The bidirectional (aka figure-8) microphone polar pattern is one of the three main polar pattern types (along with omnidirectional and cardioid). Understanding the ins and outs of the bidirectional pattern and how it's achieved in microphones will help lay a solid foundation for your understanding of mic polar patterns and microphones in general.
What is a bidirectional/figure-8 microphone? A bidirectional microphone has a figure-8 polar/pickup pattern. It is equally sensitive to sounds from the front and back while rejecting sounds from its sides (ring of silence). The sound captured from the front side capture is opposite in polarity to the sound captured to the rear side.
In this in-depth article, we'll discuss the bidirectional microphone polar pattern in great detail to answer any questions you may have about bidirectional/figure-8 microphones!
The Bidirectional Polar Pattern
A picture is worth a thousand words. Let's start with a diagram of the bidirectional/figure-8 microphone polar pattern:
The bidirectional polar pattern (often referred to as figure-8) is symmetrically sensitive to sounds from the front and back while rejecting sounds to the sides. As we can see above, the polar response pattern graph looks like a figure-8.
The bidirectional polar pattern is based on the truest form of the pressure-gradient principle. This basically means that both sides of the microphone diaphragm are equally exposed to external sound pressure.
So a sound wave from the front of the mic would hit the front and then back of the diaphragm the same way an equal sound wave from the rear or mic would hit the back and then the front of the diaphragm.
The only difference between the front and back of the bidirectional diaphragm is the polarity of the eventual mic signal. Sounds react with the front of the diaphragm in positive polarity while reacting with the rear of the diaphragm in negative polarity.
Sounds coming from the direct sides of a bidirectional mic (90° and 270° on the polar pattern graph) hit both sides of the diaphragm with equal amplitude and opposite polarity and cancel each other out. This explains the null points and “ring of silence” around the side of a bidirectional mic.
The ideal bidirectional pattern has an acceptance angle of about 120° directly on-axis in the front and back. This means that, ideally, sound will only start dropping off (by about 6 dB) once a sound source is 60° off-axis. This is also where off-axis coloration becomes apparent, and the mic's frequency response specification becomes compromised.
Bidirectional Microphone Generalities And Characteristics
- Symmetrically sensitive to sound from the front and the rear (2 lobes of sensitivity)
- The front captures sound with positive polarity while the rear captures sound with negative polarity
- Null points to the sides (90° & 270°)
- Ring of silence around the sides of the bidirectional diaphragm. The ring of silence is consistent across the entire frequency response
- Exhibit the greatest amount of proximity effect of any polar pattern
- Sensitive to vocal plosives
- Sounds fairly natural at a distance from the sound source
- Low gain-before-feedback if a monitor is directly in front or behind
- High gain-before-feedback if the monitor(s) are to the sides
- Becomes more directional at higher frequencies (narrower pattern that remains symmetrical between the front and rear)
- Standard polar pattern of ribbon microphones
- Only achievable in side-address microphones
- Truest form of the pressure-gradient principle in single-diaphragm microphones
- Achieved by combining two back-to-back cardioid diaphragms/capsules at equal amplitude and opposite polarity in dual-diaphragm multi-pattern microphones
Symmetrically Sensitive To Sound From The Front And The Rear
The bidirectional microphone is designed with both sides of its diaphragm equally exposed to sound pressure. Therefore, the bidirectional mic is equally (symmetrically) sensitive to sounds from the front and the back, according to its diaphragm's position.
The Front Captures Sound With Positive Polarity While The Rear Captures Sound With Negative Polarity
The positive amplitude of a sound wave (an increase in sound pressure) at the front of the bidirectional diaphragm will cause a positive amplitude in the mic signal. Conversely, the positive amplitude of a sound wave (an increase in sound pressure) and the back of the bidirectional diaphragm will cause a negative amplitude in the mic signal.
This helps explain the null points or “ring of silence” at the bidirectional microphone's sides. Sound waves from the sides of the mic hit both sides of the diaphragm with equal amplitude and opposite polarity.
Related article: Microphone Polarity & Phase: How They Affect Mic Signals
Null Points To The Sides (90° & 270°)
As we've just touched on, the sounds that come from the sides of a bidirectional microphone will end up hitting both the front and the back of the diaphragm at the same time and with the same phase/amplitude.
Imagine two equal but opposite forces pushing on the diaphragm. The sound waves cancel themselves out, effectively creating a “null point” at the sides of the bidirectional microphone.
These side directions show up as 90° and 270° on the 2D polar response graph.
Another way of thinking about this is that the front of the diaphragm yields positive polarity mic signals while the rear of the diaphragm yields a negative polarity mic signal. Sound that hits the mic from the side applies equal amplitude but opposite polarity, thus resulting in no mic signal at all.
Ring Of Silence Around The Sides Of The Bidirectional Diaphragm
Applying the above two points to 3D space, we find that, in reality, there are no null “points” or axes. Rather, there is a ring of rejection of a “ring of silence” around the typical bidirectional microphone.
Sounds that originate and are directed toward a bidirectional mic from the ring of silence will effectively hit both sides of the diaphragm at the same time and cancel out.
Exhibit The Greatest Amount Of Proximity Effect Of Any Polar Pattern
Generally speaking, bidirectional microphones exhibit more proximity effect than any other microphone polar pattern.
This is because both sides of the microphone are equally open to external sound pressure. The difference in a sound wave's phase between each side of the diaphragm is reduced relative to the difference in amplitude of the sound wave.
This causes a great increase in bass response at lower frequencies as the sound source gets closer to the microphone.
For a full explanation of the microphone proximity effect, check out my article What Is Microphone Proximity Effect And What Causes It?
Sensitive To Vocal Plosives
The bidirectional mic works on the pressure-gradient principle. Since both sides of the diaphragm are open to sound pressure, the plosive energy from human speech can easily overload the microphone and cause “popping.”
When plosive energy leaves someone's mouth, it creates a large amount of pressure followed by a sharp decrease in pressure (like a gust of wind). As the plosive passes around the microphone, the difference in pressure between the front and back of the mic diaphragm increases sharply and causes the overload and “pop.”
For more information on vocal plosives and how to eliminate them in your microphone recordings, check out my article Top 10 Tips For Eliminating Microphone Pops And Plosives.
Sounds Fairly Natural At A Distance From The Sound Source
The bidirectional microphone sounds quite natural at a fair distance where the proximity effect does not overly colour the sound.
Its rear pickup captures a lot of the ambience of the acoustic environment. If the bidirectional mic is pointed at its intended sound source, the rear pickup will often capture the important initial reflections of the room. This adds to the “realness” of the sound.
The typical bidirectional mic is also very consistent in its polar response. This means it lacks off-axis colouration, which ultimately makes the microphone sound more natural.
Low Gain-Before-Feedback If A Monitor Is Directly In Front Or Behind
Bidirectional mics have symmetrical lobes of sensitivity to the front and the back of a bidirectional mic. Because of this, these mics do not fair well with the classic “cardioid positioning” in live sound reinforcement.
Placing a monitor behind a bidirectional mic is a surefire way to get copious amounts of microphone feedback.
To read more about microphone feedback, check out 12 Methods To Prevent & Elminate Microphone/Audio Feedback.
High Gain-Before-Feedback If The Monitor(s) Are To The Sides
Placing monitors or loudspeakers directly to the sides of a bidirectional mic, however, will work amazingly well for feedback rejection, giving the mic great amounts of gain-before-feedback.
Side placement of monitors is not an overly effective method, though. Therefore bidirectional mics aren't generally used for live vocals.
Becomes More Directional At Higher Frequencies
Like all microphones, bidirectional mics become more directional at higher frequencies.
That being said, this microphone pattern is typically very consistent. We'll see examples of this in our bidirectional microphone examples section.
Standard Polar Pattern Of Ribbon Microphones
The vast majority of bidirectional microphones and ribbon mics and the vast majority of ribbon mics are bidirectional.
The typical ribbon element design features a true pressure-gradient ribbon diaphragm equally exposed to sound pressure at its front and back.
For an in-depth guide on ribbon microphones, head over to my article Dynamic Ribbon Microphones: The In-Depth Guide.
Only Achievable In Side-Address Microphones
Because both sides of the bidirectional microphone's diaphragm need to be evenly exposed, the microphone design must be side-address.
There is simply no way to create a perfectly symmetrical diaphragm openness with a top-address design.
Related article: What Are Top, End & Side-Address Microphones? (+ Examples).
Truest Form Of The Pressure-Gradient Principle In Single-Diaphragm Microphones
All directional microphones work on the pressure-gradient principle, where both sides of the mic diaphragm are open to sound pressure.
However, the bidirectional polar pattern is the “truest form” because both sides of the bidirectional diaphragm are equally open to external sound pressure.
Related article: Pressure Microphones Vs. Pressure-Gradient Microphones.
Achieved By Combining Two Back-to-back Cardioid Diaphragms/Capsules At Equal Amplitude And Opposite Polarity In Dual-diaphragm Multi-pattern Microphones
Note that this only applies to dual-diaphragm multi-pattern microphones.
How Is The Bidirectional Polar Pattern Achieved?
The bidirectional (figure-8) polar pattern, in general, is achieved with the true form of the pressure-gradient principle.
Pressure-gradient is an acoustic principle that has both sides of the microphone diaphragm exposed to external sound pressure. The diaphragm movement, therefore, is a result of the difference in pressure between the front and back sides of the diaphragm.
In other words, the amplitude, phase, and angle of incidence of external sound waves play a role in the directionality of the pressure-gradient microphone.
The ideal bidirectional polar pattern results from the truest form of the pressure-gradient principle, where both sides of the mic diaphragm are equally exposed to external sound pressure.
Therefore, the typical ribbon dynamic microphone works on the true pressure-gradient principle and has a true bidirectional polar pattern.
How Is The Bidirectional Polar Pattern Achieved In Multi-Pattern Microphones?
In multi-pattern microphones, it's typically not practical to add in a true pressure-gradient element only to achieve a bidirectional option.
The truth is that the vast majority of multi-pattern microphones utilize a dual-membrane capsule with back-to-back diaphragms or, alternatively, two back-to-back capsules. These diaphragms/capsules will have cardioid patterns.
So in order to acquire a bidirectional polar pattern from two back-to-back cardioid elements, the multi-pattern mic is wired to output the two mic signals at equal amplitude but opposite polarity.
Combining the mic signals in this way effectively yields a bidirectional/figure-8 polar pattern by nullifying the sounds to the sides (90° and 270°) and capturing the sounds symmetrically in the front and back but in reverse polarities.
When Should You Use A Bidirectional Microphone?
Bidirectional mics, in my opinion, are under-utilized. The main reason I see bidirectional microphones used is that ribbon mics are used, and the bidirectional pattern just so happens to be standard in ribbon mics.
Whether you're looking for reasons to use that bidirectional ribbon mic (other than the benefits of using a ribbon) or to engage the bidirectional switch on your multi-pattern mic, here are few applications for the bidirectional microphone:
Best Applications For Bidirectional Microphones
- When maximal side rejection is required.
- When recording a conversation with one microphone of individuals sitting directly across from one another.
- For maximal proximity effect.
- For capturing less room, but still recording initial reflections from the rear of the mic.
- For recording “side” information that completely cancels out when the stereo mix is summed to mono.
There are also situations where bidirectional mics will not fair so well. These applications include the following:
When Shouldn't You Use A Bidirectional Microphone?
- In front of foldback monitors in live sound reinforcement situations.
- When the proximity effect is not wanted.
- For close-miking/isolating single sound sources in noisy environments.
List Of Miking Techniques With Bidirectional Microphones
Here is a list of miking techniques that include bidirectional microphones:
- Close-Miking (mono)
- Blumlein Pair (stereo)
- Mid-Side (stereo)
- Faulkner Array (stereo)
- Ball Boundary Technique (surround sound)
- Double Mid-Side Array (surround sound)
- Hamasaki Square (surround sound)
- OCT-Hamasaki (surround sound)
- Omni+8 Surround (surround sound)
For more information on stereo miking techniques, check out my article Top 8 Best Stereo Miking Techniques (With Recommended Mics).
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Bidirectional Microphone Examples
The Royer R-121 is the flagship microphone from Royer Labs and one of the better-known ribbon microphones on the market today. Like most ribbon mics, the R-121 has a bidirectional polar pattern, and like all bidirectional microphones, it's side-address. The Royer R-121 has become very common in studios worldwide for its incredible natural sound on nearly all instruments (and especially on electric guitar cabinets).
The Royer R-121 Polar Response Graph
As we can see above, the Royer R-121 has a typical bidirectional polar response. Note that its side ring of rejection is consistent with null points at 90° & 270° at all given frequencies.
As expected, the R-121 becomes slightly more directional as the frequencies of sound increase. Overall, though, this Royer mic exhibits a very consistent polar response.
The AEA R84 is the Audio Engineering Associates' first microphone and is the top recreation of the legendary RCA 44-BX. It is a side-address ribbon microphone with a bidirectional polar pattern. With the rise of clean/bright digital recording came the resurgence of warm, natural-sounding ribbon microphones. The AEA R84 brings back the legend of the original RCA 44-BX in a time where ribbon microphones are wanted more than ever.
The original RCA 44-BX is featured in the following My New Microphone articles:
• 50 Best Microphones Of All Time (With Alternate Versions & Clones)
• Top 12 Best Vintage Microphones (And Their Best Clones)
The AEA R84 Polar Response Graph
The AEA R84's polar response graph is truly a work of art. This microphone exhibits pristine consistency from 200 Hz to 10,000 Hz.
As with all true bidirectional polar patterns, there are null points at 90° & 270°, and the front and rear sensitivities are perfectly symmetrical.
The Coles 4038 is a legendary ribbon microphone designed by the British Broadcasting Corporation in the 1950s. It is a side-address bidirectional ribbon microphone. The 4038 is still in production by Coles Electroacoustics and is still cherished as a broadcasting microphone and as a workhorse in the studio environment.
The Coles 4038 Polar Response Graph
The Coles 4038 is a microphone with the above polar pattern graph. Because the general bidirectional pattern is very consistent, the vintage Coles 4038's polar pattern was perhaps never measured across various frequencies by the BBC or Coles Electroacoustics.
AKG C 411
The AKG C 411 is marketed as an instrument pickup for acoustic guitars, violins, mandolins, and other stringed instruments. This “instrument pickup” is a side-address electret condenser with a bidirectional polar pattern. The C 411 is one of the few non-ribbon, non-multi-pattern microphones that has a bidirectional polar pattern.
The AKG C 411 Polar Response Graph
At first glance, this polar response is awfully confusing. It is a split graph showing 125 Hz – 1 kHz on the left and 2 kHz – 16 kHz on the right.
Interestingly, this graph does not have front/back symmetry, and there is no legend for the broken line. Why is this?
Well, the C 411 instrument pickup has a condenser diaphragm that, unlike a standard ribbon diaphragm, does not work on the truest form of the pressure-gradient principle (both of its sides are not equally exposed to external sound pressure).
Therefore, even though the C 411 exhibits a bidirectional polar pattern, it is not symmetrical. The broken line represents the backside of the instrument pickup, while the solid line represents the front side.
Sennheiser MKH 30
The Sennheiser MKH 30 is one of the few bidirectional condenser microphones on the market. It is a side-address mic that features a symmetrical RF capsule. It brings the best qualities of a condenser mic (extended frequency response and accurate transient response) to a true pressure-gradient bidirectional microphone.
The Sennheiser MKH 30 Polar Response Graph
The Sennheiser MKH 30 polar response is also featured on a split diagram. We see the lower frequencies on the left and the upper frequencies on the right.
The MKH 30 exhibits a beautifully consistent bidirectional polar pattern, even though it's not a true pressure-gradient ribbon mic.
All The Different Microphone Polar Patterns
Here's a list of all the different polar patterns you'll likely encounter when using microphones:
By clicking the links of each polar pattern title, you'll be brought to a My New Microphone article that focuses on that specific polar pattern.
- Omnidirectional: picks up sound equally in all directions.
- Bidirectional: picks up sound symmetrically in the front (0°) and back (180°) with equal sensitivity but opposite polarity. Bidirectional patterns have null points at their sides (90° & 270°), which yields a “ring of silence” in 3D space. Their polar pattern looks like a figure-8 in 2D.
- Cardioid: unidirectional pattern with a null point at the rear (180°) and roughly 6 dB decrease in sensitivity at its sides (90° & 270°) compared to on-axis (0°).
- Supercardioid: unidirectional pattern with a narrower on-axis response than “regular” cardioid. Null points at 127° & 233°, which yields a “cone of silence.” There's roughly a 10 dB decrease in sensitivity at its sides (90° & 270°) and a rear lobe of sensitivity with 10 dB less sensitivity (at 180°) compared to on-axis (0°).
- Hypercardioid: unidirectional pattern similar to supercardioid with a narrower on-axis response than “regular” cardioid. Null points at 110° & 250°, which yields a “cone of silence.” There's roughly a 12 dB decrease in sensitivity at its sides (90° & 270°) and a rear lobe of sensitivity with 6 dB less sensitivity (at 180°) compared to on-axis (0°).
- Subcardioid/Wide Cardioid: A unidirectional pattern with a wider response than “regular” cardioid. Subcardioid can be thought of as a midway point between cardioid and omnidirectional.
- Shotgun/Lobar: An extension on the supercardioid and hypercardioid polar patterns. The use of an interference tube in front of an already highly directional capsule yields the extremely directional shotgun/lobar pattern. These patterns generally have a rear lobe of sensitivity and sometimes even have small side lobes of sensitivity.
- Boundary/PZM (Hemispherical): The hemispherical polar pattern found on boundary and pressure zone microphones. These patterns are achieved by placing the mic capsule flush to a flat surface and then placing the microphone itself at a surface/boundary within an acoustic space. The capsules themselves can be any polar pattern, though omnidirectional capsules are often preferred.
For an in-depth definition of all the polar response patterns listed above (and much more), check out my article The Complete Guide To Microphone Polar Patterns.
What is a unidirectional microphone? A unidirectional microphone is most sensitive to sound in one single direction, known as its “on-axis.” These mics are less sensitive to and may even completely reject off-axis sounds. The cardioid is the most popular unidirectional pattern, though there are many others, including:
What is the polar pattern of a shotgun microphone? Shotgun microphones are the most directional mics on the market. Their capsules are typically either supercardioid or hypercardioid. However, these capsule patterns are enhanced by interference tubes, which narrow their polar patterns to what is known as lobar/shotgun polar patterns.
To learn about the lobar/shotgun microphone polar pattern in greater detail, check out my article The Lobar/Shotgun Microphone Polar Pattern (With Mic Examples).
To learn more about all the microphone polar response patterns, check out my article The Complete Guide To Microphone Polar Patterns.
Choosing the right microphone(s) for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Microphone Buyer's Guide. Check it out for help in determining your next microphone purchase.