Instruments and other sound sources are characterized by their frequency output, by their directional output, and by their dynamic range.
Frequency Output
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.
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
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.
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.
Dynamic Range
Dynamic range is the range of volume of an instrument from its softest to its loudest level.
The dynamic range of an instrument determines the specifications for sensitivity and maximum input capability of the intended microphone. Loud instruments such as drums, brass and amplified guitars are handled well by dynamic microphones which can withstand high sound levels and have moderate sensitivity. Softer instruments such as flutes and harpsichords can benefit from the higher sensitivity of condensers. Of course, the farther the microphone is placed from the instrument the lower the level of sound reaching the microphone.
In the context of a live performance, the relative dynamic range of each instrument determines how much sound reinforcement may be required. If all of the instruments are fairly loud, and the venue is of moderate size with good acoustics, no reinforcement may be necessary.
On the other hand, if the performance is in a very large hall or outdoors, even amplified instruments may need to be further reinforced. Finally, if there is a substantial difference in dynamic range among the instruments, such as an acoustic guitar in a loud rock band, the microphone techniques (and the sound system) must accommodate those differences. Often, the maximum volume of the overall sound system is limited by the maximum gain-before- feedback of the softest instrument.
An understanding of the frequency output, directional output, and dynamic range characteristics of musical instruments can help significantly in choosing suitable microphones, placing them for best pickup of the desired sound and minimizing feedback or other undesired sounds.
Instrument Loudspeakers
Another instrument with a wide range of characteristics is the loudspeaker. Anytime you are placing microphones to pick up the sound of a guitar or bass cabinet you are confronted with the acoustic nature of loudspeakers. Each individual loudspeaker type is directional and displays different frequency characteristics at different angles and distances. The sound from a loudspeaker tends to be almost omnidirectional at low frequencies but becomes very directional at high frequencies.
Thus, the sound on-axis at the center of a speaker usually has the most “bite” or high-end, while the sound produced off-axis or at the edge of the speaker is more “mellow” or bassy. A cabinet with multiple loudspeakers has an even more complex output, especially if it has different speakers for bass and treble. As with most acoustic instruments the desired sound only develops at some distance from the speaker.
Sound reinforcement situations typically require a close-mic approach. A unidirectional dynamic microphone is a good first choice here: it can handle the high level and provide good sound and isolation. Keep in mind the proximity effect when using a uni close to the speaker: some bass boost will be likely.
If the cabinet has only one speaker a single microphone should pick up a suitable sound with a little experimentation. If the cabinet has multiple speakers of the same type it is typically easiest to place the microphone to pick up just one speaker. Placing the microphone between speakers can result in strong phase effects though this may be desirable to achieve a particular tone. However, if the cabinet is stereo or has separate bass and treble speakers multiple microphones may be required.
Placement of loudspeaker cabinets can also have a significant effect on their sound. Putting cabinets on carpets can reduce brightness, while raising them off the floor can reduce low end. Open-back cabinets can be miked from behind as well as from the front. The distance from the cabinet to walls or other objects can also vary the sound. Again, experiment with the microphone(s) and placement until you have the sound that you like!
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