Abstract: A method and apparatus for detecting leaks within a vacuum system by narrow band detecting and amplifying a particular ultrasonic frequency peculiar to such leaks is disclosed. The device is formed as a hand held probe having a transducer and amplifier both peaked to the particular frequency sought along with a visible display and audible alarm for indicating the level or intensity of the particular frequency sought and audibly notifying the user when that sound level reaches some prescribed intensity. Appropriate shielding and filtering to prevent spurious actuation of the device are also disclosed.