In terms of physics, what is actually going on to make the choppy voice?
There isn’t much doppler effect going on. Maybe a little bit because of the lateral speed of the fan blades, but that isn’t the main effect.
The robot voice effect happens because your voice alternates between passing through the empty space between the blades and bouncing off of the blades themselves. This results in a rapid change in apparent volume: LOUD-soft-LOUD-soft.
Try talking into a fan and then turning it off while keeping your voice level constant and as the fan slows you will hear the beats slow down.
The blades are tilted with respect to you, so as they pass, your voice (as well as flying unreflected through the gaps, as ivn1188 said) is effectively rebounding off a surface that is rapidly changing its distance from you - this means that your voice takes a little less time to come back when it’s reflected off the closer, trailing edge of each blade than it does when it bounces off the leading edge of the blade, which is a further away (assuming you’re talking into the front of the thing, against the airflow).
Although the distance is quite slight, the modulation is quite quick, and I think it does generate a noticeable sort of ‘chorus’ effect.
But, hypothetically, if the fan blades were curved as to have a constant velocity toward you with time, and if they overlapped eachother to not create any breaks, wouldn’t they just permanently shift your voice higher without any staccato?
I think it has to do with the blades causing reflections of your voice to your ears in some configurations but not others. (Sort of like what inv1188 was saying, but more complex than simply blade=loud, gap=quiet.)
There’s actually an audio illusion of this effect. I’ll have to look it up.
Here it is: Shepard Tone
When you drive by a guard rail you can hear the outdoor sound keep changing rythmically, I think because the flat faces of the support posts reflect back different sounds according to their relative position to you and your car. This is a test case of on-and-off reflection without doppler.
Thanks for the answers. I thought this question might be trivial.