What is physical modelling synthesis?
Physical modelling synthesis creates sound by simulating the physics of vibrating objects. It models an excitation, such as a pluck, strike or breath, and the body that resonates in response, so the sound behaves the way a real instrument does as you play it.
How it works
Most physical models divide an instrument into two parts. The exciter puts energy in, as a bow, hammer, mallet, plectrum or breath does. The resonator shapes that energy, as a string, membrane, tube, plate or body does, each with its own natural patterns of vibration, called modes.
Waveguide synthesis models a vibrating string or air column as waves travelling back and forth along delay lines, losing a little energy each time they reflect. Modal synthesis describes a body by its modes, each a decaying sine wave with its own frequency and damping. Mass-spring and finite-element methods go further, simulating the material itself point by point.
Because the model responds to how it’s excited, playing harder, softer or in a different place on the virtual instrument changes the sound as it would on a real one. Models can also go beyond anything that exists, with strings made of glass, bells as large as buildings or materials no instrument maker could work with.
Voices
The voice can be modelled in the same way. Glottal oscillators model the vocal folds as they vibrate, a model of the vocal tract shapes that vibration as the throat, mouth and lips do, and formant synthesis sculpts the resonances that make each vowel. Breath and consonant sounds add air and articulation.
What it sounds like
Physical modelling excels at expressive acoustic-style instruments, such as plucked and bowed strings, mallets, bells, flutes and drums, and at sounds that respond to every nuance of how they’re played. It also makes instruments that couldn’t physically exist, with the same natural response.
Key terms
- Exciter
- The part of a model that adds energy, such as a pluck, strike, bow or breath.
- Resonator
- The vibrating body that shapes the exciter’s energy, such as a string, plate, tube or membrane.
- Mode
- One of the natural patterns in which an object vibrates, each at its own frequency.
- Damping
- How quickly vibrations lose energy and die away.
- Waveguide
- A model of waves travelling along a string or tube, built from delay lines.
- Formant
- A resonance of the vocal tract that shapes a vowel.
In The Resonant Star
The Resonant Star is Xionia Audio’s physical modelling synthesizer. It’s designed around the relationship between excitation, resonance and expressive control, and models struck and resonant bodies and expressive, human-like voices, shaped from breath, vocal folds and a modelled vocal tract without relying on vocal samples.