Re: Clarke - two questions

tmcd@[EMAIL PROTECTED]
 (Tim McDaniel) writes:

> In article <g46dnVl3Ksy_-X_anZ2dnUVZ8vudnZ2d@[EMAIL PROTECTED]
>,
> Mike Dworetsky <platinum198@[EMAIL PROTECTED]
> wrote:
>>Obligatory A. C. Clarke story reference in the news:  Who remembers the 
>>"White Hart" story about how Harry Purvis prevented the evacuation of 
>>southern England?
>
> I remember reading the absurd White Hart story about noise canceler --
> impossible!  Years later, I got noise-canceling headphones.
> Fortunately, I lost them before they blew my head off.

Did the noise canceler in the story cancel just noise in a region next to 
your ears, as the current noise-cancelling headphones do, or did it cancel
noise in a much wider area?  

I remember thinking about this while reading Flint and Weber's _Crown of 
Slaves_, where a noise-cancelling device is a (minor) plot point in one
scene.
I'm not sure I buy the notion that you could make a device that could
listen
to the ambient sound and accurately produce counter-waves sufficient to 
almost perfectly cancel all sound through, say, an entire room.  Not even 
given that Solarian or Manty mollycirc hardware is probably as much faster
than
current DSP hardware than that hardware is compared to the first Intel
4004
microprocessor.   I don't buy that good-enough cancellation over a wide
area would be possible, given the finite propagation time of sound waves.