On a Roll

by Randy Thom
It’s a little-known fact that the sound of a rolling object is caused by impacts with the surface it is rolling on.
The more frequent the impacts, the smoother the sound. This is even the case with a hard sphere rolling on a rigid, flat surface. Though if it’s a near perfect sphere and the surface is near perfectly flat and hard, the sound generated will be minimal, because the impacts are minimal.
How is knowledge of this phenomenon useful to sound artists?
Let’s say you’re trying to produce the sound of a walnut rolling on the ground. Literally recording that action can render something useful, but realizing that what is really making the sound is impacts, AND DOESN’T REQUIRE THE WALNUT TO BE ROLLING AT ALL, opens the door to creating a more tailored, stylized sound that can be far more expressive and “musical” than what you are likely to obtain by literally rolling the walnut across a surface.

How Do You Simulate a "Roll"

To simulate the rolling sound while giving yourself a lot more control: don’t roll the walnut. Simply tap it on a given surface several times. This simulates what is physically happening during a roll. The taps should usually vary in intensity, the intervals between the taps should vary, and the distance from the microphone should vary, so that the result will sound naturalistic.
To give it more character and make it more “musical,” tap the nut on a series of different surfaces to simulate a more complex-sounding journey. You would have to do lots and lots of takes to get a similarly diverse and interesting a sound by merely rolling the nut.
By the way, varying how tightly you hold the walnut as you tap it will render a wider variety of sounds. Holding it more loosely will allow it to resonate, like a loosely held wine glass will ring way longer than one that’s held tightly.
For the Super Mario movie, I edited a sequence of exaggerated punch sounds to create the sound of Donkey Kong rolling himself rapidly along an i-beam. I’m using a similar approach for a fun sequence in the upcoming “Ultraman: Rising” movie.
This same approach applies to using library sounds to simulate a rolling object. The sound of a giant boulder rolling can be created by editing a series of explosion attacks closely spaced. The editing can either be done manually on a timeline (which is the way I usually work) or by using a sampler. Be sure to quickly fade down the decay of each explo so that it doesn’t mask the attack of the next explo. The SOUND IDEAS libraries are filled with great material for building this kind of sound design. Happy Rolling!