# Resonating glasses

When we collide two glasses they produce sound like not from one collision but from multiple. And frequency changes over time. Is this because they resonate with each other? But how this resonase happens?

• What happens, if you hear two frequencies, which are similar? – Semoi Dec 4 '19 at 21:06
• If resonance happend then how would changing the frequency? I think if resonance happend then it should be a particular frequency.If the collision make change the structure i.e change the resonant frequency .Then Why are you so sure about resonance? – baponkar Dec 5 '19 at 3:11
• "not from one collision but from multiple" Do you get this mostly when you collide them very gently? "frequency changes over time" A glass will vibrate with multiple frequencies ("harmonics"). The higher ones will die down quicker. Could that be what you are hearing? – Keith McClary Dec 5 '19 at 3:32

I believe the question can be rephrased (more clearly) as follows: "when we collide two glass balls (or two steel balls), we hear multiple collisions. The intervals between collisions change from long to short. Why?"

Take a look of this video https://www.youtube.com/watch?v=k1id4a4EU4M (Pay attention to time 1:01) when he casually collided the balls you hear such a phenomenon.

You can reproduce the phenomenon with two cups, to bowls, etc. Just hold them loosely in two hands so they naturally droop under your hands. Now move them close to make gentle collisions.

Why? Because when we collide two very hard objects, the first collision bounces both of them back a little bit so they separate from each other, only to allow the moving hands to push them back in to make the second collision. As they are pushed closer and closer, the intervals between collisions are shorter and shorter.

A vibrating object like a wine glass or a bell is a very complicated system. It is possible for a wine glass, for example, to exhibit multiple resonance modes in which the modes are weakly coupled i.e., energy in one mode is slowly shared with other modes. This means the quality of the perceived sound will shift perceptibly on a timescale of ~seconds.

Note also that the initial strike which sets the glass vibrating is a sharp impulse function which contains many harmonics. This means that many of the fundamental modes of the glass may get excited by the strike.

These two things mean that the decay of the sound emitted by a struck glass will be correspondingly complicated!