In a 30' x 30' frame building, which motion detector type is least effective?

In a 30' x 30' frame building, which motion detector type is least effective?

• A. Photoelectric
• B. Passive infrared
• C. Microwave
• D. Ultrasonic

Answer: (C)

Microwave motion detectors are least effective in a small, cluttered room because they rely on transmitting microwave energy and analyzing Doppler shifts from moving objects. In a compact 30' x 30' space, the waves bounce off walls, furniture, and fixtures in many directions, creating complex interference patterns called multipath. That multipath makes it hard to distinguish real movement from reflections, leading to unreliable detection, more false alarms, or missed movement in certain zones. By contrast, passive infrared detectors sense body heat and work well in typical indoor spaces; ultrasonic detectors use sound waves that can cover open areas reasonably well and aren’t as prone to the same reflection problems in a small room; and photoelectric beam detectors (when used as beams) require a clear line across the space, which can be arranged for perimeter coverage but isn’t inherently better for reliable interior motion in a tight room.

Microwave motion detectors are least effective in a small, cluttered room because they rely on transmitting microwave energy and analyzing Doppler shifts from moving objects. In a compact 30' x 30' space, the waves bounce off walls, furniture, and fixtures in many directions, creating complex interference patterns called multipath. That multipath makes it hard to distinguish real movement from reflections, leading to unreliable detection, more false alarms, or missed movement in certain zones.

By contrast, passive infrared detectors sense body heat and work well in typical indoor spaces; ultrasonic detectors use sound waves that can cover open areas reasonably well and aren’t as prone to the same reflection problems in a small room; and photoelectric beam detectors (when used as beams) require a clear line across the space, which can be arranged for perimeter coverage but isn’t inherently better for reliable interior motion in a tight room.