Will painting an acoustic panel lower absorbency?

Hi J:
Hi Sean
Nice question.

Have to agree with Sean on this one. Make no claims of any expertise but will pass on a couple things. F. Alton Everest makes the following statement about mid/high frequency absorption.

“ The absorption efficiency of materials depending on the trapping and dissipating of sound energy in tiny pores can be seriously impaired if the surface pores are filled so that penetration is limited. Course concrete block, for example, has many such pores and is a fair absorber of sound. Painting that block fills the surface pores and greatly reduces sound penetration, and thus absorption. However, if spray painted with a non-bridging paint, the absorption may be reduced very modestly. Acoustical tile painted at the factory minimizes the problem of reduced absorption. Under certain conditions a painted surface can reduce porosity but act as a diaphragm that might actually become a fair absorber on a different principle, that of a damped vibrating diaphragm.” (Everest, Master Handbook of Acoustics, p 189)

He makes no mention of paint in his short section on fiberglass panels. He does however say, “semi rigid boards of glass fiber do not excel cosmetically, hence they are usually covered with fabric.” (p190) He does not say to paint them.

From this I conclude that he has no problem with paint as such. If he did wouldn’t we all be in trouble. However, it becomes a problem if your rigid panels depend “on the trapping and dissipating of sound energy in tiny pores.”

A short piece from an acoustics supply site I used before reads:

“7: Painting over sound absorbers. Many high-frequency sound absorbers work as a function of their surface porosity. It is intuitively apparent that compressed fiberglass panels and cellulose fiber coatings work in this way and few people would question the fact that these surfaces should not be painted. Not so obvious (judging by the number of people that paint them) is the fact that suspended lay-in acoustic tile, acoustic plaster and concrete block should likewise not be painted. The miniscule pores in each of these materials allows sound to be absorbed into the material, changing sound energy to heat. Painting tends to close up these pores so that absorption cannot proceed. Actually, these surfaces can be painted but only with thinned, water-based paint and by those with enough experience to predict the result.”
http://www.wconline.com/CDA/ArticleInformation/features/BNP__Features__Item/0,3299,16422,00.html

Both tend to say the same thing. Possible, but a touchy proposition. If you do it thin the paint and use a sprayer.

Rives posts here lately. He is an acoustics professional and maybe he can add something.

I remain,

HI J:

Well, from a simple question about paint to general room acoustics.

Rives is a pro at this and has lots of hands on experience so my $.02 isn't much. But I think he is right on re asking about how much fiber board you want to put in the room and pointing out that it might be best to paint some of it. I would like to hear more from him on practical points of "how to." I'll throw a couple things out if nothing more than to get a response.

I agree with Rives re the over damping. I know some folks actually like the dead anechoic chamber sound but not me. Usually the rationale is that you are listening to direct sound but I just do not care for it.

A few point to consider.

-You will need additional devices to deal with the major low frequency resonances and even the mid frequencies. You can break down how sound acts in a room by frequency. Lower frequencies act like waves, higher like rays, and there is a middle section that is really neither and both. The size of your room determines the frequencies where these regions are. Your fiber panels will not do it all. The bottom line is that, like Rives said, you will want some additional devices in the room to control the frequencies that your fiber panels do not absorb. Stuff like the devices talked about in the Risch site Sean mentioned.

-There are ways to calculate all this (what your room reverberation time is, total absorption at different frequencies, ect.....) but I have not been that lucky with that approach. I use the calculations to get “ball park” figures and then experiment. I really recommend the book I quoted above (F Alton Everest “Master Handbook of Acoustics). It is really well written and a good $35 spent. It is a little work reading and has a lot of good info. On the other hand it is really clearly written and you do not have to be a physicist to take a lot away from it. It’s easier to read than it is to tear down fiber board!

Room calculations seem to have limitations because all speakers have different off-axis response and some “cohere” 6 feet from the speaker and some you need to sit 15 feet away, ect. Read the articles by Toole below and what Linkwitz has to say about a speaker's radiation pattern and room interaction. I tend to think of it as room/speaker interaction and every speaker (or at least each type of speaker)has it's own particular characteristics.

Also the furniture plays a big part as you can see from Rives excellent site. You can, however, fairly easily calculate the resonant frequencies that will dominate your room and tune traps for those as Rives pointed out.

My approach would be to get an idea about the additional traps / diffusers you might want and actually place some of those in the room before you line the whole thing with fiber board. This way you aren’t going to extremes in one frequency area before you even start in another. Just seems more balanced. Then start placing the fiber board. You can get a good idea about the traps needed and where the crucial areas typically are for placing the board from the Everest book or the Risch site.

Lots of work but the room is 50% of the sound. I hope you can take your time on the project because it’s worth it. You know how it takes some people many years to tweak a system to taste? I think it can take a lot of time to tweak a room too. Don't be in too much of a hurry and don't do things you can't undo w/o lots of work.

Anyway...that is all too much advise for not even knowing the dimensions of your room! Take it for what it's worth. Let us know how you do. If not in a post drop an e-mail. I like to learn form other's in this area.

If you are going through all the trouble of a dedicated room you might find these sources interesting.

Two of the best web sites on audio acoustics that I really found helpful.
Art Ludwig’s site. http://www.silcom.com/~aludwig/
Linkwitz Lab. http://www.linkwitzlab.com/rooms.htm

A few articles on line.

Two articles by William Dudleston:

http://groups.yahoo.com/group/Thunderstone_technical/files/Room%20Acoustics/Dudlestone%20-%20Acoustical%20Sense.pdf

http://groups.yahoo.com/group/Thunderstone_technical/files/Room%20Acoustics/Dudlestone%20-%20Stereo%20Effect.pdf

Two articles by Floyd Toole:
http://groups.yahoo.com/group/Thunderstone_technical/files/Room%20Acoustics/F%20Toole%20Room%20and%20Speaker%20Working%20Together.pdf

http://groups.yahoo.com/group/Thunderstone_technical/files/Room%20Acoustics/F%20Toole%20Maximising%20Loudspeaker%20Performance.pdf

Let us know how you come out!

Cheers,