Improving sound isolation between rooms in an existing cinema

[Marcel Birgelen]

If you're going to design a building from scratch, there are many things you can account for in the design phase. One such things is the sound isolation between individual rooms in a cinema.

In existing buildings, achieving sufficient isolation between adjacent rooms is far more challenging. We're currently facing such a challenge as one of our customers is aiming at acquiring and renovating a triplex that faces such a challenge with two rooms, built directly adjacent and the division seems to be only a double cinder-block wall with only some basic treatmens and/or paneling on either side. There certainly is a lot of leakage between those two adjacent rooms. While we've not done any measurements yet, by just listening to it, it's clear this needs to get attention in a future refurbishment. It's not only the deeper basses that are problematic, but also the higher volume mid tones.

We're not complete rookies regarding sound isolation, but I'm curious about how you would approach the problem at hand and what solutions there are available around the world and what experiences you have with them.

[Steve Guttag]

Having a double block wall is much better than I normally start with. I, often only get a double staggered stud drywall. A key thing to remember is solids transmit sound better than air so, while the block can stop some frequencies quite well, they can transmit others to the other side like a conduit.

If they rooms are on a common slab, cut it and put in an expansion joint and do likewise on the common inside/outside walls. Decouple the rooms as much as possible. Ensure that the wall is from roof to floor and that there are no flanking paths. I've seen where the space above the booth is open to each room...letting the booth be a sound connection path.

Naturally, there should be sound absorption (fiberglass or like material) between the walls and all penetrations should be well sealed. In US terms, an STC of 75 is ideal despite STC being a non-ideal measurement (STC really was developed for office spaces and has a curve the resembles the A-weighting. It has been my experience that anything under STC-65 will yield some complaints...as one approaches STC-75, the complaints, like the noise fades away. Mind you, STC-75 generally means walls on the order of 1m thick but built up in layers, asymmetrically. I was on a site this year where the septum wall was effectively doubled by adding layers of studs and wall with fiberglass and all flanking paths sealed up. It made a HUGE difference.

[Marco Giustini]

If you get mit-tones, I wonder if there are gaps in the walls which let the sound through. The implementation of the treatment is as important as the treatment itself. You can invest in insulation, drywalls and stuff but if the builders cut corners and forget to seal gaps between materials, it's pointless.

When I asked someone to make a sound-proof door for me, I told them it had to be water-proof. Somehow a simpler concept to understand - I hope - than airtight.

As Steve suggests, try to de-couple anything you install. There are fixings for these purpose with some elastic material in them. Obviously the best outcome is the "room in the room" but that is a bit expensive I guess? What about ceilings? Is there any insulation there?

[Mark Gulbrandsen]

Nothing wrong with double staggered studs as long as one wall has an even amount of layers of drywall and an odd number of layers on the other side and there is at least 14" between the two sets of studs. Two on one side and three on the other works very good. Also isolating each floor from the next and having the wall on it's own foundation helps a lot. Many multiplex's I worked on were done this way. I never cared for block walls because the block itself is way too porous. Takes a lot of goopy paint to seal the block. And these days painters would rather spray than roll. So the stone block may not get well sealed.

[Steve Guttag]

Most double staggered stud walls measure in in the low 60s...even with fiberglass. It is sort of the bare minimum.

[Marcel Birgelen]

If you get mit-tones, I wonder if there are gaps in the walls which let the sound through. The implementation of the treatment is as important as the treatment itself. You can invest in insulation, drywalls and stuff but if the builders cut corners and forget to seal gaps between materials, it's pointless.

I suspect there will be some holes in the wall for infrastructure, which could be the culprit of the mids leaking through. But there is close to zero insulation on both sides of the wall right now.

In this case, both rooms seem to have a complicated history. The smaller room seems to be built into an existing building and is probably the original room. The neighboring room was built as an annex to the original building and probably has its own floor slab. The double-cinder-block wall also seem to be two separate walls with a cavity in between, of about 2 inches / 5 cm.

We're now having a bit of a discussion if it's better to fill this cavity with insulation foam or to leave it open as an air-gap, besides applying several layers of insulation on both sides of the wall of course.

[Mark Gulbrandsen]

Most double staggered stud walls measure in in the low 60s...even with fiberglass. It is sort of the bare minimum.

Again, depends on the number of layers of drywall, distance between the walls, and the floors being properly isolated from each other. Theaters that have continous poured floors are going to have sound leakage no matter what wall design they put up. I even did one Plex that had three drywall walls between each room. That worked really well, but was no doubt expensive to do.

[Steve Guttag]

I covered that the slabs had to be separated...that does not change the fact that a double-staggered stud drywall is in the low 60s for STC. Put 2-5 layers of gyp on each side...you are at 65ish. until that wall gets to be on the order of 3-feet thick, you are not getting to the mid-70s and you'll have a masonry septim in there too.

[Mike Blakesley]

The theater down the road from us, which recently converted the back half of their auditorium into two separate screens, has a staggered studded wall in between with (I think) two layers of drywall on each side. But the concrete slab does run all the way across the building and the wall looks to be only about 8" thick at most, so I'm sure they're seeing some sound leakage. I haven't been there myself to listen. I also wonder how much leakage they get from the original screen... they have a hallway running all the way across behind the "new" screens but it's got doors and walkways through it, as expected.

[Mark Gulbrandsen]

The theater down the road from us, which recently converted the back half of their auditorium into two separate screens, has a staggered studded wall in between with (I think) two layers of drywall on each side. But the concrete slab does run all the way across the building and the wall looks to be only about 8" thick at most, so I'm sure they're seeing some sound leakage. I haven't been there myself to listen. I also wonder how much leakage they get from the original screen... they have a hallway running all the way across behind the "new" screens but it's got doors and walkways through it, as expected.

I would assume they are not going to have massive sound systems in there. He seemd to be on the cheap side. If he does, he will for sure have leakage.

On large projects I always used Geller & Associates in Denver to either verify a design by an architect, or do do it from the ground up.

[Steve Guttag]

This is pretty "typical" of a dual staggered stud drywall with internal insulation (fiberglass) and with not only double-rock but with a sound attenuation layer on each side. My numbers didn't agree with the published STC rating of the product but mine was an actual Field Measurement rather than in lab conditions. It is FSTC-60 (and I measured three different demising walls at that site...the best was FSTC-62. It is evident that the product did well with the higher frequencies but didn't do so well with lower frequencies. How STC/FSTC is calculated is governed by ASTM. The problem with STC is that it stops at 125Hz because it is geared towards an office space, not cinemas. Tom Holman and THX came up with some desired noise reduction figures from walls that would reduce noise pollution enough that it would tend to not bother people in the quieter theatre. That is, though experimentation it was determined that while no noise transmission between auditoriums is financially impractical, if one acknowledges that both auditoriums are creating their own noise, one needs to really protect against one auditorium being in a quieter scene while the other is in a louder one.

So, here is the FSTC of the rooms:

Screen Shot 2022-10-01 at 6.52.36 PM.png

And here is the same rooms with the attenuation (and going down to 31.5Hz)

Screen Shot 2022-10-01 at 7.00.44 PM.png

There is a very noticeable drop as one goes down to 125Hz and it is quite deficient at 63Hz.

I'll tell you another thing that helps...baffle walls! For 1, they keep the bass off of the exterior wall that is often common between all audioria. By making the LF section of the speaker (and subwoofers, obviously) more efficient (at least 3dB), you are putting less energy into the system and letting the efficiency of it send the sound to people's ears rather than having to, often double the power to make up for the lack of efficiency. Baffle walls are cheaper than typical walls as they are often only 1-sided (open studs on the rear) and put 3-layers of either gyp or a sandwich of gyp/ply/gyp on the face (plus black duct liner). Baffle walls also greatly improve HF quality by stopping the HF from bouncing off the screen...going back to the outside wall...bouncing back through the screen as a delayed interference audio signal.

[Marcel Birgelen]

Building an efficient sound system in order to reduce leakage into surrounding spaces is probably something that's often forgotten in this business, while it has been pretty standard for big venue PA systems for a while now. Any energy you don't need to put in will also not cause problems anywhere else...

Meanwhile, we've come to the conclusion to put isolation foam between the two cinder brick walls, as this will seal most remaining air gaps and will add mass to the construction. In the end, to properly filter out those deeper frequencies, you need mass rather than volume.

Most of you seem to prefer fiberglass, whereas we prefer mineral wool (rock wool), because it's a bit more efficient and requires less precautions when handling it. So.I guess this is more of a cost decision than a functional one, or are there other aspects why you may prefer one over the other?

[Steve Guttag]

I have rockwool in my house for sound isolation between the basement and upper floors. It can do a good job but it is a touch-once item. Once you place it, you can never touch it again or it just crumbles to nothing. I'd say that in the USA, fiberglass is a bit more ubiquitous and construction will use Rockwool if that is the cheapest/most available product. Construction is always going to gravitate towards the lower-cost products unless it is in the spec to use a particular product and those costs to get to the the particular site are factored in. I like black duct-liner for all things on the face of a wall. It holds together much better and does a great job of sound deadening and, comes in black!

[Gordon McLeod]

Also make sure that the walls extend to the roof deck and no air ducts bridge the auditoriums

[Mark Gulbrandsen]

Also make sure that the walls extend to the roof deck and no air ducts bridge the auditoriums

Actually, one continuous roof deck is also bad for sound leakage. You want a separate roof deck over each auditorium.