Synthesizing Sounds From Physically Based Motion

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Synthesizing Sounds From Physically Based Motion

• Authors J. Obrien, P. Cook, G. Essl
• Presented by Ricardo Lent

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References

• Synthesizing Sounds from Physically based motion.
  J. OBrien et al. ACM SIGGRAPH 2001
• Animating Fracture. O'Brien, J. F., Hodgins, J.
  K., Communications of the ACM, July 2000, Vol.
  43, No. 7, pp. 68-75.
• Graphical Modeling and Animation of Fracture.
  Ph.D. Thesis J. OBrien (Chapter 3), GIT. August
  2000

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Sound Modeling

• Acoustic waves (longitudinal waves)

20-20K Hz
Moderate intensity
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Paper Idea

• Approximate sounds generated by the motions of
  solid objects.
• Based on previous work in animation using
  deformable objects.

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Sound Generation

• Analyze motion and surfaces.
• Determine how the motion will induce acoustic
  pressure waves in the surrounding medium.
• Computes propagation of waves to the listener to
  generate sounds corresponding the objects.

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1. Motion of Solid Objects

• Basic criteria
• Temporal resolution 20 - 20Khz (approx. time step
  10e-5 sec)
• Dynamic deformation modeling (elastic
  deformation) in addition to rigid body motion
• Explicit representation of the objects surfaces
• Physical Realism

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Physically Based Modeling and Fractures

• Numerical simulations of physical systems to
  generate synthetic motion of virtual objects
• Model for object deformation

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Continuous Model

• Describe behavior of material as it deforms
  (based on continuum mechanics approach)
• System determines fractures

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Material World Coordinates
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Continuous Model

• Strain measures total local deformation of the
  material
• Greens strain tensor (invariant to rigid body
  transformations)

Strain tensor
Strain rate tensor
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Strain Tensors

• Strain strain rate tensors provide information
  about internal elastic and damping forces.
• Tensors are invariant to rigid body
  transformations and orientation of material.
• They do not account for properties of the
  material.

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Stress Tensor

• Stress tensor combines strain and strain rate
  with material properties (elastic)
• Material isotropic

rigidity
resistance to changes
Elastic relationship
Damping properties
Kinetic energy dissipation rate
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Traction _at_ any point
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Mapping

• Finite element method to map an object from some
  reference state to its deformed configuration
• Mapping uses tesselating tetrahedral elements

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Tesselating
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Finite Approximation

• Tetrahedral 4 nodes (position in material and
  world coordinates, and velocity
• Continuous functions for strain, strain rate and
  stress are described at the nodes
• These forces determine the accelerations, and the
  motion of the deformable object

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2. Surface Vibrations

• Pressure for ds of ?

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Discretization

• Approximate pressure field as being constant over
  each triangle in ?

Pressure fluctuation
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Removing Aliasing Artifacts

• Low-pass filter
• DC-blocking filter

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Digital LP Filter
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Windowing
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3. Wave Radiation and Propagation

• Pressure wave from ? sum results of many simple
  waves (Huygens principle)
• Pressure wave from a single triangle

- Spherical wave - No reflection/diffraction
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Delay

• Sound speed ltlt light speed
• Doppler shifting
• Stereo sound using multiple receivers
• Delay is computed in addition to s and stored

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Examples
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Clamped Bar
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Square Plate
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Spectrogram for Plate
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Real vs. simulated vibraphone
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Swinging Bar
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Final Comments

• Time step may be too small for fast rendering
• Rigid body simulations are excluded for sound
  generation
• Reflected and diffracted sound is not considered
• Listener is omni-directional
• Model is also useful for debugging