Noise-canceling magnetic microphone

[Sparks]

Can any microphone experts please tell me if this type of microphone had any advantage or superiority over more commonplace moving coil microphones ? In the Astrolite headsets made by Racal/Amplivox, they were almost always used in Air Traffic Control applications. I've recorded speech with a magnetic microphone using a simple cassette recorder; the result is quite nasal-sounding, trebly and relatively low volume, if any of that makes sense. A moving coil microphone gives an audibly superior result.

I haven't a clue but a hope someone else has. Maybe these technical specifications will help. Thankyou.

[Simondm]

The usual way 'noise cancelling' used to be done was to have two microphone transducers, one close to the mouth, and one some distance away, wired up out-of-phase with each other (i.e. one would be wired back-to front with respect to t'other). Sound entering both mics equally would thus cancel out, whereas sound entering the mouthpiece mic wouldn't, because of the big signal level difference. This may have been augmented with filtering, but in the inexpensive ones I've seen it was just two capsules.

This works quite well, and the idea was used by many manufacturers (I didn't know Astrolite used it, but it doesn't surprise me). Shure used it in some of their handheld PTT (push-to-talk) models. A similar approach was also used to make directional microphones for other uses - wiring up two directional capsules together, so that sound from unwanted directions cancels out, whereas wanted sound doesn't.

Noise cancelling works best at lower frequences, where the ambient noise waveform would be closer in phase arriving at both microphones. As the frequency rises the phase difference increases, so there's less cancellation effect. This is true of digital systems too. So they work really well in situations where there is a large low-frequency component, which includes aircraft interiors.

It's not relevant that these are magnetic microphones however. You can achieve the same thing with all sorts of transducers. Magnetic capsules were usually chosen because they need no external power, are pretty rugged, and can accept loud sounds (i.e. close-mic'd speech), without undue distortion.

Hope this helps...

[Radio Wrangler]

Not all aeroplanes were created equal. The cockpit of a single-engine prop job is a noisy and high vibration place, just a foot or so behind a straining 200+ HP engine, and getting all the prop-wash. Occupants need over-ear phones and noise cancelling mikes just for the intercom to hear each other. Twins move the engines further away and are less frantic places to be, while jets are much quieter.

Glider pilots tend to prefer loudspeakers on their radios, and a gooseneck flexi mounted microphone sticking out in the general direction of their mouth.

Some noise cancelers have twin capsules, one facing your gob, the other away from you, wired in opposition. Others have a single capsule with the pressure of the back of the diaphragm arranged to match and cancel the sensitivity to ambient noise.

I've seen plain and noise cancelling mikes in all flavours. Early Airlite headsets had carbon mikes as an option.

Most modern powered planes use electret capsules. Those glider pilots really like mocing coil dynamic types and I had to do a phantom-powered adaptor for them to work with radios expecting electret mikes.

Audio responses are deliberately far from hifi in order to optimise intelligibility. Ground or air mis-hearing the other can have nasty consequences. The bandwidth of transmitted and received audio is rather limited, and evenmore so with the move to 8.33kHz channel spacing - remembering that channels may contain three or up to five offset transmitters, with filters in receivers to kill carrier to carrier heterodyne whistles. While flying, what you get to hear can be difficult to follow. Rigid adherence to procedure gives you some expected landmarks for you ear to start copying.

David

[Sparks]

Thankyou both. I thought the magnetic microphone might have been a good compromise for ATC speech systems, which use both AM and FM radio and telephone and intercom networks. Or, as suggested, it might have been their toughness that was attractive. Modern ATC headsets from the likes of Sennheiser and C3 - Clement Clarke use traditional dynamic moving coil or electret microphones. But then voice switching technology and radio and land telecommunications have come a long way in the last fifty years so perhaps microphone choice isn't too critical now.

By the way, an Astrolite headset I have recently acquired is almost certainly from an ex-British Airways airliner, with British-made avionics. Most likely a Trident but possibly a BAC 1-11 or Viscount. The NATO plug is a clue, plus the microphone (yes, magnetic with Mu Metal screens) which isn't amplified to carbon level. Perhaps designed for Marconi radios. The earphones are 300 ohms each but wired in series, with a pair of resistors in the circuit. Beautifully assembled.

[G6Tanuki]

The two-microphones-back-to-back was also the way the Airlite-62[*] aviation headsets were configured.

The CAA have some rather specific requirements for frequency-response of microphones - this does give a somewhat non-linear frequency/output curve which will sound 'thin' to people used to full-frequency-range microphones - but for 'communiucations' applications there's not much information contained in the voice below about 400Hz or above 3KHz. So if you cut out these frequencies at-source it means you then don't have to do it in the first few audio-stages of the transmitter.

[*] As modelled by Brian Trubshaw for the test flights of Concorde back in the 1960s... the Airlite-62 noise-cancelling microphone really doesn't play well with Clansman army radios though!

[mark_in_manc]

I seem to remember that some noise-cancelling designs were set up to a cardioid response - as well as being relatively insensitive to the 'rear', they work out to have quite a different response to a spherical wavefront (very near sources), as to a plane wavefront (far sources). This effect is frequency-dependent, and equates to a bass boost which rolls off at (again, based on memory) 6dB / oct, for close sources. So if you equalise this response back to flat, you get a corresponding cut for far sources. Speaking into them from any distance would produce a response noticeably lacking in low-end. I think this is how sport-commentator 'lip' mics worked - I have an ancient book on it somewhere, perhaps this one

https://www.bbceng.info/additions/20...Robertson).pdf


(Wow - online in a pdf. This was like rocking-horse doo-dah when I needed it for work 20 years ago).

[Sparks]

Reading between the lines, it's possible that Amplivox designed that particular magnetic microphone to a very particular CAA specification, i.e. tailor it for speech and configure it so no other sound can 'get through' the diaphragm. "Close loud speech", to quote the company literature will effectively negate anything from the other side of the microphone.

As for the noise cancellation itself, I remember these microphones making a better job of it than the dynamic moving coil types. When listening to the airbands (I know but folk do it...) now, you can hear the controllers moving in their chair or the clatter of the strip boards. With the magnetics, ambient noise was almost inaudible.

By the way, are these microphones constructed in the same way as a rocking armature earphone, such as a type 4T from a GPO telephone?

[mark_in_manc]

Sorry to make reference to my own post, I had a dig into my ref above - Chap 10 p171 is where that idea is laid out.

[Chris R]

I recall in the 1960's, that police radio operators in main control rooms used Astrolite headsets. There would be considerable "noises off", which were generally not heard on transmissions. Also, as mentioned, frequencies outside the 400 c/s to 3Kc/s (we still used those then!) were not required, as they would be filtered out in any case.

Chris R

[Sparks]

The Astrolite came along in 1969 but its predecessor, the Jetlite, had been around for some years prior. It used the same magnetic microphone. This headset was also used in ATC and in some aircraft.