Source:setAirAbsorption
Sets the amount of air absorption applied to the Source.

By default the value is set to 0 which means that air absorption effects are disabled. A value of 1 will apply high frequency attenuation to the Source at a rate of 0.05 dB per meter.

Air absorption can simulate sound transmission through foggy air, dry air, smoky atmosphere, etc. It can be used to simulate different atmospheric conditions within different locations in an area.
Source:setAirAbsorption( amount )
amount number The amount of air absorption applied to the Source. Must be between 0 and 10.