When designing an intrusion detection system, you want to cover all points of potential penetration. By that, we mean you want to identify every opening in the structure and give the alarm panel some way of determining when a person has used that opening to enter. This is how the alarm panel knows to trigger sirens or dial central station.
The two most common types of building penetrations are doors and windows. We have discussed methods in other videos for monitoring doors by mounting a magnetic contact on the frame and leaf of a door.
In this video, we discuss monitoring windows using glassbreak detectors >>
In many cases, the fastest and easiest way for an unauthorized user to gain entry to a structure is to simply shatter the glass in a window and walk or climb straight in.
One of the ways to detect such an intrusion is with an acoustic glassbreak detector. An acoustic glassbreak detector has a microphone inside, which listens for the sound of breaking glass.
Glass makes a unique sound when it breaks. Bosch acoustic glassbreak detectors have built in analytics, allowing them to distinguish between the sound of breaking glass and other similar sounds like jangling keys for example. This helps cut down significantly on false alarms.
Glassbreak detectors need to be mounted in the spot where they will be most reliable. Remember, the detector uses a microphone to pick up the sound of breaking glass and analytics to distinguish glass breaking from other kinds of noise. Therefore, mounting the detector too far away or mounting it in a spot where it can’t hear the glass renders the entire exercise pointless. So, let’s talk a little bit about mounting.
First, the best mounting spot is 10 to 20 feet from the glass, in a straight line with the center of the glass, on a wall opposite, and facing, the glass. You can also mount a detector on the ceiling over the glass.
The detector MUST be no more than 30 degrees from the center of the glass in order to be protected.
The detector covers a bubble 25 feet across. A good way of visualizing this is to tape a 25 foot long string to the spot where you plan on mounting a detector. If you can touch every part of the glass plate, the window is protected. If you cannot, you need to find another spot to mount the detector. If you have multiple sheets of glass to protect, the string test may end up showing that you need two or more detectors to cover all the windows. Remember that the detector needs to be within 30 degrees of the center of the glass to be effective. If you’re looking at floor plans in some kind of CAD program, it’s much easier to calculate the number and locations of glassbreak detectors, using a virtual “string”.
If you do need to use multiple detectors to protect the same plate of glass, like in applications with a huge picture window longer than 20 feet for example, space the detectors evenly apart, with not more than 20 feet of distance between them.
There are lots of factors that can reduce the detector’s effectiveness. Basically, anything that can prevent a true sound signal from reaching the detector should be avoided in the design phase. Materials that can absorb sound like carpeting, drapes, or plants will lower the range of the detector. Never use a glassbreak detector if there is an obstruction between the glass and the detector.
- Do not mount the detector closer than 5 feet from the glass, or the detector will not be able to properly evaluate the sound of the glass breaking.
- Do not mount a glassbreak detector on the same wall as the glass being protected- sound doesn’t travel that way.
- Do not mount the detector on hard, sound reflecting surfaces like marble or ceramic tile.
- Do not mount a detector near a vent. If cold or warm air will blow on the detector, find another mounting spot.
Try not to use a glassbreak detector in an acoustically “live” environment. Areas subject to significant echo areas with high ceilings and lots of tile or marble or stone are not ideal locations for acoustic glassbreak detectors. High temperatures and humidity actually affect the way sound waves travel, which may confuse the analytics and generate false alarms. For both of these reasons, kitchens and bathrooms are not ideal spots for glass break detectors.
Most importantly, a glassbreak detector can only detect the sound of shattering glass caused by an impact. It will not alarm on a gunshot if the glass does not shatter. It will not alarm if the glass shatters spontaneously without an impact. It will not alarm if an unauthorized user cuts or removes the glass. For these reasons, glass break detectors work well when used in conjunction with motion detectors.
Use glassbreak detectors to get an alert instantly upon an intrusion and use motion detectors to cover travel paths or areas of interest, so that an intruder that manages to evade the glassbreak detector can still be picked up as they move around inside the building.
As you can see, a glassbreak detector can be extremely useful when installed properly and worthless when installed improperly. Bosch makes the DS1110i glass break tester. This helpful tool mimics the sound of breaking glass and allows you to confirm that the area of detection covers the entire pane of glass.