How cry detection works, and when it doesn't
“Cry detection” covers two very different things. One trips on a lorry going past. Knowing which one you have — and which iOS permission it needs — decides whether you get woken by your baby or by a bin lorry.
A loudness threshold is not cry detection
The simple version measures volume: when the room passes some level, alert. It is easy to build and it is why so many monitors cry wolf. A door closing, a car outside, a sibling shouting downstairs, the heating coming on — all of it clears a volume threshold, and none of it is your baby.
Real cry detection classifies the sound, not its level. Slumbi runs a classifier that is looking for the shape of an infant cry, which is why it can stay quiet through a loud room and still fire on a quiet whimper.
Both have their place, and Slumbi shows you both: a live noise meter you can watch and judge for yourself, and cry alerts as a separate thing you turn on. Motion is a third, independent signal, taken from the video rather than the audio.
Why it keeps working on the fallback link
Detection runs on the audio as it is captured, not on a perfect stream, so it survives a drop from Wi-Fi to Bluetooth. That matters more than it sounds: the moment your network gets worse is not the moment to stop listening for a cry.
The setting that decides whether you hear the alert
This is where most disappointment with any iOS monitor comes from, and it is not really the app's doing. iOS has two separate permissions:
- Notifications — the app may send you an alert at all.
- Critical Alerts — the alert may make a sound through the silent switch and through Focus modes, including Sleep Focus.
Without the second one, a perfectly detected cry lands as a silent banner on a phone that is face down in Do Not Disturb, which is to say it lands nowhere. Slumbi asks for both and shows a warning on the parent screen while either is missing — but iOS only lets you grant them, so the last step is yours.
Check the parent phone, not the nursery one. Alerts are delivered to whichever phone is acting as the Parent Unit, so its permissions are the ones that count. Getting this backwards is the single most common "alerts don't work" report we see.
What it will and will not do
Being straight about the limits:
- It detects the sound of crying. It does not diagnose why, and it is not a medical device — nothing here monitors breathing, and no phone app should claim to.
- A cry that starts as a snuffle may take a few seconds to be recognised. The noise meter usually moves first.
- A second child crying in the same room will trigger it. There is no way for a microphone to know which baby is yours.
- If the nursery phone's audio is taken by a phone call or Siri, detection pauses until the interruption ends.
Sensible settings for a first night
Turn cry alerts on, leave motion alerts off, and watch the noise meter for one evening before you decide anything. Most people find the meter alone tells them what their room's baseline is, and then they know whether they want the motion alert as well or whether the cat is going to set it off.
Slumbi can also respond by itself instead of only telling you: auto-soothe starts a lullaby or white noise the moment a cry is detected, and escalates to alerting you only if the crying continues. For the stirring-in-the-night case that is often all that was needed.