In a spiral galaxy in Universe Sandbox, the spiral arms are created by orienting the orbits of the Galaxy Nebulae objects in the Galaxy to form a spiral structure. To simulate real galaxies with imperfect spiral structures, the orientation of these orbits can be randomly distributed across some range called the Spiral Arm Noise. Galaxies with zero Spiral Arm Noise will have perfect spiral structures, while the orbits of nebulae in a galaxy with a Spiral Arm Noise of 10% will vary as much as 18 degrees from the orientation that would create perfect spiral arms.
The Spiral Arm Noise property is located in the Spiral section of the Galaxy tab in the galaxy's properties panel. This section is only visible for spiral galaxies.