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Re: GSBN:Re: STC of Straw bales


This is an excellent summary of acoustic design.  We've designed two
straw-bale buildings with high acoustic requirements:  the vault at Joshua
Tree which is actually a studio for an eminent classical composer; and Lotus
House, a 13-room retreat house at a Buddhist center.  The requirements were
somewhat different for each.

For Lou Harrison's, we sound proofed the individual rooms, so that he will
not be distracted by the sounds of others in the house (he has ears like an
owl that pick up anything).  We used bale walls between all the rooms (they
also form the buttresses for the vault).  The proportions of the main room,
the vault, were very carefully laid out, and the room is very bright--hard
floor, hard walls.  Lou's choice was to start with a bright room and use
tapestries hanging from the ceilings and on the walls to shape the sound
(piano, wind and acoustic) to his liking.

In the monastery, we went to great technical lengths to isolate each room
from the others.  Outside noise is not really a problem, since the land is
large, spacious and quiet.  I'd emphasize what you said--it's most important
to not have leaks through cracks, doors, floors, ceilings, etc.  Seemingly
insignificant leaks can nullify all the work you did to have a wall surface
that's sound-proof.

It's interesting that the characteristics that make straw-bale an excellent
thermal material--lots of airy insulation in the straw combined with
significant heavy mass in the plaster--also make it an excellent acoustic
material that resists both high-frequency and low--and also make it an
excellent seismic material (a stiff shell with a ductile core).


----- Original Message -----
From: "Rene Dalmeijer" rened@...
To: "GSBN" GSBN@...
Sent: Wednesday, October 16, 2002 2:15 PM
Subject: Re: GSBN:Re: STC of Straw bales

> Andre,
> I suppose you also read John Glassfords mail with ale in hand and decibel
> meter in other. Although his measuremnts seem crude he did very well by
> taking note of ambient sound levels. I expect a reasonably executed SB
> without acoustic defects like the ones described by Andre will perform in
> the region of 60dB and upwards.
> The fact that SB walls are a poor sound insulator is a vicious rumor. I
> presume that Arnoud Cauwel is a promoter of pisee and other earth
> techniques. Heavy mass like a meter of concrete are necessary for very low
> frequencies ie <60 Hz above this most simple building structures, even
> quite light ones, can dampen sound quite effectively if executed properly.
> It is even possible to reach -60dB damping with a few not too thick panes
> of glass (like in sound studios).
> Besides mass stiffness and de-coupling are very important for acoustic
> sound insulation. The relatively low stiffness of a SB wall with earthen
> plaster are ideal. The fact that the cavity between the two outer stucco
> shells is filled with straw is excellent acoustic damping. Beware to be
> careful and fill all cavities and voids with straw clay, avoid any direct
> mechanical contacts between inner and outer shells, these will seriously
> degrade sound damping performance. Contrary to what you would expect
> loosely packed bales will perform better then very tightly packed (rice is
> ideal). Pay a lot of attention to all openings and edges these are the
> points. An air leak of only a 1mm^2 will seriously degrade performance.
> Door openings and windows are literally acoustic holes in the wall these
> need special detailing and attention to even remotely approach the
> performance of the walls. Doors even double ones have a poor performance.
> The gaskets and seals in the doors should be double or even triple but
> then there is the problem that over time the seals will degrade and leaks
> will occur. The type of doors you are aiming for is more like a steel
> watertight door in a ship then a house door with multiple closing bolts
> tightening clamps.
> In conclusion I would like to add due to the nature of a SB wall
> continuous surface) the wall is not the problem but the connections
> the wall and all other structures incorporated or surrounding it. I
> strongly suspect that most sound insulation tests executed on SB walls are
> measuring the defects of other structural components or mistakes in the
> test procedure (a non calibrated sound source, Background noise etc.)
> Andre regarding room acoustics. Here are some simple rules of thumb
> depending on the type of acoustics you want ie very lively to very well
> damped. Soft acoustic instruments require a live room. Loud amplified
> a dead room. The single most important parameter is the reverberation time
> and level. The harder the surfaces the livelier the sound. A bathroom is
> lively hence your drive to sing even if you can't. The opposite is
> on top of snow bound hillock. The bigger and harder the room the longer
> reverberation time ie a cathedral. Next the relative dimensions. An oblong
> box (like Concertgebouw Amsterdam) approaches the ideal. Preferably the
> dimensions relate to each other approximately in the following manner
> (I don't have the exact figures at hand at the moment but this ratio will
> avoid predominant harmonic resonances and standing waves) the exact ratios
> depend on the size and acoustic reflectiveness. I personally prefer rooms
> without parallel surfaces thus avoiding standing waves. I think if you
> build a room with clay stucco and wooden flooring and a well pitched
> ceiling you will have quite acceptable acoustics for acoustic
> If its too live you can always add some damping afterwards