How to fade in an audio effect?

I need a part in my audio to gradually get more muffled? I know how to make it muffled by not how to fade it in? I don’t know if anyone can help me with this or if it’s even possible.

crossfade between muffled & non-muffled versions

Thanks that worked perfectly.

Crossfade (Crossfade Tracks - Audacity Manual) works much better for me. Crossfade gives a smooth transition rather than an abrupt jump.

@ merlinpendragonx

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