Physical Modelling

Calculate the resonance of strings and tubes in real time from physical rules.

Related

TimbreClassical GuitarViolinContemporary

The question

Without sampling or sines, can a synth sound like a string or tube? Physical modelling records no sound; it writes the mechanism as a real-time model.

How it works

Most models split into excitation and resonator. For a plucked string, a noise burst enters a delay line; the delay length sets pitch, and a low-pass in the feedback path sets how fast highs die away, the basic Karplus-Strong form. Wind instruments use waveguides to model pressure in a tube, where length sets pitch and openings shift resonances. Damping, tension, breath and body filtering refine the result. Closer models must handle nonlinearity and stability.

Settings and what you hear

More damping shortens the tail, like a hand muting a string; less damping lets it ring. Excitation decides pluck, strike or breath; body resonance sets low-mid weight. Tension and stiffness brighten the attack; breath adds air to a wind model. At extremes, a model can self-oscillate or run away.

What to listen for

Hear nylon and muted guitar alternate, noting the hardness of the attack and the length of decay. Then hear the shamisen, short and tense, with a clean tail. Finally hear the koto, where pressing a string changes pitch and resonance together, a detail samples struggle to reproduce.

Common misconceptions

That physical modelling is sampling, when it computes a mechanism; that a model is more real, when wrong parameters make it faker; that it only handles acoustic instruments, when cabinets and metal plates can be modelled too.

Sources

• Julius O. Smith, Physical Audio Signal Processing — waveguide and Karplus-Strong models checked, not quoted
• Curtis Roads, The Computer Music Tutorial — physical modelling overview checked, not quoted

Used for fact-checking only; all prose is original.