how can I make the back wall transparent?

Your photo or a stock photo on a panel.
http://www.acoustimac.com/acoustic-panels/acousticart-panels/

I agree, Tom, I think the op is talking about the front wall.

I'd recommend treating the room with bass traps / absorption panels and diffusors. I'm new to room treatments as I just hung my first four panels (gik 244 broad band traps) but I'm totally sold on room treatment now. I think you can gain what your looking for by treating the first reflection points and going from there!
Good Luck!

not knowing what speakers used u might want to try diffusers instead of using traditional absorbtion...many articles on the net covering this..good luck

I had that issue once and physically moved the speakers out when listening and back when done. It kinda sucked but preferable to hanging weird things on the wall since it was in our living room. So it was worth the effort to me given the huge sonic benefits but maybe not for you. Or maybe there's some product that makes speakers easy to slide without adversely affecting sound?

Other thought was if you may have the flexibility to try the speakers in the corner where the corner itself is the midpoint between the speakers. I've seen manufacturers do this at shows where the room was otherwise too small and seemed to work pretty well. Just a different thought. Best of luck.

Thank for all the feedback - seems I got a few things to try :-)

The speakers are Gershman Acoustic Sonograms - which are very forgiving and very transparent.

I'm very fortunate in that I have not experienced many of the problems others have re: traps and reflected sound - I did have an echo behind the speakers, but a couple of tall panels fixed that.

I tried hanging some material behind the speakers, which helped a little with reflected sound, so maybe something thicker like a rug might be more effective ?

The speakers are of the standard forward firing drivers type, so I assume that if there is some reflected sound coming from the wall behind them, then it would be first be coming from the wall behind the listener?

The speakers do have quite a wide dispersion angle, but not enough to reflect off of the wall behind them.

If I were to look into some kind of deflection panel, which direction should I aim the deflected sound?- up, down, outwards, inwards ?

Would the deflectors need to be the height of the room?

In one hi-fi store I visit, they have "scatter reflection panels" which are long strips of wood at 45 degrees to the wall - can anyone vouch for the effectiveness of something like this?

Many thanks