Modal Synthesis
The material is the spectrum. A bell sounds like a bell because of its specific inharmonic partial ratios and decay rates. You don't synthesize a bell by adding harmonics. You choose the right mode ratios and excite with an impulse.
Material = Spectrum
Hit a wooden bar and a metal bar with the same stick. Different materials produce different mode ratios and decay rates. The excitation is the same; the resonator defines the sound. A string has harmonic modes (1×, 2×, 3×...). A circular drumhead has inharmonic modes (1×, 1.59×, 2.14×...). A bell has its own unique set. Change the ratios and you change the material.
Mode Inspector
Each mode is an independent resonator with its own frequency, amplitude, and decay time. Higher modes typically decay faster. This is why a struck bell starts bright and ends with a pure fundamental hum. Solo each mode to hear what it contributes, then listen to all of them combined.
Try it: select a material and hit Trigger. Use the mode solo buttons to isolate individual partials. Try "Tubular Bell" and solo modes 1 through 5. Notice how the inharmonic ratios create the metallic shimmer.
References
- Vibrational modes, Wikipedia
- Physical modelling synthesis, Wikipedia
- Physical Audio Signal Processing, Julius O. Smith, Stanford CCRMA