Acoustics, Absorption Coefficients, & Shag

[Harris Ueng]

There was a bit of discussion on carpeting as acoustic treatment recently. So I thought I'd share some absorption coefficient data just so people can get a better appreciation of acoustic treatments and various capabilities. The following only deals with absoption, not diffusion.

MBI Products has a great acoustics cheat-sheet in the form of an article article on "Room Acoustics" found at http://www.mbiproducts.com/room/ ]. The following data is quoted from their coefficient chart (p. 2 of the article).


Coefficients measured at fixed frequencies (Hz) as follows:
-----------------[125--250--500--1000--2000--4000]

Bricks-----------.03---.03---.03---.04---.05---.07

Carpet----------.02---.06---.14---.37---.60---.65
on concrete

Wood-----------.15---.11---.10---.07---.06---.07

Glass------------.18---.06---.04---.03---.02---.02


Compare these numbers to a purpose-built absorber that's designed to abate low-mid frequency in-room build-up, such as Ethan Winer's Real Traps (from the Real Traps site: http://www.realtraps.com/data.htm)

Mini Trap-------1.66---1.10---1.19---1.10---0.98---0.86
in corner

Other mfg's of broadband absorption products like RPG, Sonex, and others have products that perform well also.

The point is well-designed absorption/diffusion treatments isn't just a matter of putting up carpet or egg crate. Especially, since carpet does most of its absorption at the frequencies of speech, while small-room studios most often need acoustic control in their bass to low-mids. (ever come home with great sounding speakers from your local superstore like Circuit City to find that it doesn't sound the same at home?)

Also, diffusion is best handled by purpose-built products rather than relying on carpet to create diffusion since any useful diffusion they may perform is far above the critical range for studio acoustics and in a narrow freq. range due to the regularity of the pattern. A well-placed bookshelf filled with randomly-placed odd-sized books would perform considerably better than carpet. And, if you're an avid book reader, it's free.

While a small-room studio can benefit GREATLY by having even a few hundred dollars worth of acoustic treatment installed. It's even more important that the treatment selected solves the problem that the room presents. A person about to embark on treating room acoustic problems would benefit greatly from hiring a consultant for even a couple of hours give guidance, or if you can't afford it, pick up a book on acoustics. F. Alton Everest's "Master Handbook of Acoustics" may seem like a large daunting book, but it's accessible, even to non-engineering types.

If you don't want to think that hard, or spend money on a consultant, you could opt for a "Studio In A Box" acoustic treatment, like what RPG produces (http://www.rpginc.com/products/studioinabox/index.htm). It comes with directions on how to install it. I don't have much experience with the "in a box" products that have sprung into the market in the last few years, but they've garnered some decent reviews in popular media.

If you're into DIY, Ethan Winer has a great article on acoustic treatment from his site: http://www.ethanwiner.com/acoustics.html. If you read through that, you could start building your own treatments and just experiment. Most often this is better than nothing at all, as long as you don't go overboard and plaster every surface with absorption material. ;-)