Sensorship That Makes Sense

January 31, 2008 SSI Staff

The sound of breaking glass is very complex and consists mainly of two key parts: the normal higher frequency crashing sound we all hear, and a lower infrasonic sound shock wave that is created by the flexing of the glass when broken. Together, both sounds create a unique sound signature that is specific to the type of glass being broken, such as plate, laminated, film coated, reinforced (wire mesh) and tempered (safety).

While these types of sensors are very versatile, they need to be installed closely following the manufacturer’s guidelines. There is always a fine line of distinction between configuring glass detectors for good detection and minimal false alarms.

One technical item learned very early on when installing acoustical glass-break detectors is that sound, barring some echoing, is very directional. The Honeywell FG 1025Z with “time of arrival” (TOA) signal processing of the glass-break sounds has two microphones, and due to the directional nature of sound, it can tell if the sound came from the expected window protection area or another nonalarm area. This is due to how the sound wave arrives at the microphones mounted on opposite sides of the sensor.

Ultrasonic, Microwave, PIR Sensors Guard 3-Dimensional Areas

The primary application intrusion sensors are designed to handle is detecting suspicious activity in three-dimensional environments. These security devices are often referred to as “volumetric” detectors. Through the years, three main volumetric detection technologies have prevailed: ultrasonic, microwave and PIR.

Ultrasonic — These intrusion sensors detect motion throughout an area using what is referred to as the doppler effect. These devices send out a high frequency sound wave that is just above the human hearing range. As these sound waves bounce around objects and walls in an area, they set up a signature pattern. When a person walks through the area, the sound wave pattern changes with the reflection off the moving person and is detected by the sensor’s electronics.

Due to the development of newer technologies, ultrasonic-only motion sensors are almost nonexistent today; however, the technology is still used for building automation, mainly light control and automatic door openers. One of the main problems with this technolog
y was adjusting and positioning these devices so that certain environmental conditions would not cause false alarms.

However, the little-known upside to ultrasonic-only sensors is that they can completely fill an enclosed area with irregular boundaries, and depending on how well the sound patterns might fill that area, the devices could be used to detect the slightest movement.

Microwave – These motion detectors work similar to ultrasonic technology; however, the energy transmitted is at a considerably higher frequency and is sometimes referred to as “radar” technology for detecting movement. Again, once very popular, just a few microwave-only intrusion sensors can still be found now.

Positioning of these sensors must be carefully planned as they can detect through a building’s walls and windows. It is best to position a microwave-only detector to look down at an area of detection. If not properly positioned, reflected energy from nearby personnel and vehicular traffic can trigger a false alarm.

There is, however, an upside to using microwave-only detectors in high-security applications. Because the technology can detect movement through walls and ceilings, the device can be positioned in a stealth mode so that personnel cannot visually detect its presence. GE Security’s SRRCRxx Series can be optioned for mounting in the radar-only high-security stealth mode.

PIR – This is another very popular type of volumetric detector. This device is typically only passive and establishes a complete pattern of lookout and down sensors that can detect the heat differential from a human body walking at a certain rate through the detection fields. A PIR-only detector, as with other sensors, can have false alarm activations from problematic heat sources if not positioned properly. The device can also be susceptible to being compromised in high-security applications by physically covering the PIR elements.

One technique to reduce PIR false alarms is to establish an alarm circuit in which several PIR detectors need to be tripped simultaneously to be considered an actual alarm. This is sometime referred to as “cross-zoning” and is a standard feature in UL-Listed CP-01 alarm controls. Small objects such as rodents or insects might trip one PIR sensor but not both.

An interesting product that uses this application is the new BX-80NR outdoor PIR detector from Chino, Calif.-based Optex. This sensor uses dual beams to provide perimeter intrusion detection.

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