What Is a Bidirectional Microphone? (Explained)

If you’re a music producer, sound engineer, or just someone who spends a lot of time recording audio, you’re likely familiar with bidirectional microphones. They’re often used in recording studios and by musicians who want to capture the ambiance and spatial audio.

In this post, I’ll answer some questions about bidirectional microphones and give you an overview of their uses.

What is a bidirectional microphone? A bidirectional microphone, also known as a figure-of-eight microphone, picks up sound equally well in front and rear.

What is a bidirectional microphone?

A bidirectional microphone, also known as a figure-8 microphone, is a type of microphone that picks up sound from two opposite directions, typically from the front and the back of the microphone. This pattern is called a bidirectional or figure-8 polar pattern because the pickup areas resemble the shape of the number “8” or a figure-8 when viewed from above.

Bidirectional microphones are often used in recording studios and for broadcast applications where it is desirable to capture sound from two distinct sources or directions while minimizing pickup from the sides. They are also commonly used in stereo recording techniques, like the Blumlein configuration, where two bidirectional microphones are positioned at a 90-degree angle to create a stereo image.

The bidirectional polar pattern is mostly found in ribbon and some large-diaphragm condenser microphones. It’s important to note that a bidirectional microphone will have reduced sensitivity to sounds coming from the sides, which can be both an advantage (in isolating specific sound sources) and a disadvantage (in capturing a wider soundstage) depending on the recording situation.

How do bidirectional microphones work?

Bidirectional microphones, also known as figure-8 microphones, work by capturing sound from two opposite directions, typically the front and back of the microphone, while rejecting sound from the sides. The internal design of the microphone and the type of transducer used (such as ribbon or condenser) play a key role in achieving this polar pattern.

Here’s a brief overview of how bidirectional microphones work based on the two most common types of transducers:

How ribbon microphones

n a ribbon microphone, the bidirectional polar pattern is achieved through the use of a thin, corrugated metallic ribbon suspended between two poles of a magnet. When sound waves hit the ribbon from the front or back, it vibrates, inducing an electrical current proportional to the sound pressure.

Because the ribbon is open to sound from both sides, it responds to the pressure difference between the front and back, effectively capturing sound in a figure-8 pattern. Sound coming from the sides will have equal pressure on both sides of the ribbon, resulting in no movement and, thus, no output signal.

How condenser microphones

Bidirectional condenser microphones use a diaphragm and a backplate assembly to create a variable capacitor. The diaphragm is usually made of a thin, flexible material like metal-coated plastic or Mylar.

To achieve a bidirectional polar pattern, two diaphragms are used, placed back-to-back with a shared backplate between them. Each diaphragm forms a capacitor with the backplate. When sound waves hit the diaphragms, they cause changes in the capacitance, which is then converted into an electrical signal.

In a bidirectional condenser microphone, sound coming from the front and back will cause one diaphragm to move closer to the backplate while the other moves away, resulting in a difference in capacitance that can be measured. Sound coming from the sides will cause both diaphragms to move simultaneously, either closer or further from the backplate, but the difference in capacitance will remain the same, resulting in no output signal.

In both cases, the bidirectional microphones are designed in such a way that they only respond to sound coming from the front and back directions, minimizing the pickup from the sides, and creating a figure-8 polar pattern.

What are the characteristics of a bidirectional microphone

There are several distinctive features of a bidirectional microphone.

1. Symmetrically sensitive

A bidirectional microphone has a diaphragm that is shaped to get the same sound pressure from each direction. Because its diaphragm is in the middle, the bidirectional microphone can pick up sounds from the front or the back.

2. Positive polarity and negative polarity.

If the sound pressure in front of the bidirectional diaphragm increases, the microphone signal will also show an increase in volume. When a sound wave’s amplitude is positive (when the sound pressure increases), the bidirectional diaphragm at the microphone’s back will produce a negative amplitude.

3. Null points to the sides

Imagine a diaphragm being pushed on from both sides by pressures that are equal in magnitude but opposing in direction. A “null point” is created at the sides of a bidirectional microphone because sound waves cancel each other out.

On a two-dimensional polar response diagram, these directions look like angles of 90 degrees and 270 degrees, respectively. The front of the diaphragm makes microphone signals with positive polarity, while the back makes microphone signals with negative polarity. The sideway sound is just as loud as the frontal, but it has the opposite polarity, so the microphone can’t pick it up.

4. Ring of Silence

A normal bidirectional microphone has a “ring of silence” around it, which means that the signal is being rejected. All sounds from inside the “ring of silence” that is aimed at a bidirectional microphone will cancel each other out because they will hit the diaphragm from different directions at the same time.

5. Greatest proximity effect impact

The proximity effect is usually most noticeable with bidirectional microphones, compared to other microphones. This is because sound pressure might enter the microphone from any side. The difference in amplitude between the two sides of the diaphragm is smaller than the phase difference. As the sound source moves closer to the microphone, the bass response at lower frequencies improves dramatically.

6. Sensitive to vocal plosives

The pressure-gradient idea is at the heart of the bidirectional mic’s operation. Since the diaphragm is porous on both sides, the plosive energy of the human voice can easily cause “popping” by overloading the microphone.

When plosive energy is released from the mouth, the pressure goes up and down quickly, like a gust of wind. Overload and “pop” happen when the pressure on the microphone diaphragm changes quickly and dramatically as the plosive moves around the mic.

7. Realistic sounding

At the right distance, when the proximity effect isn’t very noticeable, the bidirectional microphone makes a very realistic sound. If the bidirectional mic is pointed toward the sound source it is supposed to pick up; it will often pick up the early echoes of the room. The sound becomes more “genuine” as a result of this.

The polar response of a standard bidirectional microphone is also highly stable. This means the microphone doesn’t pick up unnatural sounds when used off-axis.

8. Low gain feedback

Bidirectional microphones’ front and rear sensitivity lobes are mirror images of one another. This means that traditional “cardioid placement” in live sound reinforcement is not ideal for these microphones. Putting a screen in front of a bidirectional mic will cause massive feedback.

However, placing monitors or loudspeakers directly to the sides of the mic will provide massive amounts of gain before feedback, providing the mic with massive amounts of gain before feedback. However, placing screens out to the side is not very effective. Due to this, bidirectional microphones are rarely used for performing vocals in a live setting.

When should you use a bidirectional microphone?

Below are some situations where a bidirectional microphone could be useful:

• When strong rejection from both sides is a must.
• Captures a discussion between people across from each other
• When recording in a small space yet wanting to get the sound reflections.
• Whenever you want to record “side” information, that will be fully canceled out when you sum the stereo mix to mono.

When shouldn’t you use a bidirectional microphone?

There are some scenarios where a bidirectional mic will not perform properly. Among these are:

• Live performances.
• When the acoustic effects of closeness are undesirable.

If you want even more tips and insights, watch this video called “Cardioid vs. Omni-directional vs. Bi-directional Mics, Which Should You Buy?” from the Podcastage YouTube channel.

Frequently asked questions (FAQ)

Do you still have questions? Below are some of the most commonly asked questions about bidirectional microphones.

Conclusion

A bidirectional microphone is an excellent tool for enhanced clarity and sound quality. As the name suggests, it can help you get higher-quality audio with minimal noise when recording from different directions. This microphone is not meant to replace your standard microphone, but it can give a good boost if you need it to record better audio quality.

However, just as you can’t become an expert cyclist by reading a book, you can’t become a great music producer by reading articles alone. It’s time to take action! Go and put what you have learned into practice.

This article covered what a bidirectional microphone is, how it works, and its characteristics. Here are some key takeaways:

Key takeaways

• A bidirectional microphone’s polar and pickup patterns are shaped like in figure 8.
• Bidirectional microphones can record from the front and rear.
• If the sound pressure in front of the bidirectional diaphragm rises, so will the loudness of the microphone signal.

So, do you use a bidirectional mic? And did I cover everything you wanted to know? Let me know in the comments section below (I read and reply to every comment). If you found this article helpful, share it with a friend and check out my full blog for more tips and tricks on music production. Thanks for reading, and never stop making music.

Helpful resources

• Microphone Types & Directionality 
• Choosing a Microphone for Recording Vocals | Tips | WhisperRoom, Inc.™ 
• What Are Microphone Polar Patterns? The Ultimate Guide • Filmmaking Lifestyle