Acoustic diffusers are specially crafted sound treatment structures that diffuse an incoming sound evenly.
Typically, a sound that strikes a surface is absorbed, reflected, or transmitted. Figure 1 graphically displays what is happening in each different situation to the sound.
The rate of each depends on the surface’s acoustic properties. A diffuse reflection occurs when a significant amount of the reflected sound is spatially and temporally dispersed equally. Diffusers are typically used in concert halls, performance venues, and recording studios. The choice of building material to make a diffuser is endless. This article will examine the properties behind a good diffuser and conclude with the best material choice.
Why use them?
Three hundred years ago large concert halls and grand palaces were the main locations for hearing musical performances. These structures had ornate aesthetic engineering including statuaries, relief work, and engravings which provided ample surface scattering for a more diffuse sound field. Modern architecture has replaced these concert settings with large flat surfaces and precise engineering leading to more exacting specular reflections. As a result of these undesirable acoustic settings there has been an increased interest in scattering surfaces within concert halls.
This interest first began from the use of Quadratic Residue Diffusers (QRD) developed by Schroeder which was one of the first acoustic diffusers to exhibit reliable characteristics.
Further notable research was done by Marshall and Hyde in the Michael Fowler Centre in New Zealand and the work of D’Antonio who developed the Live End Dead End (LEDE) and Reflection Free Zone (RFZ) working in small rooms.
Acoustic anomalies such as image shift, colouration, and echoes can be removed by using diffusers. The diffuser will suppress the strong first order reflections in small spaces.
Characteristics of a Good Diffuser:
A well-made acoustic diffuser will successfully generate spatial and temporal dispersion. The design must scatter and diffuse sound so that the reflection is not planar. The diffuser needs surface irregularities a quarter wavelength or larger at the frequency of interest. Small surface unevenness is advantageous. A study conducted by Lee came to conclusion that reflections from modulated surfaces are preferred to those from simple curved surfaces. The number of slots in a QRD designed diffuser must be based on a prime number.
Characteristics of a Bad Diffuser:
A poorly made acoustic diffuser will have critical or flat plate frequencies at which the surface behaves like a flat plate and produces no dispersion. Diffusers should also not be narrow as they will have performance limited by period width. A bad diffuser will have optimal diffusion at discrete frequencies which is not the same as optimal diffusion across a wide bandwidth. Grating lobes all with the same energy is not the same as having even scattering in all directions unless there are a large number of lobes. Diffusers should also not be absorbing and covering the diffuser in fabric will result in absorption.
Further Investigation into Unwanted Absorption
When Schroeder was initially working on QRD diffusers, rumors spread that the QRD design would cause significant absorption which would make for a bad diffuser. Schroeder’s diffusers contain quarter wave resonant structures, and consequently it would be expected that some absorption would occur at and around the resonant frequencies. Studies by Fujiwara and Miyajima measured absorption coefficients ranging from 0.3 to 1. Further studies by Commins, Auletta, and Suner measured coefficients at about 0.5. Both papers results were in high contrast to random incidence absorption coefficients measured on commercial samples. Poor construction was later determined to be cause of high absorption coefficients. Provided QRD diffusers are well sealed and made from non-absorbing material, there is no reason why absorption should be excessive. An important take away from these findings is that the surfaces of the diffuser should not be covered. Excess absorption will occur due to high particle velocity near the well entrances. See Table 1 for sound absorption coefficients between a regular diffuser and one with a cloth covering.
Optimizing Building Choices
When building a QRD diffuser it becomes clear that the less the material is able to absorb sound, the better diffuser it will make. Table 2 shows possible building materials and their absorption coefficients.
When considering the weight of a finished diffuser, the ease of working with the material, and its absorption properties, some of the less absorbent materials (concrete and marble) will be too difficult to work with and too heavy to easily hang on a wall. Woods contain the optimum ratio between weight, ease to work with, and non-absorbent properties. Out of the woods tested, Balsa wood appears to have the least absorptive properties. Pine would be another good choice.
One important aspect to consider is the role of moisture content in absorbing sound in wood. A very low moisture content dramatically increases the absorption coefficient while a high moisture content decreases the coefficient.
- Trevor Cox and Peter D’Antonio, Acoustic Absorbers and Diffusers: Theory, design and application. Taylor & Francis, London and New York. (2009). 1.
- M. R. Schroeder, “Diffuse sound reflection by maximum-length sequences,” J.Acoust.Soc.Am., 57(1), (1975). 149-150
- A. H. Marshall and J. R. Hyde, “Some practical considerations in the use of quadratic residue diffusing surfaces,” proc. 10th ICA, Sydney, paper E7.3, (1980).
- P. D’Antonio and J. H. Konnert, “The Reflection Phase Grating Diffusor: Design Theory and Application”, J.Audio Eng.Soc., 32(4), (1984).
- Mendel Kleiner, Acoustics and Audio Technology (3rd Edition), J Ross Publishing, (2011).
- E. J. Lee, “Effects of surface textures of choral reflectors,” proc. 16th ICa Seattle WA, III, (1998). 2149-2150.
- Cox, “Acoustic Diffusers: The Good, The Bad and The Ugly. Proceedings of the Institute of Acoustics”, Salford University. Section 6.
- Ibid. Cox, Acoustic Diffusers.
- web_absorption_data_eng.pdf obtained from (http://www.akustik.ua/) (2017).
- Mizi Fan and Feng Fu, Advanced High Strength Natural Fibre Composites in Construction, Woodhead Publishing, Sawston, Cambridge, (2016). 341.