Q:

Tuning: beats easier to hear on guitar than on cello

I find it easier to hear beats on a guitar (when two strings do not form a P4/P5) than on a cello when bowing. Is there a reason for this, or is it just me?

A:

Beats are easier to hear when the strings are vibrating freely and with sustain -- e.g. two guitar strings, or two adjacent piano strings.
With the cello, the bowing action doesn't let the strings vibrate freely, but continuously forces them into a particular type of induced vibration. And if you pluck the strings instead of bowing them, there isn't nearly as much sustain as with a guitar or a piano.
Perhaps you can experiment with either plucking the strings and paying a lot of attention to the sound even if it quickly decays in volume, or by giving them a short bowing and then letting them vibrate freely, and see if you can hear the beats a little better in those brief moments that follow.

