Swinging Sword and Sound
Name: CJ G.
Why does a sword make noise when you swing it through the air?
Hi, Christine !!
When a sword moves against the air it compresses the
air molecules ahead of it. The air becomes like a compressed
coil, that tends to recover its original state. And it does, in fact !!
When this happens, other molecules of air - also ahead - become
compressed, and tends to recover its original state...and finally
this wave reaches our ear. And you can hear the sound.
This happens in all directions, and the sound wave travels even
with greater velocity than does the sword. And so, it is possible
to hear the sound before the sword reaches the place where
it is going to. In the case of the CONCORDE, this airplane is
so fast, that it can travel ahead of the wave sound !!! Inside the
airplane dominates the silence...
Another interesting point is what happens when a train travels
in OUR direction, and the driver sounds the horn. At first, you
hear a shrill sound (high frequency) and when it GOES AWAY,
the sound is muffled (low frequency). You can understand
frequency, in this case, as the number of waves that reach
our ear in a lapse of time. This is called DOPPLER EFFECT.
And - last but not least - the same happens with the light !!!
In this case, what changes is the colour of the star : shift to red,
it the star goes away; shift to the blue, if the star travels with
high speed to us !! And Hubble - the astronomer - discovered
that the light of the stars tend to ... BLUE, what means, the
universe is expanding. Interesting, isnt ??!!
Without getting into too much of the physics, the sound originates from
The first is the sword compresses the air in front of it and de-compresses
the air behind it. This produces a compressional sound wave. You can do the
same thing with a rope or a long stick. The second is that the sword itself
acts like a "tuning fork" and vibrates perpendicular to the face of the
blade. This sound will depend upon the size and shape of the sword as well
as the material from which it is made.
The sword smashes into air molecules in its way, pushing them out of the
way. When these molecules move back together after the sword moves out of
the way, they bang into each other. All this sudden movement causes
vibrations of the molecules. These vibrating air molecules make the
neighboring molecules vibrate. The vibrations continue on like ripples on
water continue if you tap the surface. These "ripples through the air" are
what make up sound waves.
If the sword moves slowly, the waves ripples slowly and with little energy.
Your ears cannot hear this. As the sword moves faster, the waves ripple
fast enough to sound like a low-pitched hum. As the speed of the sword
builds up even more the rate of rippling (called frequency) gets high enough
to sound like a squeal. Faster swords also produce more volume.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012