Microphones 101

If you’ve done any recording at all, chances are you’ve used a microphone of some sort. Simply put, a microphone detects sound waves and converts them into an electric current – if you didn’t know already, now you have a basic understanding of what it does!

But there are many types of mics, at widely varying prices. So how do I select the right mic for what I need, and keep within my budget? This article discusses the basics any microphone user should know in order to make a good selection.

We have microphones everywhere today…

• in your telephone or mobile phone handset
• on stage at a rock concert
• on the table in TV chat shows
• clipped to the news reader’s shirt
• hidden in the spy’s hat(!)
• in your sister’s baby alarm
• and of course in any recording studio

Without microphones we could not amplify the speeches in an auditorium, or mellow the sound of Aunt Annie’s attempts at karaoke; we couldn’t hear film or TV soundtracks, or radio broadcasts…

So how did these every-day items come about?

• 1876 - Alexander Graham Bell transmits intelligible words (“Mr Watson, come here, I want to see you”) across a wire from one room to another using a liquid transmitter, and electro-magnetic receiver. (Basic technology of the dynamic mic.)
• 1876 – first commercially practical carbon microphone invented by Thomas Edison to improve Bell’s telephone. Developed it into the basis for the traditional phone model that we know today. Emile Berliner also built a transmitter for voice based on “loose contact” between 2 metal electrodes.
• 1878 – an improved carbon microphone was developed by David Edward Hughes
• 1942 – following the invention of the radio, the broadcast ribbon microphone was created.
• 1964 – saw the birth of the electro-acoustic transducer, used to create the Electret that revolutionized the microphone due to its reliability, affordability and smaller size
• Throughout the 1970’s these dynamic and condenser microphones were developed to provide much higher sensitivity and a clearer recording sound.

Microphone technology is continually advancing but through this brief history you’ll have a better idea of how we got to where we are today.

How it Works

As you know, a microphone is a device that senses sound information. This arrives in the form of air pressure patterns, and is converted into an electric current, to become the audio signal.

All microphones have some kind of diaphragm – the microphone’s equivalent of our ear-drum. When a sound arrives at the diaphragm, it vibrates in sympathy with the sound. It is this minute mechanical vibration that is converted into an electrical current, and sends on the audio signal to a mixer or amplifier.

So if you are a recording engineer, you are mostly interested in the accuracy of the transformation that occurs when the sound vibrates the diaphragm.

The exact process of converting micro mechanical movements into audio signals depends on the various microphone technologies used. Here are the main ones you’re likely to come across:

• dynamic – in a dynamic microphone, the diaphragm is attached to a wire coil. Sound waves arriving at the diaphragm move the wire coil within a magnetic field, which induces a varying electric current in the wire. Probably the most popular dynamic mic ever produced is the Shure SM58 Mic .
• ribbon – in a ribbon microphone, a moving (metal ribbon) conductor in a magnetic field induces varying current in the conductor
• condenser or capacitor – in a condenser microphone, varying the distance between 2 electrically charged plates results in varying capacitance, with corresponding electrical current variation. In order to sustain the required electrical charge, condenser mics need a power supply. In a traditional mic design, this means using the “phantom power” source on the mixing desk or mic pre-amp. Electret mics are condenser microphones with a permanent electric charge built in during manufacture.
• piezzo-electric – in a piezzo-electric transducer, varying compression creates corresponding electrical potential variation in some materials. (More often used in direct contact with musical instruments eg as under the saddle acoustic guitar pickups)

To see some good diagrams of the different types of microphone, click here: Microphone Diagrams from University of Salford

If you are just getting started in home recording, you have 2 broad options

• use the onboard microphone, or
• the external microphone

If you have one, the onboard mic is already built into your computer, so there are no additional costs associated. However, these generally are poorer quality, more susceptible to feedback and they do not have the ability to pick up the finer details of your sound source to give you professional quality results.

For these reasons I usually prefer to use a external microphone, although the convenience and quality of some high-end laptop mics make this less of an issue than it used to be. For example, my MacBook Pro (June 2007) records one-one interviews well enough for general use, since it has an excellent onboard microphone.