has to do with the STIFFNESS, and therefore RESONANCE, of the cement board, as compared to the original particle board. I can't put my finger on why I'm thinking this, but I got this hunch as I was listening to the voices coming up from the room below. So, I stood a piece of the cement board on its end, and tapped it with my fist...it rang like a bell, or a like drum with a long decay. I unfortunately don't have any of the original particle board around, it was tossed out...but if I did the same "ring test" to a piece of it, and it sounded more like a "thud" than a "gong", would that meant that the old particle board is acting somewhat like a damper in the sub-floor assembly? And that the new cement board is adding resonance to the assembly, and somewhat "amplifying" the sound that originates in the room below? Thanks for taking the time to read through all of the above. I look forward to reading your responses, and answering any questions you might have.

It's a combination of still only being a dual but coupled wall system and flanking. Dense heavy stuff conducts sound really really well. The downstairs single plaster layer acts like a diaphragm and conducts all the noise into the structure and your now enhanced massive structure is the speaker... Since your renting, I would suggest considering making the room into a big isolation booth with inside out panels on isolation mounts which also supports a decoupled ceiling. Use a floated floor inside the "booth" to decouple - 2x 1/2" plywood over 1" rigid insulation and separated from the isolation walls. What you will suffer is triple leaf effects unless you can make the booth smaller so it's a couple of feet from the existing walls. Not overall ideal but it's what you have to work with. Another unrest option would be to put 2x 1/2" plywood layers over rigid insulation on all surfaces - using decoupled anchor screws (kinetics for example) to hold the wall and ceiling coverings in place. Cannot say it will be completely effective but may be sufficient.

gullfo wrote:

It's a combination of still only being a dual but coupled wall system and flanking. Dense heavy stuff conducts sound really really well. The downstairs single plaster layer acts like a diaphragm and conducts all the noise into the structure and your now enhanced massive structure is the speaker...

Thanks for your response, Glenn. So, its possible that my replacement of the particle board with the denser, heavier cement board created a slightly "better" speaker?

gullfo wrote:

Since your renting, I would suggest considering making the room into a big isolation booth with inside out panels on isolation mounts which also supports a decoupled ceiling. Use a floated floor inside the "booth" to decouple - 2x 1/2" plywood over 1" rigid insulation and separated from the isolation walls. What you will suffer is triple leaf effects unless you can make the booth smaller so it's a couple of feet from the existing walls. Not overall ideal but it's what you have to work with. Another unrest option would be to put 2x 1/2" plywood layers over rigid insulation on all surfaces - using decoupled anchor screws (kinetics for example) to hold the wall and ceiling coverings in place. Cannot say it will be completely effective but may be sufficient.

I understand the effectiveness of this design, but I don't think it would fly with building management during the twice-yearly interior inspection...even though it would all be "temporary", it would look like a structurally altered room, unless I sized it down to a pretty small booth. In the short term, do you think going back to particle board (or MDF) for the sub-floor top layer would make a less-efficient speaker? I considered using Green Glue between the planks and top layer but thought that would be a waste because the plank layer is not very flat and smooth, and I did not think I would get decent contact between planks and top layer...what about topping the planks with a damped layer, for example, 2x 1/4" plywood with GG, or 1/2" + 1/4" with GG? Is the MLV+foam, sold for use as a "isolating" carpet padding, worth considering? Thanks, Ron

the bonded foam under the MDF on the floor would also be effective. i think you may be stuck with a booth for work where you need to isolate the voice recording and the rest of the room can be set up for mixing where a little bit of noise won't impact the recording...

gullfo wrote:

the bonded foam under the MDF on the floor would also be effective.

Thanks Glenn....so, just so I'm clear, by "bonded foam", you mean foam that is bonded to mass loaded vinyl, and you would place it between the planks and the particle board, not between the particle board and carpet? Ron

agglomerated foam, bonded foam, heavy carpet padding etc tend to be similar where polyethylene foam is shredded and re-bonded.

I see, thanks again, Glenn... I would have though that separating two "rigid" layers with foam like that would be creating a triple leaf...why is that not so? Ron

no, it's technically a triple leaf but what options do you have? sometimes you can only make a best effort and the foam (or rigid insulation) approach is probably as good as you're going to get without serious construction (which you cannot do).

Thanks Glenn, yes, my options are limited. I have pulled out the cement board...so, would the assembly you are suggesting look like this? Carpet and pad ------------ 3/8" particle board or MDF ------------ Bonded Foam ------------ 1x6 diagonal planks ------------ Floor Joists I noticed on the Kinetics website that for the products that they sell to float a floor, the top plywood layers do not get mechanically attached to the structural floor, which makes sense. If I have your suggestion correct above, would I be screwing the particle board down into the planks, through the bonded foam, or letting it float? What function is the foam doing in this design? ron

let it float on the foam. but use 2 layers of 1/2" MDF and stagger and seals the seams.