it be loud enough to wake up someone who sleep with earplug . on occasion the alarm do not go off . I remember one time in particular . something jolt I awake but the alarm be not beep . it go off for a split second at the proper time but do not keep beeping . I remember a few other time in which I wake up late but notice that everything on the alarm be set correctly . sometimes it do not go off , or only go off for a split second . to have the back-light not flicker , you have to press on the big button on the top really hard and put pressure on just the right spot . use it as a back-up alarm .