A door contact knows one thing with near-perfect reliability: whether two parts are touching. Nothing else can convince it otherwise. A glass break sensor listens to a room and makes a judgement, and every strength and every weakness it has comes from that difference.
Understanding which one you are installing is the whole job, because they protect against different events and they fail for unrelated reasons.
Two completely different mechanisms
The contact on a window is a magnet and a reed switch. The window opens, the magnet moves away, the circuit opens. There is no ambiguity, no tuning, and essentially no false alarms. What it cannot do is tell you anything about the glass itself. Slide the sash and it triggers. Smash the pane without opening the frame and it stays silent.
The glass break sensor is a microphone plus a decision. It has to hear an event and decide whether that event was glass breaking. That decision is the product, and it is made under conditions the manufacturer cannot control: a room with a television, a kitchen with dishes, a house with a dog.
Why the good ones need two sounds
Breaking glass makes two distinct sounds in quick succession: the low-frequency thud of the impact, then the high-frequency shatter of the pane letting go.
Cheap detectors listen for the shatter alone and are easy to fool, because a dropped mug, a set of keys, or a film soundtrack contains plenty of high-frequency transients. Better detectors require both components inside a short window, which removes almost all of those.
That two-stage requirement is the single most useful specification on the datasheet. If it is not stated, assume the simpler design.
What still fools them
Even a good detector has a list, and it is worth knowing before mounting one.
Dogs bark in the same band as the impact component, and a large dog standing near the sensor is a recurring false alarm source. Dish and glassware noise in a kitchen is the classic one — which is why a sensor pointed at a kitchen sink is a bad idea. Balloons, and children's toys with sharp transients, pop convincingly.
The fix is placement rather than settings. Point the microphone away from the kitchen and the television, keep it out of reach of a dog, and remember that coverage is a cone, not a sphere — the microphone has a preferred direction, and mounting it facing a wall or behind a curtain costs you most of the room.
What makes them miss
Undetected breaks are rarer than false alarms but they are the ones that matter, and they cluster around three causes.
Distance and soft furnishings. Heavy curtains, a bookshelf in the line, or a sensor at the far end of a long room all attenuate the high-frequency component first, which is exactly the component the decision depends on.
Glass type. Laminated glass, which is increasingly common in ground-floor windows, does not shatter. It cracks and sags, and the acoustic signature is nothing like the one the detector is listening for. Security film does the same thing. A window that is deliberately hard to break through is also hard to detect breaking.
The third cause is coverage arithmetic. One detector is rated for a room, and a house with six windows on two floors needs more than one. The rating assumes line of sight to each pane.
Neither one replaces the other
The most useful way to think about it is by event.
An intruder lifts a window that was left unlatched: the contact fires, the acoustic sensor never will. An intruder breaks a pane to reach the handle and is interrupted: the acoustic sensor fires, the contact never will, because the frame was never opened.
Running only one of them leaves a real gap. That is the argument for contacts on the opening frames that matter and acoustic coverage in the rooms with the most glass behind them, rather than choosing between the two.
What to ask before buying
Four lines settle most of it. Is detection dual-stage, requiring both impact and shatter? What is the coverage distance, and is it stated as line-of-sight to the pane or as room area? Is sensitivity adjustable, and in how many steps? And how is it tested — a real tester that emits both components, or a phone app that plays a shatter sound?
That last one is a practical trap. Playing a recording of breaking glass into a single-stage detector proves nothing about how it will behave with a dog barking in the room.
Acoustic and contact sensors are both covered in the sensors and alarms range, and the detection platforms behind them are described on the door and window sensor manufacturing page.
Programme details including certification and MOQ are on the factory site, and specification questions go to the factory team.
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