Instruments of complex metallurgic societies
As instruments they are idiophones (that is, they vibrate when a—usually curved— surface is struck with a second instrument made of the same material: wood, metal, pottery) whose vibration needs metal (bronze or iron) which dates their origins to a relatively recent historical period, around two millennia before our era, probably the Bronze Age, in complex societies with communication needs which were more sophisticated than those of tribal groups of hunter-gatherers or early farmers.
Since their beginnings, the sound of a bell (or hand-bell) seems to have been associated with calling for attention and summoning people both in religious and everyday life in complex metallurgic societies.
We know of the use of bells in Roman times and before, although we only know small bells in the form of hand-bells and rattles, the tintinabulum. We do not know whether larger ones existed, as metal was expensive and therefore reused. Until recently, bells were smelted to make new ones. In the Museums of Roman Art in Mérida and Tarragona there are Roman hand-bells.
The idiophonic sound of a bell clearly differs from the whistle/hiss of aerophones or wind instruments like cornets, which were used for communication in war. The whistle seems to be an older system of communication than the bell, as the latter require the percussion of two metal objects, and metal appeared late in History. Even today the whistle is used to summon and coordinate small groups with a common goal, and cornets are used to mark large-scale military manoeuvres, while bells and other idiophones summon and order festive events for society in general.