Improving phase grating and absorption grating diffusers

Dadiotis, K

Abstract

This thesis investigates room acoustic diffusers based on number sequences, exploring their
shortcomings and presents improvements.
Standard Phase Grating Diffusers display frequencies where they act like flat plates and fail
to diffuse. To overcome this, two new sequences (Luke and power residue) are introduced.
The diffusers based on these sequences display extended frequency range compared to
standard ones such as Quadratic Residue and Primitive Root Diffusers. Their performance is
studied using Boundary Element Modelling which shows that they can avoid flat plate
phenomena in the audible frequency range. Furthermore, it is shown that by taking advantage
of their inner symmetries Quadratic Residue and Primitive Root Diffusers can be created
from smaller components thus allowing for the flat plat effect to be mitigated.
Next, Absorption Grating Diffusers are investigated. They consist of ideally absorbing and
reflecting elements. For their implementation heavily damped Helmholtz Resonators are
investigated showing that they give an approximation of the required distribution of
admittance on the surface.
Then the performance of ideal Absorption Grating Diffusers is investigated using Boundary
Element Modelling. Even with idealised completely absorbing elements, the performance of
the diffuser is shown not to achieve substantial diffusion. This arises because edge diffraction
from the reflecting elements weakens at high frequencies. At frequencies where smaller
elements are creating substantial scattering, larger elements are producing specular
reflections. Furthermore, due to the lack of cancellation, the specular reflected lobe is
insufficiently attenuated, because it can only be changed through absorption.
Improvements to the original design are suggested. By changing reflective elements to
reactive ones, scattering can be extended to higher frequencies. This allows for a range of
frequencies were more reflecting elements display substantial dispersion. Also, implementing
the absorbing elements using porous material in a shallow well allows some reflection,
resulting in cancellation in the specular reflection lobe due to interference.
Measurements of the scattered pressure distribution of absorption grating surfaces are carried
out and then compared to Boundary Element Modelling simulations using surface admittance
data measured in an impedance tube. The agreement between measurement and simulation is excellent proving the accuracy of this simulation method for these applications. The results
show that the samples tested perform as two level Phase Grating Diffusers, with some energy
loss, while their diffusion characteristics are shifted to lower frequencies. This arises because
of the lower speed of sound in the porous medium. This implementation is shown to absorb
50% of the incident sound while the rest is scattered uniformly but only over a limited
bandwidth.

Citation

Dadiotis, K. Improving phase grating and absorption grating diffusers. (Thesis). Salford : University of Salford

• Thesis Type: Thesis
• Deposit Date: Oct 3, 2012
• Publicly Available Date: Oct 3, 2012
• Award Date: Jan 1, 2010