Almost total silence: acoustic absorber cancels 99.7% of sound


We all need a bit of quiet in our lives sometimes, but have you ever took a minute to ponder what ‘total silence’ might feel like? It’s scary. Every bodily function, otherwise unnoticed, now sounds like a freight train. Feels like it, anyway. You can even hear your heart beats. Though not exactly ‘perfect silence’, a team of researchers at Hong Kong University of Science and Technology have come mighty close.¬†They report 99.7% absorption of low frequency pressure waves (sound) using subwavelength structures or materials.

Their sound absorber is a dissipative system composed of two resonators, both tuned to the same frequency. Their impedance also matches that of the environment, or air in our case, which keeps reflections to a minimum. Overall, the resulting¬†system basically destroys sound. “Owing to its subwavelength dimensions, acoustic waves incident from any direction will be completely dissipated,” the researchers write in their paper.

Motherboard explains: “The set-up then was to use a thin absorbing material in conjunction with a hard reflecting layer, with a super-thin pad of air in between. The idea was that waves would leak through the weakly absorbing material, bounce off the reflective surface, and then collide with the incoming waves in such a way to create interference and neutralize the sound.”

How this is works, very briefly, is that when sound hits the first material it will naturally want to scatter, only it will do so – this time – at the natural frequency of the resonator. The second resonator is tuned to just the right frequency, though, enough to create a destructive pattern. So, the scattered waves are canceled and almost everything is absorbed (99.7%). That’s 50 dB noise reduction, which is a heck of a lot. Can you think of any worthwhile practical applications to this?

4 thoughts on “Almost total silence: acoustic absorber cancels 99.7% of sound

  1. jimmy kraktov

    I’ve had a thought similar to that for a while now. When I first set up my Home Theatre system about 10 years ago my first ‘system test’ was a little disappointing. About five minutes into things, my neighbor called to say that the dishes in her dining room cabinet were rattling. My system was in my basement so I removed the windows and replaced them with insulation between high density particle board. That worked but, what if I had been on the ground floor? You can’t just fill in all the windows. I like this idea and it could work if there was a way to ‘surround’ the room with noise cancelling frequencies. Apartment dwellers would pay big for this one.

  2. Hal Wagner, Indigo Sound

    I could use this system right now in my sound design studio. It is of the up most importance to be able to hear the soundscape that I am working on that is not distorted by room reflections, combing, etc. The concept is to not correct in the digital or analog workstation, but to have the room acoustically correct. How can I get in touch with the folks that are working on this product. Thank You Hal

  3. Teresa Kuhl

    There are a number of jobs such as construction and landscaping which are often prone to high levels of noise, enough that the worker is at risk of becoming deaf. Perhaps if this technology was included in the ear muffs the noise won’t be such a problem.

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