Sound Proofing Opinion -Airstream mobile Meditation Studio

[brobes05]

Hi,

This is my second post and the first project that is actually going to get built! I am working for an architecture firm that has an unusual project designing a 'Sound Shala' which happens to be a Chakra Meditation studio. The studio is to be built in an Airstream and to be completely custom. Our budget is rather high, due to the fact that the client wants custom parametric diffusers as well as lights hooked to sensors that activate as meditators sing certain notes.

That being said, there is an inner layer of 87 plastic diffusers that wrap around the airstream's interior like whale bones. Inside of the diffusers (which are half tubes) there are LED strips of lights. My main question is how can I cheaply add sound insulation to the interior of the airstream's shell. Our goal is to have no sound from the exterior enter, but to have the interior be live enough for the room to have light to medium reverberation. Definitely not looking for a dead vocal booth. There will be Indian Meditation bowls and people singing so some reverberation will be necessary for the proper experience.

Attached is the plan, detail, as well as renders. We are working out a budget for this right now so finding a proper solution for sound reinforcement is our first step. The dimensions of the airstream are: 28.5', 7.9', 7.14'
The area of the manifold that needs to be sound proofed is: 617 SF (this is the inner shell that will need to be covered with sound proofing.

Any suggestions would be more than appreciated!
Thanks so much.

Erik

[brobes05]

I have done some research and it seems like a solution like this may be my best option. Since the treatment will most likely have to cover the doubly curved interior of the airstream shell.
http://www.audimutesoundproofing.com/noise-control.aspx

[Soundman2020]

When you say "custom parametric diffusers", you are talking about light diffusion, correct? Not sound diffusion? Because you won't get much acoustic diffusion from those, except at very high frequencies where it isn't needed anyway. If you want acoustic diffusion inside the trailer, then this is the book you need to read to learn how to do that: http://www.amazon.com/Acoustic-Absorber ... ox+antonio

However, you did say that you want the space to be highly reverberant, so obviously you can't have any acoustic diffusion in there at all!

My main question is how can I cheaply add sound insulation to the interior of the airstream's shell. Our goal is to have no sound from the exterior enter...

Simple answer: you can't! Isolating sound (preventing it from getting in or out) requires mass and airtight seals. Insulation has no useful mass, and is not airtight at all. The more isolation you want, the more mass you need, and the better your airtight seals need to be. The equation for calculation how much isolation you will get from a certain amount of mass with a single-leaf structure such as yours, is very simple. It goes like this:

Tl = 14.5 * log (Ms * 0.205) + 23 dB

Where:
Tl = "Transmission Loss" (how much sound is blocked, in decibels)
Ms = Surface density (or surface mass: how much each square meter of your wall weighs)

If you do the math, you'll find that you need a huge amount of mass to get "have no sound from the exterior enter".

So you cannot accomplish what you want with a single leaf wall, and you cannot accomplish it "cheaply".

You can get reasonably high levels of isolation by building a 2-leaf wall (in other words, two separate "walls" made of heavy materials, with an air gap between them), but that isn't cheap either, and even thou it needs much less mass than a single leaf wall, it still needs quite a bit. Probably more than the Airstream chassis and suspension can handle. You would likely have to reinforce the chassis and floor, beef up the suspension, and use larger wheels and tyres to support that amount of weight. You'd also need a pretty big vehicle for towing the trailer.

You will likely have to reduce your goal of "have no sound from the exterior enter" to something more reasonable, if you don't want to modify the Airstream itself, and also don't want to spend a lot of money.

Our goal is to have no sound from the exterior enter, but to have the interior be live enough for the room to have light to medium reverberation.

Those are two very different things, and there is no relationship between the. First is "isolation", which is stopping sound from getting in or out, and second is "treatment", which is what you do inside the room to make it sound good. They are two entirely different aspects of acoustics, and are not related to each other. You can have a well isolated room that is not treated at all, and you can have a well treated room that is not isolated at all, and you can also have a room that is both well isolated and well treated.

The things that you do to treat the room don't have much effect at all on what you do to isolate it.

Definitely not looking for a dead vocal booth. There will be Indian Meditation bowls and people singing so some reverberation will be necessary for the proper experience.

That's all about treatment inside the room, not about isolation. But it's a very long narrow room, with a very low ceiling, so the acoustics can never be great in there. If the entire inner wall is completely reflective (acoustically), then the best your RT60 time would be about 1.6 seconds. That's the very best you can hope for with natural reverberation inside. But just by having a few people in there, that automatically reduces the RT60 time, since people absorb sound, and clothing absorbs sound. Put 5 people in there, and your RT time is down to about 1 second. If you have carpet on the floor too, then you are down to about 0.6 seconds. But your walls are not 100% reflective, they are curved, they will be slightly diffusive, and you will have furniture and fittings in there, so I doubt that your reverberation time will be more than about 300 milliseconds, best case.

finding a proper solution for sound reinforcement is our first step

What will you be using for the sound reinforcement system? From what you said, it seems likely that your sound reinforcement system that will consist of a series of microphones, equalizers, amplifiers, and speakers, as well as some playback devices that are triggered by specific tones. Is that correct? All of that equipment will also affect the room acoustics, so it is important to take that into account. Even though sound reinforcement has nothing at all to do with acoustics technically, the simple fact of having speakers and equipment in a small room will affect the acoustics in the same way as all the other furniture: it will reflect to a certain extent, and diffuse to a certain extent. As long as you don't put a lot of large speakers, amps and other equipment in there, it should be OK, but it is important to consider those.

The area of the manifold that needs to be sound proofed is: 617 SF (this is the inner shell that will need to be covered with sound proofing.

It seems as though you are a bit unclear about how acoustic isolation works: you cannot just add something to the wall surface to create "soundproofing"! There is no such material. Acoustic isolation is not something you buy in a store and glue on the walls. Acoustic isolation is a system that is designed to produce the desired goal. It is a system that is made up of many parts, and is tuned specifically to do the job. Part of that system is the existing metal shell of the trailer, which might already include some for of sound isolation. If so, then that will need to be removed first, leaving just the outer metal shell and structure of the trailer, then the new high-performance isolation system can be built inside. It will require mass, damping, resilience, and sealing. And since this entire room moves (it is an Airstream trailer, so presumably it will be towed from place to place), that needs to be taken into account in the design of the isolation system: it must be able to handle the shaking, bending, twisting, bumping, acceleration and deceleration that a trailer typically is subject to, and it must be able to do that without losing any of its acoustic isolation capacity.

So, to summarize: what you want to do is basically not possible, in the way you defined it originally. It is possible to get a reasonable level of isolation in such a trailer (perhaps 40 dB or so), but it is not possible to "have no sound from the exterior enter". Getting that reasonable level would require careful design of the isolation system, and careful construction of the walls, floor and ceiling that you would need in order to do that. Weak spots will be (as always) your doors, windows, electrical and ventilation systems.

And every time you open the door, you have no isolation at all, of course, no matter what you do to the rest of the trailer.


- Stuart -

[Soundman2020]

I have done some research and it seems like a solution like this may be my best option. Since the treatment will most likely have to cover the doubly curved interior of the airstream shell.

That won't do much. It is basically MLV. According to the technical specifications on that website: "addition of Peacemaker (3.2 mm) will increase the STC rating by estimated 4-5 points". That implies a barely perceptible change.

Here's a table of what STC ratings really mean, in relation to speech:

STC What can be heard
25 Normal speech can be understood quite easily and distinctly through wall
30 Loud speech can be understood fairly well, normal speech heard but not understood
35 Loud speech audible but not intelligible
40 Onset of "privacy"
42 Loud speech audible as a murmur
45 Loud speech not audible; 90% of statistical population not annoyed
50 Very loud sounds such as musical instruments or a stereo can be faintly heard; 99% of population not annoyed.
60+ Superior soundproofing; most sounds inaudible

The website for that product that you are interested in, says that you can expect the total isolation to be about STC-19. So you would not even be on the above chart at all with that. However, that estimate seems a little too low.

In real world terms (not STC ratings): The aluminium shell on an Airstream trailer is about 0.02" thick, (0.5mm). The density of alumium is about 2800 kg/m3, so the surface density of that shell is about 14 kg/m2. According to the empirical Mass Law equation, you should get roughly 29 dB of isolation from that. Adding a layer of MLV would increase that to about 31 dB of isolation. That's about the same as a typical house wall (a layer of 1/2" drywall on each side of a stud frame). That's not much at all. You would need far more than that to make it useful. Plus, don't forget that the walls are only a small part of the total isolation. As I mentioned before, windows, doors, ventilation and electrical system are usually sources of big problems in isolation.


- Stuart -