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Glassbreak
sensors
Glassbreak
sensors "listen" for the sound of breaking glass
to detect an intrusion. The glassbreak sensor has a small
microphone that picks up audio from the room and feeds into
a signal processor designed to listen for the "audio
signature" of breaking glass.
There
are two typical uses for glassbreak sensors. They are sometimes
used as early detection devices. For example, they might be
installed in the window of a retail store so that the alarm
would be triggered when a thief broke the window rather than
waiting for the motion sensor to be triggered when they actually
walked into the store. Glassbreak sensors are also used when
designing systems that can be fully armed even when people
are still present inside the building. An example of this
would be a large office building where a lot of staff work
after hours at random times. With a typical system using motion
sensors, it is very difficult to determine when the alarm
system can be turned on because you have to check the entire
building to make sure you are the last person to leave. In
buildings of this type we sometimes use a combination of door
switches, glassbreak sensors, and card access to create a
system where the facility can be secured and still be occupied
by staff 24 hours a  day.
The
original glassbreak sensors simply listened for the loud,
high frequency sounds typically associated with breaking glass.
These were very prone to false alarms as there are many other
things that can generate these audio frequencies. Newer sensors
use more sophisticated signal processing to look at the shape
of the audio waveform to determine the frequencies generated,
the rise and fall times, amplitude, and other factors to determine
if the sound was most likely caused by breaking glass.
Despite
the improvements in technology glassbreak sensors still have
a significantly higher rate of false alarms than other sensors
in certain situations. In particular installation near windows
adjacent to busy streets can be problematic as they are in
close proximity to sounds created by emergency vehicle sirens,
screeching tires, and people banging on the windows. There
are a large number of factors that affect the sound of breaking
glass, including the size of the window, the type of object
that struck the window, the type of glass and whether any
"window film" was in place, whether the windows
are covered by curtains or blinds, and what the acoustic properties
of the room are. Each of these factors can influence the likelihood
that the sensor will trigger if the glass is broken and there
is no way to fully test the sensor (without smashing all of
the windows in the building!). For these reasons glassbreak
sensors should be used with caution, and a clear understanding
of their limitations.
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