ETC that can resolve the 4.3Hz
Thinking about this a bit more, you are only modulating a reflection with the fan - not the direct sound.
So what you measure is the interference between the direct sound and the ceiling reflection. But you are applying a phase modulation to the ceiling reflection (the reflection occurs either off the fan or the ceiling, and partially inbetween due to finite width of sound beam).
If someone wants to write out the equations for interference with a phase modulated interferer, it should represent what you are measuring.
Thinking about this a bit more, you are only modulating a reflection with the fan - not the direct sound.
So what you measure is the interference between the direct sound and the ceiling reflection. But you are applying a phase modulation to the ceiling reflection (the reflection occurs either off the fan or the ceiling, and partially inbetween due to finite width of sound beam).
If someone wants to write out the equations for interference with a phase modulated interferer, it should represent what you are measuring.