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RE: Room Lens Question

Posted by David Aiken (A) on May 26, 2010 at 13:19:36

In Reply to: Room Lens Question posted by AudioDwebe on May 26, 2010 at 08:11:03:

The suggested basic placement for the original Argent Room Lens used 3 units with one placed to the outside of each speaker but quite close to the speaker and a central one placed close to the wall midway between the speakers. I'd say you've got enough room for them.

You say "I've only got anywhere from 2 to 3 feet from the speakers to the side walls, where there's currently 1st reflection point damping." If by "damping" you mean absorption, that will work against the room lens. The room lens diffuses using 2 mechanisms. The first is reflection from the curved surface of the pipes which reflect sound over a wider angle than a flat surface would. The second is by diffraction of sound passing through the gaps between the pipes. Diffraction spreads the soundwaves over a wider angle as they exit the gap so they then reach a wider area of the wall behind for reflection over a wider angle also (angle of reflection = angle of incidence so widening the angle of incidence also widens the angle of reflection). Sound reflected from the wall towards the room lens is also diffused in the same way. Placing absorption on the wall behind the room lens is going to reduce the amount of sound diffracted through the gaps to the wall which gets reflected and also reduce the amount of sound reflected from the wall towards the room lens which gets diffracted on its way back into the room.

In my experience with my DIY room lens units, the original recommended placement wasn't always the most effective. When I tried them years ago in my current L-shaped room I found they didn't help particularly well in the standard locations and part of the reason for that was the fact that I had a lot of books providing absorption on one wall behind the left hand unit, an open archway into a hall behind the right hand unit, and heavy curtains behind the central unit. In other words none of the surfaces behind them were reflective. By experimenting I did find useful placements elsewhere in the room but they were obviously heavily determined by the asymmetrical shape of the room and where I had reflective surfaces. If you're going to place them in front of absorption you may not get much benefit (I suspect a lot will depend on the size of the wall area covered by the absorption and how that area relates to the way the lens diffracts sound along the wall and also how it relates to how the wall reflects sound towards the lens). In that case you're really going to have to experiment with placement elsewhere, picking areas where you do have reflective surfaces and trying them to see what effect they have. It will vary with location.

You also said you were considering "a DIY room lens" and the use of the "a" rather than a term like "three" or "some" tends to suggest a single room lens to me. If you're going to use a single lens only, then I wouldn't be placing it near the speakers but rather on the centre line between the speakers close to the wall behind the speakers or the wall behind the listening position.

Further comments for DIY construction: units with more than the original 3 pipes are more effective because of their greater width which increases the amount of diffusion provided. If you're going to use more pipes, then I think there's a fair amount of room for experimentation with pipe geometries other than placement in a straight line. I originally made 4 of the standard 3 pipe units but later dismantled them and used the pipes for 2 units, an 8 pipe unit with pipes based on a pattern determined by quadratic residues, and a 4 pipe unit based on a primitive root sequence. I felt the 8 pipe unit was more effective than 3 of the standard 3 pipe units placed side by side in the same location. Those are the only non-straight line geometries I used and I make no claim that they are the most effective in normal use. I was playing with odd placements in my asymmetrical room when I decided to try them but I do think a symmetrical unit like the 8 pipe unit would be far more effective than a single 3 pipe unit if you're only using one unit placed on the room's central line. It certainly was in that location on the wall behind my speakers but I also found that while it produced a stronger effect I did not like that effect. I ended up with it placed on the wall behind me where I thought it's effects were quite positive but that wall was in the toe of the L-shape and I don't know that it would have the same effect in the centre of the rear wall of a normal rectangular room.

I eventually stopped using them and now rely solely on absorption in my room.

So basically I think they work, I think bigger units work better and there's a lot of room for playing with different pipe geometries to make them even more effective, and I find them extremely placement sensitive. They definitely work more effectively in front of an untreated wall.



David Aiken

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