MadSci Network: Physics

Re: Is it air pressure or sound absorbtion?

Date: Wed Mar 5 11:16:05 2003
Posted By: Aurelio Ramos, Grad student, Computer Engineering
Area of science: Physics
ID: 1045716493.Ph

When holding a balloon near a person's ear, the person cannot feel a 
change in pressure, since the pressurized air is contained inside the 
balloon. So the only principles in action are sound absorption, 
reflection and diffraction.

Absorption is when energy from a wave is lost as heat as the wave 
encounters a boundary.

Reflection is when the wave energy is not absorbed, and instead bounces 
off the surface.

As a guideline: Hard smooth surfaces (like a tiled bathroom wall) reflect 
almost all sound energy. Softer and flat surfaces (like wood walls) can 
absorb low frequency energy because they can flex a bit, and rough 
surfaces (such as fibrous material, hair, etc) can absorb high 
frequencies very well.

Diffraction is when a wave changes direction as it encounters an obstacle 
whose size is comparable to the wavelength. So, given an obstacle, like a 
balloon, wavelengths much longer than the balloons diameter are sure to 
be diffracted (to change direction as to make it all the way around the 

A balloon alters sounds approaching the ear in three ways:
1. Absorbing energy of waves that hit it, mostly midrange frequencies
2. Diffracting waves that go around it, mostly frequencies around and 
under 1kHz (about one foot of wavelength)
3. Reflecting energy of waves that hit it. Mostly very high frequency.

1. Because of its surface elasticity, waves of medium frequencies can be 
absorbed at the surface, and the energy disipated as heat.

2. Waves of low frequencies have no trouble at all passing around the 
balloon's curvature, and are relatively unaffected

3. Waves of very high frequencies are partially reflected at the surface 
of the balloon and bounce at an well defined angle, just like light 
bouncing off a mirror. This is because, to high frequency waves (with 
very short wavelengths) the balloon seems like a pretty flat wall. 
However, because of this mirror like behavior, no high frequency waves 
due to reflection are ever heard, because the human ear happens to be the 
one thing directly in front of the balloon, and the human ear will not 
produce any significant sound (*note: the human ear is capable of making 
very weak sounds under laboratory conditions)

So, from this information we know that the balloon will form a pretty 
good filter letting all frequencies under about 1kHz pass unaffected 
(assuming a 1 foot diameter baloon), and all frequencies above will be 
absorbed or reflected away. This will give a person a very weird 
sensation, and some people might even describe it as a "pressure", simply 
because people tend to describe new experiences in terms of familiar 
ones. But in fact, people could not feel the pressure *inside* the 
balloon, for obvious reasons (the balloon surface is in the way), and the 
pressure outside the balloon is always the atmospheric pressure, the 
balloon does not affect it.

Hope this answers your question!

your mad scientist,

-Aurelio R. Ramos

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