UK Registered Charity Number: 1154107
Conservation by Re-use
Helping churches acquire surplus and/or redundant bells to be hung for
English-style full-circle bell-ringing.
Sound of Bells – Loudness
As the sound intensity is dependent on the surface area of the bell it follows that smaller bells have a lower intensity. This would be a significant problem if it were not for the human ear's frequency response. For sound pressure levels (SPL) between 40-80dB (approximately equivalent to the sound levels of talking and that of a passing car) the human ear has an equal loudness/ frequency response which increases from 20Hz to a peak at 450Hz, then decreases to 1200Hz where it once more increases with a peak at about 4000Hz8. For ringing bells with a strike note less than 450Hz (a rough approximation is bells of weight 8cwt or more), the relationship shown in equation 3 gives a reasonable equal loudness response. For ringing bells with a strike-notes between 450-1200Hz, the sound intensity needs to be increased for the bells to be clearly heard.
The relative amplitude of each partial is dependent on the location of the impulse. The normal strike point is on the soundbow, but totally different and often un-harmonious sounds can be heard by striking the bell in different places. A typical example would be striking the bell on the waist whereby the Quint and Upper Third partials would be very strong and the resulting sound quite unpleasant. The Quint partial has little, if any bearing on the sound if the bell is struck on the soundbow.
The relative amplitudes are also dependent on the clapper mass, material and contact time. A shorter contact time results in stronger high-order and weaker low-order frequencies, conversely a longer contact time results in the high-order frequencies being dampened. Increasing the mass of the clapper should result in a longer contact time and a change of clapper material to one with a smaller elasticity moduli will also result in a longer contact-time.
To increase the sound strength the diameters of the lighter bells are increased; Sometimes those of the tenors are reduced to decrease their intensity. To compensate for an increase in diameter the thickness has to be increased otherwise the bell will have a lower note than required – see equation 1. Although this is usually adequate for rings of eight or less, the proportionate increase of thickness for lighter bells in rings of ten or higher may result in tonally poor bells.
One of the problems is by increasing the thickness more energy is required to sustain the vibrations and as a result the decay time is shorter and the higher partials are attenuated giving an inferior sound. To compensate the proportion of tin to copper may be reduced from the usual 23-24% to 20% to produce a softer alloy. Another technique is to increase the weight by increasing the height of the trebles (elongating the waist) but this does have the disadvantage that it alters the harmonic spectrum. The graph shows typical diameter to frequency curves together with the linear relationship as per equation 2. It can be seen that the bell diameters have a linear relationship until a strike note frequency greater than 440Hz, where they deviate. The deviation curve, which is directly related to the increasing thickness scale, is usually a straight line to avoid a bell having a disproportionate size and weight when compared to its neighbours.
The graph gives examples of different weight profiles ranging from Canterbury Cathedral (also Llandaff cathedral), Stockton-on-Tees (also Ormskirk, Dalton-in-Furness, St Dunstan-in-the-East, Tewkesbury Abbey) and finally Highbridge (also Footscray).
Diameter/ Frequency relationship
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