Bim-Bam Chimes originally were called Normandy chimes. Instead of one chime on the hour, there were two and it was a musical dual tone, instead of a single, (often harsh sounding) single hourly strike. Nowadays, it is a descriptive term for the two-note chimes which sound only at the half hour and hour. With most clocks, mechanically-driven hammers strike the two tuned chime rods to produce the distinctive tone.
Listen to Bim-Bam Chime