Microphone Basics

Week 1: Microphone Basics -- Type, Frequency Response, and Polar Pattern.

Microphones are transducers which convert acoustic energy into electric current. The sound source (e.g. a vocalist or an acoustic instrument) makes the air vibrate, and these waves of compressions and rarefactions propagate in the medium. When these waves reach our ear, we hear them as sound. A microphone works quite like a human ear: the sound waves pass through the grille and the windscreen of the mic (~outer ear); reach the membrane (~ear drum) and set it in motion (~middle ear); the motion of the membrane is converted into voltage/current variations (~inner ear) by a permanent magnet; these variations pass through the microphone cable (~nerves) and finally they are processed by the audio interface (~brain).

There are several types of microphones.

• Dynamic microphone is quite a solid type, as she can take the extreme conditions of being used on stage with ease. Should an exhibitionist vocalist grab her by the cable, swing her above his head, drop her accidentally, or even pour some beer on her, a dynamic mic may handle all this impoliteness well. Also, she provides a nice, warm sound suitable for vocals, as well as bass and middle oriented instruments. In other words, her frequency response isn't flat: she tends to emphasize frequencies between ~2kHz and ~8kHz, and suppress trebles above ~10kHz. Bass can also be suppressed a bit (below ~100Hz) -- however, it can be (over)compensated by the proximity effect.
• Condenser microphones are more suitable for use in professional and home studios, as they tend to have a more natural and crisp sound. This type is more demanding than a dynamic one, as she needs careful handling and also phantom power (48V) from the audio interface (or for some types, from a battery). Condenser microphones are more sensitive; with this type you can record more details of the sound environment, i.e. reverberations of the room, but background noises as well. She is better at recording sound sources with a lots of treble (percussions, cymbals, brass instruments, etc.) on stage, but she can be used to record vocals or acoustic guitars as well -- or almost everything in a studio. The frequency response of a condenser mic is pretty flat in general, so she "hears" bass and treble equally well, and there are hardly any colorations in the sound. This makes her recommended to studio quality recordings -- however, a dynamic mic of good quality can provide a more live or "hot" vocal sound.
• One can also meet ribbon microphones and piezo microphones, but they are far less widespread in studios or on stage. The former ones can be rather vulnerable but are said to sound great on vocals, while the latter ones are used mostly in mobile phones as they are small and cheap to produce, although their sound quality is questionable. However, piezo mics are used as contact microphones in acoustic guitars quite often, as they can take vibrations of solid bodies of instruments well.
• Fun fact: a dynamic microphone works just like a reversed speaker. If you sing loud enough into a speaker or a set of headphones, you might be able to record your voice with it when the system is assembled accordingly. This method is becoming more and more popular being used in front of kick drums, as a large speaker (woofer) can record bass well and suppress treble coming from the cymbals at the same time. (You can try this in small when connecting two pairs of earphones to a Y-adapter, wearing one pair and tapping the earpieces of the other (or simply letting them collide).) You can also listen to music via a dynamic mic, although there are way better methods to do this (and it can also be pretty dangerous for the mic).

Both the dynamic and condenser microphones can have different sensitivity in different directions. It means that a particular mic can "hear" perfectly in one direction and worse in another. This direction dependent sensitivity is plotted on the polar pattern (see the figure below), and this can define the possible applications of the microphone.

Fig.1: Polar Patterns in 2D
(source: https://jwoodardvt.wikispaces.com/file/view/polar_patterns.jpg/425698960/polar_patterns.jpg)

As you can see, there are circles, buns, upside down hearts, double buns, dragonflies or just funny blobs or spots on the figure. Actually, they represent how sensitive the microphone is in a particular direction, when the main axis of the mic is pointing upwards. The more the spot approaches the outer circle (dotted line), the more the microphone hears in that direction. Maybe the following figure can help a little:

Fig.2: Polar Patterns explained in 3D
Note: the labels "Cardioid" and "Supercardioid" had been switched accidentally!
(source: http://soundtech.files.wordpress.com/2009/02/mics-polar-patterns0101.png)

Microphones of different polar patterns can be used in different situations.

• An omnidirectional mic hears in every direction around herself, so she can be useful when recording the sound of the whole room, all the reverberation, or even several speakers or vocalists around the mic.
• A cardioid mic hears well from the front (on axis) but hears almost nothing from the back. She can be useful when one wants to record only a single vocalist or instrument and not the others (also not too much of the reverberation), and on stage the vocalist can turn the rear of the mic towards his stage monitor to prevent feedback. Proximity effect is also to be considered, as when a cardioid mic is close to a speaker or the vocalist's mouth, its bass sensitivity rises sharply, and one can use that to their advantage.
• Supercardioid and hypercardioid mics are quite the same as the cardioid ones, but they hear a bit more from the back.
• A bi-directional mic hears well from the front and from the back, but not from the sides.
• A shotgun microphone hears almost only on axis, so it can be useful to record a particular speaker or sound source from afar.

That's all for now, we will continue next week :)