Redesigned Window Stops Sound But Not Air, Say Materials Scientists
The design is relatively simple and relies on two exotic acoustic phenomenon. The first is to create a material with a negative bulk modulus.
A material’s bulk modulus is essentially its resistance to compression and this is an important factor in determining the speed at which sound moves through it. A material with a negative bulk modulus exponentially attenuates any sound passing through it.
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An important factor in this is how efficiently the sound can get into the chamber and here Kima and Lee have another trick. To maximise this efficiency, they drill a 50 millimetre hole through each piece of acrylic. This acts as a diffraction element causing any sound that hits the chamber to diffract strongly into it.
The result is a double-glazed window with a negative bulk modulus that strongly attenuates the sound hitting it.
Kima and Lee use their double-glazing unit as a building block to create larger windows. In tests with a 3x4x3 “wall” of building blocks, they say their window reduces sound levels by 20-35 decibels over a sound range of 700 Hz to 2,200 Hz. That’s a significant reduction.
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