The 3F bell designer is inspired by Jean-Claude Risset's early insight that the inharmonic spectra of bells can be considered as chords and used as material for musical composition; chords and spectra are only different manifestations of a single phenomenon. Risset's early description of a computer music "bell" sound is widely available now (for instance in Dodge and Jerse, Computer Music) as are his writings about the use of inharmonic spectra in composition.
Another inspiration is Philippe Manoury's piece, Jupiter, in which, in sections 6 and 12, an oscillator bank plays frequencies that are derived from theoretical FM spectra given various carrier and modulation frequencies. This technique also strives to generate believable and listenable spectra from generating frequencies provided by a composer or other musician.
The 3F bell designer attempts to choose partials to make up an inharmonic spectrum that carries an internal harmony controlled by specifying three component pitches, so that the spectra so created will mix pleasingly with harmonic, instrumental sounds played at those pitches. The three pitches are converted into frequencies (call them f, g, and h) whose linear combinations (f+g, f-2g+3h, etc) become a spectrum. Because spectra created in this way can easily become overly dense, a random percentage (between 3 and 100 percent) of these frequencies are chosen, counting upward from low-order combinations to higher-order ones, until a fixed number of frequencies is obtained.
Although the original three pitches themselves will not in general be prominent in the mixture, they will occur in many possible ways as the difference between two frequencies in the calculated spectrum; this is what makes the resulting sounds fit in with the original pitches, despite the very inharmonic nature of the resulting sounds.
The patch runs in Pd version 0.41 or newer. Select an output level (the dB control in the output~ box, top right; set to 70 or 80 to start with). Then click on any of the "examples" boxes and you should hear the sounds.
To make your own sounds, enter numbers for the five parameters, named pit1, pit2, pit3, prob, and seed. The first three specify (in MIDI units) the three frequencies from which to build the spectra. The "prob" parameter controls the probability of hearing any individual calculated component, in percent; if set at 100, the first 16 computed partials will be taken, if 50, each one will be taken with probability 1/2 (and other, later candidates will be taken to arrive at 16 components in all). The "seed" is 0 or a positive number. If it is positive, it will seed the random choices so that the same spectrum comes out each time; if zero, the seed will be replaced randomly each time.
To actually make the sound, either click on the "trigger" button or turn on the "auto" toggle, so that any changes in parameters automatically recalculate and replay the spectrum.
The toggle "hear pitches" at center top allows you to hear the three component frequencies; these should be reminiscent of the "bell" sounds but at the same time very different.
A pair of pitch shifters is provided to soften the sounds up -- you'll need this if you are spending more than a couple of minutes searching for sounds. They are turned on by default.