How your security alarm glassbreak detectors work

AlarmTech - How your glassbreak detector works

Glass Break Detectors

The original method used to protect glass required the application of a conductive metallic tape (foil), directly to the glass. The foil was varnished onto the glass 2-3 inches from the edge and glued around the perimeter of the glass. If the glass was broken, the cracks would cause the foil to break. While this was a reliable method of detecting glass breakage, it wasvery labor intensive to install and prone to extensive service from scratches and breaks in the foil.

Today almost all glassbreak sensors are acoustic devices, although vibration detectors are still being used. There are many advantages to using acoustic detectors. They are visually attractive, less expensive to install and maintain, and current detectors are very good at tripping to only the sound of breaking glass. New glassbreak units are less prone to false alarms.

Types of glass:

It is very important when selecting glass protection that the type of glass be considered. Some detectors are designed to only work with certain types of glass and in other cases the type of glass will affect the range and performance of the unit. The most common types of glass are:

Plate Glass: The most common which contains few impurities.

Tempered Glass: Which breaks into rounded grains instead of jagged shards.

Laminated: Resists shattering, generally a composite of two sheets of glass with an intermediate layer of transparent plastic.

Vibration detectors:

Vibration detectors fall into 2 categories; those that mount on glass and those that are frame, wall or ceiling mounted. The glass mount type may be specifically tuned to the vibration of breaking glass or a basic vibration detector. The units that are tuned to sense only breaking glass will not trip if someone simply knocks on the glass. The optimal mounting location for these detectors is in a corner approximately 2 inches from the frame. The reason for this is that the shock waves tend to concentrate in the corners. Basic vibration detectors will sense any vibration within their range. They are often connected to a control/analyzer which allows the installer to control the strength, duration, and time interval that will activate the alarm. These units can also be used to protect wood, concrete or other materials being attacked. The typical range of vibration detectors is a radius of 10 feet, although you should have 1 detector for each window unit being protected regardless of size.

Acoustical detectors:

Acoustical or sound discriminators work by "hearing" the noise of breaking glass. Most units today are tuned to react only to the specific frequency of glass breaking. There are some units still on the market that react to any loud noise. Be aware of the environment that you put an acoustical in. Loud music, high background noise levels, ceiling fans or other machinery (can generate harmonics), and very high humidity such as in bathrooms, can adversely affect the performance of sound detectors. Newer detectors are mostly immune to loud noises unless they are extremely close the frequency of breaking glass. A major advantage of these units is their ability for one detector to protect a medium sized room with several windows. The detection range can extend up to 40+ feet. Window coverings must also be considered, drapes, curtains, and shutters can all affect sound transmission, so it is important that the installer test the units thoroughly.

Dual technology units:

Several manufacturers have combined both vibration and sound detectors into one unit that will not trip an alarm unless both technologies have activated. These units may be used where the normal conditions would cause a single technology detector to generate false alarms or for enhanced false alarm protection. Dual units are very stable and not prone to false alarms.