Microphones – the basics

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16 June, 2011 by ehauke

For a lot of audio recording, you can get away without using a stand-alone microphone. Computers often have microphones built-in and audio recorders have increasingly good quality microphones on-board. But having a hand-held microphone in the field, or sidling up to a stand-held microphone in a studio, really is the business. Nothing says you know what you’re doing, like a nice bit of kit. But knowing when having a quality microphone is really necessary, choosing the right one and having a feel for how much to spend on a microphone  is really important. I mean, there’s a difference between looking the business, and looking like a flash twat.

So. Where to start. At the beginning, of course.

How does a microphone work?

A microphone converts mechanical sound waves in the air, into electrical impulses that can be digitally recorded or amplified. These electrical impulses can be turned back into mechanical sound waves in a speaker. So the main function of the microphone is this conversion – it is acting as a transducer – turning acoustic energy into electrical energy. Different types of microphone work in slightly different ways and include:

  • carbon
  • dynamic
  • ribbon
  • condensor
  • crystal

Most microphones use a diaphragm to ‘capture’ the sound waves of the acoustic energy and ‘present’ them to the rest of the components of the microphone for conversion into electrical information. You mainly need to know about the dynamic and condensor microphones, but it’s quite interesting to know about the others too, so we’ll go through them one by one.

Carbon microphones

These are the oldest type of microphone, and this same technology also used to be used in telephones. The sound waves enter the microphone and vibrate the diaphragm. Now on the other side of the diaphragm, is a layer of carbon dust, and as the diaphragm vibrates, this carbon dust gets compressed. The compression varies the electrical resistance of the carbon. An electrical current is passed through the carbon, and this variation in resistance alters the flow of current. That variation represents the electrical version of the ‘sound information’.

Dynamic microphones

In a dynamic microphone, a magnet takes the place of the carbon dust. So when acoustic energy vibrates the diaphragm, the magnet is moved with it. Now when a magnet moves past a wire or coil of wire, electrical current is induced to flow in that wire. So the vibrations of the diaphragm create little bursts of current that represent the ‘sound information’.

Ribbon microphones

In a ribbon microphone, a thin metal ribbon is suspended in a magnetic field. The sound waves vibrate the ribbon, which moves it in relation to the magnetic field, and again current is induced to flow.

Condensor microphones

The sound waves vibrate the diaphragm, which moves one plate of a capacitor. As this plate moves in and out, the capacitance of the capacitor changes. These changes are amplified to create a measurable signal. The important thing to know about condensor microphones is that they require power. Some will take a battery, others will require plugging into a device that will supply ‘phantom power’. The power is needed to create a small current across the capacitor, so that the capacitance changes can be detected. Another thing to remember is that as condensor microphones are drawing power, they should really be used in a stand – you shouldn’t hand-hold them.

Crystal microphones

We’re back with a diaphragm again in this type of microphone. But instead of carbon dust or magnets, this time the diaphragm is pressing against a crystal. Now, as you know, some crystals change their electrical properties when they change shape, and that is what is exploited here.

Basically speaking, the dynamic microphone is the most versatile and the most robust. However, condensor microphones are more sensitive, so if you’re capturing quiet sounds and you’re working in a good acoustic environment, you might want to invest in a condensor microphone. If you’re recording in your house though, beware getting a microphone that is too sensitive, or you will hear the motor in your five-doors-down-neighbour’s fridge clicking on and off.

Now. When you start reading about particular microphones, the next thing you might come across is directionality. Different microphones ‘collect’ sound from different directions. So let’s figure out the different types.

Omnidirectional

This collects sound information from every direction relative to the microphone.

Cardioid and Hyper-Cardioid

These microphones collect sound information from a ‘heart-shaped’ area in front of the microphone. The hyper-cardioid collects from a much tighter area than the cardioid and is therefore more directional. If you are using one of these microphones for an interview, you need to be sure that each speaker is speaking into the sensitive part of the microphone or the voices will be recorded at different levels and qualities.

Bi-Directional

This microphone has two areas of sensitivity – one in front and one at the back. This is idea for recording a two person interview. It is relatively insensitive on the sides, so will help to minimise any background noise that might be a problem.

It is traditional to illustrate the microphone sensitivity on a polar graph like these.

You’ll find that individual microphone manufacturers will plot the exact ‘shape’ of each microphone they make, but it will fall into one of these broad categories.

If you’re not sure what you need, go for an omnidirectional microphone as this will give you the most versatility and is the easiest to record with.

First-time microphone buyers

Finally a word for first-time microphone users and buyers. You’ll probably get a clearer idea of what type of microphone is best suited to your needs with experience, but you won’t get that experience unless you get your hands on a microphone. For that reason, it is probably best to get an omnidirectional dynamic microphone for less than £60. Definitely do not go over £100. It’s probably also worth scouting out second-hand microphones on eBay in the first instance. You’ll save a bundle, and also get an idea of whether you’ll be able to re-sell it if you change your mind!

I’ll be posting further on microphones in the future, including microphone and recording technique, and some microphone reviews based on what I’ve had the good fortune to use so far.

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One thought on “Microphones – the basics

  1. Nice Work clearing things up for the prospective microphone users!

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