Microphone Theory Part I

Testing 1… 2… 3….

Mics are the beginning link of your audio chain, but most of the time are ignored until they don’t work. This two part article will start with some different microphone types, and finish up next time with selection for specific situations. Let’s get started.

•    Carbon mics were the first microphones and consisted of a small button of carbon powder connected to a metal diaphragm. A battery was applied across the “button” and as sound flexed the diaphragm , the carbon grains changed their electrical resistance, and produced a varying current. In fact, this is how the first telephones were constructed, and many phones to this day still use the idea. There are no music mics using this technique, since poor frequency response and bad signal-to-noise ratios would make you sound like a telephone conversation.

Ribbon Microphone
From “The Acoustic Musician’s Guide”

ribbon1

•    The next invention, the ribbon mic, is still in use today. It uses a thin piece of metal foil suspended in a magnetic field. As sound moves the ribbon, it produces a small current in the foil which is stepped up to standard levels by a transformer. The classic example is the RCA 77DX, which you can see on reruns of “You Bet Your Life” or currently on the David Letterman Show. Ribbon mics can sound great in a studio, but generally aren’t suited to the rigors of the stage. A strong wind or breathy vocalist can blow-out the ribbon and cause an expensive repair. However, Beyer has been making stage ribbon mics for years which they claim are as rugged as a dynamic mike. I’ve used them with success, and they are indeed blow-out proof.

•    Everyone has used dynamic mics. They are built like a tiny little speaker in reverse. A plastic diaphragm pushes a coil of wire suspended in a magnetic field, which generates a current. The old workhorse is the Shure SM58. Dynamics are the most universal of all mikes since they require no batteries, are extremely rugged, and tolerate abuse better than a road manager. They can sound anywhere from terrible to great, depending on how much money you spend for them.

Condenser Microphone 
From “The Acoustic Musician’s Guide”

capmikeb

•    Condenser mics are relatively new on the scene. They use a thin metal diaphragm that is suspended next to an electrically charged surface. As the diaphragm moves in and out, it changes its electrical capacitance. A circuit converts and amplifies this signal to a usable level. They always require some kind of power, either “phantom power” from the mixing board, or from a small battery in the case. In fact the housing for the electronics can be much larger than the mike itself. Country Man Associates make a line of condensers that are hardly larger than a stick of Dentine gum. Early efforts at condenser mics sounded pretty brittle nowadays especially when recorded on digital tape. The electronics during the 1970’s left a lot to be desired in terms of slew rate and saturation. But now condenser mics sound great and compete with dynamic mics for stage and studio work Besides the technology of the element itself, mics also have directional characteristics. The two main types are omni-directional, and uni-directional (generally referred to as a Cardioid).

•    Omni pattern mics pick up sound equally well from all directions. They are generally not used on stage because they not only pick up in front of them, they also hear the sounds on the sides and back just as well. They are used a lot in studios where monitor speakers and stage noise don’t exist. A good omni mic will always have a smoother frequency response than a Cardioid mic, since it doesn’t have all the resonant chambers to contend with.

•    Cardioid pattern mics can be had in Wide, Super-, and Hyper- Cardioid pickup patterns. They reject sound from the sides and rear, and have better sensitivity from the front than an omni-directional mic. You must sing straight into them though, since sound coming in off-axis is usually pretty bad. Their frequency response can be either good or bad depending on how much you pay. (Isn’t capitalism great!) There are a few other types of patterns such as bi-directional and pressure-zone effect types, but they aren’t used too much for live stage work. In Part II we’ll go into what makes a mic great for a specific application. See you then.

Copyright Mike Sokol 1992/2016

 

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