Microphone polar patterns explained: Bi-directional

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Directionality is the sensitivity of a microphone to sound, relative to the angle or direction from where the sound is coming from. Several directional patterns are available, and these are represented in graphs and are called polar patterns. The graph of a polar pattern shows the change in sensitivity 360-degrees around the microphone. This is as long as the microphone lies at the center and the 0-degree point is in the front.

There are three basic types of microphone polar patterns, these are the unidirectional, omnidirectional, and bi-directional. So, in this post, we are going to be looking at the bi-directional polar pattern.

What is the bi-directional polar pattern?

bidirectional-microphone-pattern

The bi-directional polar pattern is otherwise called Figure 8. It is a pattern that is symmetrically sensitive to the sounds projected from front and back, while the sounds coming from the sides are rejected. Diagrammatically, the diagram of the bi-directional polar pattern looks or is similar to Figure 8.

The pressure gradient principle forms the foundation upon which the bi-directional polar pattern is based upon. This simply means that both sides of the diaphragm of a microphone are exposed equally to the pressure from external sound.

A sound wave coming from the front of the microphone hits the front and the back of the diaphragm. Just like an equal sound wave coming from the back of the microphone would hit the back and therefore the front of the diaphragm.

There’s only one difference between the front and the back of a bi-directional diaphragm, and that is the possible polarity of the microphone signal. There is a positive polarity reaction when sounds hit the front of a diaphragm, on the other hand, sounds react with negative polarity when they react with the back of the diaphragm.

The sounds that come from the direct (or opposite) sides of a bi-directional microphone (at 90° and 270° on the polar pattern graph) hits both sides of the diaphragm. Each with the same or equal amplitude and the same opposite polarity on the two sides of the diaphragm and they cancel each other out. This is why there are null points and “rings of silence” around both sides of a bidirectional microphone. 

The ideal bidirectional pattern comes with an acceptance angle of approximately 120° directly on the front and rear axis. What this means is that sounds ideally starts to drop (at about 6 dB) when the source of a sound is 60° off-axis. The off-axis coloration of the microphone at this point starts to become obvious because the frequency response specifications of the microphone are compromised.

What can I use a bi-directional microphone for?

Bidirectional microphones are commonly used in various applications where there is a need to capture sounds from either front or back of a microphone (or both), but not from the sides of the microphone. One typical example of this application is during a question and answer session of a lecture, where a professor (or class teacher) gives a lecture to his or her audience and actively accepts questions from them at the same time.

Consequently, the microphone is needed to pick the teacher’s speech from the front and also the questions from behind, among the audience or students. This is one of the most practical examples where a two-way or a bidirectional microphone is required.

Best applications for a bi-directional microphone

There are various applications for a bi-directional microphone. And some of these applications include the following.

  1. Maximum rejection of sound from the side is required.
  2. Conversation of people directly sitting in front of themselves is to be captured or recorded using one microphone.
  3. There is a need for maximum proximity effect.
  4. There is a need to capture less room, but still needs the initial reflections on the back of the microphone to be captured.
  5. To record “side” information which will be completely canceled out if the stereo mix is summed to mono.

Pros of bi-directional microphone

  • The use of a bidirectional microphone is the same thing as using a two-in-one microphone. The bidirectional mode offers the advantage of using a microphone between -for instance, two singers, to record them rather than using two different microphones. The two singers can face each other at the two ends of the microphone. It is more practical and with a better advantage too because the two singers can see the other person to know the cues, timing, and the other necessary communications.
  • When you’re recording a singing guitarist and an instrumental guitarist, the simple solution is to record the two separately. But then, this doesn’t always happen to be the best solution when the guitarist is singing. If the performer is comfortable doing both at the same time, a bidirectional mic will be of help. With a bi-directional microphone polarity pattern, it is possible to place a microphone between the artist and the guitar to record both sounds.

Cons of bi-directional microphone

  • One of the potential problems of using the Figure 8 pattern is the recording of unwanted sounds, which may include simple things like computer sounds, a standing fan, or even a snoring dog. Of course, some microphones support multiple patterns, so there may be a switch to turn off the undesired side. If this is the case, always have it at the back of your mind that the switch must be turned on or off as the case may be so as not to pick up unwanted sounds. 
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