Scientists make animated collisions sounds realistic

August 21, 2012 By Bill Steele
The sounds accompanying all these computer-generated events will be more realistic with now sound-synthesis software developed at Cornell.

( -- Bang. Clatter. Tinkle. Jingle. When solid objects collide in the real world we hear a sharp impact sound, sometimes followed by a ringing aftershock. Creating sounds like that to accompany computer animation has long been a challenge. In a movie, you can plug in pre-recorded sound -- although it doesn't always fit perfectly. In virtual reality for games or training the sound must be created on the fly, based on what's happening in the animation.

Cornell have devised a new method, based on an overlooked bit of physics, to make these sounds more realistic. The results were reported at the 39th International Conference and Exhibition of and Interactive Techniques (SIGGRAPH), Aug. 2-9 in Los Angeles, by Doug James, associate professor of computer science, and graduate students Jeff Chadwick and Changxi Zheng.

The usual approach is to use the to calculate how objects will vibrate when they collide with the velocity and (imaginary) mass portrayed in the animation, and how those vibrations would produce in air around the object. James, a specialist in computer-generated sound, has been using that approach, but he hasn't been happy with it. It just didn't sound right. "We realized that just simulating the vibrations of objects to get the sound was flawed," he said. With large objects like a bowl or a garbage can, the ringing sound tends to be the dominant contribution, he explained. With small, rigid objects like or rolling dice, the ringing sound can be too high-pitched for human hearing, and the algorithm would generate no sound at all.

What's missing, James explained, is acceleration noise. When rigid objects collide, as Newton's third law of motion tells us, there's an equal and opposite reaction: The objects are briefly accelerated back the way they came, pushing back at the air behind them and creating a we perceive as sound. With large objects the ringing noise from vibrations can overshadow acceleration noise, he noted, but adding acceleration noise still increases realism.

Physicists have already worked out equations to calculate how much the acceleration pulse would be. The sound it creates depends on the shape of the object. A flat coin falling on the floor makes a different sound from a ball bearing. With a large, hollow object like a mug or a bowl, there is some unique acceleration noise from the inside as well as the outer surface. To make real-time computation possible, the Cornell researchers give the shape to the computer in advance so it can pre-compute how sound will propagate from that object, and then simply plug in the information about the acceleration pulse to synthesize the sound at runtime.

Using the familiar physics of how sound travels through air, a "virtual microphone" can be placed anywhere -- usually the point from which the action is seen. Place two microphones and you get stereo sound.

The work was supported by the National Science Foundation, fellowships from the Alfred P. Sloan Foundation and the John Simon Guggenheim Memorial Foundation, a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship, and donations from Pixar and Autodesk.

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1 / 5 (1) Aug 21, 2012
An energy distrubution solution - as far as information is concerned.
Barnett solved this in 1962 and submitted his paper to the American Journal of Acoustics Society.

I'll cite the exact source with the next posting here.
And of course Mindy Loo Garber's MIT paper submitted to fullfill her Master's science requirement - written in 1982 with Fortran.

'Computing Modelling of a Vibrating Piano String'
Your welcome. :)
not rated yet Aug 21, 2012
Sounds nice -- but links to example audio files would be cool.
5 / 5 (1) Aug 21, 2012
They're easy to find at Doug James website.
not rated yet Aug 24, 2012
I would be interested which equations they used. In my PhD thesis in 1984, Noise emission from rock impacts, I developed a computer algorithm to do just this. So I would be interested to see who's work they quote ..

Dr A Doyle
1 / 5 (1) Aug 30, 2012
Journal of Sound and Vibration (1972)20(3), 407-417
On Vibrating Strings and Information Theory
T.W. Barrett

Computer Modeling of a Vibrating Piano String
Mindy Lu Garber
Partial Fulfillment towards mechanical engineering Bachelor of Science.
June 1982

Standing in the shadow of giants and following the footprint paths they took.

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