### 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
Message:
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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
balloon)

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.

-Aurelio R. Ramos

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