[Elecraft] Noise Cancelling Headphones

[Jim Brown]

On 12/26/2013 10:39 AM, Mark Petiford wrote:
> Re:  "Is there such a thing as a noise cancelling black box you can plug in aheadset such as the CM-500 and get good noise canceling? No knobs. Just anON-OFF switch like the headsets have."
>
> Well, sort of, Tony.  In the aviation world,  conversions kits have been marketed for popular (in aviation circles) headsets to convert them to noise canceling.  They weren't "black box" plug-in devices, but rather a kit of microphones and electronics mounted in each earpiece and required some disassembly of the existing earpieces.  I believe they were made for David Clark and Flightcom headsets, and maybe a few others.  The result was a significant improvement over the original passive headset

Right.  Successful noise cancelling requires that the resulting acoustic 
output AS COUPLED TO THE EAR CANAL precisely match the acoustic noise 
that GETS THROUGH THE HEADPHONES TO THE EAR CANAL (caps added for 
emphasis) in both magnitude and phase, and that the two acoustic signals 
are out of polarity. That means that the mic picking up the noise be 
very close to the headphones, the distance between the mic, the 
transducer, and the ear must be compensated, and the relative response, 
both magnitude and phase, must be precisely compensated.  That's a VERY 
difficult design problem, and a really good solution is only practical 
when everything is built as an integrated package, with dedicated mic, 
headphone transducers, and electronics.

73, Jim K9YC