[mrb]

I was in Hawaii 2 weeks ago, and they were broadcasting these types of alerts to my girlfriend's phone (Android 4), to warn about a risk of flash floods. Android was even using text-to-speech to read the alert out loud, and automatically, which woke us up during the night!

[ConstantineXVI]

Think how exactly the alerts happen is up to the OEM, beyond 'loud noise'. The iPhone uses something similar to the EBS tone in 1s bursts (it was 1AM when mine fired, so I can't say with certainty if it was the same tone or not). I've heard similar from other Android users, but I've shut off the alerts on all my phones since.