Geometric structures
Geometric Structures: The Physics of Sound Refraction
In room acoustics, geometric structures are deliberately used to diffusely reflect sound waves instead of reflecting them sharply back like a light beam. The goal: to scatter incoming sound in many directions. However, not every uneven surface serves this purpose.
The golden rule: A structure only acts as a diffuser if its dimensions are on the order of the wavelength of the incident sound. If the structure is too small in relation to the wavelength, the surface appears acoustically smooth.
The optical illusion: Why woodchip wallpaper fails
A common misconception is the perceived benefit of small irregularities in everyday life. To our eyes they appear rough, but to sound they are a smooth surface:
- Textured wallpaper: The wood chips (approx. 1 mm) correspond to a wavelength of 340 kHz. Diffuse scattering therefore only occurs in the ultrasonic range. In the audible range, the wallpaper acts like a sound-reflective surface that directly reflects sound.
- Mixing consoles: Control knobs (approx. 1 cm high) only become diffuse from about 34 kHz upwards – far above the range we perceive as music or speech.
Strategic planning in studio and hall
Acoustic planners make targeted use of knowledge about wavelengths. For example, large acoustic sails are used in concert halls. For a sail to direct sound precisely instead of scattering it, it must have a certain minimum size. Below this cutoff frequency, it loses its directing effect and becomes a scattering element itself.
Solution 1: Reduce the area
By keeping reflective surfaces so small that they fall below the relevant wavelength, diffuse reflections are inevitably created.
Solution 2: Targeted inclination
Surfaces (such as windows or rack surfaces) are angled so that geometric reflections are directed away from the listening position.
Structural forms and the structural period (g s )
The shape of the structure determines its bandwidth. A key concept is the structural period g s – the length after which a pattern repeats itself.
- Rectangular structures: These are usually only effective within a very narrow frequency range.
- Prismatic and cylindrical shapes: Offer a significantly more uniform and broadband scattering behavior.
Conclusion: Geometric structures are powerful tools, but require precise planning. While classical approaches often remain narrowband, modern HSA3 technology, through microperforation, enables much finer control over reflection and absorption in a very small space.