### Re: why does a balloon make a loud bang sound when popped

Date: Tue Sep 26 13:02:44 2000
Posted By: Steven Miller, Undergraduate, Mechanical Engineering, San Diego State University
Area of science: Physics
ID: 967419117.Ph
Message:
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Hi Lizzy,

The question you ask is a very good one - and I couldn't find any
resources to answer the question directly for us so we'll have to do it
the hard way.

Our ears detect sound, but sound is actually a wave of pressure which
is transmitted through the air around us.  As the wave encounters our ear
it transmits pressure changes to our inner ear.  Our ear is very, very
sensitive and can detect extremely small changes in pressure.  It
interprets these pressure changes as the sounds we hear.  The higher the
pressure (or the harder it pushes against our inner ear), the louder the
sound.

Normally some mechanical action creates the pressure waves we hear
(much like when you move your hand [a mechanical action]in a swimming pool
or bathtub and create a wave in the water).  Some examples of mechanical
actions that create sound waves in the air are vibrating violin strings,
the impact of a hammer against a nail or wood, or the vibrations of our
own vocal cords.  In the case of the popping balloon no mechanical action
is required because the pressure wave is bottled up inside the balloon

A balloon contains pressurized air which presses out against the
sides of the balloon, stretching the sides and allowing the balloon to
keep its nice shape.  The more air we put into a balloon, the harder we
have to blow (the air already inside the balloon is pushing out against
the air we want to add to the balloon).  Because the air on the inside of
the balloon is always pushing out in all directions, when we pop the
balloon this pressure escapes.  This high pressure "wave" would travel
away from where the balloon was to our ear.  Our ear would sense this
change in pressure - in this case a relatively large change in pressure -
and interpret it as a loud sharp "POP" sound (remember, the higher the
pressure the louder the sound we "hear").

Another interesting version of this problem is what causes the
thunder we hear after lightning strikes.  Britannica.com (Encyclopedia
Britannica) gives a good description here:
http://www.britannica.com/bcom/eb/article/0/0,5716,74210+1+72323,00.ht
ml

Additionally, they discuss sound waves and our sensitivity to pressure
here:
http://www.britannica.com/bcom/eb/article/5/0,5716,117555+2+109557,0
0.html

Some of the math is difficult but the pictures of wave representations may
be helpful.  Acoustics is a very challenging area of engineering but can
be a lot of fun.  Try to learn all the math you can while you are in
school because math really is power!

I hope this helps.

Sincerely,
Steven Miller
San Diego State University

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