A sonar-based contactless monitoring system comprises a sonar system (308), a contactless sensing assembly (310), and a controller (302) configured to read out measurements transmitted by the sonar system and the contactless sensing assembly and calculate posture and activity of a subject. The sonar system may include a microphone (314) and a speaker (316), wherein the microphone is configured to sense a first acoustic signal in a frequency range associated with the sound and/or motion made by the subject, and a second acoustic signal in a frequency range associated with the reflection of an acoustic signal transmitted by the speaker. The contactless sensing assembly senses at least one of vital and environmental conditions, such as a heart rate, respiratory rate, activity, snoring, subject's position, and subject's movement, or noise level, weather condition, light exposure, time and radiation level.