Abstract: Damage to sheet glass is detected by monitoring with a piezo-electric sensor vibrations at ultrasonic frequencies propagated in the glass. Cutting, scoring or chipping of glass generates a frequency spectrum having high frequency components at greater than 100 kilohertz and particularly at about 300 kilohertz. By selective detection at these frequencies cutting, scoring and chipping are distinguished from lower frequency vibrations due to wind and traffic rumble. Cutting and scoring are distinguishable from chipping because the high frequency components occur in numerous bursts in the former against but few bursts in the latter. Shattering of the sheet glass is detected by the high energy of much larger amplitude high frequency components produced thereby. Apparatus to carry out these functions is described and finds particular use in an intruder alarm system.