An active volumetric sensor emits energy (such as sound waves or electromagnetic fields) into a defined volume and detects changes caused by movement or presence within that space. Ultrasonic sensors are a common example:
* They transmit high-frequency sound waves and measure the reflections to detect objects or intruders in a monitored area.
* Infrared sensors are often passive (detecting heat) or line-of-sight, rather than volumetric.
* Capacitive sensors detect changes in an electric field but are typically used for proximity or tamper detection.
* Vibration sensors detect mechanical disturbances rather than volumetric intrusion.
Active volumetric sensors are widely used in intrusion detection systems for rooms, corridors, or other enclosed spaces because they monitor an entire volume rather than just a single point or surface.
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Before an enterprise risk analysis can be properly conducted, the security professional must first identify the assets to be protected. These may include people, property, information, and operations. Only after defining what needs protection can threats, vulnerabilities, and risks be effectively evaluated.
A (Identify threats) comes after asset identification.
C (Classify risks) and D (Determine security adequacy) are later stages in the risk management process.
References:
PSP Study Guide - Enterprise Risk Management Process
POA Manual - Asset Identification and Risk Assessment

Alarm monitoring systems are categorized into central station systems, local systems, proprietary systems, and auxiliary systems. Each system involves different methods of signal transmission and monitoring, including third-party monitoring centers or direct links to authorities.
References:
ASIS International, Protection of Assets (POA), Chapter: Alarm System Technologies ASIS PSP Study Manual, Domain 2: Physical Security Systems