Auditorium design generally includes gently curving surfaces, diffuse reflection and controlled absorptivity of sound. Large flat surfaces with strong, specular reflection are avoided. Still, auditorium design is only partially scientific; a great deal is left to trial and error. Our perception is sensitive to reverberation, echoes, timing, intensity, phase and other clues about the direction and distance to a sound source. These effects have to be taken into account in order for a performance to sound natural, especially when electronic amplification is used.