Acoustic Sensors

As home security devices, acoustic sensors shine as “glass break detectors,” focusing in the aural aspect of things, sensitive to sound condition changes in a specified detection area environment.

The fist generation glass break detectors (not utilizing the benefits of acoustic sensors), consisted of metal foil encasing circuits wrapped around (or adhered) to the designated glass to watch over. In the event of glass rupture (in other words, something breaks the glass), the circuits in the foil would trigger the alarm system, detecting the glass break. This was the older version for sound and motion detection.

You could imagine just how troublesome setting this up is, which actually was, considering one would have to physically stick the alarm to the glass (window, doors, etc). Also requiring the device precise adherence to the window surface came to be unappealing aesthetically. In addition, a separate device has to be installed for every window pane (or door, or whatever needs to be monitored).

An advance in technology sired the birth of smaller, more non-noticeable glass break devices. Commonly called bugs or piezos, these devices are sensitive to vibrations. Obvious logic would dictate that in the event of a glass break, the sensors would register vibratory changes, thus activating the alarm system.

The downside to these devices, though more appealing compared to its bigger “eye catching” predecessors, is that they register every form of vibration, leading to false alarm scenarios.

Acoustic sensors stood to be another advance in technology for this type of security device, and to date, are the best as glass break monitors. Acoustic sensors are calibrated to detect the sound of breaking glass, and are a lot easier to install, considering that only one sensor is needed for a single room, eliminating the hassle of sticking one device per window (or door). As the sound from the breakpoint travel outward at a high frequency, the sensors would eventually register this, and transmit the sound to a filter which triggers the alarm, thus functioning as intended.

First generation glass break detectors (utilizing acoustic sensor) had a detection area of thirty five feet. Though they still called similar false alarms (as the Piezos did), the devices could be pre-programmed to “ignore” reoccurring sounds which posed as no threat to security.

A more recent advance in the field of glass break sensors combines two types of sensor technology, eliminating false alarms. These sensors utilize a microphone (for aural detection, which the first generation acoustic sensors ran on) and a filter attuned to two sounds. A close examination of the physics involved in a glass break would reveal a process. When an object strikes glass, a relatively dull, low frequency sound of a flex wave emanates from the breakage point, outwards. From then, a five kilohertz high frequency sound follows. With this breed of acoustic sensors, the two sounds are required to be detected in order for the system to detect the sound as an actual glass break.

Though quite effective, acoustic sensors are not infallible. Items like window shutters, blinds or drapes may obfuscate sound and inhibit detection. As easy as they are to install, the act of doing so may entail compromising aesthetic considerations for one’s home, but are not as limited with Piezo sensors.

Background noise, like radio interference, running machinery, or sudden impact noises may also trigger false alarms. These sensors also loose optimum operational performance if the glass is cracked.

Glass cutting instruments, and the act of cutting the glass, circumvents these sensors’ detecting system. Muffling the break sound also causes the system to fail.

The utilization of these devices must come along with knowledge of these limitations, and the use of them in conjunction with other security devices would increase the effectiveness of one’s home security system.