How to Get the Most Out of Your Alarm System’s Glass Break Detection

Learn what might have gone wrong for one Tennessee couple whose glass break detection system did not activate during a robbery.

A couple in Tennessee was asleep when their windows were smashed as burglars entered their home. The damage to the windows was substantial, but the couple did not realize anything had happened until the morning. So why didn’t the home’s glass break detection sensors activate?

First, let’s establish some background. An audio glass break detector is designed and manufactured to listen for the sounds of breaking glass producing a certain frequency, and the microphone built into the intrusion detection device activating once the threshold of this preset alarm condition frequency is exceeded.

Similarly made detectors utilize dual technology, where the detector must sense both ultra-low and ultra-high frequencies within a certain time frame before an alarm condition will be generated. This helps detect when an intruder bangs the glass (an ultra-low frequency) followed by the ultra-high frequency of the intruder breaking the glass.

What’s troubling is the window treatments covering the glass can absorb and/or impair the sounds of breaking glass from reaching the detectors microphone, causing the expected sounds to never be heard by the audio glass break detector. As a result, no alarm condition will be generated. 

The thicker the window shade and/or treatment, the more likely it is that glass breakage sounds of forcible entry will not be heard by this intrusion detection device, which is generally placed on the home side of the protected premises and not in between the glass and the window treatment itself.

If the audio glass break detector is mounted too far away from the glass, or if it is not properly adjusted to its sensitivity to detect, the audio glass break detector will not function as intended and the subscriber will likely never know of this material defect until it is too late.

Detector sensitivity and placement is yet another mission critical factor in being able to reliably allow for this intrusion detection device to hear the sounds of breaking glass over the preset alarm condition threshold, and as a result activate the alarm system. In other words, if the detector is not adjusted properly to its microphone’s sensitivity or if the detector is located too far away from the glass to be protected, forcible glass breakage entry will not be detected.

Subscribers who rely on alarm companies to follow and comply with the equipment manufacturer’s specifications and industry standards are simply not equipped to test these detectors. That’s especially the case if they do not understand or do not have the proper testing equipment to be able to quantify system reliability and its ability to hear the sounds of glass breakage intrusion.

In other words, if the audio glass break detector is mounted too far away from the glass, or if it is not properly adjusted to its sensitivity to detect, the audio glass break detector will not function as intended and the subscriber will likely never know of this material defect until it is too late.

The system has no way to warn the subscriber from its keypads if the detectors has been properly adjusted, placed and tested. Therefore, the subscriber can arm their alarm system and have no idea that these mission critical intrusion detection devices are actually working or not working.

Alarm companies need to fully ensure that what they select and install will function reliably and as intended. Otherwise, the results could have been catastrophic to this family since not one but both of the germane audio glass break detectors at issue were unable to function as intended, and it is an undisputed material fact that the perimeter glass was broken by the perpetrators.

I would be curious to see if the audio glass break detector activates from behind the window treatment using a factory manufactured glass break simulator in order to see how or if the audio glass break detector[s] will activate. I’d also like to see what the distance and sensitivity of each of the audio glass break detectors was set at in order to quantify if the distance, location and sensitivity of same was set within factory specifications. I would try other forensic testing as well in order to further quantify the most likely reasons why both of these detectors failed, which could include what I have elaborated to above and further expand into the way in which these detection devices were programmed by the alarm installer.

Having said that, both of these detectors could be programmed to be shunted when the system is set to stay, all the while the on-board light emitting diodes of the audio glass break detectors would still function, which would have nothing to do with the respective protective loop circuits being bypassed based on the way the system was programmed, or better yet, improperly programmed.

Glass break sensors can warn you when an intruder is in your house, but they can’t warn you if they weren’t installed properly.

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About the Author


Jeffrey D. Zwirn, CPP, CFPS, CFE, FACFEI, CHS-IV, SET, CCI, FASI&T, MBAT, writes Security Sales & Integration’s “Security Science” column. He is also president of IDS Research and Development, an alarm and security consultation, expert witness and training authority providing nationwide services on all issues related to alarm and security matters. He can be reached at (201) 287-0900.

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