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Re: why is burlap so recommended?

Posted by Jon Risch (B) on November 20, 2005 at 21:58:16

In Reply to: Ethan...why is burlap so recommended? posted by gonefishin on November 15, 2005 at 21:15:19:

The orignal jute based burlap, which is still readily available from most of the larger Wal-Marts with a fabric department, and most decent sized fabric stores, is NOT acoustically transparent, it would NOT make a good speaker grille.

What it does do, is to avoid reflecting mid and high frequencies, due to the fact that the large yet loosely yarned fibers actually seem to absorb sound compared to most other materials used to make cloth, such as polyester. This may have something to do with the natural fiber being so convoluted on it's surface,(similar to real long hair wool for T-lines, it has a highly convoluted surface compared to synthetic fibers).
The burlap is still a fairly inexpensive material compared to most other fabrics.

Many polyester and poly-cotton fabrics tend to reflect HF's, and some of the thicker ones can even reflect some of the upper midrange too. Thick and heavy velours, which seem like the fuzziness would be a good absorbing surface, tend to reflect a significant amount of upper midrange and lower HF's.
Straight cotton is usualy too densely woven to avoid reflecting some of the mids, upper mids and lower HF's.

The only reason any of this matters, is that for sound absorbing wall panels, (or for bass traps that you want to be tunable by placing a reflecting strip down one side, the other side has to be fully absorbing), you want maximum sound absorption across the whole frequency range.

For a fabric on a bass trap to be used ONLY for bass absorption, most any cloth can be used, but I have found that many folks are surprised at how much reflection they get from fabrics that seem like they would be "OK", and they are also surprised at how much sound ends up reflecting (albeit diffused) from the corners with such fabrics.

Jon Risch

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