Table 2 - Comparison between device-free sensors Type Piezoelectric * easy to extract data * detection computationally simple * arrangement of sensors is important for higher accuracy * impractical for some activities due to vibration Pyroelectric Infrared (PIR) * usually array of PIR is needed * easy to use * noise sensitive * low range of detection Infrared Array * detection in darkness * mostly one sensor is adequate * preserves privacy (low resolution) Acoustic * can be used in darkness * not affected by dust or high-moisture environment RFID * sensitive to mounting angle * only distance of the nearest object can be measured * array of tags is needed * variety of activities can be detected * high accuracy * adjustment is needed in different environments * computationally complex; feature extraction needed * reduced performance in complex environments Wi-Fi * cost-efficiency: almost everywhere is already equipped with Wi-Fi * high accuracy * acceptable performance in complex environment 48 * computationally complex; feature extraction needed * sensitive to the noise interference * adjustment is needed in different environments * fine-grained health-related posture detection in sitting and standing in different styles [13] * sensitive to thermal noise * dementia detection by travel patterns classification [8] * recognition of health-related actions such as walking and sitting [9] * fall detection [10] * recognition of health-related actions such as standing, sitting and falling by audible sound [11] or ultrasound [12] * low price (5pcs IRA-S230ST01 are about $3) Advantages Disadvantages Usage in well-being assessment * fall detection monitoring system based on sensitive floor equipped with piezoelectric sensor [6] footstep pattern identification by smart flooring [7] * low price (10pcs 35 mm ceramic piezoelectric discs are about $10) Typical Price ($US) * low cost (AMG8833 8×8 infrared array sensor is about $27) * low cost (HC-SR04 is about $4) * high cost (Impinj speedway R420 UHF RFID reader (4 ports) is about $1585 - RFID antenna is about $150 - Passive RFID tag (Monza R6) is about $50) * real-time fall detection system [14] * human respiration monitoring [15] * low cost (typical wi-fi router is about $50 - Intel Wi-Fi Link 5300 card is about $20) IEEE Instrumentation & Measurement Magazine September 2021