Soundproofing rooms usually involves sealing them - even a small slit allows the sound to leak inside. In the recording studio have to use special low noise ventilation system. South Korean scientists have developed a design that breathes freely, but it weakens the sound at frequencies 400 - 5000 Hz at 30 - 35 dB. Soundproof wall consists of a hollow square blocks of Plexiglas with a through hole in the middle, which are long been used in architectural acoustics Helmholtz resonators, specially designed to absorb sound in the mid-range where our ears are most sensitive.
Resonators are very simple device - a parallelepiped 150h150h40 mm with a wall thickness of 5 mm. In the center of each block has a through hole 20 or 50 mm. When a hole diameter of 20 mm, the sound is extinguished is much stronger, but less breathable structure. At 50 mm - on the contrary, when the size of insulation efficiency decreases to about 20 - 35 dB in the range of 700 - 2200 Hz, but the air flows more freely.
The blocks may have internal partitions that separate them into two or four parts. The holes are separated from the internal volume of the resonator damping air filter. Absorbing transparent wall consists of three layers of these blocks, each layer dampens its sub-band frequencies. The resulting structure has a negative Bulk modulus at certain frequencies, i.e. resonance arising therein actively quenched corresponding vibration frequency, instead of skipping it. At the same time, air passes through openings in the blocks easily.
Similar windows and walls can be widely used in architecture, providing a convenient compromise between the requirements on sound insulation and ventilation. Their design is very simple, and changing the size of the resonator, it is possible to configure them on almost any desired frequency range. Preprint of the paper with the description and characteristics of the design can be freely downloaded from the website arXiv.org