Re: Mt. St. Helens and sound

[palmer@sfu.ca (Leigh Palmer)]

>        Greetings!  I was watching a documentary of the latest eruption
> on Mt. St. Helens.  They were interviewing one of the survivors from
> (point zero) from the eruption who was a lumber jack on the side of the
> mountain during the eruption.  During the explosive eruption, people from
> hundreds of miles away could hear the explosion.  What really caught my
> attention was that the lumberjacks that were ON the mountain did not hear
> a thing!  I tried looking up a reason as to why they wouldn't hear the
> explosion, but with no luck.  Does anyone have an explosion as to how
> three lumberjacks that were on the volcano during the explosion did not
> hear it where as others miles away could?

My son David and I were out fishing in two canoes that Sunday morning in
Burnaby, British Columbia, 220 miles north of Mt. St. Helens. The blast
was heard here as a very loud bang, not at all dispersed or extended in
duration. I recall assuming immediately that an idiot neighbor across
the lake had chosen that lovely May morning to blast stumps, and I was
annoyed. Naturally I was astonished to learn on going back into the
house that Mt. St. Helens had blown its top. The blast was heard
throughout the Vancouver area, but it was not heard at all in Seattle,
120 miles nearer the mountain.

Clearly the sound was channeled in the atmosphere in a rather narrow
region, hence the lack of spread in arrival time at our location which
would have attended transmission through a large channel with multiple
path possibilities. I have no direct knowledge of the mechanism but I
guess that it involves temperature stratification of the atmosphere and
the phenomenon of critical internal reflection, an acoustical property
which is well known in the oceans and is the reason that global scale
acoustical experiments can be carried out in the oceans. It is also the
salvation of large cetaceans like the blue whale. These beasts can find
one another over vast distances when they get horny, so a small global
population can still maintain itself.

Sounds like I spend all my time thinking about fish.

Leigh