Re: Hiroshima...No Sound?

[Leigh Palmer <palmer@sfu.ca>]

> The freshman English classes are reading the book "Hiroshima" about the
> events surrounding August 6th 1945. In the spirit of "Interdisciplinary
> Education" I've been invited to speak to the classes briefly as a
> "technical expert." I expect I'll mostly answer some of their questions.
>
> The book mentions something about people seeing the blast but hearing no
> sound. I do not know the details of what is actually said; but, I expect
> to be asked about it.
>
> Is this a real phenomenon? I seem to remember hearing this discussed
> before--perhaps on this list. A brief look through reference materials
> and the Internet has turned up nothing.Could this be a refraction effect
> caused by heating from the blast? Anyone know about this?

If those people were far removed from the source (50 km or more) that
is quite possible. When Mount St. Helens blew its top it sounded like
a local explosion here in the Vancouver, BC area. I thought some idiot
was blasting stumps on a peaceful Sunday morning, probably within a
kilometer of me. The sound was not heard at all in Seattle, which lies
halfway between Mt. St. Helens and Vancouver. On the other hand, there
is a cannon which is fired every evening at nine PM in Stanley park in
Vancouver. Often we hear this report here on Deer Lake, 15 km from the
cannon, quite by accident while in normal conversation. On some other
occasions, with perfect quiet, and listening intently for it, we hear
nothing at all. It is clear that acoustic pathways and visual pathways
are not strongly correlated over tens of kilometers in the atmosphere
at the surface of the Earth.

Incidentally, if you visit Vancouver and you are interested in acoustic
phenomena, there is one you don't want to miss. Stand near (say 50 m)
the cannon (called "The Nine O'Clock Gun") in Stanley Park when it is
about to be fired and stop your ears with your fingers (there's a red
light to warn you). Pull your fingers out as soon as it fires and
listen to the multiple echos from the large number of tall buildings
that have appeared in Vancouver over the three decades I've lived here.
The echos are fantastic! I think that a great science fair project
could be done by sonically mapping Vancouver by taperecording the echos
at two or more spots accurately surveyed from the mouth of the cannon
and then matching echos on the tapes (after digitizing and storing them
on a hard disk) to determine building locations by a *distance* form of
triangulation. That is, determining angles from known distances rather
than the other way around. It is a pretty sophisticated project for
high school students, but all of the instruments required are consumer
electronics, and what a learning experience it would be!

Anyone who wants to take on this or a similar project is welcome to
contact me for consultation, by the way.

Leigh