Will painting an acoustic panel lower absorbency?

It has been a couple of weeks since I looked at my original question and the responses you guys have provided. I thought that there would not be anymore responses but I see three have been added since my last visit. Thanks for the continued help.

I don't believe my room is too dead, although I will be soon experimenting with removing some of the fiberglass to see if I like a more lively sound. I have been adjusting the speaker and listening chair positions to be sure I'm getting the best sound out of what I currently have. Once I'm satisfied with this, I'll experiment again with the fiberglass panels.

This week I replaced my preamp and cd player power cords with Top Guns. The jury is still out on this addition. I already was using a Top Gun power block on everything except the amp, which has a CPCC Model 11.

I am taking my time on this project and am in no particular hurry to get finished. My listening room is upstairs and away from the main level in the house, so the fact it looks like a room that is under construction doesn't matter. I guess when you get right down to it, this could be a project that may never completely end. I am looking forward to using the links that Clueless provided and appreciate your providing them to me.

Jim

WOW!!!!!!!!!!! Thanks so much Clueless. The links to the sites were fantastic. Educational and well written. I will have to print them so I can really go through them. Thanks again

Very good references Clueless. Particularly Everest's book. Although not in depth, it's a great reference book and has about 90% of the equations we use in solving problems. It's also easy to read for the non-technical person as you pointed out. I do think you need to change your moniker--it's not working with posts like this.

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,

Many recordings are very bright and harsh. Particularly CDs that were recorded in the first several years of CD. A dead room might actually help these, so if your collection has a lot of early recorded CDs, the deading effect might be welcomed. There are a number of threads here on A-gon that discuss reference listening material. You might want to take a look at some of those suggestions and buy some (or you may already own) that you would enjoy as well as being able to use as reference listening material. What I find is important is to find a good variety (unless you only listen to one type of music). I generally use a vocal, large symphony, jazz, acoustical guitar, and solo piano. I enjoy listening to all types of music, but these are my favorite types and they work well for me to evaluate a system and room performance.

Rives, thanks for your additional comments. I will take your advice and visit your listening room to see what else you have there that might be helpful. I've always had a decent quality stereo and/or home theatre in my home, but this is my first high end system with a dedicated listening room.

As I listen to various recordings, the "dead" effect we've discussed is much more pronounced on some discs or LP's than it is on others. On many recordings, I don't notice the dead effect at all. It may be that the problem is more directly related to the source material than I first thought. I am going to clock a number of hours listening to my current set up before I start making changes. I want to have a good reference point as I go forward.

Jcambron: You probably need a combination of other acoustical devices, such as diffractors. You want to get a balance of absorption, so over damping won't work terribly well, which it sounds like you've discovered. You can actually tune (build) diffractors to certain frequencies, which is what we typically do in order to get a good balance in the room. These are not trivial things to figure out. The difficulty is the whole room works together and although it might seem that eliminating reflections with fiber board would get rid of some of the nasty reflections (it will), but at the expense of over damping the room. Our website has more on the subject, but does not go into diffractors. If you go to www.rivesaudio.com and go to the listening room. You can get to the listening room either by clicking on the tag line on the 1st page (not the intro page) or by clicking on "Issues" on the first page. It's a pretty basic tutorial section, and you may already know the information there, but hopefully it's helpful to you.

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.

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.

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.

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,

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