Different Words, Same Feature
Different monitors describe this feature with different words but they all refer to the same thing: microphone sensitivity, background noise filter, sound sensitivity, sound-activated, or voice-activation (VOX). We refer to it as a "Sound Activation feature" in our reviews.
While to a 1st time parent of a newborn this feature may not seem important, it can play a key role in allowing you to sleep better. Why? Because most parents will want the monitor set to relatively loud volume to assure they hear it (and wake up) when baby cries. But, with the monitor volume turned up loud you'll invariably hear a background hiss that sounds like a radio tuned between stations combined with whatever subtle sounds exist in baby's room. This background noise can be very annoying and prevent you from getting much needed sleep. With the sound activated feature enabled, that same background hiss is reduced to absolute silence which really help you fall asleep. Yet, when baby cries, or a significant noise occurs in the room, the monitor kicks-in within seconds and you can hear everything going on in the room.
Turning on/off Sound Activation is Critical
Activation Sensitivity Level
The very best monitors, like the Philips Avent DECT, give you adjustable control over the sound activation's sensitivity level from the parent unit. This can be very helpful in practice to allow you to adjust so as to barely tune out background noises, yet assure that the slightest sound above the background level comes through on the monitor.
Many of the lowest cost baby monitors offer no sound activated feature. We recommend you avoid them. We've listed which monitors offer the feature in our comparison table. If you value your sleep, put this feature on your must-have list.