Sound exits a diffraction horn loudspeaker through a rectangular opening like a small doorway. A person is sitting at an angle off to the side of a diffraction horn that has a width D of 0.0831 m. The speed of sound is 343 m/s. This individual does not hear a sound wave that has a frequency of 10500 Hz. When she is sitting at an angle /2, there is a different frequency that she does not hear. What is it?