[Bell Ringing] If you listen to a church bell,
it always has this sort of a "Wa wa wa wa wa wa" sound to it.
Why doesn't it just slowly get quieter?
That's what this video is about.
But first I'll put this piece of coat hanger wire
in a vise and pluck it.
And you'll notice the vibrations seem to increase
and decrease in strength over time as well.
Why? The key here
is to look at it from the end. And you'll notice that
the vibrations never actually die out, in fact
they're always about the same strength, it's just they change
direction and they change from straight line to circular.
So, if I'm looking at it from just one angle,
sometimes it's vibrating like this, where seeing it from here,
it doesn't appear to vibrate and sometimes it's vibrating
like this where it looks like it's vibrating a lot.
All right, next experiment.
I've set up a simple pendulum here
and the period of the swing of the pendulum is dependent
on the length of the string but the way I have it hooked up
swinging this way, it's pivoting up here,
whereas if I swing it this way it's actually pivoting here
because the strings are tied in the line, like this.
What we're actually seeing is a pendulum swing
that's going this way plus a pendulum swing
that's going this way but they're going
at different frequencies and if we look at an angle,
like for instance from this way,
then we see that some of those which sometimes are in phase
and will be strong and sometimes are completely
out of phase and they'll cancel each other.
And getting back to our coat hanger wire,
same thing is happening here, the frequency of oscillation
this way is different from the frequency of oscillation this way
And if I plucked this wire exactly horizontal
or exactly vertical it won't make these patterns
because it's only been excited in one mode of it's oscillation
at one frequency. I don't have a bell
to demonstrate, but bowls will work as bells quite nicely too.
[Ring ring ring ring]
[Ring ring ring]
We could think of a bowl or a bell
sort of like a cylinder and it can vibrate like this.
Now, this doesn't vibrate very well,
but this is a band saw blade
and it will vibrate quite nicely.
Now you may have noticed, as I was doing this,
the points here and here don't actually move
in and out at all, but I can actually just pluck it
so that those points move the most,
at which point the points here and here and here
don't move in and out, those are actually two separate rows
of vibrations and because of imperfections in a bowl
or a bell those are not necessarily
at the same frequency. Now depending on where
I strike the bell, I may or may not excite
both modes of vibration equally and depending on
where the microphone is, I may or may not pick them up equally
so the amount of "Wow wow wow" will greatly vary
with how and where I strike the bell.
[Ring ring ring ring ring]
Now that's only covering the two major modes of vibration
of a bell, any physical object will have
a lot of modes of vibration, for instance a gong
will have many modes of vibration.
[Bang bang] This isn't actually a gong
It's a stockpot
but a bell tends to have two principal very strong modes
of vibration. [Ring]
This piece of steel pipe is not a bell either,
and you can tell that the two modes of vibration
are only slightly out of tune. For an experiment,
I'll try to make some changes to it to make the two modes
more out of tune than they are.
[Ring ring ring ring ring]
I've drawn four flat spots on the sides
corresponding to the notes of one mode of vibration
and the peaks of another. And, that effects the tuning
of then differently. [Ring]
[Ring]
And, if I strike it between those two spots,
it sounds both notes. [Ring ring]
[Ring ring]
[Ring]
But, the coolest feature is if I hit it to sound both notes,
and then spin it, you can hear the notes alternating.
[Ring]
[Ring ring ring]
[Ring ring]