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By properly selecting the right microphone and deploying it optimally, the A/V operator can avoid and even eliminate many typical problems. The best placement depends on a range of factors, but the underlying principles are constant.
For simplicity, we will assume a typical corporate sound system, with individuals as the main sound sources and a basic loudspeaker PA system for reinforcement. The main idea is to maximize intended sound (speech) while minimizing unwanted sounds. This section offers a series of pro tips designed to do just that.
Because audio follows the inverse-square law, you’ll get the best results by placing the microphone as close as possible to the intended sound source. This is the single simplest thing you can do to maximize audio intelligibility while minimizing the chance of feedback.
Similarly, it is important to ensure that loudspeakers are not aimed at any microphones. In a typical rectangular room, this is most simply done by making sure no loudspeaker is behind any microphone position. If the talkers are mobile, it’s important that they be instructed
not to walk in front of the loudspeakers, as this is very likely to cause feedback.
Knowing and utilizing the null angle of a unidirectional microphone to avoid unwanted sound is a great way of avoiding trouble. If a loudspeaker is aimed at the space where a talker is stationed, be sure the microphone is positioned such that any loudspeakers are aimed as far
off-axis as possible – preferably at its null point.
Here are a few more key factors in determining the best deployment of your sound system’s microphones.
Proximity Effect – This refers to an increase in low frequency response of unidirectional microphones at short distances. Thus, when a presenter nears the mic, the sound becomes both louder (due to inverse-square law) and more bass-intensive. When very close to the mic, the proximity effect can be quite pronounced, lending an authoritative tone to the speaker that can be used to great advantage.
Pro tip: Because extreme close-up speech also makes the vocal air stream audible to the microphone, it’s important to have adequate pop filtering in place. If the microphone is unpleasantly loud on plosive consonants (b, d, p and t sounds), consider adding an external foam windscreen to eliminate the popping.
Interference Effects – An unpleasant interaction between direct and delayed sound. There are three main types of interference effects, all caused by separate, avoidable circumstances:
Reverse polarity causes physical cancellation of sound. In balanced microphones, this can happen when two microphones and their cables are wired with opposite “hot” pins. You can test this by substituting mics and cables already known to be good and listening to the sonic result.
Multiple Microphone Pickup is another interference effect, caused by mixing two microphones at different distances from the same sound source. Due to the delay time
in traveling to the more distant mic, peaks and notches are created in the frequency response of the mixed signal, called comb filtering, causing a hollow, distant sound. The obvious solution is to avoid picking up the same source with more than one microphone. Since that is not always possible in conference situations, use the 3-to-1 Rule to ensure that any comb filtering effects are inaudible.
When using multiple microphones, the distance between any two active mics should be at least three times the distance between the microphone and its source.
Pro tip: If a presenter wearing a lavalier microphone steps up to the podium or picks up a handheld mic, make sure only one of those microphones is active! Similarly, if a podium is double-miked with two gooseneck microphones for redundancy, only one should be turned up at a time.
Reflection Pickup is the third form of interference effect. The problem is that sound arriving at a single microphone along multiple paths (and thus at slightly different times) causes comb filtering. The effect becomes audible when the mic is placed too close to a reflective surface, causing the reflected sound to be nearly as loud as the direct sound.
Reflection pickup issues are often solved through mic positioning, reorienting things so that the direct sound is more dominant. Changing the angle of the microphone so that reflections are in the null point of the pickup pattern is the most effective solution. It is also helpful to move the microphone closer to the sound source, or move it away from the reflective surface. Covering the reflective surface with sound-absorptive material can also be helpful.
It should be noted that boundary microphones address this very issue by embracing the reflections. This is done by moving the microphone element very close to the reflective surface (1/4-inch or less), making the delay so short that any comb filtering occurs only at inaudibly high frequencies.
Pro tip: Use Minimal Microphones
Multiple microphones can create other problems. Every open channel adds a small bit of noise to the system, and noise is the enemy of intelligibility. In addition, each open microphone channel adds more ambience to the mix, increasing the potential for feedback.
In general, always use the fewest microphones needed to get the job done. If the system you are using has more mics than your meeting has participants, be sure those extra channels are turned off.
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