The acoustic width is not affected by the placement of the absorbent material. The reflective surface of the wall is still the edge of the room. You will get more absorbtion of lower frequencies with thicker absorbtion placed further out from the reflecting surface. Maximum attentuation happens at the 1/4 wavelength distance from the reflective surface, where kinetic energy is highest and the fastest air movement can be forced through labyrinth obstacles (fibers) to convert to heat. It depends on what problems you are trying to fix as to how to use treatments.
If you think your room is too narrow, maybe you can make your listening triangle a little smaller to increase the perceived width. This has some other advantages too, farther away from walls, greater time difference between direct and reflected sounds, listening quieter means less amp distortion, etc. but the potential gain is limited by the speaker design, how far spread apart are the drivers. A single driver can be listened to very closely and is a lot of fun. Sometimes listening closely removes too much room distortion and reveals system deficiencies. Gotta try it to see what you like.
Listening on the long dimension of your room might be worth a try too, but in theory requires more treatment to deal with front and rear walls being closer to the speakers and the listener. Front and rear wall reflections are more important to control than sidewall reflections for stereo listening , IMO. Your electronics and speaker timing performance also determines how wide the listening angle can be before the center image gets weak.
Rich
