Will painting an acoustic panel lower absorbency?

I would venture to say that painting the panels would lower and alter their absorbency.

I would HIGHLY suggest reading Jon Risch's website regarding the sound dampening panels that he designed. To briefly summarize his suggestions, you should take and cover those panels in Dacron i.e. polyester batting used for pillows and comforters and then cover that in Burlap. Burlap can be dyed to any color that you like and will not lower the efficiency. One could even weave, sew or dye patterns into the burlap so long as only cloth or stain was used ( no paint ). Sean
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http://www.geocities.com/jonrisch/index.htm

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,

Thanks Sean and Clueless for your responses. I will read with great interest the links you provided, but the information you have already provided in your answers has convinced me to go in the direction I originally planned, which is to remove each panel, cover it with fabric, and reinstall.

I really appreciate your help.

The fabric is the right way to go. It does depend on what frequencies you are trying to absorb. Quite honestly if you are planning to cover the entire room in fiber board you are going to have an aweful lot of high frequency absorption, and while painting will reduce this, in your case it might be a benefit. Fiber board is a great acoustical tool, but needs to be used appropriately. It's frequency absorption typically starts around 640 Hz (depending on the fiber density) and is not doing very much at this frequency. Without knowing much else about your room I really can't add very much, except to be cautious with over damping even a challenging room. It will make it sound lifeless and dull. I'm actually re-engineering a dealer showroom that was 2/3 covered in fiberboard. It was a dreadful room to begin with, but the overdamping made it worse. We're going to fix it.

Rives...thanks for the comment.

The results from adding the panels have been very good. I started by completely covering the front and back walls with the 2" thick panels. I then installed panels 2" thick on the 4' tall knee walls along the left and right side of the room. Finally, I added 1" thick panels on the sloping ceilings along the left and right sides of the room. At that point is where I may begun hearing some of the effects of over damping, but the sound is much better now than ever before.

As I remove and cover the panels, I will be doing so in such a way that I can remove some panels, listen to the results, and see if the removal of a few of these panels will "brighten" the sound somewhat. Although my stereo sounds much better than ever, a little "less dead" would be better.