January 1, 2005
Problem: You place more than one microphone on an instrument, a guitar amp, or some drums, and when you listen to them mixed together, some frequencies sound thin or are attenuated almost entirely.
Cause: The signals from the mics are probably arriving at your ears (and the recorder) at slightly different times, resulting in phase cancellation. The subject of phase relationships is complex (consult any good audio handbook or Web site for details), but put very simply: When waveforms of the same type are aligned in time they reinforce each other and produce a louder and fuller sound than they do individually. When not aligned, however, the waveforms work at cross purposes and tend to cancel each other out. The closer the alignment of the waveforms, the less phase cancellation occurs.
- Reposition the microphones so that they are equidistant from the source, using your ears to evaluate the results.
- Use a different combination of microphones.
- Flip the polarity (or “phase”) invert switch on one channel of your preamp or mixer— especially if two mics are positioned facing each other on either side of the source. Or,record to individual tracks, then use recording software to invert the polarity of one track. If one of your mics has a polarity switch, inverting its polarity may also remedy the problem.
- Record to individual tracks, and then use a waveform editor to nudge one track and align the waveforms visually.
- Run the mic closest to the amp through a delay line and set the time in the sub-millisecond range.