Tips for Installing and Testing Acoustical Glass-Break Detectors
Find out why acoustical glass-break detectors will sometimes fail to trigger alarm systems and how to ensure successful deployment.
Recently, I ran across a news report about the alarm system of Fran & Joe Martin. They had come home to find their residence burglarized. Windows had been broken and several acoustical glass break detectors (AGBD) failed to trigger their alarm system. Instead, the main entrance door sensor detected the burglars and set off the alarm as they were leaving the house.
What made this report interesting is that I was able to observe the news report video of the broken windows and AGBD devices. When the AGBD devices were tested with a hand clap, the device’s LED blinked and the microphone appeared to be working. The big mystery is why they did not detect the windows being broken? To get a possible answer we must look more closely at AGBD devices – how they work, how to install them and how to test them.
Simply put an AGBD is an electronic alarm sensor that is designed to detect the sounds made from the breaking of a glass window. While devices may vary from manufacturer to manufacturer, most typically detect a sound signature. This often includes the sharp, brittle sounds we all commonly hear, along with inaudible lower frequency or infrasonic sounds. The sound sensor needs to identify a special pattern of both frequencies.
You have heard it before: The first thing you must do BEFORE installing an AGBD device is to carefully read the installation directions and understand what is expected. While most sound detectors work in a similar manner there can be important variances that need to be observed. Remember, AGBD devices may need to only work once during the course of many years to trigger an alarm system.
Realize that there are different types of glass that make different sounds when broken. Many AGBD devices have settings for different types of glass. The most common in the home is plate glass. However, you may have laminated safety glass around entrance ways as this is often required by building code. The same can hold true of large sliding glass doors, which are often tempered glass. Also, be careful if film type security glazing is applied to the glass as that will change glass-breaking sound signature. Make sure AGBD devices are adjusted and positioned, accordingly. If you have any questions, contact the manufacturer.
Simply put, AGBD devices rely on a certain loudness to activate. Many people do not realize that the loudness of the sound from the breaking window is not linear with distance, but, as with many things in nature, it is logarithmic. Sound intensity versus distance follows what is called the Inverse Square Law. So the loudness or sound press level (SPL) is one-fourth at twice the distance from the sound source and drops to one-sixteenth at four times the distance. You get the picture.
Also remember that window coverings, such as our window blinds example above, curtains, carpeting, furniture, etc., will block and absorb sound waves. Make sure to test AGBD devices when the rooms and windows are furnished.
Locate the AGBD devices per the manufacturer’s instructions. Some allow the AGBD to be installed on the wall next to the window and others do not. Nearby on the ceiling is often selected, but again check the specified distances to the window and allow for obstructions. Locations can vary so review closely.
How do I know if I have picked a good location for the AGBD? One of the best ways is to test the device. Many units today allow for a hand clap test. However, do not confuse that with a complete glass break test. The hand clap test often is just a test to let you know the battery is still good and the microphone is listening.
You may need to use the particular manufacturer’s AGBD tester to properly test for glass breaking activation. These testers simulate and create different types of breaking glass sounds so adjust yours accordingly. Make sure to place the tester behind window blinds or drapery and have them pulled closed when testing. Also make sure if you test an AGBD in a new construction environment to come back later and test when the carpets and furniture are installed.
You may also notice that when you test with a hand clap you will see the LED trip but the alarm system loop is not activated. When you test with a tester you should see the alarm system triggered and an alarm signal sent to the central monitoring station to confirm as well. Call the central station first to let them know you are testing the alarm system.
A couple extra words of caution when installing AGBDs: You can have issues with false alarms, especially in commercial locations. You may need to adjust sensor locations since a simple tapping of a quarter on the glass can activate a AGBD alarm.
Something else you will not read about the manufacturer’s documentation is the case of sound saturation. This is when the sound of the breaking window is so loud that it overwhelms the AGBD and it does not activate. This could happen in the instance of a large commercial display window. I have seen many devices in the past not function in this type of saturation breaking sound. Yes, we broke very large display windows.
A professional alarm tip: It is good practice to not rely solely on one source of intrusion detection such as the AGBD. While this device can deter further penetration of the premise’s perimeter, the installation of a strategic passive infrared motion (PIR) detector can back up a missed AGBD activation. It can also help confirm a possible AGBD false alarm when the PIR is tripped from someone walking around in the facility or dwelling.
As we can see there is more to installing an AGBD than just sticking it up anywhere. So take time to read the manufacturer’s directions, and carefully install and test the device.
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