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June 2017 Giveaway

How to Make a Vocal Mic Feed Back — and How to Avoid It

You’ve probably seen this: In the midst of emotional throes, a singer cups their hands around the pop screen of the mic. Unfortunately, that high-pitched whine isn’t the heart-wrenching high note the singer intended – rather, it’s ear-piercing feedback! Why, they ask? “Oh why, why, why, and just when I was about to break hearts and take names…”

The reason is this: A microphone’s polar pattern is largely responsible for controlling feedback. The mic’s polar pattern and subsequent feedback resistance is determined by how much sound hits the front of the mic’s diaphragm, and how much hits the rear. As you may know, sound hitting the diaphragm causes it to move. That movement is translated into electromagnetic energy. Sound hitting the front of the diaphragm causes it to move backward, whereas sound hitting it from the rear causes it to move forward. So, it stands to reason that if equal amounts of sound pressure (SPL) struck the diaphragm’s front and rear simultaneously, the diaphragm would not move and the sound would cancel out. By controlling the amount of sound that strikes the front and rear of the diaphragm, we determine the mic’s pickup (polar) pattern. This is achieved by designing the mic in such a way that specific amounts of sound energy are allowed to enter from the rear. Highly directional mics such as shotgun mics have ports built in along the length of the capsule housing for just that purpose. With your basic vocal mic, the ball-shaped grille is designed to allow a certain amount of energy to strike the rear of the diaphragm. The resulting phase cancellation determines the mic’s polar pattern (e.g. cardioid, hypercardioid, etc.), and therein, the mic’s ability to resist feedback. Omnidirectional mics pick up sounds from all directions since they have no rear ports, and thus, no way to cancel sound. When the aforementioned singer cupped their hands around the ball of the mic, the polar pattern was changed from cardioid to omnidirectional, which in our hypothetical club, picked up the sound coming from the stage monitors creating a feedback loop — hence, the ear-splitting, performance-wrecking squeal.

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