The Lowpass Resonance Q (short for Lowpass Resonance Quality Factor) determines how much the filter’s self-resonance is dampened. Higher lowpass resonance quality indicates a lower rate of energy loss, i.e., the oscillations die out more slowly.
The Audio Low Pass Filter has a Rolloff curve associated with it, making it possible to set Cutoff Frequency over distance between the AudioSource and the AudioListener.
Sounds propagates very differently given the environment. For example, to compliment a visual fog effect add a subtle low-pass to the Audio Listener. The high frequencies of a sound being emitted from behind a door will be filtered out by the door and so won’t reach the listener. To simulate this, simply change the Cutoff Frequency when opening the door.