→Harmonic partials in bells
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 (depending on the quality of the bell).
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 Taylor as well as 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 (exhibited by a wow-wow sound as the bell's sound decays).
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 during manufacture.
== Ghost Tones ==