Re: Clarke - two questions

In article <x7eja4g66l.fsf@[EMAIL PROTECTED]
>,
Richard Todd  <rmtodd@[EMAIL PROTECTED]
> wrote:
>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?

A concert hall during a performance of a new and very modern opera by
Edward English.

>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.

The problem I see is the shape of the waves.  So far as I can see, if
there's a rarefaction at time t coming out of a speaker, say, that
rarefaction propagates as an expanding spherical shell, and any
canceler will do the same, but from a different location.  With two
spherical shells growing from two different points, there will be
interference on the circle of intersection but nowhere else.

Noise cancelers today work because they are not in a situation where
sound waves move as spherical shells.  Car frame (propagation along
beams), ducts (linear movement), headphones (close-to-point
receptors).

Or so I think.  I invite correction.

-- 
Tim McDaniel, tmcd@[EMAIL PROTECTED]