Bookshelves

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eraos

eraos

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Last night, I was reading the Crutchfield.com guide to acoustics .. and they mentioned that a "bookcase filled with odd-sized books makes an effective diffusor."
Is this true?

I have my drumset set up against a wall which has a couple shelves for books. I was about to take them down and put up some mineral wool... but then I read that article. So would it be better to take the bookshelves down or leave them where they are?
 
> the Crutchfield.com guide to acoustics ... mentioned that a "bookcase filled with odd-sized books makes an effective diffusor." Is this true? <

No. This is an effective diffusor:

diffusor1.jpg


> I was about to take them down and put up some mineral wool <

Yes, you should do that rather than use a make-believe diffusor.

--Ethan
 
For what it's worth

I'm no expert like Ethan, but...

My former housemate (a trained sound engineer) took this old bookshelf (6x6.5) and filled it with the pink stuff, then covered the face of it with fabric. It may not be a perfect absorber, but I think it would be better than books.

Recently another engineer looked at it and said it would be better to repack it with 709 (the compressed pink stuff).

Just a suggestion worth looking into.
 
A mass of books can make pretty good bass traps and add a fair amount of absorbtion- but diffusion? I don't think so...

I've been in a study with all walls covered with books, a metal ceiling and carpet. It was, as Dickens would say, as dead as a doornail.
 
hm... so I was hoping to avoid having to remove all the books because I'll be forced to find a place to put them.
It's not a bookcase: the shelves are just boards and some shelving-hanger-things, so it's not a simple matter of relocation. It's a matter of where to put all the books and objects that are on the shelves.

With that in mind, is it worth it to remove the shelves or would the books fair fine with the addition of mineral wool on other spots around the kit?

It seems that everyone is at least suggesting that the books are better than bare wall, though obviously not as good as mineral wool.

So the question is whether or not it's worth the effort of removing the shelves and books when they currently have a positive effect, though it may be minimal.
 
Well...it might not be as effective as the diffusor you are showing Ethan but im sure it would have some diffusion properties. Just not very scientific. I do not however see how it could be used as a bass trap or an absorber. Im sure books reflect frequencies quite well and the different sizes would break up the frequencies. Just isnt going to do quite as much as what Ethan showed.

I doubt the bookshelf would give a real diffusion sound though.

Danny
 
eraos said:
hm... so I was hoping to avoid having to remove all the books because I'll be forced to find a place to put them.
It's not a bookcase: the shelves are just boards and some shelving-hanger-things, so it's not a simple matter of relocation. It's a matter of where to put all the books and objects that are on the shelves.

With that in mind, is it worth it to remove the shelves or would the books fair fine with the addition of mineral wool on other spots around the kit?

It seems that everyone is at least suggesting that the books are better than bare wall, though obviously not as good as mineral wool.

So the question is whether or not it's worth the effort of removing the shelves and books when they currently have a positive effect, though it may be minimal.
Click to expand...
I don't think there is anything wrong with a bookcase in your studio as long as you have other means of diffusion and absorption.
 
A mass of books can make pretty good bass traps and add a fair amount of absorbtion
Click to expand...
.I don't believe it. This is why. You can't prove it unless you test it in a lab. When you have absorption coefficients from the tests, THEN I'll believe it. Untill then, it is pure conjecture. By what principle would books absorb, especially when the cover faces outward? To be an effective RESISTANCE absorber, the air molecules must be able to enter the surface WITHOUT being reflected, and THEN friction within the fiber INTERSTICES resist molecular movement, transforming energy into heat. Personally, I doubt it books can be an effective BROADBAND absorber, let alone a bass trap. They may allow small pockets of resistance BETWEEN the books but that is highly subjective, especially considering the typical surface binding material of books. Of course, this is only my opinion. But its as good as yours, as NIETHER of us has test lab results to back it up. Thats why I usually suggest to people to use materials that have test lab results published.
Not only that, to absorb low frequencies, resistance absorbers must have a depth of at least a quarter wavelength to absorb effectively. A soundwave with a frequency 100 hz has a wavelength of 11.3'(approx). A quarter wavelenth would be approx 34". Theoretically, for a 1 square foot resistance absorber to absorb 100 persent of a 100hz wave over the whole face, it would have to be 34" DEEP. :eek: But there is much more to it. This is why absorption coefficents for a given thickness roll off at lower frequencies at a given thickness. However, there are some caveats here, but I won't go into it. Go to Ethans Real trap site for the whole story if you want.
Absorption coeffecients are the percentage of sound absorbed per square ft/frequency band. For instance,a 1' square open window will absorb 1 sabine of absorption at ALL frequencies, because a window is a perfect absorber.
An absorber material with an absorption coefficent of .55 at 125 hz, means it absorbs 55 percent of a 125hz incident sound striking the face of the absorber. This equals .55 sabines per SQUARE FT. at 125hz. I doubt if books can perform even this effectively. But like I said, no test, no proof. Just my opinion.
fitZ :)
 
Rick,
I can appreciate your doubts. I've personally been in a small mixing room that used this and all I can say is it worked to tame some low end problems. Is it the best method? No. Why does it work? I'm not even entirely sure I know. maybe it's the mass combined with the space behind the books- (if they are not pushed up tight against the wall). I can't really say other than it does get results. will 20 books on a shelve make a difference? No, but I'm talking a relatively full, floor to ceiling book rack. They also look cool and give clients something to do besides look over your shoulder.

One sure thing I've learned about acoustics is the more I think I know, the more I can be fooled into believing I know enough, and am consequently proven wrong. For instance: how the heck can a long rectangular, fully tiled men's room with only those little metal stall dividers have perfect decaying reverb with no flutter echo and no low end boom? You mean those those little dividers with no absorbtion somehow tame bass? I was also in a cement block storage room this past summer. it was maybe 15' x 40' x 9' tall and was nearly empty, yet it had no noticeable flutter echo. None. Everything in me said there should be some, but when I clapped my hands, it had a very nice natural reverb decay. Things that make you go hmm..

Trial and error- we DIY'ers without labs live by it...
 
Danny,

> im sure it would have some diffusion properties. Just not very scientific. <

It might reflect, but I don't think it would diffuse. And there's a difference. It's possible to arrange a bunch of books on a shelf in the same pattern as a real diffusor. But most books are more or less the same depth, and that prevents them from offering diffusion. Understand that a real diffusor sends different frequencies off in different directions. A normal reflector sends everything in the same direction. This is a very big distinction. But books might be a little better than a bare wall.

> I do not however see how it could be used as a bass trap or an absorber. <

Nor do I. To get a bass trap you need some sort of porous absorbing material like fiberglass, or a resonant panel or air cavity, etc.

--Ethan
 
Hey Ethan,

Yah thats kinda what i was getting at. Not a real diffusor but still breaking up the sound waves a bit so you dont get flutter echo. Nothing scientific. I think it would be about the same kind of diffusion as using a rounded surface on one wall like a bent piece of fiber board or something. Not real diffusion just spread out reflection.

Isnt that considered diffusion though. Just not the really desired skyline diffusor sound?

Danny
 
Isnt that considered diffusion though. Just not the really desired skyline diffusor sound?
Click to expand...
You need to define a DIFFUSED sound field.

According to Alton Everest in the Master Handbook of Acoustics, here it is.

The frequency and spatial irregularities obtained from steady state measurements must be negligible.

Beats in the decay characteristic must be neglible.

Decays must be perfectly exponential, i.e., they must be in a straight lines on a logarithmic scale.

Reverberation time(RT-60) will be the same at ALL positions in the room.
The character of the decay will be essentially the same for different frequencies.

The character of the decay will be independent of the directional characteristics of the measuring microphone.
Click to expand...
 
Danny,

> Not a real diffusor but still breaking up the sound waves a bit so you dont get flutter echo. <

Absorption can also avoid flutter echo. In my opinion (uh oh), small room ambience sounds pretty bad. When you see really cool looking diffusors in a pro studio, those rooms are a lot larger than what most folks have. In those large rooms diffusion can be a nice added touch. But most small rooms need absorption a lot more than they need diffusion.

--Ethan
 
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