An open window that reduces noise from the streets
If you live in a city, then you probably keep your windows closed when you get home from work. Unless you like the noise. My neighbor plays the drums in the evening, and it is annoying.
And here is the dilemma: If you live in a noisy environment, you don’t feel like opening your window because the noise goes through the openings that are allowing the passage of air.
Noise levels are getting higher as more and more people move into our cities. A recent study showed that 25 million people in France are concerned with road traffic noise, and the social cost of noise is estimated at EUR 57 billion annually. This need for soundproofing is also a major challenge for most architects who erect or renovate buildings.
My team and I in Hydro have been working on the acoustic topic for the past three years, trying to optimize the acoustic properties of doors and windows, and come up with an innovative solution. The active acoustic window is one of the most promising concepts we have come up with so far.
Together with Gamba Acoustique and the CNRS Mechanics and Acoustics Lab, we have designed a window that lets air in but leaves noise outside. With this window, you can leave it open all night and sleep anyway.
Let’s be precise:
- We don’t open the window completely. Not yet anyway. The opening is 10 centimeters – or about four inches – which is more than enough for good ventilation and in accordance with anti-defenestration rules in public buildings.
- We don’t completely cancel the noise, but we manage to reduce it sufficiently so that it is no longer a discomfort.
To achieve this amazing result, we designed a solution, implemented on a sliding window system as a first step, that combines the power of two technologies: passive and active. We use both technologies because you can’t use just one to manage the entire range of frequencies.
The passive technology is most useful with regard to loud and medium-range frequency waves. It works like this: You build an aeraulic silencer 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, will be absorbed and reduced before entering the room.
With low frequency noise, the idea is to use active noise canceling technology. This is similar to what the manufacturers of headsets use in their noise-reduction products. Microphones register the incoming noise, then a dedicated processor – a digital sound processor – analyzes it and generates a counter-noise, which is emitted by speakers positioned on the fixed aluminium frame of the window. The result is a bigger attenuation of low frequencies. What you hear and feel at a discotheque – a lot of bass – is low-frequency.
Both technologies are working in the same way, addressing the adequate range of frequency. If you only have “bass” noise, then the passive technology will not be sufficient because it doesn’t reduce bass. The active technology will do the job. And vice versa. The point is that both work at the same time to address the noise frequency. Right now, we are reducing the sound by a level of 300.
We are not finished yet. Active acoustic is not an easy field. But we are addressing the challenge and making progress.