Noise levels in cities are getting higher as the urbanisation rate increases. In France, for example, the social cost of noise is estimated at 57 billion Euro annually. The need for soundproofing is also a major challenge for architects who design new or renovate old buildings.
The objective of Hydro’s engineers was to optimise the acoustic properties to design a window that lets in the air but leaves noise outside. The engineers worked with Gamba Acoustique and the CNRS Mechanics and Acoustics Lab to develop window which opens about 10 centimeters and which reduces the noise sufficiently so that it is no longer a discomfort. At the same time, the opening size is enough for good ventilation and in accordance with anti-defenestration rules in public buildings.
The innovative solution was initially implemented on a sliding window system. The solution combines the power of two technologies: passive and active. The passive technology is most useful in tackling high- and medium-range frequency waves. An aeraulic silencer was built by adding two walls on each side of the opening slot with acoustic-absorbing material, in the thickness of the wall. The medium and high-range frequency waves, when they pass in this tunnel, are absorbed and reduced before entering the room. To address the low-frequency noise, the engineers employed an active noise-cancelling technology, similar to the technology used by manufacturers of noise-cancelling headphones. Microphones register the incoming noise, then a dedicated digital sound processor analyses it and generates a counter-noise, which is subsequently emitted by speakers positioned on the fixed aluminium frame of the window. The consequence is a bigger attenuation of low frequencies. The result achieved is sound reduction by a level of 300.