Racal-Amplivox magnetic microphone insert

[Sparks]

These were fitted to Jetlite, Astrolite and (I think) Clansman headsets from the mid-1960s up to about 20 years ago. Perhaps they are still fitted to modern headsets. My question is: was this type of microphone especially suited to AM radio systems and telephone networks? A cassette recording of speech using one of these results in a trebly, nasal sounding voice so perhaps not as suitable as a conventional dynamic or electret microphone but with more fitting applications. British air traffic control units made extensive use of the Jetlite and Astrolite for decades, in conjunction with AM radio and telephone systems, hence my question.

Thankyou.

[Radio Wrangler]

The name tells you the reason...

'Noise-cancelling' there are TWO dynamic microphone capsules in the package. One aimed at the user's mouth, the other at the environment. They are wired in series so you theoretically get what comes from the mouth but isn't in the environment. The environment mike will pick up lower frequency coice components quite well, with little delay from the direct mike. There is only a limited distance between the two, so directionality fades at low frequency, to be replaced by cancellation. This is also a good characteristic for battlefield radios.

So you get a very bass-light sound, but you get cancellation of ambient noise. For aviation you lose much of the noise at the price of losing low frequency from the voice. But aviation transmitters have a highpass function and only really transmit speech components above 300-600Hz.

So they are good communications mikes but too bass light for entertainment. Try an entertainment mike in a light aircraft cockpit and you won't be easily understood.

Glider pilots have very quiet cockpits and like to flaunt it. They like to use a cabin speaker rather than headphones, and they like to use a straight-forward dynamic mike on a flexible gooseneck. They don't need noise cancellation, they just don't have much noise to cancel. Big aircraft with two pilots tend to go for noise cancellation so there is reduced pick-up of the other pilot and they can each be talking to different ground functions.

If you're interested in recording entertainment, speech, music etc, then a noise canceller is what you don't want

[Sparks]

Thankyou RW. I understood the noise-cancelling aspect. It was the type of microphone, i.e. moving iron or magnetic and its associated speech reproduction that piqued my curiosity. Would I be right in thinking this microphone is at least similar in construction to a type 3T or 4T earphone as found in old GPO telephones?

[Radio Wrangler]

I think they are just small moving-coil types. This allows the magnet structure to be effectively closed and self-screening. Useful in a cockpit with a magnetic compass you're relying on nearby.

The highpass response is wanted for intelligibility and also filtering of noise. It's also inherent in the distance fro one mike input round to the other.

David

[G6Tanuki]

In 'aviation' grade headsets like the Airlite-62 they used a dynamic microphone with both sides of the diaphragm exposed; the idea being that 'ambient' noise would arrive at both sides of the diaphragm with little phase-difference so wouldn't cause much displacement, while close-talking speech would only appear on one side of the diaphragm so would move it.

Photo shows Brian Trubshaw on the left, during Concorde trials, wearing an Airlite-62 headset.

[ScottBouch]

Hi Sparks,

I have several headsets, boom microphones, throat microphones, flying helmets and oxygen masks of this era, with a vatiety of microphones. High fidelity is not their strong point, they all sound a bit pants.

But they are designed to cope with moisture (eapecially inside an oxygen mask), and to operate over a range of cabin pressures, and ambient temperatures.. Ie, relibility and robustness is priority.

Typically the microphones you reference will be dynamic type for British military aircraft, no phantom voltage. Now, USAF aircraft also use dynaic mics (lower impedance), but USN aircrat use phantom powered radio/intercom mic lines as you would use for carbon or electret mics, but most USN onygen masks are fitted with dynamic mics and need a little amplifier unit to convert the signal for use!

The RAF and RN Fleet Air Arm use the same standard headset and mic impedances and types. This interestingly is also a standard in the French Air Force, this standard also encompsses the use of a NATO plug (Type 671).

General Aviation is another kettle of fish, here you have "twin plugs", and elctret phantom powered mics. But GA helicopters are different again, using the same plug as tge USAF, but with a different pinout!

The microphone you reference, it would help to see a photo of it, as there were several types, you may have a headset ued by ar traffic control, which again are seen with different mics to aircraft.

Cheers, Scott

[ScottBouch]

Originally Posted by G6Tanuki

In 'aviation' grade headsets like the Airlite-62 they used a dynamic microphone with both sides of the diaphragm exposed.

This is interesting - of the microphones I have, there are two or three varieties of noise cancelling mics, with grilles or apertures on both sides. I was always taught there would be two back-to-back mic elements within, but I have never taken one apart to see. Your point puts a new spin on this.

The 2 element back-to-back microphones will pick up ambient noise in-phase with each other, and connected electrically in anti-phase, these signals would mostly cancel each other out. But voice mostly being applied to one element would get through.

I can see how the single diaphragm idea would work, as noise sound pressure is theoretically in-phase, the diaphragm should not move as it's being acted upon equally on both sides, but voice would only act on one side and move it.

I suppose where noise would pass through these systems is where ambient noise would be reflected from a person's face and appear slightly differently to the microphone to the ambient noise presented to the other side with no close-by reflective surfaces. Maybe beards could help this....

Cheers, Scott

[Radio Wrangler]

For a dynamic microphone, the coil former is a significant fraction of the total cone and cap area. The pole piece therefore masks a significant area of the rear side, unbalancing the sensitivity to front and rear, reducing the cancellation. So a pair of identical capsules connected in series opposition will give better cancellation.

With the sound level in some classic light singles, you need all the cancellation you can get.

David

[Sparks]

Thanks all. I'll come back when I can photograph the microphone in question. It is Racal part number 13750, tropicalized NC magnetic microphone. As stated, very much used in ATC Astrolite headsets and also many British civil aircraft, including Concorde and the Trident. Some of these aircraft radio installations required a microphone with a matching amplifier to give a carbon level output, some didn't!

[ScottBouch]

Hi Sparks,

This may be the adaptor amplifier you refer to, made by Airmed.

It converts a UK military headset (type 671 NATO plug and dynamic mic) for use in a typical civil aircraft (GA twin plugs and phantom powered mic). This adaptor would give a carbon or electret level output, phantom powered by the aircraft radio/intercom system.

Cheers, Scott

[Radio Wrangler]

That's a very useful adaptor even today.

The change to 8.33kHz channel spacing has forced a lot of aircraft to update their radios, and modern transceivers are almost all set for phantom-powered electrets.

The gliding fraternity are wedded to dynamic mikes on goosenecks. They have nice, quiet cockpits and want to flaunt them. So a dynamic to electret converter, especially one running from the phantom feed is rather useful. I designed one with switched gain that would go in a bump in a cable. Without the switched gain, it could be laid out to fit inside the cover of a jack plug. SMT has its benefits...

David

[ScottBouch]

This 1960s Airmed adaptor is ok, but compred to a modern Pilot PA98 adaptor is's sound quality is not as good

David, I would be very interested in learning more about your compact adaptors.

Cheers, Scott

[Sparks]

Here is the microphone in question. A very neat little unit but quite fiddly screwing the nuts on to those small studs. As you can see both front and back appear identical but I expect the differences are inside.

[Sparks]

More technical data.

[Radio Wrangler]

It was done while I was working at Trig, so the design is their property. I don't know whether it went into production. I'd got as far as making a bunch of lab prototypes. If you ever come across one, it uses a DIL switch to select a string of resistors to select the gain. I did this to avoid the uncertainties of a pot and have a circuit where switch settings could be specified for particular microphones. I designed it to have a flat response, leaving any shaping to the microphone. It's just a 2-transisrtor feedback pair with some care in choosing quiescent current to suit the phantom power of our radios and not need any other power source.

David

[ScottBouch]

Hi Sparks, ah yes, I have seen this mic type more in use on land based Army headsets - I used to have a RACAL tank drivers helmet,it had this element in its boom mic.

Not seen them in airborne headsets before. They are similar in appearance to the mics used in type P/Q oxygen masks, but these are one-sided and have a switch on the outside.

[ScottBouch]

Here are two microphoned used in type P/Q oxygen masks. One by Amplivox, then a later one by Racal. Same NATO stock number on both.

Similar hole pattern, but different kettle of fish.

[Sparks]

They're probably all much the same inside Scott. By the way, when I mentioned amplifiers for carbon level, I should also have said that the amplifiers were actually fitted inside the headset itself, inside the boom arm earshell to be exact. But not all cockpit Astrolites had the amplifiers fitted; it depended on the aircraft type and the make and model of radio fitted. There isn't much audible difference in volume between an amplified and non-amplified magnetic microphone. Just a little more bass and less hiss, which I guess is enough for carbon matching purposes. ATC radios and telephone networks didn't need this amplification.

[Sparks]

Inside of an Astrolite. The amplifier is the black object on the right. WR721 was a CAA number issued to these headsets.

[ScottBouch]

Two Airned /Clement Clarke International Airlite62 airborne headsets with two different mics. Both appear to be dynamic, noise cancelling, back to back.