11. As shown in the diagram below, a speaker plays a single frequency sound. On its way to you, some of the sound echoes (reflects) off a wall. As a result, the sound is quieter than it should be. (a) If the wall is flexible, what is the lowest frequency that creates the lowest sound intensity at your position? (b) If the wall is stiff, what is the lowest frequency that creates the lowest sound intensity at your position? speaker 4.0 m you 1.8 m wall