Monday, June 14, 2010

An Interesting Experiment

We have all studied resonance at some stage or the other. This interesting phenomenon has loads of practical examples. I wish to list one such fun way of demonstrating resonance which I accidentally discovered.

Now it so happens that everyone in my family has a mobile connection which enables us to talk free of cost to one another. This ill-conceived scheme was perpetrated by some stupid telephone company guy(who is surely out of job now) without the proper awareness of the abuse it can be put to. Needless to say, we have exploited this offer to the hilt and logged millions of hours of telephone calls between the four of us in the family.

So it happens that I was playing with my phone and my brother's one when I noticed the following phenomenon. Try this:
Take 2 cell phones with loudspeaker facility. Make a call from one to the other. Receive the call and turn the loudspeaker on in both or at least one). Bring them together(with the keypads facing each other) and then flip one of them so that the hearing apparatus of one may be adjacent to the speaker of the other and vice-versa(Its 69........for those who know what I mean.. ;)).
If you are done you will notice a ringing phoneomenon in the loudspeaker. The sound will start off as basically noise but gradually shape into sound of a much smaller spectral bandwidth with oscillating amplitude(and hence the ringing). Sometimes it does happen that you will not hear a sound. In that case make some sound near the pair of phones and this should start-up the phoneomenon.

My Explanation( which may not be right)

The cell phones act as amplifiers here. Flipping one and placing them close is like cross coupling 2 amplifiers akin to many of the oscillators we study in standard text books. The communication channel serves as a frequency selective attenuating filter here. By communication channel I mean the wireless media,repeaters etc which are part of the mobile service provider's setup. Thus when there is certain amount of noise introduced in this system the noise travels in a loop infinitely .....each time being attenuated by the channel and amplified by the cell phone. In the long run only those frequencies which are passed by the channel remain and hence the final oscillation (in steady state) comprises of them.

P.S. : for those who did understand what I meant kindly point out if I am mistaken. For those who didnt just try the experiment and have fun.

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