What causes sound waves, glass breaking, pendulum clocks and beats?

[nemzy]

what causes sound waves to travel? is it the pressure, or does sound create a vacuum created by air?

Why is it that if you sing at a really high note you can break a glass? Does it have to do something about resonant frequency of the glass?

WHy is a pendulum a good basis for a clock? Is it because a pendulum experiences no damping force?

When two waves of the same frequency combines, then do the beats dissapear?

 

[Gonzolo]

1. Sound travels for the same reasons the whole of a table moves when you push only one end.

2. Exactly. Some crystals have a resonant frequency that the human singing voice can match.

3. Whatever the amplitude or time, pendulum frequency is constant.

4. Frequency has to be absolutely identical for beat to dissapear.