Good Wall System for Noise Reduction

Most people who have been in a straw bale building have had the sensation that interior sounds somehow seem louder. Interior sounds are more distinct because they are not being drowned in background noise coming from outside. This is a clear indication that a straw bale wall works very well as an acoustic insulator. It does so because it is an almost perfect example of a damped cavity surrounded by two not-so-stiff membranes with sufficient mass — a far more effective method, weight for weight, than structures based on pure mass, like brick walls.

The anecdotal evidence of good sound insulation is supported by a test executed in the summer of 2003 at the acoustic lab of the Eindhoven Technical University. The test and the facility meet ISO 140-3 standards for testing the sound isolation of building aperture closures (i.e., windows). The test was done according to ISO 140-3, which determines the sound isolation of a building member between two acoustically separated chambers, with the test sample placed in an aperture between the chambers. Although we were aware of the limitations of the test facility for testing a wall system, we endeavored to make the test as accurate and as representative as possible. The size of the aperture (ISO standard) is 1.88m2 (18 sq. ft.). The tested bale wall section had the following configuration:

• two-string bales laid flat (density 120-130kg/m3)

• earth/clay straw plaster between 25mm and 35mm (1-1.5 in.) thick (intentionally asymmetrical plasters)

• no reinforcing plaster netting or mesh or any form of pinning

The chosen sample structure was to be representative of a normal earth/clay plastered bale wall structure, as used by experienced builder Rob Kaptein of RAMstrobouw, who was also responsible for manufacturing the test sample. The graph and table summarize the test result.

The result can be expressed as 55 decibel (dB) A-weighting, which approximates human hearing sensitivity. This result might seem low, but in fact it is very good. Most conventional wall systems — including a brick cavity wall with much higher mass — have a lower performance. Specifically interesting to note is the 2-3dB better performance at very low frequencies of the bale test sample when compared to conventional brick cavity walls. Heavy mass like a meter of concrete is still necessary for very low frequencies, i.e., less than 60 Hz.

A recipe for good acoustic isolation with a straw bale wall is: besides mass, low stiffness with sufficient mass, and acoustic decoupling. The relatively low stiffness of a bale wall with earthen plasters is ideal. The fact that the ^

more thorough studies, the calculations include harvesting, production, transportation, storage, life cycle, recycling, and disposal. But no matter how you calculate embodied energy, baled straw is one of the best materials available.

Estimated embodied energy (production only) of some common materials:

• baled straw = 0.24 MegaJoules per kilogram (MJ/kg)

• expanded polystyrene plastic (EPS) = 117 MJ/kg

S 30

S 30

z1

/

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A

125 250 500 1000 2000 4000 Frequentie (Hz)

125 250 500 1000 2000 4000 Frequentie (Hz)

cavity between the two outer plaster shells is filled with straw provides excellent acoustic damping. Care must be taken to fill all cavities and voids with very light straw/clay. Avoid any direct mechanical contacts between the inner and outer plaster shells, as these will seriously degrade sound-damping performance. Contrary to what you might expect, loosely packed bales will perform better than very tightly packed bales (rice straw, due to its floppy nature, is ideal). Pay a lot of attention to all openings and edge details; these are the weak points. An air leak of only 1mm2 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.

Here are some simple rules of thumb about room acoustics, depending on the type of acoustics you want to create. Soft acoustic instruments require a live room. Loud amplified sounds are better in a dampened room. The single most important parameter is the reverberation time and level. The harder the surfaces, the livelier the sound. A tiled bathroom is lively, hence your desire to sing (even if you can't!). Standing on top of a snowbound hillock gives the opposite effect. The bigger and harder the room, the longer the reverberation time. An oblong box approaches the ideal relative room dimensions, preferably the dimensions relate to each other at the ratio of approximately 2:3:5. This ratio will avoid the formation of predominant harmonic resonances and standing waves. The exact ratios depend on the size and acoustic reflectivity. I personally prefer rooms without parallel surfaces, thus avoiding standing waves. I think if you finish a room with earth/clay stucco on bale walls, with wooden flooring and a well-pitched ceiling, you will have quite acceptable acoustics for acoustic performances.

In conclusion, I would like to stress the following: Due to the nature of a bale wall (homogenic continuous surface), the wall itself won't be a problem acoustically, but the connections between the wall and all other structures, incorporated or surrounding, require proper detailing and careful execution. — René Dalmeijer ■

Freq.

R

1/3 oct

1/1 oct

Hz

dB

dB

50

29.6

63

33.5

30.9

80

30.5

100

34.7

125

37.4

36.4

160

37.8

200

38.1

250

34.8

36.1

315

36.1

400

43.0

500

47.8

46.2

630

52.4

800

56.8

1000

59.7

59.1

1250

62.9

1600

66.4

2000

68.2

67.0

2500

66.6

3150

68.0

4000

60.9

59.2

5000

55.8

• recycled aluminum = 8.1 MJ/kg (Source: Andrew Alcorn, Embodied Energy Coefficients of Building Materials, 2nd ed.)

As the list above shows, it can be quite an eye-opener to see the amount of energy industry expends on creating and supplying building materials. The numbers on this list show Mega-Joules of energy consumed per kilogram of material, hence the higher numbers for the lighter

1.5: Other natural building materials, including tile and wood, can perfectly compliment the feel of plastered bale walls, and lower the potential toxicity of a home.

materials and lower numbers for the heavier ones. But straw is both a lightweight material and it has a low embodied energy figure.

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