Dear Sergio! The positioning of the screen is situational, depending on location, degree of wind, location of reflective surfaces and texture of the audio desired. If you are looking to shield from wind, position the solid side into the direction of the wind. If there is an acoustically reflective surface you do not want to pick up, do the same. If you are looking for the audio reflection, position the side grill so that it will accept the input.
Hope this helps.
Ciao,
Ernst