🚀 go-pugleaf

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In sci.physics, ecmtwhk@ccu1.aukuni.ac.nz (Thomas Koenig) writes:

    Wandering around in the fog recently, I wondered wether the apparent
    dampening of sound was just imagination, because of different visual
    aspects, or real.

    I've got a mechanism for the dampening:  As sound waves move past water
    droplets, the sound - induced speed difference between the droplets
    and the air means there is a resulting force on the droplets because
    of drag effects; they also move a bit, therefore there is energy
    dissipation.

    Influences would be the concentration of liquid water in the air,
    drop size distribution and sound frequency (plus the whole range of
    physical properties of the two substances involved, which depend
    on temperature and pressure).

    Has anybody ever checked this effect?
    Is it really large enough to be noticable?
    Failing that, does anyone have numbers on fog drop size distribution
    so I could do the calculations myself?
    -- 
    Thomas Koenig, ecmtwhk@ccu1.aukuni.ac.nz, ib09@rz.uni-karlsruhe.de
    The joy of engineering is to find a straight line on a double logarithmic
    diagram.
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You're in for a heck of a fluid dynamics problem here.  Good luck!
The real droplet distribution in fog is going to range from the micron
range up to macroscopic water droplets, i.e., mist or even rain drops.  
Imagine the fog as consisting of uniform thin shells of absorber material.
Using an effective density and sound absorption factor for these "shells"
will yield a much simplified calculation.  Of course, you may find that
the effective absorption factor is strongly dependent on the specific drop
size distributions, but only experiments can give you that information,
not theory (unless you're a great mathematical physicist in which case
using certain assumptions about the bulk sound absorptivity in water, you
might be able to apply statistical theories to the problem).  Obviously,
it would be nice if the effective absorption factor only depended on
the density ratio of water droplet content to air.  Then the problem would
have one of those nice, clean results that engineers love to work with.

Don't forget that you will need to have some reflectors for the sound to
bounce back from (at least the ground).

Take care.

Alex