Existing examples of acoustic volume diffusers

Reverberation chambers often make use of suspended scattering surfaces hung within the room to promote a diffuse field by redistributing energy and breaking up modal propagation [3; 16], an example of which is shown in Figure 1.6. These may be viewed as a form of volume diffuser, for which current guidance on their application is given in Appendix A of international standard BS EN ISO 354:2003 “Acoustics – Measurement of sound absorption in a reverberation room” [18]. This recommends introducing a number of diffusing elements; specifically thin non-absorptive panels varying in size from 0.8-3m2 (for one side). These may be slightly curved and of random orientation and location. The actual number of diffusing panels to be used is essentially based on a trial and error process; a method which involves measuring the absorption coefficient of a test sample (assuming a diffuse field in accordance with Sabine’s equation [19]), and introducing diffusing panels until the average coefficient remains constant. In general the panels (considering both sides of each) should account for approximately 15-25% of the total surface area within the room.

In the context of reverberation chambers, volume diffusion can have significant advantages over surface diffusion [3]. Cox [3] for example points out that for a reverberation room to achieve a diffuse field via surface diffusion, at least three of the boundaries must be treated (e.g. at least one of each pair of surfaces directly facing one another). This implies a diffusing area of approximately 50% of the total surface area of the walls. Introducing diffusing panels that account for approximately 15-25% of the total surface area of the room on the other hand, as recommended by BS EN ISO 354:2003, equates to a volume diffusing area equivalent to 18-33% of the total surface area of the walls. Furthermore, this value considers the surface area of both sides of the panels, and so their total area considering only a single side is halved to 9-17% of the wall surface area. Whilst these improvements in efficiency do not necessarily carry over directly to more conventional spaces, they do however suggest the potential of a volume diffuser to at least pass on some of these gains.

Chapter 1: Introduction

14

Figure 1.6: Reverberation chamber with suspended scattering panels

The above guidance is given for the measurement of absorption coefficients over a frequency range of 100Hz-5kHz, which implies a maximum panel size (assumed to have a width of 31/2 ≈ 1.73m) on the order of half a wavelength for the lowest measured frequency. The use of simple rigid panels is largely due to cost and practicality reasons, though alternative shapes could potentially be used. These scattering panels are often referred to as volume diffusers, though it could be argued that they form a single volume diffuser since independently they will have little effect, and it is their combined interaction that results in a more uniform sound field. In contrast to the volume diffusers considered here, however, the sound field is sampled within this diffusing structure. It is likely though that an arrangement which results in a uniform internal sound field would also provide dispersion external to the structure when placed in a free-field environment.

15 Overhead stage canopies

Overhead stage canopies have been investigated for their effectiveness in controlling reflected sound to enhance intelligibility and clarity [20]. They are usually found suspended above a stage or audience in auditoria to reflect sound back into the space, for example back towards the musicians to allow them to hear what is being played and keep in time with one another [3]. A common criterion for their design is to achieve an even distribution of reflections over a defined area [21], and as such are similar in concept to surface diffusers. Since they are located in the volume of a space however, these may be viewed as a type of volume diffuser. Some canopies have virtually no open area and are curved to direct reflections sideways to promote lateral reflections and reduce colouration [8]. Of most interest to the work presented here however are canopy arrays, composed of a series of reflecting panels separated by intermediate gaps. By introducing these gaps the sound directed back to the stage may be controlled, whilst also permitting some sound to pass through the structure to be heard by the audience and to add to the general reverberance [3]. These arrays usually form a single layer of scattering panels, and as such are similar in concept to the 1D array volume diffusers presented later on in the thesis, in particular the 1D slat arrays presented in Section 4.2.

Rindel [22] determined that small elements were required in a canopy array so that diffracted energy was received in locations where there is no specular reflection, concluding that low frequency performance is determined by density and high frequency performance by panel size and spacing. Later Cox and D’Antonio [21] considered the effect of the density, size, shape and location of canopy elements using optimisation techniques, concluding that for high levels of support dense arrays of large simple shaped panels are best.

Much of the work above considers the near-field response of an array, though some investigations have been carried out for far-field conditions. Here at low frequency the response is dominated by the scattering characteristics of a single panel, and at high frequency by strong specular reflections, with the sidelobes of the array configuration being largely suppressed [3]. This behaviour is demonstrated for arrays of slats in Section 4.2 and is discussed further.

Chapter 1: Introduction

16 Other volume devices

Sonic crystals are periodic devices, ordinarily composed of cylinders (2D) or spheres (3D) with locations determined by a square or cubic lattice respectively. Consequently they are similar to the cylinder arrays of Chapter 6 whose arrangement is based on a periodic grid. Previous work in this area has predominantly focused on attenuation by arrays at specific frequencies, known as band-gaps [23-25], rather than on the spatial distribution of its scattered field. A typical sonic crystal will spread energy temporally, but produces inherent grating lobes and therefore makes a poor diffuser whose behaviour varies significantly with position and frequency [26]. Consequently they must be altered significantly if they are to provide diffusion.