Why is the sound produced at one end of a hollow iron pipe heard twice at the other end?
Answer: The sound produced by striking one end of a hollow iron pipe with a hammer travels through the iron of the pipe and through the air inside the pipe. The speed of sound is different in iron and air. The speed of sound is faster in iron than in air. So the first sound comes through iron and the other comes through the air. This results in two sounds being heard at the other end of the pipe.