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Sound Advice

Audio Phasing: Part II by Al Whale

Comb filtering, which produces a hollow, diffuse, and thin sound, will occur with one microphone receiving the same sound from two sources. A common example of this is shown in image E. If the microphone had been closer, the difference in the direct path and the reflected path would have been greater, thus the reflected path’s reduced level would have had less effect. Also the reflected source volume would have been less if the floor had been carpeted.

Methods of correction:
1. Keep the vocal audio mix low into the monitor.
2. Handhold or place the microphone closer to the singer.

While the monitor helps the singer, as the monitor’s gain is increased, the resulting vocal will be more muffled. Many professionals use in-ear monitors to eliminate this effect. Although not popular with the performers, using music only on the monitors (no vocal) will also minimize comb filtering. Often, the house audio suffers when trying to improve the monitoring for the performers.

This article was prompted after I attended several concerts in which the music was excellent, however the dialogue was difficult to understand. Most of the production crews knew the script so well that they were unaware of the problems. If you asked the audience, they would probably say that they thoroughly enjoyed the music. If you were more specific and asked them about the script, they probably would be unable to answer. The comb effect of excessive use of stage monitoring would mush the dialogue so that the audience (which doesn’t know the words) would be unable to understand them. If the concerts are trying to tell a story, they basically miss the goal and only provide enjoyable music.

Ideas to reduce comb filtering:
· Reduce the number of paths from the same audio source.
· Fewer microphones.
· Reduce the possibility of reflections.
· Reduce the relative amplitude of the additional paths.
· Increase the difference in path lengths, thus the secondary path will have more attenuation.
· Use absorbent material.
· Use directional qualities of the microphones.

The following sites assisted in this article: Calculations of attenuation over distance www.mcsquared.com/dbframe.htm; calculations of distances www.pagetutor.com/trigcalc/trig.html.

Al Whale is a Broadcast Technologist and Assistant Chief Engineer at CHBC-TV. He has also set up and operated sound systems and taught sound in many church settings. Reach him at awhale@chbc.com.

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