Mysteries in Sound Phenomena

One friend asked, why in the place that is an empty square covered at a sixth floor ceiling and surrounded by classrooms in a college, echos a lot? 

After some discussions, we came to a simple conclusion: they had used that place after college hours, when there is absolutely no-one in the college. During the day-time, when it is flooded with students, it doesn't echo at all! The absence of students here, is giving rise to a lack of any absorbing medium for the sound energy, and it finds it easy to bounce back and forth. This effect can be seen in any empty place, flat, hall, most famous being the hilltops, as the sound seems to come back from distant hills.

Our staircase has hollow pipes as hand railings. I was having fun striking the metal with my metal keys, and it would create quite a sound! I checked striking on different sized pipes, and the sound is so very different! When the metals being stroked the same, why did the sound differ?

The energy of the stroke gives rise to an impulse of vibration in the solid of the pipe. This impulse is a superposition of a huge number of frequencies. These all frequencies get transmitted to the air within the pipe. Due to the size of the pipe, the back and forth oscillations are sustained only at the natural frequencies of that length and radius of the pipe. So the standing waves are dependent on the radius of the pipe, thus giving different sound in different sized pipes.