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preferable to place room mics relatively close to their near field brethren. As always, use your ears! The three-to-one rule The second variable in comb filtering is the relative level between near and far microphones. If amplitude is equal between the two signals, cancellation at specific frequencies can be nearly complete, and a hollow sounding response results. If one of the mics is quieter, phase interference is minimized, and the tonal balance improves. This is typically how engineers solve the room-mic conundrum, and a simple rule has been created to help reduce phase cancellation in the recording process. Called the “threeto-one rule”, the tenet states that when placing two microphones on a sound source, the mics should be separated by at least three times the distance the closer mic is to the source. For example, if a close microphone is one foot from the source, the room mic must be at least three feet from the close mic. The three-to-one rule functions on the premise that sound level decays with distance. The room mic, farther from the source, will be lower in level and thus phase issues will be curtailed. Note that the rule does not function within in the confines of the near field though, as

sound pressure level does not predictably decay close to a source. Thus the rule is rarely effective within the dimensions of a piano or other large instrument. Like all “rules” in audio, the three-toone rule is not mandatory, but just a suggestion. I rarely follow it. For one thing, regardless of microphone separation, simply lowering the level of room mics reduces comb filtering. I also find equalization quite effective. Filtering room-mic low frequencies lessens low-end cancellations while still keeping higher frequencies of the room audible. Of course, diligently following the rule might save you from needing such solutions. The three-toone rule guarantees the minimal amount of level reduction with the maximum amount of room ambience. Polarity One last solution exists for engineers facing multi-mic phase issues. The polarity switch, found on many preamps and plug-ins, reverses the electrical direction of an audio signal. On a graph, this process completely inverts the visible waveform, flipping it upside down. A powerful tool, polarity reversal grants an alternative phase relationship without the need to move a mic or digitally adjust timing. While sometimes the results are less dramatic than others, one should always

experiment with polarity when recording or mixing separated microphones. Note that polarity reversal, and its symbol “Ø”, is often incorrectly referred to as “phase flipping”. Contributing to the problem, many manufactures incorrectly label their polarity switches as “Phase”. Although commonly misused, “phase” refers to time relationships only, not to electronic reversal of voltage. Conclusion Close miking is at the heart of modern recording, and the skilled engineer knows when and how to mic instruments for maximum punchiness, isolation, and tonal variation. Miking at a distance is essential in certain music styles, as it harks back to the sounds of earlier eras, and it imparts the unique acoustics of a performance space on a recording. To many engineers, the ideal solution is a blend of near and far microphones. Understanding the physics of sound is key to resolving the phase complexities of separated microphones. Eric Ferguson (ferguson@recordingmag .com) recently escaped Los Angeles after a dozen years of freelance audio engineering. He is currently on the faculty of the New England School of Communications (NESCOM) in Bangor, ME.

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Recording - March 2012

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