Measure noise levels with Apple Watch
The Noise app 
on Apple Watch Series 4 and later measures the ambient sound levels in your environment using the microphone and duration of exposure. When Apple Watch detects that the decibel level has risen to a point where hearing could be affected, it can notify you with a tap on the wrist.
Note: The Noise app uses the microphone to sample and measure the sound levels in your environment. Your Apple Watch doesn’t record or save any sounds to measure these levels.
Set up the Noise app
Open the Noise app
on your Apple Watch.Tap Enable to turn on monitoring.
To measure the environmental noise around you in the future, open the Noise app or use the Noise complication.
Get noise notifications
Open the Apple Watch app on your iPhone.
Tap My Watch, then tap Noise.
Tap Noise Threshold under Noise Notifications, then choose a setting.
View information about environmental sound levels
Open the Health app on your iPhone.
Tap Browse, tap Hearing, then tap Environmental Sound Levels.
For more information about hearing health, see the iPhone User Guide.
Turn off noise measuring
Open the Settings app on your Apple Watch.
Go to Noise > Environmental Sound Measurements.
Turn off Measure Sounds.
You can also turn off noise measuring on your iPhone. Open the Apple Watch app on your iPhone, tap My Watch, tap Noise, then turn off Environmental Sound Measurements.