Thanks Rowland -
You have left us in no doubt that you know your stuff - and have put it over
in a manner that most of us can understand.
My comments were based on my experience of aviation headsets rather than
scientific analysis and I dont doubt that noise cancelling mics improve the
situation considerably compared to non cancelling mics.
At least when I open my mouth next time (on the subject) I can speak with a
degree of authority.
Kind regards,
Carl & Dot
----- Original Message -----
From: "Rowland & Wilma Carson" <rowil@clara.net>
Sent: Saturday, July 28, 2007 5:17 PM
Subject: *** SPAM *** Europa-List: noise-cancelling mic [was: anr headsets]
> <rowil@clara.net>
>
>
>>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
> --
> | Wilma & Rowland Carson http://home.clara.net/rowil/
> | <rowil@clara.net> ... that's Rowland with a 'w' ...
>
>
>
|