Voice over room - making the best of a bad job

[nicklear]

I have been given the use of a small room (3x4x3m) rent free – to use as my video editing suite – on the understanding that I make it as good as it can be for voice over recording to be used a few times a month by the guys I’m renting off.

There is not a high expectation here - they understand they will have to be quite in the office during VO recording. I have higher aims that the guys giving it to me. Thus anything I can do over the basics is a bonus so no pressure here. Just want some feedback on my plan. Also, I won’t be doing any critical audio mixing in here.

I will look at sound attenuation and treatment separately. Would appreciate direct feedback to specific points for anyone who has the time, and it is much appreciated.

Sound “proofing” (attenuation)

Two walls are brick, and two are cheap steel drywall partition (stud and track?), with two single layers of drywall at about 70mm distance, with an air cavity in between. Ceiling is single layer of drywall, with ceiling lights showing a hole around them. Door is hollow door, not sealed.

There is not much I can do here as the landlady does not want me mucking around with the walls. I can’t put on any extra drywall, I can’t strengthen the structure.

- one idea is to take off one of the drywalls, and fill with gap with 50mm OC703 equivalent batts that I happen to have plenty of lying around. Or try to drop them in vertically.

Is this worth it? As I understand it, this wall is a very poor MAM system, with very little mass in the walls and the OC703 makes the “air” bit a little better, by turning some of the sound into heat as it passes through the walls. I know ideally it is mass that stops sound, but my hands are tied.

- another idea someone suggested is to pour something down from the top (the top is accessible) like a powder or a foam – acoustic company locally said it needs to have mass and that's not a good idea anyway as the steel frame wouldn’t support much extra weight, any ideas?

- ceiling: plan is to seal the lights & the glass windows, and then cover the whole ceiling with OC703. Builder also wants to make a false ceiling of insulboard (https://insulpro.co.za/insulboard/) - not sure if this is worth it, it is not heavy so it isn’t building up the mass of the ceiling and local acoustics company advised I couldn’t bulk it up much (e.g. with vinyl or more board) as the steel system wouldn’t take much weight.

- door: plan is to replace the seal with a rubber one (QL 48750) which will sit flush, and put in a bottom floor sealing unit (Planet KT). Have been told the steel stud system will not take the weight of a solid door – I really want to try to work around this if at all possible, and I have a door ready to go in. The hinge is 100m away from the brick wall so I wonder if some extra strength could be gained without too much visible work (i.e. the issue is the landlady doesn’t want to see too much difference).

- a big question I have is whether I will need a ventilation system if I manage to seal the door and ceiling really well? I do have one I can use (2xDIY silencers and a fan) but it will be a hassle installing it & heavy for the ceiling structure. The local acoustics company said I don't need it, that there is plenty of air in there and I will be in and out at least once every 2-3 hours to refresh the air. I do generally like to work with the door closed. Any feedback on this?


Sound treatment

- 2 or 3 superchunk style Bass Traps.
- 6 100mm thick 60x120cm Acoustic Panels on walls
- one diffuser that I have because why not (looks quite nice)
- some wood panels spaced apart on one wall mainly for looks, but can break up the sound somewhat

I have attached the REW charts of the empty room. What is a good rough RT60/RTopt to be aiming for? (I know RT60 isn’t totally right for small rooms, but I think it must give some info)

On another note, their boardroom was a similar room, and it was boomy as anything and I've put two superchunks in there on the ceilings and it sounds great in there now for meetings. They are really pleased with that.
Attached Thumbnails

[Soundman2020]

High Nick! Good to see you back again... it's been a while!

And you sure picked a tough one this time...

Sound “proofing” (attenuation)

You can probably improve it a bit, but with the restrictions placed on you, it's not going to be fantastically isolated.

Two walls are brick,

How thick? Are they painted or bare?

cheap steel drywall partition (stud and track?), with two single layers of drywall at about 70mm distance, with an air cavity in between.

Is that just one single metal frame with drywall on both sides? Or two frames, with drywall on only one side of each frame?

Ceiling is single layer of drywall, with ceiling lights showing a hole around them

Ouch! Any idea what is above that? Would it be possible to pop out one of those lights and take a look?

Door is hollow door, not sealed.

Are you allowed to replace the door with a solid one?

- one idea is to take off one of the drywalls, and fill with gap with 50mm OC703 equivalent batts that I happen to have plenty of

If you are allowed to take off the drywall and put it back again, then how come you can't put an extra layer on at the same time? Or replace it with thicker drywall? Your best option if you can only take it off and put it back: put resilient channel across the metal studs BEFORE you put the drywall back, and fill the cavity with insulation. And use thicker drywall for the replacement. And use two layers of thicker drywall if you can!

fill with gap with 50mm OC703 equivalent

Definitely! The insulation will help, as it will damp some of the internal resonances in the cavity, and slow down the speed of sound, and change the way air deals with heat, from adiabatic to isothermal. All of that helps. However, if this is a single-frame wall with the drywall on each side attached to the same single metal frame, then the improvement is not going to be spectacular: mild, but useful. Adding the resilient channel could make a big difference, because it partially decoupled the drywall on one side from the studs. Resilient channel plus insulation plus two layers of thick drywall, would make a very noticeable difference.

Is this worth it? As I understand it, this wall is a very poor MAM system, with very little mass in the walls and the OC703 makes the “air” bit a little better, by turning some of the sound into heat as it passes through the walls.

It is worth it, yes. You are right that it is a poor and coupled MSM system, but even the insulation alone will help a bit. Decoupling one side with resilient channel would help a lot more. Thicker drywall even more. Two layers of thicker drywall: a lot!

- another idea someone suggested is to pour something down from the top (the top is accessible) like a powder or a foam –

I can't see that helping! There are some types of insulation that can be blown into a wall with special equipment. There are both dry and wet blow types. It's hard to do right, as the blowing might leave empty cavities still in some spots, or it might create flanking paths at some points where it built up too thick... and you'd never be able to see that. In the hands of an expert operator who can get even coverage, it might be an option. Expensive, though. And not necessarily good.

acoustic company locally said it needs to have mass and that's not a good idea anyway as the steel frame wouldn’t support much extra weight, any ideas?

The steel frame for that wall would not be supporting any extra weight! The floor under it would be supporting the weight. Are you taking about a sand-filled wall? If so, the drywall itself would need to be able to handle the outward pressure. You can't go very deep in loos sand before the pressure gets substantial, and it would burst the drywall off it's nails... And even if you could make that work, a sand-filled wall is basically a single-leaf wall. Damped, true, but still single-leaf.

- ceiling: plan is to seal the lights & the glass windows,

You have windows in the ceiling?

and then cover the whole ceiling with OC703.

Including the lights and windows? If you are allowed to cover the lights and ceiling windows, then could you just take out the lights and put up an additional layer of drywall on the ceiling, then surface mount lights on that?

Builder also wants to make a false ceiling of insulboard

Seems to be rigid mineral wool insulation.... probably not much use, if you already have 703 up there.

it is not heavy so it isn’t building up the mass of the ceiling and local acoustics company advised I couldn’t bulk it up much (e.g. with vinyl or more board) as the steel system wouldn’t take much weight.

Find out what the ceiling joists are (make, model), and check their load-carrying capacity with the manufacturer. Figure out how much dead-load they are already carrying (drywall, insulation, lights, their own weight) and then you'll know how much extra dead load you can add.

- door: plan is to replace the seal with a rubber one (QL 48750) which will sit flush, and put in a bottom floor sealing unit (Planet KT).

That will help, yes.

Have been told the steel stud system will not take the weight of a solid door –

Once again, find out what the metal studs really are, and check with the manufacturer. If the framing was done properly and is typical steel framing, it should be able to handle an ordinary solid door, but perhaps not a massively heavy studio door.

The hinge is 100m away from the brick wall so I wonder if some extra strength could be gained without too much visible work (i.e. the issue is the landlady doesn’t want to see too much difference).

100m? That's a BIIIIGGGGG room! Maybe you mean 100mm? But anyway, if you can open up the wall (take off the drywall), then you will have access to the framing to beef it up any way you want, making it string enough to take the weight of a hefty door. Once option would be to replace the metal framing in that section with wood, another would be to add some wood, and a third would be to add more metal, of a thicker gauge.

- a big question I have is whether I will need a ventilation system if I manage to seal the door and ceiling really well?

Do you plan to breathe in there? There's your answer! You probably won't die from lack of oxygen (hopefully), but it will get very stuffy and unpleasant in there after a short time. Long sessions will not be nice... (Actually, you would die from CO2 poisoning long before you die from lack of oxygen. Each breath you exhale has a lot of CO2 in it, which builds up rather fast. You can die in air with plenty of oxygen in it still, if the CO2 level is too high).

I do have one I can use (2xDIY silencers and a fan) but it will be a hassle installing it & heavy for the ceiling structure.

Then install it through the walls, instead of the ceiling! And do check the ceiling load details, as I mentioned above: you might be OK.

The local acoustics company said I don't need it,

Why do I get the feeling that you might nor be talking to the right company, every time you mention them?

that there is plenty of air in there

There is plenty of air in there, yes, and you will not run out of oxygen, as I already mentioned. Actually, you would die from CO2 poisoning long before you die from lack of oxygen. Each breath you exhale has a lot of CO2 in it, which builds up rather fast. You can die in air with plenty of oxygen in it still, if the CO2 level is too high. Each breath you exhale also has a fair amount of humidity in it, so as you sit/stand and work in the room, the CO2 and humidity levels will be rising constantly. Your body also puts out heat, and so does your video editing gear, and the lights. That heat has no place to go, so the temperature will also rise slowly.

ASHRAE guidelines state tht you need about 15 CFM (cubic feet per minute) per person of fresh air to keep people alive and happy. Actually, their guideline says 15 CFM minimum, up to 60 CFM in some cases. So you need AT LEAST 15 CFM fresh air for your booth. Part of the goal here is to keep Co2 levels down, ideally below about 700 ppm. ASHRAE says under 1000 PPM, but there's several studies that show mild health effects for some people at those levels. Discomfort starts at around 700 ppm, and certainly at 800 ppm most people notice the "stuffy" unpleasant air. At 900 ppm some people start showing adverse cognition and decision-making. At 1000 ppm, some people start feeling sleepy, and I don't think you want to be napping on the job due to bad air quality! Above 2000 ppm, some people start experiencing headaches and even mildly blurred vision, plus other symptoms. The higher it goes, the worse it gets. To put the above in perspective, your exhaled breath runs at around 40,000 ppm Co2 concentration (anywhere from about 35,000 to 50,000, depending on many factors)... This does not mean that everyone experiences that! Many people are still fine at 4000 ppm, or higher. But I like to leave a margin for safety, so I usually aim for something like 700 ppm. That keeps everyone happy. To get to that target level of 700 ppm, you need about 22 CFM of fresh air per person.

So it's rather strange for that company to say that you don't need fresh air!

I will be in and out at least once every 2-3 hours to refresh the air.

Why would the air move if you just open the door? Unless there is a pressure difference, and TWO paths for the air to move (one that brings air in, the other going out), the air won't move at all: it will just sit there. If you park your care on a flat road with the brake off, it won't move. It would only move if the road slopes. Same with air: if you park it in a room with a flat pressure gradient, it won't go anywhere, even if you "take the brake off" by opening the door: it needs a "slope" to the air pressure: higher at one point and lower at another, with an open path at each of those points. Just having the pressure gradient by itself is no use, unless there is a path for the air to flow through. Your car parked on a slope but up against a solid wall is also not going to go anywhere, because there is no free path for it: the wall stops it moving, even though the slope is there.

In other words, just opening the door does nothing: that's like only having empty road BEHIND your car on the slope, but a wall in front of it. You need a SECOND path for air to get in or out of the room: that's equivalent to taking the wall away from in front of your car. And even then, even if you have two paths, the air will still not flow, unless you create the pressure difference: you have to tilt the road so your car will roll down it... IT won't roll down a flat road. Thus, you need a fan to make the air move, and two pathways...

Treatment: We can get into that later, but first look into the isolation and ventilation issues...


- Stuart -