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The frequencies that will be accentuated and attenuated can be determined by the difference of the distance between the reflective surface and the microphone (D1), and the distance between the sound source and the microphone (D2), in relationship to air velocity. The amount of reinforcement and cancellation of certain frequencies that will be perceived when the two signals are combined will be determined by their particular amplitudes. Constructive and destructive interference are most pronounced when the two signals are of equal amplitudes. As the difference in amplitude values between the two signals becomes larger, the effect becomes less noticeable.
The constructive and destructive interference of the combined signals result in a frequency response with emphasized and attenuated frequencies (peaks and dips). These peaks and dips of the frequency response curve of the sound's spectrum have been compared, in analogy, to the tines of a comb. Thus, the term comb filter has been applied when a signal is combined with itself, with a slight time delay between the two signals.
The distance of the microphone to the sound source, and other sound sources in the environment, along with the height, lateral position, and angle of the microphone, alter the sound quality of the sound source. As such, they are variables that may be used as creative elements in shaping sound quality (and the musical material), in the same way as microphone selection, above. These areas will most significantly influence the following aspects of sound:
• Environmental characteristics (secondary distance) cue:
ratio of direct to reflected sound
• Environmental characteristics cue:
early time field information created by initial reflections
• Distance cue: definition, or amount, of timbre detail
• Capturing of the blend of the source's timbre
The distance from the microphone to the sound source is the primary factor that controls the ratio of direct to reflected (indirect) sound. As the microphone is placed closer to the sound source, the proportion of direct sound increases in relation to reflected sound.
Reflective surfaces that are located near the sound source will cause inconsistencies with this general rule. If at all possible, sound sources should be placed at least three times the distance of any reflective surface, as the distance from the microphone to the sound source. The height and angle of the microphone in relation to the sound source, coupled with polar patterns, may allow the microphone to keep from picking up the reflected sound off surfaces close to the sound source (such as the floor or a gobo).
Distance cues are established by microphone placement when an audible amount of sound from the recording environment is present in the source's sound quality. While this is not usually the case with close miked sound sources, many sound sources in multi-track projects are recorded from a distance that will capture certain information from the recording environment. Of course, all distant micing techniques will capture a significant amount of the sound of the environment.
The reflection information in the early time field is especially prominent in recordings that have utilized a moderate distance between the microphone and the sound source. These reflections will often be caused by the floor or other objects immediately around the sound source. While only a few reflections may be present in the final sound, and the reflections may be of significantly lower amplitude than the direct sound, they will impart important environmental characteristics information. These reflections often provide environmental cues that conflict with the perceived stage-to-listener position distance cues.
Microphone placement location and performance characteristics will play vital roles in establishing distance cues, through defining the amount of timbre detail present in the recorded sound source.
Generally, the closer the microphone to the sound source and/or the more sensitive the microphone in terms of distance sensitivity and transient response, the more timbre detail that is captured when recording the sound source. This provides a distance cue that may place the listener of the recorded sound near the reproduced image of the recording. This may be accomplished in such a way that the listener may imagine themselves performing the instrument, located beside the performer of the instrument, or even located adjacent to the location where the instrument is sounding (the location of the recording microphone).
Distance cues are often contradictory when a high degree of timbre detail and a large amount of reverberant energy in relation to direct sound are present simultaneously. This unrealistic sound may be desired for the artistic process, but the recordist must be aware of this dichotomy to be in control of the medium.
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