1- sound striking the curved surfaces of the pipes is reflected over a wider angle than if it struck a flat surface. This effect reduces as frequency lowers—low frequencies will simply wrap around each pipe with little or no reflection.
2- sound passing through the gaps between pipes is difracted over a wider angle than if the pipes weren't there. This also is less effective as frequency lowers.
3- the hollow space within the pipe has some effect as a broadband Helmholtz resonator so it may help tame some room resonances slightly (depends a lot on the particular room resonances you have and how they relate to the frequency the pipes are tuned to).
Effects 1 and 2 provide diffusion. Both effects work in both directions. Effect 1, reflection from the surfaces of the pipes, will also block some reflections from the wall that would otherwise reach you and reflect them back to the wall where they are again reflected back into the room over a wider angle. Effect 2, the diffraction through the gap, also spreads wall reflections passing through the gap back into the room over a wider angle. The fact that the room lens works in both directions increases it's effectiveness quite a bit. Note, however, that it means you need a reflective wall behind the unit if it is going to work effectively. Put an absorptive surface behind the unit and you lose diffractive spread in both directions, leaving only the effect of reflection from the surface of the pipes. That's a large drop in effectiveness.
For the standard 2" pipes, Jon Risch suggests that the diffusion effects start at around 750 Hz and are in full play somewhere aroung 1500 Hz if I remember correctly. If those figures are wrong, it's much more likely to be my memory than Jon's calculation.
Place a room lens between speaker and wall and it will reflect some of the sound reaching it. If you're sitting somewhere where there will be a direct reflection from the room lens surface to you, you will still get an early reflection but it will be somewhat down in level compared to a wall reflection simply because the sound energy is being reflected over a wider angle than the wall would reflect it. The odds are that if the location of the room lens is along the line between the speaker and the early reflection point on the wall that it will generate an early reflection that you will hear, but it may do that also in some other locations because of the wider angle the reflections are spread over. The angle at which it is placed relative to the speaker will also play a part in determining whether or not a reflection is directed to you. You can play with angling to minimise this somewhat.
Wider pipe arrays (more than 3 pipes per unit) are going to be more effective. Pipe placements other than in a straight line may be more effective. I currently use 2 DIY units in the 'toe' area of my L-shaped room. One is an 8 pipe unit with pipes placed at varying depths determined by a quadratic residue formula. The other is a 4 pipe array with varying depths determined by a primitive root formula. I haven't done measurements but I prefer the results to what I got when I placed my original four 3 pipe units in the same area.
These 2 units are place in corners and their effects make me wonder whether what they do their is similar in some ways to what is claimed for the Shakti Hallograph devices and also for other devices like the Eighth Nerve products which claim to work by reducing the horn loading effects of corners. Argent, the manufacturers of the original Room Lens, don't make any mention of corner placement in their recommendations but I find some benefits with it and that's where I now use my units. One advantage of the room lens is that it lends itself to experimenting with placement and you're only limited by your imagination. Making your own room lens clones also gives you considerable freedom to play and experiment with unit size and design.
David Aiken