A Matter Of Diffusion Page 2

Secondly, the diffusor breaks up irritating slap and flutter echoes that often plague playback in domestic listening environments. These echoes---artifacts of reflective, parallel room surfaces---overlay the music that follows and obscure its inner details.

Thirdly, the diffusor tames comb-filter-type frequency colorations, previously addressed only by laboriously tweaking the physical position of the speakers relative to room boundaries. By decreasing the depth and increasing the density and irregularity of comb-filter notches, the diffusor makes optimum speaker placement decidedly less critical.

The fourth and potentially most remarkable benefit of diffusion is that the flooding of the listening room with diffuse sound fosters a sensation of complete envelopment or immersion in a musical event, as opposed to the disengaged puzzling over sonic minutiae that too often passes for the audiophile listening experience.

In sum, with adequate diffusion, you can theoretically get a small listening room to sound rather like a very good concert hall because, psychoacoustically, all the same basic ingredients are there: a large enough time gap between direct sound and first reflected spike; lack of slap and flutter echoes and comb-filter colorations characteristic of small rooms; and the envelopment that comes from a dense, smoothly decaying reverberation that permeates the whole room.

The trick to designing an efficacious diffusor is to get it to exhibit no prejudice for frequency, amplitude, or angle of incidence or reflection. This is not an easy task, for physical structures generally operate, acoustically, only on narrow slices of the frequency band and within a restricted range of incidence angles, making them effective only for a small percentage of the total sound energy impinging on them. To be sure, recording studios and the like have relied on large polycylindrical diffusors for years, but these have been cumbersome, restricted in frequency range, and impractical in domestic environments. The upshot is that diffusion---at least up until now---has been more a theoretical solution than a practical one.

RPG
What amounts to the first commercially viable broad-range diffusor is the main enterprise of RPG Diffusor Systems of Largo, Maryland. Working off the findings of Dr. Manfred Schroeder, the German mathematics professor and Bell Labs researcher who conceptualized an acoustic "reflection phase grating" (RPG) device a few years ago in the Journal of the AES, the RPG is a 2' by 4' by 8"-deep affair of wood and aluminum. For all the higher math involved, it is a low-tech-looking affair comprised of thin aluminum slats defining wooden "wells" of constant width (about 31/2 inches) and varying depths. The exact depth of these wells and their sequence are what, according to the literature, imbue the RPG with its nearly ideal diffusion characteristics. The RPG diffusors I tested cost $295 each, and are available through RPG directly at 12003 Wimbleton Street, Largo, MD 20772.

The Set-Up
Coached over the phone by RPG president Dr. Peter D'Antonio, I deployed 24 of his model QRD(R)-734W diffusors in my store's main soundroom, a purpose-built facility some 19' wide, 23' long, and 10' at its highest point, the ceiling being a coffered affair. The wall directly behind the speakers (hereinafter called the front wall) is a 55-ton construction of concrete, cork, and sand; it is well over 2' thick and nearly 20' high (the upper floor is used for storage). Walls and ceilings are of conventional 5/8" sheetrock nailed to 2x6 studs. The room is of the "single-speaker demonstration" variety, meaning that speakers not under direct audition are normally ensconced in a separate, acoustically isolated closet to prevent interaction with the pair being played. Over the years the room has seen a fair amount of acoustical experimentation, including full Sonex treatment on the front wall, dozens of ASC tube-traps crammed into corners, and half-rounds scattered around side walls.

For critical auditioning, 6 RPGs were elevated about 2' off the floor and screwed into the front wall; up to 7 were set along each side wall; and up to 12 were stacked in a 16' by 6' monolith a few feet behind the listening positions (see diagram). It was an imposing sight, to be sure.

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