Category Archives: Noise- cancelling microphones

World War 2 M.T.L.L.No2 Throat Microphone

M.T.L.L. No2 Throat Microphone

Found in a garden shed where it had been for at least the last thirty years, this WW2 British military throat mic looks a bit sad and dilapidated. The paint is falling off and the steel fittings are rusting, the elastic is past its best, and a hungry woodworm has been drilling into the crumbling leather. So imagine my surprise when I plugged it in and it immediately burst into life!

The M.T.L.L. No 2 was used by the RAF and by the army. The mic was worn around the neck and was designed to pick up vibrations direct from the voice box. The two leather covered sensors were pressed against either side of the throat. The great advantage of the throat mic was that it could be used effectively in very noisy environments, such as inside a tank or an aircraft in combat. Even loud sounds around the wearer would be largely rejected. Although the audio quality is not fabulous, speech has a high degree of intelligibility.

This view below is of the back of one of the sensors with the plastic cover removed. Inside there is a magnet, the polarity of which is marked on the metal case ‘N’ and ‘S’.  The wires are identified with red and blue cotton thread. (Hot and cold?)M.T.L.L. No2 Throat mic view inside the back

Below, (pinched from an ebay listing) is a photo of the front side of the mic, without its leather covering. Here there appear to be a couple of coils.M.T.L.L. No2 View inside front of throat mic

Below, the strip of steel which picks up vibrations from the throat is sitting on top of the coils and magnet. This assembly is held in place by the leather covering. The result of this ingenious arrangement is a small electrical (audio) output signal appearing at the red and blue wires.Inside throat mic M.T.L.L. No2

 

CLICK here to hear a short voice test of this unusual creation.

Conclusion.

It would be hard to exaggerate the importance of military communications during World War 2 and the crucial role played by the M.T.L.L. No 2 throat microphone.

 

Electro-Voice Rags To Riches. T45 Noise-Cancelling Microphone (1944)

In 1942 after America entered World War 2, the US military estimated that only 20% of radio communications in combat were successful. Failure in the other 80% was mainly due to the voice of the radio operator being drowned out by the surrounding cacophony of war. Like no other conflict before, success on the battlefield relied on communications. Spotting a gap in the market Al Khan and Ed Burrows, the owners of Electro-Voice, came up with a brilliantly simple, ingenious and cost effective solution to this problem. Electro-Voice T45 Box

Electro-Voice T45 Instructions 1944

Electro-Voice T45 Instructions P2Even in 1942 the single button carbon microphone was a piece of old fashioned tried and tested technology, having been in use in telephones since the tail end of the previous century. Although the audio quality of the T45 is little better than it’s telephonic predecessors it is extremely reliable and very robust. It also has a high output making it ideal for long distance communication. Even if the microphone gets wet you can simply dry it out (as per the instructions above) and it will carry on working! However, the really clever part of this design utilises 2 small holes of equal size on the front and back of the mic.Electro-Voice T45 Back hole.Electro-Voice T45 Front Hole

These allow the surrounding noise to enter the microphone on both sides of the diaphragm. The sound striking the back of the diaphragm is 180 degrees out of phase with the sound at the front. This causes a very impressive cancellation of the unwanted noise whilst the speaker’s voice, which is less than a 1/4 of an inch from the front opening, dominates the transmission.

In terms of manufacturing costs it would be hard to produce a cheaper microphone.  A carbon button is a very small tin of glorified coal dust (carbon granules) with a simple diaphragm attached.  A bit of wire and some lightweight plastic fittings and that is it! Pure genius !Electro-Voice T45 Noise-cancelling Microphone

After some initial military skepticism the product was thoroughly tested and a first order came through to Khan and Burrows  for 100,000 units! The T45 was soon taken up by all branches of America’s armed forces and  the success rate of combat communications rose to 90%.

Rags to Riches.

Prior to World War 2 Electro-Voice was a small struggling company, with 20 employees, manufacturing a handful of dynamic and velocity microphones per week. By the latter part of the war Electro-Voice had 500 employees working in 3 shifts  producing more than 2,000 T45 microphones a day! After WW2 it was also adopted by commercial aviation and remained in service for several decades.  The T45 was also used on the Mercury, Gemini and Skylab space missions.

Over the entire production run more than a million were produced placing the T45  among the highest selling microphones ever made.

During the war many small firms went out of business due to a shortage of manpower and materials, but for those involved in the war effort fortunes were to be made. In 1946 Electro-Voice moved into an impressive new factory at Buchanan Michigan where they continued to manufacture  innovative and exciting audio products  for the next 60 years.

 

Footnote 

Circuit for powering a carbon microphone.  Circuit for powering Carbon Microphones.