How to measure the air flow resistance of glass wool
In actual engineering, it is difficult to measure air flow resistance, but it can be roughly estimated and controlled by thickness and bulk density.
1. With the increase of thickness, the sound absorption coefficient of middle and low frequency increases significantly, but the high frequency changes little (high frequency absorption is always larger).
2. When the thickness remains unchanged, the bulk density increases, and the sound absorption coefficient of medium and low frequencies also increases; but when the bulk density increases to a certain level, the material becomes dense, the flow resistance is greater than the optimal flow resistance, and the sound absorption coefficient decreases instead. For centrifugal glass wool with a bulk density of 16Kg/m3 and a thickness of more than 5cm, the low frequency 125Hz is about 0.2, and the sound absorption coefficient of the medium and high frequency (>500Hz) is close to 1.
When the thickness continues to increase from 5cm, the low-frequency sound absorption coefficient gradually increases. When the thickness is greater than 1m, the low-frequency 125Hz sound absorption coefficient will also be close to 1. When the thickness remains the same and the bulk density increases, the low-frequency sound absorption coefficient of centrifugal glass wool will continue to increase. When the bulk density is close to 110kg/m3, the sound absorption performance reaches the maximum, 50mm thick and close to 0.6-0.7 at a frequency of 125Hz. When the bulk density exceeds 120kg/m3, the sound absorption performance decreases, because the material becomes dense, and the medium and high frequency sound absorption performance is greatly affected. When the bulk density exceeds 300kg/m3, the sound absorption performance decreases a lot. The sound-absorbing glass wool commonly used in architectural acoustics has a thickness of 2.5cm, 5cm, 10cm, and a bulk density of 16, 24, 32, 48, 80, 96, 112kg/m3. Usually use 5cm thick, 12-48kg/m3 centrifugal glass wool.
The sound absorption performance of centrifugal glass wool is also closely related to the installation conditions. When there is an air layer behind the glass wool board, the sound absorption effect is similar to that of a glass wool board with the same thickness without an air layer. In particular, the sound absorption performance of medium and low frequency will be greatly improved than the material is attached to the hard bottom surface. The sound absorption coefficient will increase with the thickness of the air layer, but the effect will not be obvious after increasing to a certain value.
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