DIY Sound Absorption

K

kjones9999

Member
Hi.

Typical story-- a school system-- poorest in the state- great arts programs but terrible acoustics in the auditoriums.

Cant afford to hire, so thats a no go. I plan to do something to counter the massive flutter echo that we have in our 700 seat theatres. My first instinct is to build sound panels out of roxul and built frames.

I would really appreciate any advice however, hiring a consultant is not a possibility. Oh and also there is no way it could make it worse.

Any suggested - hopefully cheap approaches? we have bare 25 foot walls that run the entire length and width of the house.
 
Since advising against this is likely futile, search CB for acoustic panels, acoustic treatment, or similar. I've never heard of roxul; it looks like a typical sound-deadening insulation. On here somewhere is a recommended product that comes in 4x8 sheets and doesn't need to be framed or covered; just screw it/glue it to the wall and it's not unattractive. Might have been this thread: https://www.controlbooth.com/threads/sources-for-acoustic-panels.32831/ .
 
As far as a consultant, my experience has been an overall savings or null increase a by hiring one.

Anyway, if I had to do what you suggest, I would be measuring room responses for dB, frequency, etc. in different areas, and then research products that meet the needs of your space. Sometimes areas of rebound can be determined visually, sometimes they can't. Knowing the original design intentions can help also, as a lot of older rooms were not designed with modern sound reinforcement in mind.

I'm assuming your issues have to do with the sound system? Sometimes it doesn't take a lot to reposition or replace speakers to better match existing acoustics. This is what we ended up doing, and I'm glad, because the system works great, and we still have good acoustics for live, unreinforced performances. We did hire a consultant however.

Can you add pictures?
 
@derekleffew , Roxul is an Canadian brand of stone wool insulation. I expect some time around 2050 scientist will tell us it's worse than asbestos, but in the meantime it's a pretty nifty product.

Getting back to the poor persons' guide to acoustics...

Terrible acoustics is not very descriptive. Does it have a reverberation problem, lack of any reverberation, uneven amplitude across the seating area, or something other? I'm going to assume it's got a reverberation problem based on your proposed remedy.


Regarding sound absorbing (or reflecting) materials:
  • Different materials have different absorptive properties and most manufacturers can provide them. Google can help with common materials (wood, curtains, etc.)
  • Fire rating matters, especially in a theatre.
  • Baffles typically absorb twice as much sound as wall mounted panels because they have twice the surface area.
  • The orientation of baffles matters, especially for voice and acoustic performances so you typically want to orient them to allow free passage of the primary sound but soak up reflections, so thin edge pointed at the stage.
Installation
  • Nobody wants people injured by falling materials, whether due to fire, earthquake, or rogue basketball.
  • Don't rely on adhesives. Some mechanical fasteners are required.
  • Don't block, shadow, or obstruct in any way sprinkler systems, fire exits, smoke vents, or the like.
  • Think about what happens to the material when it gets wet. Your building inspector and/or fire marshal will.
  • To be effective, about 1/4 of the surfaces need to be covered. That's why you find many gymnasiums have a band of acoustic panels near the ceiling.
  • Remember the ceiling is a surface too. A lot of reflections can be bouncing between the floor and the ceiling.

Attached are some photos of the amendments I did for our rehearsal room. The wall panels are cloth covered fiberglass batts 2' x 4' x 2" thick. The ceiling batts are plastic covered (think cheap picnic table plastic) 4' x 8' x 1" thick fiberglass batts grommeted. Note the wall panels are only on 2 of the 4 walls. About 60% of the ceiling is covered.

Prior to the amendment the primary reflection mode was floor-to-ceiling and decay time was about 4 seconds. Now it's slightly over 1 second. Cost was about $2000 back when the exchange rate was more favourable.

IMG_0912.JPG IMG_0914.JPG IMG_0911.JPG
 
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One consideration for the DIY approach is that you need to ensure all the materials and glues used are fire rated. Not only the rigid fiberglass material, but also the fabric. That means using upholstery fabric. You need the spec sheets for the fire marshal when they inspect, otherwise all of the work might wind up in the dumpster.

I have used a Acoustic Sciences Corp. to evaluate a space and provide custom treatment materials. Not the cheapest, but excellent results. In my case, it was a recording studio.
http://www.acousticsciences.com/
 
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Fire ratings are very important here, because if you do this without proper documentation, you're probably going to have to take it all down. And it'd be unsafe, and what's the point of that? Another thing mentioned above are sprinklers: if you install on or suspend from the ceiling, do not obstruct the flow path or spray pattern of the sprinklers.

For walls and ceilings, the materials you install will typically need to meet ASTM E-84 Class A. This is the same as UL 723 Class 1, and NFPA 255 Class A. Some organizations use 1,2,3, some use A,B,C. This is a test for surface burning characteristics and measures flame spread (how fast will the fire move across this material). Note that there are several testing methods that use a Class 1, Class 2, Class 3 system. You want ASTM E-84.

Depending on how they are installed - for example, if you install rigid panels that are a few inches away from the wall - the fire inspector may ask for testing reports for NFPA 701, which is the test for drapes, curtains, decorations, etc. The key is there is airflow on both sides of the item that could burn. If they're not mounted directly on the wall, this would be appropriate test in my opinion, but they might still ask for ASTM E-84, which is more common.

In either case, professional suppliers of acoustic dampening panels or fabrics have usually already done this testing and can provide the lab test reports. It might be on their websites, or you might have to ask for it. If you follow the DIY approach with something like 1" fiberglass or other types of panels, with say, burlap that you purchase and cover yourself, be sure to ask for the certification from the fabric supplier before you purchase the burlap. Note that I'm not suggesting fiberglass and burlap. I'm not a sound person or acoustician.

Work out a design of what you want. Take a trip to your fire department or building department to get input and approval before you go ahead. They can make recommendations about the safety side of this.
 
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This is a low resolution pic -- best I have now- showing about half of the house-- but the other half is like the visible part of the pic. This is all concrete block construction. There is no sound treatment anywhere except on the very back wall. 1 in sound panels which do very little. I did have a consultant in to take a quick look and he said what he would do is break up the wall space with absorption panels, so I am not flying completely blind here.... I have designed around 100 high school productions in many different spaces so I have some knowledge especially with regard to codes. I do appreciate the words of caution. But it cant be made worse. There's no way. And I am trying to find a way to make it a little better because I strongly believe that our students shouldnt have to suffer for 30 years because of an architect who had major deficiencies in knowledge. Oh and we have TWO of these theatres identical at two concurrently built high schools.

As far as code goes would a wood frame around roxul covered with IFR cloth be acceptable? http://www.rosebrand.com/product3572/7-oz-Polyvel-IFR.aspx?cid=555&idx=1&tid=1&info=IFR+Fabrics

proxy.php
 
sk8rsdad said:
@derekleffew , Roxul is an Canadian brand of stone wool insulation. I expect some time around 2050 scientist will tell us it's worse than asbestos, but in the meantime it's a pretty nifty product.

Getting back to the poor persons' guide to acoustics...

Terrible acoustics is not very descriptive. Does it have a reverberation problem, lack of any reverberation, uneven amplitude across the seating area, or something other? I'm going to assume it's got a reverberation problem based on your proposed remedy.


Regarding sound absorbing (or reflecting) materials:
  • Different materials have different absorptive properties and most manufacturers can provide them. Google can help with common materials (wood, curtains, etc.)
  • Fire rating matters, especially in a theatre.
  • Baffles typically absorb twice as much sound as wall mounted panels because they have twice the surface area.
  • The orientation of baffles matters, especially for voice and acousitc performances so you typically want to orient them to allow free passage of the primary sound but soak up reflections, so thin edge pointed at the stage.
Installation
  • Nobody wants people injured by falling materials, whether due to fire, earthquake, or rogue basketball.
  • Don't rely on adhesives. Some mechanical fasteners are required.
  • Don't block, shadow, or obstruct in any way sprinkler systems, fire exits, smoke vents, or the like.
  • Think about what happens to the material when it gets wet. Your building inspector and/or fire marshal will.
  • To be effective, about 1/4 of the surfaces need to be covered. That's why you find many gymnasiums have a band of acoustic panels near the ceiling.
  • Remember the ceiling is a surface too. A lot of reflections can be bouncing between the floor and the ceiling.

Attached are some photos of the amendments I did for our rehearsal room. The wall panels are cloth covered fiberglass batts 2' x 4' x 2" thick. The ceiling batts are plastic covered (think cheap picnic table plastic) 4' x 8' x 1" thick fiberglass batts grommeted. Note the wall panels are only on 2 of the 4 walls. About 60% of the ceiling is covered.

Prior to the amendment the primary reflection mode was floor-to-ceiling and decay time was about 4 seconds. Now it's slightly over 1 second. Cost was about $2000 back when the exchange rate was more favourable.

View attachment 13379 View attachment 13380 View attachment 13381
Click to expand...

Hi... thanks for the response. We have what I have been is told is flutter echo. Decay is around 2.5 sec on average.
 
AudJ said:
As far as a consultant, my experience has been an overall savings or null increase a by hiring one.

Anyway, if I had to do what you suggest, I would be measuring room responses for dB, frequency, etc. in different areas, and then research products that meet the needs of your space. Sometimes areas of rebound can be determined visually, sometimes they can't. Knowing the original design intentions can help also, as a lot of older rooms were not designed with modern sound reinforcement in mind.

I'm assuming your issues have to do with the sound system? Sometimes it doesn't take a lot to reposition or replace speakers to better match existing acoustics. This is what we ended up doing, and I'm glad, because the system works great, and we still have good acoustics for live, unreinforced performances. We did hire a consultant however.

Can you add pictures?
Click to expand...


Thanks Jeff. The main issue is ANY amplification -- and I have tried every position of both speakers and 31 bands of parametric EQ- results in a very difficult listen. So we amplify on the high side to try to overcome the echo. We really dont have a choice. Low amplification results in muddy sound. So as you can imagine there is a lot of ear exhaustion taking place. We have been in these spaces 20 years. I did receive a quote of 20,000 to correct. Being the poorest county in the state, its just not a possibility for us....
 
I should also mention that I work at a third theatre that is a similar size and shape. The main difference is the clouds are made in different shapes depending on their focal point, and every parallel wall surface is covered from ceiling to floor with 2" sound absorbing material. Its a dream to work in- especially compared to the other two. Because of that experience, I am convinced that repeating the absorbing material approach will make things better.
 
I looked up Roxul 'Safe-n-Sound' and it has the testing you need. You'll need to check with Rose Brand if they have testing for ASTM E-84. I'd guess it has NFPA 701 testing. The wood frame might be an issue, but it's a small percentage of your installation, this would be a question for the fire dept.
 
Carpet glued to the walls will reduce bounce and is fairly cheap. Be sure to check the flame rating for the particular purpose. I used some in a Gym-auditorium about 25 yrs ago, a 12 ft tall strip all the way around the upper 1/2 of the walls. Sprayed adhesive on the wall (blocks) and the back of the carpet, bonds quickly. Reduced the slap from 9 sec down to 3 sec. I have seen auditoriums do carpet with stained wood 1x2 vertical every 2 ft around the back wall and 1/2 of the side walls.

I have built sound panels 2x2 frames with luan and wrapped around with carpet the edge. 2 ft wide and 8 ft tall. Use Roscoe flame treatment on the wood.
 
Tom Andrews said:
I looked up Roxul 'Safe-n-Sound' and it has the testing you need.
Click to expand...

We used Roxul Safe'N'Sound extensively in our theatre in Ontario. Excellent sound reduction and thermal insulation. But, it did need to be covered by a non-flammable material. Not sure why, as it has a flame-spread rating of zero. Hey, it's rock.

We used double Roxul-filled walls with 3/4" Type-X drywall over it for a nominal 3/4 hour (actually closer to 1.5 hour) fire sep and acoustic reduction (with the building's concrete block walls) in excess of 90 dB. Rock band inside; faint thumping outside.
 
Curtains are often least expensive. Look at multi-cinemas - curtain fabric tacked to nailers top and bottom with fulness. If there was cheaper, they'd use it.
 
You have to be very careful about curtains. Depending upon the fabric, they may or may not produce the desired effect. Typically, they have little to no effect upon lower frequencies. I don't know for sure, but my ears tell me that the fabric on cinema walls covers additional batting of other materials.
 
The distance between curtain and wall is important too. The further from the wall the lower the frequency as I recall. 100% fullness is good also. This is most common absorption used in major venues as well - though it is usually storable in those instances. Look at accustac for test data. Wenger/Clancy may have data also for their banner.
 
Regarding fabric on movie theater walls: Most have fabric that is installed with knife pleats over sheet rock. There's no batting underneath. Though I've seen a tendency, at least in the Regal Cinemas and some AMC's in the northeast, to have premanufactured panels of sound absorption wrapped in fabric that matches the rest of the theater placed in the back of the auditorium.

Other theaters may have sheetrock walls with fabric applied flat, plus have a series of large square or rectangular panels set out from the wall a couple inches. These panels would be fabric over sound absorption material.

In a multiplex, you might see a difference between the theaters with large screens and/or 3D compared (greater investment overall) to the other 'regular' theaters.

Next time you're in a movie theater, check out the wall behind the fabric.
 
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