Paintbrush selection needs an adjustable feathering option
We have Size and Opacity which are fine, but for the purposes of selecting a frequency-steady background noise, it would be much better if the painted selection could be made with a feathered edge, just like a brush in Photoshop, such that any changes made to the selection would decrease towards the outer edges of the selection.
This would allow for smoother changes, as currently a change will effect 100% of the selected area, and anything outside will be 0% changed. This can result in a harsh, unnatural transition between frequencies.

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Ted Satterfield commented
I would say that this should be an option with the Time, Marquee, and Lasso selection tools in Adobe Audition. Being able to feather the outside edges of a selection would be incredibly useful--especially in situations where I am wanting to process an effect in the Effects Rack. Let's say a Reverb effect, with a feathered selection, would fade in instead of kicking on and off. This is for the track Waveform editor, not the Multitrack editor. In Multitrack, it can be ramped on and off using keyframes, but creating a whole multitrack project file just for that purpose is time consuming and feels unnecessary.
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This is sort of what the Opacity control is for. The lower the opacity of the selection, the less processing is applied. I believe there is an "auto-feather" item in the backlog, though. Let me take a look and update it with your comments.