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  • Carbon Microphone

    What is a carbon microphone?

    The carbon microphone is seen as a simple device to turn sound into electronic signal. Some examples of their use where in telephones, radio broadcast systems and the popularity was at a peak around the 80’s. The Carbon Microphone was eventually replaced by more powerful and less noisy microphones like the dynamic or condenser microphones. Some carbon microphones are still used today due to their durability and working in low power environments.

    Why do people tap carbon microphones?

    The tapping of microphones came from carbon microphones as the carbon atoms would get stuck together sometimes and so needed to be hit to separate them to ensure the carbon microphone worked properly, this is still carried on today even though there is no advantage compared to just talking into it. Condenser microphones can get damaged with this practice.

    How a carbon microphone works

    With reference to the diagram below. The Carbon in the carbon microphone is a resistor, which isn’t very efficient at conducting electricity as it tends to resist, hence being a resistor. A current runs from the diaphragm which is the first plate, through the carbon to the other plate. The carbon molecules normally resist it somewhat, lowering the power flow.

    When a sound wave pushes down on the top plate, however, it squeezes the carbon molecules more tightly between the two plates. This increases their conductivity, creating more electric current. As the backplate moves up and down with the sound wave, the current increases and decreases, creating an electric wave in the shape of the sound wave.

    carbon microphone
    Grey Circles = Carbon Granules – Diaphragm (flexible electrode) moves, changing signal output strength

    You will rarely come across this type of microphone and its not really got a place in location sound recording, but a nice piece of microphone history. Comment on what you have learnt on the carbon microphone below the related posts.




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Matt Price

Matt Price

London Sound Recordist

I am a London Sound Recordist and Sound Editor for many types of projects around the film genres. This covers creative content right through to feature films. I am passionate about sharing my journey and knowledge to help the rest of the community. If you need anything from me, feel free to get in touch with me at London Sound Recordist.

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