Dear Robbie,  I am always impressed with the fine and technically
accurate information that is provided by MMD.  However, the editorial
comment you made about ANC technology needs to be corrected.

Noise canceling headsets do not identify any constant frequencies to
cancel out, like the drone of a plane engine.  They actually respond
instantaneously to the pressure rise and fall in the earcup and servo
the driver in the earcup to maintain zero pressure change.  The sounds
that you are supposed to hear such as your radio communication signal
are then added on top of this cancellation signal so that you can hear
them loud and clear.

Instantaneous sounds like firing a gun or clapping your hands are
canceled up to the limits of the system.  In the case of the Bose
headsets, this cancellation is 20 to 25 dB and works up to somewhere
between 500 and 1000 Hz.  Sounds above this frequency range are reduced
in amplitude by the presence of the earcup, which just happens to start
being effective in the 500 to 1000 Hz range.

If you are in an environment with a lot of low frequency background
noise, like an aircraft cabin or a machine shop, you can hear people
talking to you more clearly with the headsets in active mode than you
can without the headset.  The reason for this is that the high
frequency portion of the speech comes through to your ear, while the
confusing background noise is significantly reduced in volume.

It is a common misconception that these headsets identify constant
background sounds, "learn" them, and then cancel them out.  This is not
the case and I hope that this message will clear this up, at least in
our MMD community.  As I have said in MMD in the past, I am one of the
R&D engineers at Bose that developed both the worlds first noise
canceling headset in 1989 and the most recent versions mentioned at
the Bose web site.  Thank you for posting this correction and keeping
this forum open and accurate.

Jack Breen
Southboro, MA

 [ Thanks for the new information, Jack.  I guess it's nearly twenty
 [ years since I evaluated the technology for Lockheed!  -- Robbie