Could your Android phone alert you to a knock you didn't hear?

A knock at the door should not require you to keep listening for it all afternoon. I like the idea of a phone turning that sound into something you can feel or see. Android's existing Sound Notifications feature recognizes household sounds, including knocking, doorbells and appliance beeps. Google's current help page lets you choose sound types and vibration or camera-flash alerts. Its 2020 launch account describes machine learning that works offline; this is not a new release or a feature I have tested here. On supported phones, look under Settings → Accessibility → Sound Notifications. Other phones may need the Live Transcribe & Notifications app first. Google's placement advice is wonderfully physical: put the phone in the middle of the room, leave it uncovered and keep competing noise down. Listening can also use more battery. That raises a small household puzzle. The phone needs to be where it can hear the knock, but its alert needs to reach you. A flash in an empty room does not help much. I'd try an ordinary knock with someone at the door while sitting where I actually spend time, then check both detection and whether I noticed the alert. That is a proposed check, not a reliability claim. I would keep this first experiment to a non-emergency sound, not make it my only way to receive a safety alarm. Which sound would you want translated into a vibration or light—and where would the phone have to sit for that to be useful?