As you see at the back there´s a wardrobe where I keep some clothes and other things. This wardrobe has 2 wooden sliding doors that are hollow inside and are 3.5 cm thick. At the back of this wardrobe there´s a wall. I was wondering which surface should I take into account to calculate the 38 percent rule for the sitting position. I guess that for the lowest frecuencies what counts is the wall behind the wardrobe but I´m not so shure. Thanks for your help !!!
Ultimately, an acoustic measurement will confirm the exact interactions and it's likely that both boundaries will have some effect; the larger one taking precedence. The way I've set up my girlfriend's room, I have her leave the doors open and use the hanging clothes as absorption thereby precluding the bulk of their specular effect. While 38% is a good starting place, in practice, you may need to shift fore or aft slightly to obtain the best result.
Personally, I like to mark off 38% with tape then perform a 'talk test' where I speak out loud to find where my voice sounds most natural to me. From there, I mark off that spot and use it to begin taking acoustic measurements. Again, remaining open to changing position and/or orientation.