16 years ago RPG introduced a new type of scattering surface called a reflection phase grating. These surfaces consist of a series of wells or cells separated by dividers, having uniform width and depths based on mathematical number theory sequences. Today, tens of thousands of these useful surfaces have helped to enhance critical listening and performance spaces. There are some applications in which a limited depth is available and there is a need for a flat panel. To fill this need RPG developed a new class of diffusors that scatter sound by changing the amplitude, rather than changing the phase of the incident sound. These surfaces are called amplitude gratings.

Figure 1. Planar binary amplitude template. Black areas are reflective and white areas are holes that provide access to absorption

Acousticians have known for some time that surface treatment consisting of patches of reflective and absorptive areas provide some measure of sound diffusion. The problem has always been that the normally used patch size of roughly 2' x 2' only provides diffusion up to a frequency whose wavelength is approximately equal to the width of the panels. Thus a 2' x 2' panel only provides scattering up to about 500 Hz. Higher frequencies are specularly reflected from the reflective areas. Therefore, if one were to reduce the size of these patches to roughly 5/8Ó one could raise the upper limit of useful scattering. The question is what is the best distribution of reflective and absorptive areas?

The answer is easily understood once a mapping is done in which the reflective areas are a one in a binary sequence and the absorptive areas are a zero. Once this digital mapping is recognized, it is apparent that one approach would be to use an optimal binary sequence of zeros and ones. In this case one would obtain a flat power spectrum and achieve uniform scattering. If omnidirectional scattering is desired one would need to derive an optimal 2D binary sequence. The 2D sequence of zeros and ones define the locations of the reflective and absorptive areas. In 1998 RPG developed the first binary amplitude template shown in Figure 1.

Binary Amplitude Diffsorber

Figure 2. BAD panels consist of a binary template over an absorptive core, upholstered with fabric

The binary amplitude diffsorber or BAD™ panel is created by placing this template over a porous absorptive panel. This can then be upholstered with a decorative fabric. The BAD panel solves the problems of the traditional absorptive fabric wrapped panel, by minimizing high frequency absorption, increasing low frequency absorption and introducing sound diffusion above 1000 Hz. Thus the BAD panel provides reflection control without overly deadening the space, the goal in many designs.

Figure 3. The graph illustrates how the BAD(tm) Panel offers an increase in bass absorption over a standard upholstered 1" panel below 1000 Hz, and a decreased absorption above this frequency for a 5/8" diameter and the standard 1/2" diameter hole. The binary template allows the sound above 1000 Hz, which was conventionally lost to absorption, to be uniformly diffused providing reflection control without destroying the room's ambiance.

Figure 4. The thick solid line illustrates how the BAD™ panel distributes sound away from the specular direction into non-specular directions compared to an absorptive and reflective panel

Figure 5. The diffusion coefficient illustrates that the BAD™ panel essentially scatters sound uniformly above 1000 Hz.

Home: Research & Development: Research Topics: BINARY AMPLITUDE DIFFSORBER PART I