MadSci Network: Physics

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

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 

     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 
already, waiting to be released.

     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. (Encyclopedia 
Britannica) gives a good description here:,5716,74210+1+72323,

Additionally, they discuss sound waves and our sensitivity to pressure 

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.

Steven Miller
Undergrad - Mechanical Engineering
San Diego State University

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