Phase grating
Phase grating: Acoustic scattering by Helmholtz resonators
A phase grating is a periodic surface structure that selectively scatters incident sound instead of simply reflecting or absorbing it. This is achieved through a precise variation of the wall impedance (acoustic resistance).
Operating principle: Reflection and phase
The effect of such a grating is based on the complex reflection factor , which provides two key pieces of information:
- Amount: Indicates the proportion of reflected energy (1 = total reflection, 0 = total absorption).
- Phase: Describes the time offset (phase shift) of the sound wave upon reflection.
An effective diffuser minimizes absorption, but influences the phase of the reflected waves so massively that they are distributed evenly in different directions.
Helmholtz resonators as "phase shifters"
Helmholtz resonators are ideally suited for phase control. Near their resonant frequency, the phase of the reflected wave can theoretically rotate by up to 180° .
- Low damping: The less damped the resonator is, the lower the absorption and the wider the effective range of phase shift.
- Combination: By placing differently tuned resonators side by side, the necessary phase differences between adjacent wall areas are created.
Construction options
Based on research by Meyer, Kuttruff, and Rischbieter, strip-shaped arrangements are most commonly used in practice. There are two ways to vary the resonant frequencies:
- Variable volumes: Identical hole pattern at different cavity depths behind the strips.
- Variable perforation: Identical cavity depth with different hole patterns (diameter or spacing) in the front panel.
Optimal dimensioning for maximum dispersion
| parameter | Recommendation |
|---|---|
| Frequency spacing | The resonant frequencies should be approximately 3 to 4 thirds apart. |
| bandwidth | Good dispersion is achieved over a range of about 2 octaves . |
| geometry | The highest scattering effect occurs when both resonator strips have the same width . |
Definitions:
• Third octave: Frequency ratio of $1 : 2^{1/3}$ (approx. $1 : 1.26$).
• Octave: Frequency ratio of $1 : 2$.
• Third octave: Frequency ratio of $1 : 2^{1/3}$ (approx. $1 : 1.26$).
• Octave: Frequency ratio of $1 : 2$.