Answer to Question #63684 in Other Physics for aastha thapa
people usually prefer knocking at the door instead of speaking to call someone inside a closed room .why ?
If a man speaks to call someone inside a closed room, then his friend does not hear him because most of the sound waves is reflected and the energy of his sound is not enough to cause vibration of the door. As a result, very less sound passed through the door which causes very less air to cause vibration which may or may not be perceived by person inside room. But, when we knock at the door, the door is set into vibration which causes the air of the closed room to vibrate and the sound can be easily heard by person inside room. Hence, people usually prefer knocking at the door instead of speaking to call someone inside a closed room.