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[g'bye, Dick Rees]

Thanks to all for your ideas.  I agree that an analysis would be the way to go, but that would require a clearly defined goal, which doesn't exist here!  My boss came to me and said "the foam is coming off and needs to be replaced".  I asked him what specific issues they were trying to solve and the best answer he could give me was simply reflectivity from the roof back to the stage.  I think the issue is mid-high mid freqs, but even that is vague.  I'll direct him to this thread and hopefully get some better answers.

Denny....

It has already been mentioned, but I think it's worth repeating:

Diffusion as opposed to (or in conjunction with) absorption.

This is something to check out as it may be a definite option in taming the on-stage sound without "deadening" it and seems to be a reasonable alternative allowing more robust materials which will yield a longer useful life.

DR 

[Andre Vare]

Fran's and Thomas' Acoustics In Motion is a more  appropriate forum for your query.

Not really as that is part of the R/E/P forums and seems intended specifically to address studio acoustics.  There is no dedicated PSW forum related to large room and live sound acoustics.

Agreed it is not the intent of AIM.  It is not the intent of this forum either.  There are several acousticians on AIM that do large space acousitcs who frequent AIM, including one of the moderators.

Andre

[Dan Mortensen]

We have an outdoor amphitheater here that has a metal roof over it.  It was previously treating with some sort of absorptive foam, but that's falling apart and coming off.  The facilities guy here has asked for some recommendations as to what to use to replace it, and I could use some suggestions.  Has anybody ever dealt with this, or have any ideas based on places you've worked?  Here's a pic of the venue...

http://www.thecuthbert.com/gallery.html

Hi Denny,

I was a consultant for an outdoor amphitheater 3 or so years ago, and the acousticians came up with a perforated metal-covered material that looks and sounds good. The ultimate client drastically scaled back their operations just after this thing was completed, so I haven't done any shows there, but I visited the completed product and clapped my hands onstage, and it was pretty dead. I don't know what it is, or where to get it, but perhaps you can show the pictures to somebody and they will recognize it.

Good luck,
Dan

Well, this is interesting; I see how to browse for attachments, but don't see a way to upload an attachment. There is a "Post" and a "Preview" button, but no "Upload". I have two pictures for you, but aren't able to post them. Help?

[Dan Mortensen]

Well, this is interesting; I see how to browse for attachments, but don't see a way to upload an attachment. There is a "Post" and a "Preview" button, but no "Upload". I have two pictures for you, but aren't able to post them. Help?

Gee, the picture appears in the post but not the preview. What do you know? Never mind about the help.

Here's another:

[Brad Weber]

Agreed it is not the intent of AIM.  It is not the intent of this forum either.  There are several acousticians on AIM that do large space acousitcs who frequent AIM, including one of the moderators.

Getting way off topic but it has been suggested in the past that maybe there should be a forum that relates more to acoustics typically associated with live sound, AV and installed sound/contracting applications.  Now that the R/E/P and Live Sound forums are apparently back to requiring separate registration that seems to both practically and functionally make AIM even more focused on studio applications and a less viable option for other purposes.  Maybe some of those same people involved in AIM would be interested in a forum for such a purpose.
 
Dan, there are many manufacturers of similar products but those panels looks similar to some of products Kinetics and IAC offer.  For exterior use they often use a fill material that is sealed in mylar or similar.  One of my more interesting experiences with exterior absorber/barrier panels was integrating them into a new outdoor habitat for gibbons.

[Gordon Brinton]

Most studio designers and acousticians laugh at the "all popular" acoustic foam because it has very limited uses in controlling reflections. That is, it does an ok job of absorbing upper-mid and high frequencies. It does little to nothing on lower-mid and lows, which are the most problematic frequencies when it comes to reflections and standing waves. The laws of physics dictate that in order to absorb energy from lower frequencies you need a thick dense material that is porous. (Foam is usually cut thin and has almost no density.)

Many pro studios are now using 4" to 6" thick rockwool panels for broadband absorption because it outperforms most other products and has a safer MSDS report than fiberglass. It is extremely fire resistant and will not support the growth of mold. It is only a fraction of the cost of acoustic foam to boot.

However, it would surely add a lot of weight to the structure.

Just saying.

[Charlie Zureki]

Most studio designers and acousticians laugh at the "all popular" acoustic foam because it has very limited uses in controlling reflections. That is, it does an ok job of absorbing upper-mid and high frequencies. It does little to nothing on lower-mid and lows, which are the most problematic frequencies when it comes to reflections and standing waves. The laws of physics dictate that in order to absorb energy from lower frequencies you need a thick dense material that is porous.

Just saying.

   Hello,

   Some Foam Diffusive products do very well at attenuating reflections and standing waves for mid and high frequencies. 

   For low frequency energy, one would need some material that has Mass (which is probably the least of their concerns at this Outdoor Shell)

   Hammer

[Brad Weber]

It seems like there is some vagary regarding whether the goal is absorption versus transmission loss andwhat "foam" products are being referenced (closed cell, open cell, spray-on fibers, expanding foam, etc.), both of which appear to relate directly back to the early posts regarding defining the general goals and intent along with knowing more about the existing material.
 
Keep in mind that this type of venue is likely considered a public assembly space and some approaches and products that may be perfectly acceptable in home or even commercial studios may not be acceptable in public spaces.  Of course being a permanent outdoor application adds other factors specific to that application.
 
Also consider that the size and shape of the space as well as the application are significant factors and in this case differ greatly from a typical studio.  This is purely guessing based on just the photos on the site linked, but given the open back, open sides at stage level and shape of the roof there appears to have been little, if any, intent to reinforce the natural stage sound to the audience or to limit sound transmission from the stage and that much of the intent of the acoustical treatment of the roof was probably primarily to reduce the impact of the concave roof for those on stage.  However, this is eactly the type of assumption that probably should be confirmed as it could be erroneous for one or more aspects.

[Denny Conn]

Once again, thanks to all for your ideas.  It's great to have a forum like this to share experiences!  Dan, what/where is that structure in your pictures?

[Dan Mortensen]

Once again, thanks to all for your ideas.  It's great to have a forum like this to share experiences!  Dan, what/where is that structure in your pictures?

Hi Denny,

It's called the Willis Tucker Amphitheater in Marysville, WA.

http://www1.co.snohomish.wa.us/Departments/Parks/Park_Information/Amphitheater.htm

Regarding the various materials used for sound absorption and what parts of the frequency spectrum they do or don't affect, my big summer concert series went to a new stage last year that had a 3/4" plywood flat back wall ~28' x ~12'. There was a nasty slap coming off of it from monitors/band gear/backspill from line array, and simply putting a flat theater curtain (no pleats) across it made enough of a difference that the reflections were down in the murk rather than being front and center like they were without the curtain. Sometimes a little is enough, IMO.

Good luck,
Dan

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