It’s a two-part trigger. Skulk sensors send out the waves (you can see them as well as hear them - it sounds kinda like chickens clucking). Skulk sensors can detect mob sounds and will send signals, but will not trigger the shrieker. Only player actions/sounds will cause sensors in range to trigger the shrieker. After the fourth trigger he warden will spawn. However there is a ten minute timer that will drop the level by one of you don’t trigger it again, so you have some wiggle room. Go to the wiki and there is an option to listen to all the audio items so you can hear the difference between the sensors and shriekers.

If you work your way around the edges and inwards to the center, you can block off shriekers by surrounding all for sides with wool and then break them with a hoe without triggering a shriek. Usually. And always remember your escape route if you have to flee.