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Old 20th November 2019
Here for the gear

Soundproofing an apartment workshop


like many of you have bedroom studios, I have a bedroom workshop. Actually, it all comes down to my own designed and built CNC router. Being on hobby level, I cannot afford to rent a space. Building a machine enclosure is not an option neither. But now I have possibility to move the machine from bedroom to a dedicated room in my apartment.

I have been reading about soundproofing and acoustic treatment for couple of weeks to get a basic idea. As this won't be recording or listening room, I do not care about acoustic treatment at all. On the other hand, I like to sing during my routing sessions, but because of the high rpm spindle and ear protectors on my head I don't hear anything of my lovely voice. So, my plan is to make the room dead. Treatment done.

My goal is to soundproof this room as much as possible. Thing is, this is an apartment in reinforced concrete building with neighbours below, above and next to me so I must handle floor, ceiling and walls. Lets ignore one window and one door to make it bit easier. More simplification - no ventilation as well as there are no long routing sessions. It looks to me that room within a room concept is suitable for my scenario. The room itself is 4 x 3 x 2,65 m (13 x 9 x 8.7 ft). The biggest problem is the floor load.

Yes, yes, I have talked to structural engineer. He told me three things:

#1 It is no problem to hang additional weight on concrete walls as this weight is transfered directly to building foundation and building will not even notice it.

#2 The code says 150 kg/m2 (30 lb/ft2) usefull load for floor. This omits a safety factor of 1.5 to 2 at least.

#3 Considering weight of my machine (300 kg or 650 lb) and it's footprint (1.3 m2 or 14 ft2), he recommended to place steel RHS (rectangular hollow section) beams (joists) under the machine to distribute the load, towards floor edges i.e. to the vertical structure.

With these rules kept in mind, I came to following idea.

1) Starting with rule #3 - if placing floor joists under the machine, why not to place them on remaining floor area as well? I want to create a joist grid as big as possible but leaving a small gap between concrete walls and grid for decoupling. Grid would be placed on rubber pads. Grid cavities filled with fiberglass insulation. More rubber pads on the top of the grid. MDF or plywood or OSB placed on top. Laminate floor on top (this is a must for easy cleaning sawdust, shavings and occasionaly cutting fluids). This step increases the floor load.

2) Applying rule #1 , adding mass to concrete walls. Attach resilient clips on concrete walls, fill air cavity with fiberglass insulation, attach channels to clips and put two layers of drywall. This mass would just hang, not touching new floor, no floor load increase.

3) Adding mass to concrete ceiling. Similiar to previous but horizontal orientation. This mass increases floor load of the apartment above but this is a small increment, no issue here.

4) Finally, my wish, the room within a room, placed on new floor, decoupled from surrounding walls, with fiberglass insulation between studs and one layer of drywall on studs, ideally two layers of drywall, ideally green glued(?). New ceiling placed on RWAR walls. RWAR weight is the major increment to floor load. Considering the floor distribution grid and distance between RWAR walls and existing concrete walls, this should not be a problem as most of the load will transfer to vertical structure.

I did not mention the noise produced by machine. I am not able to qualify it. What I think, most of it is a air borne noise. Machine in operation produces small vibrations that can be considered as impact noise, right? When I touch machine frame with my finger, I can feel gentle vibration. I definitely cannot see it, machine does not move, objects placed on working table do not jump, glass of water is still. Machine is on four rubber feets and when I touch floor with my finger, that gentle vibration in frame is almost gone or hardly to notice.

I have recorded machine sound with Zoom H4n recorder in one hand and measured dB level with SPL meter in other hand about 60 cm (2 ft) from machine. Ambient noise of 38 dB, machine turned on but idle 55 dB, machine turned on and routing wood 90 - 92 dB. I ran FFT on recorded audio to get idea on frequencies, but again, I am not able to fully interpret the results. What I think I see is that most of the noise is above 200 Hz. I generated 50, 100 and 150 Hz sine waves to verify H4n recording capabilities in low frequency range and they were all captured. This is the link to recorded audio (7 MB).

With approach described above, is it possible to get 30 dB transmission loss? There is no triple leaf situation; can the construction be done more efficient?

I am very not happy about hanging mass on concrete walls as this added mass is not fully decoupled, but it is the only option to add serious mass to a room. But if I am right with the air borne noise characteristics, that mass might do its job. Do I need RWAR at all? Other thing - the floor - mass is fine, but the weight limit...

Please, I would love to read any comments, criticism or advices from you, what would you do different, what wouldn't you do at all...

Thank you.