External websites we've worked on:
Room Acoustic Active Diffusers
Milestones achieved:
- Development and implementation of various active impedance systems to allow the control of surface impedance for single and multiple wells;
- Design and implementation of active diffusers based on various number theoretic sequences and Bessel functions;
- A series of measurements proved the performance of the diffusers;
- A set of application-realistic simulations were carried out and evaluated the worth of the diffusers.
A project funded by the Engineering and Physical Sciences Research Council (EPSRC), GR/N39685. Value £120k, Rated "Tending to Outstanding".
What is a diffuser?
It is common to control the acoustics of a space by treating the boundary surfaces with appropriate combinations of reflective, absorptive and diffusive material. In recent decades, there has been increased interest in and use of diffusers to improve room acoustics. Diffusers are used in spaces where acoustics is a critical requirement [1]. They can be used to improve speech intelligibility in railway stations, theatres and teleconferencing rooms. Diffusers are also used in auditoria, music practice and listening rooms where the quality of music is important. However, there is still an on-going debate in academia and consultancy about where diffusers should be applied, and how much is appropriate.
The development of modern diffusers can be traced back to the work of Schroeder in the 1970s [2]. This work initiated the design of diffusing surfaces with defined acoustic properties. The Schroeder diffusers have found favour in many applications, and have been commercially exploited and developed into many different designs [3]. New diffusers based on other geometries, such as curved and fractal diffusers, have also been produced [4]. Read more about passive diffusers here. So far, diffusers have been limited to passive devices. This study began the development of a new generation of diffusers using active technologies.
What are active diffusers?
By placing control surfaces (instrumented loudspeakers) in the diffuser structure (see 2nd picture in right sidebar), it is possible to have the surface move and react to the incoming sound. By moving the control surface appropriately, it is possible to absorb sound incident on the surface, or simply change the phase of the reflected wave. This can enables the reflected wavefront to be broken up, and the reflected sound can be scattered (or diffused).
While there is a large body of knowledge on the use of active control to alter acoustic environments, publications have, however, in general centred on the cancellation of noise and the addition of reflections to enhance reverberation, rather than the spatial dispersion of reflections. While there has been work previously on the active control of surface impedances, the actual development of active impedance has concentrated on active absorption. While the possibility of making active diffusers has been mentioned [5], this project actually broke new ground by trying to develop active diffusers.
In the late 1980s, Nicholson and Darlington[6] and Nelson and Orduna-Bustamente[7] identified the primary techniques for active impedance. More recent research on active absorption has concerned the construction of hybrid acoustic impedances, where a passive porous element is used in conjunction with an adaptively controlled loudspeaker in order to achieve wider bandwidth absorption (e.g. Furstoss et al [8]). This technique has been exploited in this project make active hybrid absorber-diffusers. The controllers used by Nicholson and Darlington were the starting point for the other diffusers made in the project. However, the problem of achieving the desired surface impedance for most active diffusers is inevitably more problematic than is the case for active absorption, as shall be described later.
Why active diffusers?
The active devices created in this project offer significant advantages over passive devices:
- They allow diffusion over a wider bandwidth by extending the response of the diffusing surfaces to lower frequencies. This is useful because the space available for diffusers is usually limited. To achieve good diffusion, a passive diffuser must be significantly deep compared to the wavelength of sound; at low frequencies building space costs generally limit the depth of the diffuser, leading to compromised performance. This research project showed that using active technologies it is possible to extend performance by 2-4 octaves in comparison with passive devices.
- The project produced active diffusers which can not be physically realised
using passive technologies: for example a surface where the well depth is frequency
dependent. This enabled the active surfaces to out perform passive devices.
Another potential advantage is that active devices allow variability. Many rooms have to be multi-purpose, and active elements have the potential to enable the acoustics of a space to be easily changed. However, the variability can only be achieved electronically over the bandwidth that the active controller is working, which in our implementations is about 3-4 octaves at low-mid frequencies.
Why active diffusers might not be used
They are difficult and expensive to make
Links
References
[1] P. D’Antonio and T. J. Cox, “Diffusor Application in Rooms,”
Applied Acoustics, 60, 113-142, (2000).
[2] Schroeder, M.R., 'Binaural Dissimilarity and Optimum Ceilings for Concert
Halls: More Lateral Sound Diffusion', J. Acoust. Soc. Am. 65, 958-963 (1979).
[3] P D’Antonio and T J Cox. Two Decades of Sound Diffusor Design and
Development. Part1: Applications and Design. J.Audio.Eng.Soc. 46(11) 955-976.
(Nov 1998).
[4] T. J. Cox and P. D’Antonio, “Acoustic absorbers and diffusers,”
Spon press (2004).
[5] D. Guiking, K. Karcher, and M. Rollwage, “Coherent active methods
for applications in room acoustics,” J.Acoust.Soc.Am.78 1426-1434, (1985).
[6] Nicholson, G.C., Darlington, P., ‘Smart Surfaces for Building Acoustics',
Proc. IOA 13(8) (1991) pp 155-164
[7] Orduna-Bustamante, F., Nelson, P.A., ‘An adaptive controller for the
active absorption of sound', J.Acoust.Soc.Am. 91 (1992) pp 2740-2747
[8] Furstoss, M., Thenail, D., Galland, M.A., ‘Surface impedance control
for sound absorption: direct and hybrid passive/active strategies', JSV 203(2)
(1997) pp219-236
Principle Investigator: Prof Trevor Cox; Co-Investigator: Dr Mark Avis; Research assistant: Dr L. Xiao
Publications
- T. J. Cox, M. R. Avis, L. J. Xiao, ‘Maximum length sequence and Bessel diffusers using active technologies,’ J. Sound Vibr., 289 (4-5): 807-829, (Feb 2006)
- L. J. Xiao, T. J. Cox and M. R. Avis, "Active diffusers: Some prototypes and 2d measurements," J. Sound Vibr., 285(1-2), 321-39, (2005).
- M. R. Avis, L. J. Xiao, T. J. Cox, “Stability and sensitivity analyses for diffusers with single and multiple active elements,” J.Audio Eng.Soc 53 (11): 1047-1060 (Nov 2005).