為何超聲波運動感應器可探測範圍不能太遠/太近

Ultrasound motion detector (超聲波運動感應器) emits and receives ultrasound wave (超聲波), which is mechanical wave (機械波) travelling in air. Because of air absorption (空氣吸收), the reflected wave signal (反射波訊號) from a moving object is weak and hence the detector cannot be used on long distance (unless the wave is of a very high intensity, like that used in submarines).

If the moving object is very close to the detector, the time lag between the emitted and received ultrasound signals is extrememly small, and which may not be accurately resolved by the detector.

太遠就感應唔到啦,但係太近都唔可以,因為感應器探測範圍要有一定既空間,唔可以太前。