Typically the frequency of the AC electrical current feeding the bell. This can be 50CPS or 60CPS depending on the country. Sometimes bell manufacturers put a diode in series with the bell circuit that lops off half the current cycle creating either a 25cps or 30cps bell ringing frequency. Old electromechanical central offices used to ring the bell on party line phones with different frequencies. Each subscriber had a capacitor tuned to the bell coil that was "excited" at a different natural frequency allowing up to 4 people to share a single phone line and be individually notified of a call for them. The frequencies they used were 20, 30,40, 60 cps.
The frequency at which the current changes direction is the frequency of the sound that the speaker produces.