Arnold Ambient Occlusion tutorial
From TOI-Pedia
Rendering with ambient occlusion
Assign an AiAmbientOcclusion shader to all objects (geometry) in your scene.
Samples
Controls the number of rays. Increasing the number of samples will reduce the noise and increase the quality.
Falloff
Controls the exponential falloff rate for the occlusion along the ray distance. Increasing the falloff rate will decrease the width of the 'shadow' in the corners. The number depends also on the size of the objects in your scene. Try different settings to get it right for your scene.
Source
https://support.solidangle.com/display/A5AFMUG/Ambient+Occlusion