Tech Tip:How microphone patterns work
By Peter L. Janis; President - C-Tec . Radial . Tonebone . Primacoustic . Gefell . Zebracase
The other day, while in a Nashville restaurant, the band invited an audience member up to sing. During the performance, the sound system began to feedback and howl. It was working fine for the band. What changed? Why feedback all of a sudden? The answer: the singer was holding the microphone with his hands cupping the grille ball. What he was doing was effectively altering the feedback eliminating unidirectional pattern and turning that cardioid mic into an omnidirectional mic. It occurred to me at that instant that I had to write a tech note on this subject.
Phase control, or more precisely phase cancellation, is the key to controlling the microphone's pattern. But before we get into how the directionality or polar pattern of a mic is created, we must first understand how a microphone works. There are essentially two types of microphones: dynamics and condensers.
microphones work exactly like backwards speakers in that they employ a moving coil mounted on a magnet. Sound vibrates the diaphragm, forcing the coil to move in and out of the magnetic field, creating current which in turn is preamplified by a mixer or mic preamp and then supercharged by the amplifier that drives the speaker.
Condenser microphones employ a power source to charge a metallized capacitive diaphragm. Sound hitting the diaphragm causes the capacitive value to change as the diaphragm moves in and out, creating a differential voltage and current.
In both cases, sound (or air vibrations) is picked up with a diaphragm. This is where it gets interesting. When sound reaches the front of the diaphragm, it makes sense that the diaphragm would vibrate in one direction. But if sound were to hit the diaphragm on the opposite side, it would make sense that the vibration would be reversed. Herein lies the magic: By controlling the amount of sound that hits the front and the rear of the diaphragm, we can control the pickup pattern. If 100% of the energy were to hit the front and the back at the same time, with one pushing in one direction and the other pushing against it, the sound would cancel out. They would be 180 degrees out of phase.
By limiting the sound hitting the back side, we can control how much cancellation we want. This is why most cardioid vocal mics have a ball grille. This ball is designed in such a way to allow energy to strike the front while being partially open at the bottom to allow a bit of energy to strike the back. The resulting phase cancellation is used to create the cardioid or hypercardioid polar patterns that make it possible to resist feedback. Because the guest singer was cupping his hands around the bottom of the ball grille, he was closing the rear ports that create the pattern and effectively turning the cardioid mic into an omnidirectional mic. Omnidirectional mics have no rear entry ports for sound, thus they do not have any way to cancel sound. Omnis pick up sound from all directions, making them great for lavalieres or for recording in controlled environments. But because they pick up sound from all directions, the sound from the stage monitors was being picked up, causing a feedback loop.
Condenser mics work the same way except that instead of having an access port on the back side, they employ a second plate or diaphragm that is charged and polarized. By changing the voltage and polarity on the back plate in reference to the front sound-receiving plate, one can create cardioid, omni, and figure-eight patterns. So the lesson is simple: If you cup a cardioid mic with your hand, you are basically closing off the phase-cancelling ports that actually create the directional pattern. The result is feedback. Have fun!