I like the audio approach. It should make it possible to measure rotation speed under load. Even a Vitamix slows down a bit when powering through stuff. This might provide a way to compare different blenders in terms of quality. A good blender and a cheap blender might have the same top speed, but the good blender will have more torque and slow down less when meeting resistance.
I’m not sure if you saw it, but the last figure in the post is a spectrogram of the motor under load. I'd say the load results were the most interesting because they revealed differences in the speed control of the different models.
