Sound (speed = 343 m/s) exits a diffraction horn loudspeaker through a rectangular opening like a small doorway. A person is sitting at an angle a off to the side of a diffraction horn that has a width D of 0.060 m. This individual does not hear a sound wave that has a frequency of 8100 Hz. When she is sitting at an angle a/2, the frequency that she does not hear is different. What is this frequency?