When a bell is struck, it rings. Easy, right? That is, however, an overly simplistic description of the acoustical vibrations that emanate from a struck bell. Bells are deceptively complex musical instruments. Each sound comprises dozens of slightly inharmonic partial tones that die out quickly, giving way to more dominant partials which together form the note immediately perceived by the human ear. This ‘note of first impression’ is called the strike tone, or sometimes also called the fundamental or tap note.
How we hear a bell ring
The strike tone of a bell is a subjective note because the musical acoustics are dependent on four aspects:
- The Physical: When a bell is struck by a clapper or hammer, the energy imparted causes complex vibrations at each part of a bell’s structure. Different frequencies across the shape of a bell send different pulses into the air.
- The Molecular: The to-and-fro vibration of a bell imbues energy into the air, which sets molecules in motion. These waves convey the sound of the note to our ears, with factors potentially interfering as they travel.
- The Physiological: Your eardrum senses this sound, vibrates concurrently with the incoming waves, and passes the ripples of information through the inner-ear bones and fluid and onto the auditory nerves.
- The Psychological: Your brain interprets the electrical signals communicated by the auditory nerve, piecing together the bits of information gathered and synthesizes it into the musical note we perceive.
Across these four stages, the human ear loses much of the distinction between partial notes and provides you with the simplest answer: one perceived tone. But if you listen closely, the trained human ear can distinguish some of the more dominant partials.
Five principal tones of a bell
Acousticians who analyze a bell’s frequency spectrum record more than 50 partial tones each time a bell is struck. As we’ve learned, what humans perceive as the strike tone is rather a perception built up by overlapping harmonics, generally composed of the five most dominant partials. Because campanologists study these tones so closely, ever trying to create the perfectly-tuned bell, we have names to distinguish these main partials.
- The Hum: The lowest of the five partials is called the hum. The hum can be heard after the intense strike tone dissipates and is the slowest overtone to decay.
- The Prime: An octave above the hum in the pitch of the strike tone is the prime. The prime is the most prominent tone heard when a bell is struck and is the tone for which the bell is named.
- The Tierce: A minor third above the prime is the tierce, which is distinctive to bells and imparts their somewhat plaintive or melancholy sound.
- The Quint: A fifth above the prime is the quint, which imparts a glassy effect, particularly in many old-style bells.
- The Nominal: An octave above the prime and two octaves above the hum tone, the nominal is the overtone with the highest frequency of the main partials, with a short burst of intensity.
The clarity and purity of the strike tone may be amplified if additional high-order partials are present. In a well-tuned bell, the strike tone aligns closely with the pitch of the prime.
How is a bell tuned?
As one of the most complicated musical instruments in the world, a bell must be finely tuned. A bell founder will cast a bell slightly thick, as the casting process does not allow sufficient precision to produce a perfectly tuned bell profile right out of the pit. This allows extra material for the bell founder to whittle away on a specialized lathe, which shaves metal from the inside surface.
Millimeter by millimeter, the tuner adjusts the thickness of the bronze across the profile of the bell to achieve the desired partial tones. Each partial is tuned separately by removing metal from a different area of the bell. Once tuned, a well-kept bell never needs retuning. The pleasing harmonic fusion of partial tones lasts for centuries.
Factors influencing the propagation of bell sound waves
As is now very clear, the acoustical properties of a bell are complicated. A bell founder spends a lifetime fiddling with the timbre, or overall sound quality, of his or her bells. There are many factors to account for, as more than just shape influences how a bell sounds. Other determinants may include the composition of bronze (the proportion of tin to copper), casting temperature, external ornament and artwork (like inscriptions or filigree), or damage from elements like water and fire. Each may subtly or dramatically influence the strike tone.
Soft notes, intense flavors, layers of complexity, and a lingering finish – the strike tone of a bell is comparable to a fine wine. We like to spend an evening with both.
