Ambient noise reduction option missing on iMac MacOS High Sierra

Using the internal microphone for Facetime or Skype conference calls (etc.) on a 27" iMac, the "echo chamber" (empty room) effect (not feedback) is a serious problem, with complaints from attendees.

• I tried setting the "ambient noise reduction" in System Preference->Sound input settings for it, but it is not there.
• I've tried a good external microphone, but unless I put it right at my lips the problem persists. It doesn't look good on conference calls anyway, more like I'm on a game show. I want to use the internal anyway.
• I've disabled Skype's "automatically adjust volume" option, and tried different MacOS input sound levels

How do I enable "ambient noise reduction" on my iMac internal microphone?

I read a 3rd hand comment in one thread that iMac doesn't get that option, but that is ridiculous. I hope.

iMac (Retina 5K, late 2015) running MacOS High Sierra (10.13.4)