How Microphones Work ?

2

2infamouz

New member
I recently wrote this article on how microphones work, and the differences between the 3 main common types (dynamic, condenser, ribbon). I'd love to get some feedback on it. Also I'm going to include a section from it in the post below, wondering what you guys think about my description of transduction, if it's easily understood by reading it, and if you think anything should be changed:
Microphone Transduction​
The way dynamic and ribbon microphones transduce energy is through the electromagnetic induction principle, while condenser microphones use an electrostatic capsule. As intense as it sounds, I’m going to attempt to explain it in relatively simple terms.

Electromagnetic Transduction Principle: In regards to microphones, electromagnetic transduction is simply a coil moving back and forth near a magnet. In dynamic mics the diaphragm is connected to the coil, and causes the coil to move when the diaphragm vibrates. The coil then starts to generate an electric current as it’s moving near the magnet, which travels out through wires as an audio signal. In ribbon microphones the diaphragm actually is the “coil”, but the same principles apply.

Electrostatic Transduction: Regarding microphones, electrostatic transduction is caused by the distance between two plates changing. These 2 plates have a voltage between them. The back plate stays stationary, while the other (which happens to be the diaphragm in this design) moves when struck by sound waves. When the diaphragm moves towards the back plate the voltage is charged. When the diaphragm moves away from the back plate the voltage is discharged. This configuration creates what’s called a capacitor. Since the capacitor needs a voltage to function, condenser microphones typically require either 48v phantom power or an internal battery, although some use other sources w/ different voltage.
 
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You are part way there - but you really need to read up and understand how microphones work as, from what you have written, it's not clear that you really do understand how they work.

a "moving coil vibrating in a magnetic field" is much better than "moving near a magnet".

And you really *do* need to understand the difference between AF and RF condenser microphones before you write about this subject.

This is something that I did that explains the difference between AF and RF condensers:-

Basically, AF capacitor microphones use the capsule as a capacitor to store charge. With one fixed plate and the other free to vibrate in sympathy with the sound, the capacitance varies, and the charge moves in or out of the capsule accordingly. This is measured by the head preamplifier and an audio signal results. All well and good, but the capsule is inherently in a high impedance circuit (over 1GigaΩ) – it has to sit there with stored charge until the diaphragm moves and any changes in the charge are perceived as audio. In a humid atmosphere the stored charge finds it easier to escape on water molecules in the air rather than through the input of the pre-amplifier, hence noisy and reduced output, and misery all round. The high biasing voltage also attracts dust particles to the diaphragm, reducing its efficiency and linearity.

The RF system (as used in Sennheiser MKH microphones) uses the capsule (a low impedance capsule) in a completely different way: as a tuning capacitor for an RF oscillator – which inherently employs it in a low impedance circuit where a high frequency signal is being passed through the capacitor all the time. Changes in capacitance (caused by sound moving the diaphragm) alter the resonant frequency of the circuit (circa 8MHz) and so its frequency becomes proportional to the audio signal. A simple RF demodulator restores the output to a conventional audio signal. More complex and sophisticated (but still very rugged), this system is highly immune to the effects of humidity and is thus the preferred design to be used out of doors (or when moving from outside to inside on a cold day!).
 
John Willett said:
You are part way there - but you really need to read up and understand how microphones work as, from what you have written, it's not clear that you really do understand how they work.

a "moving coil vibrating in a magnetic field" is much better than "moving near a magnet".

And you really *do* need to understand the difference between AF and RF condenser microphones before you write about this subject.

This is something that I did that explains the difference between AF and RF condensers:-

Basically, AF capacitor microphones use the capsule as a capacitor to store charge. With one fixed plate and the other free to vibrate in sympathy with the sound, the capacitance varies, and the charge moves in or out of the capsule accordingly. This is measured by the head preamplifier and an audio signal results. All well and good, but the capsule is inherently in a high impedance circuit (over 1GigaΩ) – it has to sit there with stored charge until the diaphragm moves and any changes in the charge are perceived as audio. In a humid atmosphere the stored charge finds it easier to escape on water molecules in the air rather than through the input of the pre-amplifier, hence noisy and reduced output, and misery all round. The high biasing voltage also attracts dust particles to the diaphragm, reducing its efficiency and linearity.

The RF system (as used in Sennheiser MKH microphones) uses the capsule (a low impedance capsule) in a completely different way: as a tuning capacitor for an RF oscillator – which inherently employs it in a low impedance circuit where a high frequency signal is being passed through the capacitor all the time. Changes in capacitance (caused by sound moving the diaphragm) alter the resonant frequency of the circuit (circa 8MHz) and so its frequency becomes proportional to the audio signal. A simple RF demodulator restores the output to a conventional audio signal. More complex and sophisticated (but still very rugged), this system is highly immune to the effects of humidity and is thus the preferred design to be used out of doors (or when moving from outside to inside on a cold day!).
Click to expand...
I appreciate the feedback and additional information, thanks for taking the time to read it. I hope the real message of the article is clear, in how different microphones can be used for different applications and what their strong / weak points are, more so than thoroughly explaining the ins and outs of the mechanics in microphones. The target audience is people with little to no understanding of the subject rather than those with an extensive comprehension like yourself. I feel it's an audio engineers job to know how to utilize their tools, and some of the basic principles of how they work in regards to real application, but not necessarily to be electricians and physicists. Do you think that any of the information is incorrect or misleading, or simply not thorough/in depth enough?
The comment about moving "near a magnet" was merely to simplify the subject for the reader. Are there other sections that you feel could be clarified?
 
2infamouz said:
I appreciate the feedback and additional information, thanks for taking the time to read it. I hope the real message of the article is clear, in how different microphones can be used for different applications and what their strong / weak points are, more so than thoroughly explaining the ins and outs of the mechanics in microphones. The target audience is people with little to no understanding of the subject rather than those with an extensive comprehension like yourself. I feel it's an audio engineers job to know how to utilize their tools, and some of the basic principles of how they work in regards to real application, but not necessarily to be electricians and physicists. Do you think that any of the information is incorrect or misleading, or simply not thorough/in depth enough?
The comment about moving "near a magnet" was merely to simplify the subject for the reader. Are there other sections that you feel could be clarified?
Click to expand...

I fully understand about making it simple - but don't go too simple as to make it silly.

I have done presentations about microphones, explaining it all in simple terms - so I do know what this is all about.

You *do* need to be technical, but it is possible to explain this in simple terms - I have done this many times.

You not only need to explain microphone types, you also need to explain microphone directivity.

A dynamic mic. makes its own power - a condenser needs power.

An electret only needs low power to power the amplifier at the capsule voltage it permanently in the electret material.

A proper AF condenser needs high volts on the capsule (Neumann use 60V, Gefell use 80V for example) and this is derived from the 48V phantom power.

You should explain the difference between phantom power and plug-in power.

You do need to describe the difference between pressure and pressure-gradient microphones and the resulting polar patterns and how all the patterns other than omni and fig-8 are a combination of these two. It was only when I understood this that everything clicked into place as to why polar-patterns are the way they are and really helped me to understand and get the best out of microphones.

I often liken a pressure mic. to a baked beans tin with cling-film on the top and a pressure gradient to the same tin but with the bottom sawn off so sound can reach both sides of the cling-film - it's a good visual aid that people can easily visualise in their heads.

But *you* really need to thoroughly understand the ins and outs of microphones in order to write it all down in a way that the uninitiated can understand.

My power-point on the subject took me many many hours of work to get it right - in man hours, probably a couple of months or so of solid work if I tot it all up.

This is not an easy task you have set yourself.
 
Just wanted to chime in with credit for both of you.

2infamouz - Good job. It's good that you're trying to compile information for people and (presumably) expand your own understanding of things.

John - Can't fault what you're saying at all. Those were some good tips about visual aids etc.
 
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