A few weeks ago we posted a news article reporting new research into sound absorbers which suggested that it was no longer the case that acoustic treatment which could adequately absorb low frequencies had to be impractically thick. French researchers published results which, while not ready for any products to be produced, did offer the strong possibility that thin, full range, passive bass trapping could soon be a practical proposition.
While absorption is always the more necessary treatment in typical studios, diffusion is a complementary and often essential acoustic treatment which has always suffered from the same wavelength issue.
As we know, flat surfaces reflect sound waves much like a mirror reflects light. It is commonly held wisdom that a bookshelf on the rear wall of your studio can be acoustically helpful because of the scattering effect the books and CDs present to sound waves hitting the rear wall. There is of course a more thought through alternative in purpose built diffusors. Common variants being Quadratic or Schroeder diffusers.
Rather than being random, these diffusers present a surface with a series of varying heights which are based on a number sequence which ensures an identical, random scattering of sound regardless of the angle from which it arrives.
While there are varying designs, all of these share a familiar issue. As anyone who has seen the spectacular diffusion treatment in Blackbird’s Studio C will understand, to achieve the desired phase delays the thickness of a diffuser could be as much as 69cm at 250 Hz. For nearly everyone full range diffusion is out of the question.
Ten Times Thinner Diffuser Design
The new diffuser design, instead of having "wells" of identical area but with varying depths, has semi enclosed wells of a fixed depth around a tenth the depth of a traditional design but with varying apertures in the front which vary across the panel in a similar way to the quadratic residue sequence found in conventional diffusers. By designing an acoustic metasurface which breaks down this fundamental physical link between wavelength and thickness the team from Nanjing University have produced a practical diffusor which is an order of magnitide thinner than conventional Schroeder diffusers.
Read more here.