>IMHO noise cancelling microphones dont seem to cancel out very much
>noise - maybe someone with a superior technical understanding might
>volunteer an explanation
Carl - they certainly don't work perfectly, but if you compared a
recording made with the usual aviation-type noise-cancelling mic
side-by-side with one made using a conventional microphone, I think
you would notice the difference! I know they do work; I use a
Sennheiser MD430 "close-talking" mic to record speech (for
announcements, etc) in normal domestic locations, and that frees me
---From having to worry much about intrusion of external noises.
{As an aside, I recently swapped headsets with my instructor to try
out his ANR set. At first it didn't seem dramatically quieter than my
own Sennheiser HME 100, but switching the NC off & on again soon
convinced me that there was a big difference, particularly in the low
frequencies. Just being exposed to the engine noise while swapping
headsets had disturbed my own threshold enough to make the comparison
difficult to make. (Incidentally, we did this on the ground!) The
moral is that a direct comparison with a NON noise-cancelling mic
would be needed to judge the effectivenes of the noise-cancelling
mic.}
Many "noise-cancelling" microphones work on the principle of
accepting spherical pressure wave-fronts and rejecting planar
wave-fronts.
The idea is that a sound source very near the mic will produce a
spherical wave, expanding in all directions from the source. The
wave-front from a distant sound source will be much closer to a plane.
Designers take advantage of this in various ways, often involving
labyrinth construction, so that the pressure variations from plane
waves arrive at the transducer out of phase, and thus tend to cancel
out, whereas pressure variations from wavefronts differing
significantly from planar do not cancel and may actually achieve a
reinforcing effect. Of course the effectiveness of this will be quite
frequency-dependent.
MIcrophones of this type are uusually referred to as
pressure-gradient types; those which respond equally to sounds from
all directions and distances are called pressure (or omnidirectional)
types. The most common ype of pressure-gradient mic is the cardiod
(or unidirectional) type, typically seen on stage for singers or
announcers, which responds much more sensitively to sounds from the
front than from the back or sides. However, the standard cardiod is
not noise-cancelling; it will respond equally to all sounds arriving
along its axis of sensitivity. The true noise-cancelling mic is
rarer, and (apart from the ones in aviation headsets) the most likely
place to see one is on a PA system where the announcer is in the room
served by the sound system (eg at airports). It does require the
talker to be very close to the mic inn order to work properly. That's
why you need to get your headset mic as close to your mouth as
possible, but without putting it in the direct path of the breath
puffs from plosive sounds (words with syllables starting with P, B,
T, D etc). Hence the advice often seen to position the mic at the
corner of your mouth.
One of the finest examples of the noise-cancelling mic technique was
the STC 4104, sometimes referred to as the "Raymond Glendenning"
model because he was seldom pictured without one. It had a small pad
which was placed against the upper lip (or moustache as the case may
be) in use, thus ensuring accurate positioning of the lips in the
location intended by the designer. It was designed for radio use,
before the days of sound-insulated commentary boxes, but was latterly
also used by TV commentators on Saturday afternoons for reading the
football results live from a (very noisy) teleprinter. Very
occasionally one can be seen still, typically with a reporter wearing
ear-muffs and standing beside a running jet engine or other very
noisy artifact.
My own Telex 66C aviation mic (acquired way back before the universal
use of headsets, when I couldn't be sure the mic in the flying school
aircraft would always be working, and carried until recently as an
emergency backup) is very similar in principle, having a protruding
ridge that you can rest on your top lip for accurate positioning. It
also benefits from the slight non-linear sensitivity of the
carbon-granule transducer. (That's why old-fashioned phones with
carbon mics tended to discriminate in favour of the speech from the
user and somewhat attenuate lower-level sound, whether speech or
other; of course the carbon granules introduced lots of noise of
their own, but that's a different issue!)
I think the common approach for aviation headset boom mics now is to
have matching orifices front and rear, each communicating with
opposite sides of the transducer diaphragm. Thus plane pressure waves
(ie those from relatively far away) will tend to be displacing the
diaphragm both forwards and backwards at the same time, so cancelling
out. The distance from the talkers lips to the nearest orifice should
be of a similar scale to the air-path round to the orifice on the
other side of the mic. Thus (over a restricted frequency range) the
user's speech can generate positive pressure on the front of the
diaphragm, accompanied by a reduction in pressure at the back of the
diaphragm (and vice versa).
Sorry, this is probably much more info than anybody wanted, but hope
it helps to understand the principles.
regards
Rowland
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