What Is a Bell? A Shape Tuned by Removing Metal From the Inside
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A bell produces several notes at once and its character depends on their relationship, which the founder controls by shaving metal from the inside after casting. That process, tuning, is why a good bell sounds like a note rather than a clang, and it was only properly understood in the nineteenth century.
The partials
Striking a bell excites many modes of vibration simultaneously, and unlike a string or a pipe these are not in simple whole-number ratios, which is why a bell does not sound like an ordinary musical note. Five partials dominate in a well-made bell and each has a name. The hum note is the lowest and sounds an octave below the note the bell is said to be. The prime or fundamental is the nominal pitch. The tierce sits a minor third above it, which is why a bell has a faintly minor quality even when nothing else is minor. The quint is a fifth above, and the nominal is an octave above the prime and is in fact the partial the ear identifies as the pitch of the bell, which is a surprising result since the bell is named for a note that is not the loudest or the lowest. The strike note that a listener hears is a construction of the ear from these partials rather than a physical frequency present in the sound.
How a bell is made
The process has changed remarkably little and each stage matters:
- •Designing the profile, which is the curve of the bell's wall thickness from lip to crown, and which determines the relationship between the partials before any metal is poured
- •Building a mould in two parts, traditionally from loam, representing the inner core and the outer shape
- •Casting in bell metal, an alloy of roughly four parts copper to one of tin, which is chosen for its acoustic properties and is hard and brittle
- •Cooling slowly, since a large bell takes days and cooling too fast cracks it
- •Tuning on a lathe, cutting metal from specific zones of the inner surface, with each zone affecting particular partials, which permits them to be brought into the intended relationship
- •Hanging, since the headstock, wheel and fittings determine how the bell swings and therefore how it sounds in use
Change ringing
English full-circle ringing is a practice with no real equivalent elsewhere and it is a mathematical activity as much as a musical one. Bells are mounted on wheels and swung through a full circle, which gives the ringer control over timing but not over pitch, and a tower's bells sound in sequence. Because the timing between successive strikes can be altered only slightly, a band cannot play melodies, so instead they ring changes, meaning permutations of the order in which the bells sound, moving each bell one position at a time according to rules. A full extent on seven bells is five thousand and forty changes and takes around three hours to ring without repetition, and larger numbers rapidly become impossible within a human lifetime. Methods are named systems of permutation learned by heart, and the practice is a substantial living tradition connected to both the mathematics of permutation groups and to the maintenance of several thousand towers.
What bells were for
Before clocks and broadcasting, a bell was the only way to reach everybody within a few kilometres simultaneously, and the uses reflect that. They marked the canonical hours and later the civil hours, structuring the working day for people with no timepiece. They summoned to worship, to work, to market and to assembly. They warned of fire, flood, invasion and approaching storms, with distinctive patterns for each. They marked births, marriages and deaths, with a tolling pattern in many places conveying the age and sex of the deceased. They were rung at curfew and at the opening and closing of markets. Civic bells were objects of enormous local significance and were removed or destroyed by conquerors precisely because of it, and bells have been melted for cannon in several wars and cast from captured cannon in others, which is a recurring exchange that says a good deal about what the metal was worth.
The takeaway
A bell sounds several partials at once that are not in simple ratios, and tuning means cutting metal from specific zones of the inner wall to bring them into relationship, which is why a good one reads as a note. The pitch the ear reports is a construction from those partials rather than a frequency present in the sound. English change ringing permutes the order of the bells because the pitches cannot be varied.