Physical modelling synthesis is the synthesis of sound by using a set of equations and algorithms to simulate a physical source of sound. Sound is then generated using parameters that describe the physical materials used in the instrument and the user's interaction with it (e.g., by plucking a string, covering toneholes, etc...)

For example, to model the sound of a drum, there would be a formula for how striking the a drumhead injects energy into a two dimensional membrane. Thereafter the properties of the membrane (mass density, stiffness, etc...), its coupling with the resonance of the cylindical body of the drum, and the conditions at its boundaries (a rigid termination to the drum's body) would describe its movement over time and thus its generation of sound.

Similar stages to be modelled can be found in instruments such as a violin, though the energy excitation in this case is provided by the slip-stick behavior of the bow against the string, the width of the bow, the resonance and damping behavior of the strings, the transfer of string vibrations through the bridge, and finally, the resonance of the soundboard in response to those vibrations.

Although physical modelling was not a new concept in acoustics and synthesis, having been implemented using finite difference approximations of the wave equation by Hiller and Ruiz in 1971, it wasn't until the development of the Karplus-Strong algorithm, the subsequent refinement and generalization of the algorithm into the extremely efficient digital waveguide synthesis by Julius O. Smith III and others, and the increase in DSP power in the late 1980s that commercial implementations became feasible.

Yamaha signed a contract with Stanford University in 1989 to jointly develop digital waveguide synthesis, and as such most patents related to the technology are owned by Stanford or Yamaha.

The first commercially available physical modelling synthesizer made using waveguide synthesis was the Yamaha VL1 in 1994.

While the efficiency of digital waveguide synthesis made physical modelling feasible on common DSP hardware and native processors, the convincing emulation of physical instruments often requires the introduction of non-linear elements, scattering junctions, etc... In these cases, digital waveguides are often combined with FDTD, finite element or wave digital filter methods, increasing the computational demands of the model.

Examples of physical modelling synthesis:

• Karplus-Strong string synthesis
• Digital waveguide synthesis
• Formant synthesis

References

1. L. Hiller and P. Ruiz, "Synthesizing Musical Sounds by Solving the Wave Equation for Vibrating Objects," Journal of the Audio Engineering Society, 1971.
2. K. Karplus and A. Strong, "Digital synthesis of plucked string and drum timbres," Computer Music Journal, 1983.
3. J.O. Smith III, "Physical Audio Signal Processing," August 2004 Draft, [1] (http://ccrma.stanford.edu/~jos/pasp/)

External links

• Julius. O Smith III's A Basic Introduction to Digital Waveguide Synthesis (http://ccrma-www.stanford.edu/~jos/swgt/swgt.html)
• Music synthesis approaches sound quality of real instruments -- Stanford University's 1994 news release (http://www.stanford.edu/dept/news/pr/94/940607Arc4222.html)