A man in front of the wall facing the voice shouting the wall if he needs to cause an echo should stay away from the walls sound much faster rate in air 341 m.
One man shouted in front of the wall facing toward the wall if I want to echo what should stay away from the wall. The sound of the air speed of 340 m.
A man in front of the wall facing toward the wall shouting sound if he wants to cause resonance should stay away from the wall to afford. The sound speed is in the air 340.