Listener's perception differs widely between being a spectator in a recital and listening to a CD at home.  Lack of visual support for auditory signals when one listens to a recording creates a set of problems for performer on the CD.  In recording the artist confronts challenges with volume of sound, texture, nuance in phrasing and general sound quality, which were never an issue in live performance.  Recording versus live performance can be compared to acting on stage versus acting in motion pictures.  On stage one needs to exhibit everything to extreme, while in the movie a slight change in facial expression can have a powerful effect.

It's easy to say that a clear sound, good intonation, musicality and attention to details will guarantee a good recording, but unfortunately that is not the case.  The set up of microphones, performer's distance from the microphones, acoustics of the room, and sound production by the artist all play a role in recording experience.  Understanding of physical properties of sound and capabilities of microphones will be of help in creating a successful recording.

Physical properties of Sound

When sound is produced it incites a series of compressions and rarefactions of the air molecules, spreading in all directions.  One complete cycle of compression and rarefaction is called a sound wave.  The frequency with which sound waves pass through a specific unit of time (such as a second) is what we refer to as pitch. 

We are all familiar with A=440.  This sign represents that the frequency of 440 complete wave cycles per second is the pitch we hear as A.  Notes within a scale are based on ratios between different frequencies of sound waves.  For example when notes are one octave apart, the upper octave is exactly double the number of frequencies of the lower octave.  A above middle C is 440 waves per second (also known as Hz).  An A above that is 880 Hz.  Same applies to octave below (an A below the 440 Hz is 220 Hz).

It is important to think in three-dimensional terms rather then one dimensional when trying to envision sound waves.  In fact sound waves are often compared to circles on the surface of water, when stone is dropped into it.  Although that gives an excellent visual for the wave pattern, it presents a view of sound waves which is linear rather then spherical.  

 When a note is sounded we hear the actual pitch, for example C at 66 Hz.  Along with that pitch, called the fundamental, there is a range of multiples of this frequency, called overtone series, which is produced through the vibration of the string.  Those multiples are double, triple, quadruple, etc. of the original fundamental.

The unique color of the sound we produce on the viola is a reflection of how dominant or secondary specific frequencies are within the overtone series. 

 Sound production of viola is very complex and rich in nuance unique to each specific instrument.  The wood, strings, tuning, quality of the bow, all effect resulting sound.  However there are a few general things that universally apply to sound production on the viola.  The hollow body of the viola, coupled with sound board, and F holes forms a resonator (called Helmholtz resonator).  That is what makes it possible for viola to project.  After the sound is initiated (via bow or pizz.) the sound waves spread in all directions.  The note frequency as well as general approach to the string from both bow and left hand are responsible for the range of overtones, and color of sound.

 Below is a chart of directional characteristics of the sound of the viola at different frequencies.

This illustration is an approximation in both frequencies, and directional patterns, but it does give a general idea of directional patterns at low, middle and high frequencies.

Microphones: types and capabilities

 In response to the physical properties of sound, microphones have been designed to react to auditory stimulus in a way that reflects the spherical qualities of the sound waves.  All microphones posses a membrane or plate or other device whose sole responsibility is to respond to the volume of sound present. 

Below is an illustration of response of one such membrane.

Main patterns in which the sound is picked up by microphones are omnidirectional, bi-directional and unidirectional.  Unidirectional microphone picks up only sounds arriving in front of it, and rejects sound from sides or back. 

Bidirectional microphone picks up sound from front and back of it.

Omnidirectional microphones pick up sound evenly from all directions.  

Stereo microphones are made up of a capsule encasing two omnidirectional microphones set at 120 or 90 degree angles (typically adjustable by user).

Below is an actual directional pattern from Sony Stereo Microphone model# ECM-959A

Since viola reaches frequency of around 3,000Hz, we do not need to be concerned with top frequencies not coming through, since the level of reaction to volume drops after around 10,000 Hz.  On the other hand we do need to be concerned with pick up of lower frequencies, because a lot of our action in first position on the C string (between 130 Hz and 196 Hz) falls within low response zone indicated above.

For your general reference, below is a chart of how frequencies correspond to specific pitches in equal temperament tuning system:

Getting your musical vision across: playing and recording

With greater familiarity of the physical properties of sound, and microphones comes a reasonable question of how to put all this information to practical use in a recording situation. 

First point that needs to be addressed is frequency versus volume.  Since the scope of frequency response goes beyond the actual needs of a violist, while volume response is significantly below typical level used by violists, one needs to come up with a solution for keeping wide dynamic range while not sacrificing the genuine qualities of sound by moving microphone significant distance from source of sound (viola).  The solution to this problem is to use less pressure, and get the string to vibrate to its maximum ability at instances when high volume is desired.  What that accomplishes is an illusion of greater volume through increased scope and resonance of the overtone series above each fundamental note.  

The next point is how low is too low, when it comes to dynamics.  When the microphone is set close to the player (1 to 5 feet) within recording setting one can use a palette of colors typically unavailable when playing in a hall for a large number of people.  Occasionally so called "air" in the sound becomes a commodity rather then a defect when one aims to get a whispering soft sound out of the instrument.  Sound that is very focused and direct often becomes one-dimensional when recorded, and looses roundness and sense of space.  Sound in soft dynamics doesn't always have to have a solid core to it.  The string needs  the space to resonate and vibrate in softer passages, and a more relaxed, even breathier approach to producing a sound can bring unexpected and wonderful results. 

Vibrato is another Achilles hill when it comes to recording.  Often vibrato, without player even realizing that, is too narrow or tight.  In recording it doesn't come through as a tone enhancing tool, but rather makes the sound simply flat, and dull.  For recording vibrato needs to be on a slightly wider side, and frequent enough to be warm and noticeable, but not so frequent as to sound frantic.  The tip of the vibrating finger needs to be loose in order for string to have maximum chance to vibrate on its own.  If vibrating finger presses the string too much it cuts down on string's natural flexibility and vibration, and hence vibrato's effect is minimized because the string is not going to respond favorably to force.   

In conclusion, recording is a highly personal journey.  Information listed above will help in resolving many practical matters in your recording process, but with all theoretical knowledge and self-criticism, it is important to never lose sight of what you set out to do from the start of recording process.  Ultimately the goal is to share your musicianship with the audience, and in the process grow and learn a little bit more about yourself.