Bell Harmonic Partials

Any one of a series of tones which usually accompany the prime tone (fundamental) produced by a string, an organ-pipe, the human voice, etc. The fundamental is the string tone produced by the vibration of the whole string, or the entire column of air in the pipe; the partial tones are produced by the vibration of fractional parts of that string or air-column. Harmonic tones such as these are also obtained, on any stringed instrument which is stopped (guitar, violin, zither), by lightly touching a nodal point of a string.

Partials in Bells

Various types of bell sounds are differentiated by their partials. You will see in bell (and competitive literature) various ways of illustrating this. Some use a musical staff with middle C being the fundamental, and the partials being shown above and below C.

It has been established that there are five harmonic partials that impart a basic recognizable bell tone. These are named:

1. Hum
2. Prime
3. Minor Third or "Tierce" (Major third bells have seen some limited production)
4. Perfect Fifth or "Quint"
5. Nominal or "Octave"

The Hum, Prime and Nominal should be the same note in their respective octaves. The strike tone of the bell (apparent musical pitch) is always one octave below the Nominal, but not necessarily in tune with the Prime.

For many years this basic bell theory was lost until being rediscovered after World War II by an English clergyman named Canon Simpson, who worked with the Taylor and Gillett and Johnston bell foundries to improve the sound of their bells. It is therefore sometimes called the Simpson five-tone harmonic principle, even though we know that 16th century Flemish founders François and Pieter Hemony made use of the principle, but failed to teach it to their successors before their demise.

A bell's strike tone is what determines the actual pitch of the bell, and is often influenced by higher partials. In some bells, it may be a difference frequency between the nominal and octave above the nominal that occurs in the listener's ear. Especially in larger bells, if the higher partials are not also tuned, there may be beats.

Some campanologists think that the Quint is so short lived (in decay time) that the octave quint is more important for tuning (especially in larger bells).

Smaller bells will often only exhibit two to four partials that need tuning, simplifying the process to the point that an automated lathe can be employed.

Ghost Tones

The strike tone is not directly measurable but is definitely perceptible. Generally it can be found by setting an oscillator to match the perceived note of the bell. Multiply the oscillator's frequency by two and you will generally find that the result matches the measured frequency of the Nominal partial. We have tested this theory even on really bad sounding bells. It somehow always works.

When the ear-brain hears combinations of high tones, we think we hear a single lower tone. For instance a combination of a Nominal with an octave-Quint sounds like a single tone lower than them both. This explains why Big Ben may sound to most listeners like an A even though it was cast to be an E.