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Yes, but I have read that higher density acoustic insulation (like OC705) is better for low-end absorption.

That’s simply not true.

Owens Corning has thoroughly tested the absorptive properties of both 703 and 705 in acoustic laboratories and found that the less dense material [703] better absorbs low frequencies. Here are the manufacturer’s published absorption results for these two materials;

You can review an extensive compendium of absorption coefficients for various materials here: http://www.bobgolds.com/AbsorptionCoefficients.htm

The common misconception that higher density acoustic insulation makes for better low frequency absorbers arises from a fundamental misunderstanding of how the physics of porous acoustic absorption works in the real world [as opposed to the world of internet chat forums].

Density is not a defining characteristic of a porous absorber. Density is instead a by-product of the characteristics which are defining.

In a porous absorber there is an impedance jump when the sound's pressure wave transitions from traveling through air to traveling to through the absorber, but after making that transition the intrinsic gas flow resistance of the medium comes into play.

When comparing two version of a specific acoustic insulation material [such 703 and 705] it is these two properties which make the difference. 703 and 705 are made from the same base fiberglass material. The properties of entrance impedance and gas flow resistance vary with the porosity of the board. In the process of manufacturing the board, as the material is compressed to a greater degree, density rises as impedance climbs and porosity declines. The task of choosing the proper core material is one of matching the impedance and porosity of the core material to the thickness of the panel and its application.

With too high a porosity [and low impedance] the sound can easily enter the material but the sound waves may pass all the way through the panel before they are fully absorbed – with too low a porosity [and a high entrance impedance] the sound will tend to bounce off rather than enter the absorption panel [especially at grazing angles].

Use plain boards of 3 lbs. pcf density semi-rigid fiberglass boards [703 or equivalents] for making broadband panels [either 4” or 6” thickness]. The porosity and impedance properties of 703 is excellent for panels of this thickness. The entrance impedance is low enough to allow the sound waves to enter the panel, and the porosity is high enough to absorb the wave before it leaves the panel.

Choosing a higher density fiberglass board such as 705 instead of 703 for a broadband panel will yield lower performance at low frequencies as the gas flow resistance and impedance of this material are less than optimal. If a different material such as rockwool or cotton or hemp fiber boards are used to make a porous absorber, you must ignore the density properties of 703 [3 lbs. per cubic foot] and instead seek a material with similar gas flow and impedance properties in order to match performance with 703.

If you want to explore a better understanding of how the gas flow resistance properties of a material define its absorptive properties you can use this porous absorber calculator to model the performance of various materials: http://www.whealy.com/acoustics/Porous.html

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This question was added to our store on Thursday 21 June, 2007.
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