From: Bret <>
>
> --- Walter Knapp <> wrote:
>
>>>5. Why does a dish amplify a freqency at 1/8 wavelength?
>>
>>I think the problem here is a error in thinking about sound waves.
>>Sound
>>waves in air are not transverse, but longitudinal. And they are the
>>movement of particles that have mass. Working from a longitudinal
>>waveform with such particles, a reflecting surface down even to
>>molecular size will reflect some sound. We cut off what we call
>>amplification when the effect the dish produces becomes too small for
>>easy measurement. It's a arbitrary point.
>
>
> I agree.
> With sound waves, it is the movement of compression and rarefaction of
> air molecules that are longitudinal waves.
>
> I say this so that someone doesn't confuse what Walt said to think that
> air molecules are moving from the sound source to the ear or mic. They
> are not. The air molecules move to a degree longitudinally
> (compressing the air in direction the sound is moving), and then the
> air molecules move backwards (rarefaction) in a cyclic fashion. The
> compression wave travels through the medium, but the air molecules
> themselves do not travel the distance.
Actually we have a name for when the air molecules travel long distances
on a path, instead of just moving back and forth a little as they do
with sound, it's called wind.
It's the distance between air molecules that determines if the air is
compressed or not, there is no separate air to compress, only the
molecules.
This is very clearly graphically demonstrated and explained in the link
I gave. Anyone who is still confused, take a look:
http://www.gmi.edu/~drussell/Demos/waves/wavemotion.html
There is a even more simplified animation here:
http://www.cbu.edu/~jvarrian/applets/waves1/lontra_g.htm
I should note that a call from a animal propagates in a dispersing
pattern, the first link above contains some more patterns linked off it.
And there are even more if you start from it's base page though not all
are sound:
http://www.kettering.edu/~drussell/demos.html
Even though these patterns are more complex, it's still longitudinal
waves for sound.
Walt
________________________________________________________________________
________________________________________________________________________
|