Abstract: A burglar alarm system of the non-wired perimeter protective type for detecting and indicating break-ins at an entrance or opening, such as a window or door, of a building, employs as a sonic signal generator a transducer that is both triggered and powered by the break-in. The sonic signal is of a specific selected supersonic frequency and is received by a remotely located local receiver within the building which in turn may actuate an alarm. Protection against false activation by sonic noise is provided by altering the gain of the receiver in response to received sonic noise adjacent to but different from the selected frequency, so that noise containing even a strong signal at the selected frequency, as well as at adjacent frequencies, does not result in an alarm being raised, but a weaker signal at the selected frequency, without signals at adjacent frequencies, such as is produced by the transducer, does result in an alarm being activated.