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There are three main pick-up patterns used to mic voices, instruments, and a variety of other sounds.  The particular pick-up pattern that’s needed is heavily dependent on a number of factors.

First, you must consider the source you wish to mic.  Frequency plays a major part in this, as does the ambiance of the surrounding area.  Velocity is also another determining factor.  This is because certain microphone pick-up patterns work better with low frequencies and high velocities for instruments such as drums, while others are better suited to higher frequencies and lower velocities for smaller instruments or vocals.  The three main pick-up patterns are: cardioid, figure-eight, and omnidirectional.  Though other patterns exist, these are the three most commonly used and are the basis from which the others exist.

Cardioid

The cardioid pattern is used to pick up sounds from one direction.  Because of this, it is often referred to as “unidirectional”.  The name cardioid comes from the heart-shape of the actual pattern.  This pattern is popular for sound reinforcement, or when recording concerts and other live events where audience noise could possibly interfere with the show if picked up by the mic.  This pattern also works well when miking vocals.

The concept is to have a mic that picks up sounds from the source at which it’s directly pointed.  However, one problem with this type of pattern is that sounds from the back of the microphone are not completely rejected.  Generally, the smaller the diaphragm of the mic, the better this works.  The reason is that larger diaphragm mics have too much diameter, which only serves to flatten the cardioid pattern.  This is heavily due to frequency.  The sounds coming from the back are actually reduced 10 to 30 DB.

The second problem, which can tend to be more problematic, is that the actual shape of the pick-up pattern will vary with frequency, causing it to be uneven.  This can add undesirable noise to the end result, whether live or recorded in the studio.  Another effect is that microphones with this pattern emphasize the low frequency components of any source very close to the diaphragm.  This is known as the “proximity effect.”  This can serve as an advantage to singers and radio announcers whose voices are light instead of boomy, and will help compensate for that lack of boominess by accentuating the lower frequencies needed to carry those voices.

Bi-directional

A bi-directional pattern is one that only picks up sounds from both the front and rear of the diaphragm, while rejecting sounds from the sides.  This pattern is also known as a figure-eight pattern, and can work well in a number of situations.  Some drummers prefer to use bi-directional microphones when miking certain drums because it eliminates the sounds of symbols and other drums from bleeding through, thus making it much easier to properly EQ each individual drum.  It also works great when miking two singers who are performing live in a duet situation.  In this case, the mic is placed an equal distance between both singers who stand on either side of it.

One problem with this type of pattern is the difficulty of using it in a variety of situations.  Primarily, this is because it accepts sounds from both ends, and therefore, it’s usually placed above an instrument.  A bi-directional mic, however, is not often placed directly in front of an instrument, because it would not achieve the desired effect, and other unwanted instruments may be picked up as well.

Omnidirectional

The omnidirectional pick-up pattern is the simplest one, because it picks up all sounds in the vicinity, regardless of the direction the mic is pointed.  These mics are very easy to use, and often have good frequency response.  Omnidirectional microphones are great for miking choirs and ensembles, and they work well as overhead mics, too.

There are numerous types of microphones, each with a specific polar pattern.  These patterns are important both in live and studio situations, and play a significant role in the overall sound of the finished product.  You should take this into consideration before choosing a mic.