[GSBN] SB & Acoustics
mfhammer at pacbell.net
Sun Nov 14 10:00:31 CST 2010
Some years ago a strawbale highway wall was constructed in the state of
Nevada. I¹ve seen photographs and text written about it, but I can¹t put my
hands on either, even after doing a moderate internet search. I did find
reference to this system of construction being used as an acoustic barrier
in a Nevada Department of Transportation report. You can see it via the
following downloadable link:
On page D-2, ³Hay Bales² are described as one of many ³Previously Submitted
Soundwall Systems². Further on there is a matrix evaluating the different
systems according to an array of criteria.
I¹ve often thought that strawbale walls would be great acoustic barriers on
the side of highways. I don¹t know how the Nevada wall has performed and
weathered, but the challenges for these walls seem to be
1. that the wall¹s height be adequately restrained against lateral loads
from wind and earthquakes
2. that the bales remain dry enough to avoid deterioration
I have seen strawbale landscape walls by others where I know the bales
rotted (a 10 year old wall with lime-cement plaster) because I did
conclusive probing, or where I suspect the bales rotted because of a hollow
sound. I¹ve thought for some time that putting a ventilating ³roof² on a
plastered bale landscape wall would greatly improve its longevity. Or one
can simply accept that the bales are just formwork for ³structural² plaster
and let them deteriorate, but that wouldn¹t work so well for tall walls or
The acoustic issues for a highway sound barrier are worth carefully
considering. Some were mentioned by Derek and Graeme. I agree with most of
their points. I¹ll add a few things.
* Mass is an important element in acoustic barriers, but it attenuates
mostly the lower frequencies. Transmission of higher frequencies can be
diminished by less massive materials, such as straw bales. Here we¹re
talking about transmission through the barrier, not above or around it (as
Derek points out). So extent of the barrier is important.
* The surface of the barrier, both its texture and it absorptive/reflective
qualities, is important. So on a straw bale wall, a roughly textured clay
plaster finish would absorb more sound, and the sound that was reflected
would be more dispersed, as compared with a dense, smooth, cement plaster
* There is a difference in acoustic performance between having an acoustic
barrier on one side of a highway versus both sides. Having the barrier on
one side creates a worse situation on the side without the barrier, because
it imposes additional reflected sound on that side. Having the barrier on
both sides creates back and forth reflectance. That can make the sound in
between the walls (on the highway and its shoulders) rather unpleasant.
Although I suppose we don¹t really care (or care much less) about the
acoustics on the highway itself. Having an acoustic barrier on both sides,
with back and forth reflectance, can change the acoustic dynamics and the
reach of the sound on either side of the highway, because the location and
height of the ³source² can change as it bounces off the walls.
I do think a plastered strawbale wall provides a good balance of mass and
insulation/absorption (if that¹s an accurate way to state it) for an
acoustic barrier, in a similar way that a plastered strawbale wall provides
a good balance of mass and insulation for thermal performance.
Those are my thoughts on the subject.
Martin (bouncing off the walls) Hammer
On 11/11/10 6:42 PM, "Sarah Johnston" <sarahjohnston at ihug.co.nz> wrote:
> We've had an inquiry from a local road works company about using straw bale as
> a landscape wall for acoustical reasons along the main highway (keep in mind,
> in this neck of the woods...the main north-south road is two lanes only...).
> They have requested acoustical research and data. We have the information in
> Bruce's book, but I thought I would check to see if there has been any new
> testing or research done recently that you could share with us?
> Does anyone have experience with landscape walls along busy highways?
> I'm questioning the overall long-term durability of this type of landscape
> wall. Your thoughts and input are all welcome.
> Many thanks!
> Sarah Johnston
> Sol Design, Ltd.
> 50A Connolly Street
> Geraldine New Zealand
> 03 693 7369
> sarahjohnston at ihug.co.nz
> GSBN mailing list
> GSBN at greenbuilder.com
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