[Leica] why do bombs whistle?

[marcsmall at comcast.net (Marc James Small)]

At 05:05 PM 11/1/2008, Eric Korenman wrote:
 >My son and I were playing today -- we were making bomb whistle
 >noises.. y'know, standard father and son stuff
 >
 >I suddenly realized I couldn't tell him WHY bombs whistle with that
 >falling pitch sound.
 >I'm certified comp sci, physics nerd too..
 >Google didn't help either!
 >


What is "comp sci"?  I am rather hostile to 
cutsey phrases like this:  if you want to say something, say it.

There are various causes for various noises made 
by incoming ordnance.  If you are asking about 
the whistling sound so beloved of Hollywood for 
bombs dropped from airplanes, this is more an 
artifact of the past than of the 
present:  current bombs are far more aerodynamic 
for the most part and therefore a lot quieter.

In World War II and Korea, the standard 
heavy-bomber weapon was just a drum filled with 
explosives.  The British called this a "cookie" 
for reasons which remain obscure.  A drum will 
tumble as it falls, thus generating a fair amount 
of wind noise.  Even bombs which were externally 
carried and which were therefore given some 
regard to aerodynamic efficiency, ended up being 
rather crudely designed to accommodate production 
efficiencies.  So, yes, all of these bombs did 
produce some noise while falling.

The change in pitch is produced in most cases by 
"dopplering" -- a bomb falling some distance away 
will compress sound waves as it builds up speed 
and then will decompress them as it factors away 
from the listeners' location.  A Wikipedia or 
Google search for "doppler" might be 
illuminating.  The same effect explains that drop 
in the pitch of a railroad whistle as the train 
passes by.  (I have lived in railroad towns for 
decades -- the 3:15 CSX freight this morning gave 
a perfect example of this as it passed about 
three miles from my house. this morning.

I do know some folks with strongly technical 
backgrounds who study these things -- most of 
them play around with explosive devices for a 
living, some with gummit jobs and some working 
for private industry (that is, "merchants of 
death", in terms familiar to an older 
generation).  I will be happy to ask some of them 
about this if you contact me off-line but be 
warned that their responses will probably soon 
turn into mathematics incomprehensible to those 
who do not regularly deal with such matters.

But, the short answer is that the mass production 
of bombs carried internally led to the use of 
drum-shaped bombs which were not aerodynamically 
sophisticated and so which produced a lot of 
noise while falling.  And the Doppler effect 
generally explains the change in pitch as the bomb falls.

Marc




msmall@aya.yale.edu
Cha robh b?s fir gun ghr?s fir!