Instruments and other sound sources are characterized by their frequency output, by their directional output, and by their dynamic range.
Frequency output is the span of fundamental and harmonic frequencies produced by an instrument, and the balance or relative level of those frequencies.
Musical instruments have overall frequency ranges as found in the chart below. The dark section of each line indicates the range of fundamental frequencies and the shaded section represents the range of the highest harmonics or overtones of the instrument. The fundamental frequency establishes the basic pitch of a note played by an instrument while the harmonics produce the timbre or characteristic tone.
It is this timbre that distinguishes the sound of one instrument from another. In this manner, we can tell whether a piano or a trumpet just played that C note. The following graphs show the levels of the fundamental and harmonics associated with a trumpet and an oboe each playing the same note.
Instrument frequency ranges.
The number of harmonics along with the relative level of the harmonics is noticeably different between these two instruments and provides each instrument with its own unique sound.
A microphone which responds evenly to the full range of an instrument will reproduce the most natural sound from an instrument. A microphone which responds unevenly or to less than the full range will alter the sound of the instrument, though this effect may be desirable in some cases.
Directional output is the three-dimensional pattern of sound waves radiated by an instrument.
A musical instrument radiates a different tone quality (timbre) in every direction, and each part of the instrument produces a different timbre. Most musical instruments are designed to sound best at a distance, typically two or more feet away. At this distance, the sounds of the various parts of the instrument combine into a pleasing composite. In addition, many instruments produce this balanced sound only in a particular direction. A microphone placed at such distance and direction tends to pick up a natural or well-balanced tone quality.
Instrument spectra comparison.
On the other hand, a microphone placed close to the instrument tends to emphasize the part of the instrument that the microphone is near. The resulting sound may not be representative of the instrument as a whole. Thus, the reinforced tonal balance of an instrument is strongly affected by the microphone position relative to the instrument.
Unfortunately, it is difficult, if not impossible, to place a microphone at the “natural sounding” distance from an instrument in a sound reinforcement situation without picking up other (undesired) sounds and/or acoustic feedback. Close microphone placement is usually the only practical way to achieve sufficient isolation and gain-before-feedback. But since the sound picked up close to a source can vary significantly with small changes in microphone position, it is very useful to experiment with microphone location and orientation. In some cases more than one microphone may be required to get a good sound from a large instrument such as a piano.