Some mini-rings are build from ceramic bells, otherwise known as clay bells or flowerpots. These are commonly available, cheap, light in weight and have a distinct musical note.
Here is a youtube video of the clay bells at Clay Bottom, Bristol recorded by Nick Bowden. The first peal of clay bells was probably the one at Liss Campanile in Hampshire, constructed in the early 19080s. Carillons of porcelain bells cast by the Meissen factory date back much further. Here is the porcelain carillon at the Fraunkirche, Meissen created in 1929. The Meissen factory produced a quantity of porcelain carillons and chimes. Carl Scott Zimmerman’s website has a comprehensive list.
This page documents a brief investigation into the acoustics of clay flowerpots. The instruments investigated include 11 flowerpots and planters in my local garden centre, five pots of various shapes in my garden, and six peals of flowerpots:
the Clay Bottom ring linked above
the installation at Aylsham Bellflower Mews
the campanile at Branston, Lincolnshire
the Dunkirk Red Light ring, probably the same pots moved from Clay Bottom
the campanile at Chedburgh, Lincoln
the campanile at Overton, Hampshire.
A total of 56 pots were investigated.
Some of the author’s ceramic bells.
The investigation involved measuring the partial frequencies of all 56 pots, estimating their strike pitch by comparing the sound with a sine tone, and measuring the diameter, rim thickness and height of the garden centre and garden pots. The diameters of those measured ranged from 11.4cm to 40.4cm and the notes ranged over a little more than 2 octaves.
Here is a typical spectrum of a flowerpot. Despite the range of sizes and shapes, including pots with thin and very thick rims, all the spectra look similar, with, in order from the lowest, two single partials, a block of three closely spaced partials, and then a set of wider spaced higher partials. Here are a typical set of partials and intervals, for a pot 15.2cm in diameter:
| Frequency (Hz) | Interval to lowest (cents) | Ratio to lowest |
| 610.5 | 0.0 | 1.00 |
| 1659.5 | 1731.2 | 2.72 |
| 2988.0 | 2749.3 | 4.89 |
| 3069.0 | 2795.6 | 5.03 |
| 3180.5 | 2857.4 | 5.21 |
| 4094.5 | 3294.7 | 6.71 |
| 4801.0 | 3570.3 | 7.86 |
| 5584.5 | 3832.0 | 9.15 |
| 6194.0 | 4011.4 | 10.15 |
| 8437.5 | 4546.5 | 13.82 |
The partials do not form a harmonic series, suggesting that flowerpots do not give rise to a virtual pitch. This is confirmed by estimation of the strike pitch. The pitch of all the pots tested is determined by the lowest partial alone. Here is a plot of the estimated strike pitch against the lowest partial frequency showing the close correspondence:
The frequency of the lowest partial is determined primarily by the diameter and rim thickness of the pot. A regression of log(frequency) against log(diameter) and log(thickness) gives a formula f =20575 * d-2.296 * t0.8654 where the frequency f is in Hz and the diameter d and thickness t are in metres. When the diameter increases, the frequency goes down. When the thickness increases, the frequency increases. So tuning a flowerpot by filing or grinding the rim will lower the frequency, just as with a bronze bell. A regression of frequency against diameter alone gives a good fit for 12 of the 16 pots for which measurements were available. The remaining four pots had very thick or very thin rims.