A figure-8 microphone, also called a bidirectional microphone, is a microphone whose polar pattern picks up sound equally from the front and the rear while rejecting sound arriving from the sides. Plotted on a polar diagram, its sensitivity forms two touching circles that resemble the digit 8. Among microphones, it is the classic pressure-gradient design, and its deep side nulls and opposite-polarity lobes make it a specialist tool for stereo techniques, duet recording and isolating sources.
The pattern occurs naturally in ribbon microphones and can be produced electronically in multi-pattern condensers. It is the only common first-order pattern with a rear lobe as sensitive as the front, which is both its main strength and its main source of mistakes.
How the figure-8 pattern arises
A figure-8 microphone has a diaphragm open to the air on both sides. It responds not to absolute sound pressure but to the difference in pressure between front and back, the pressure gradient. Sound arriving on axis from the front or rear creates a large difference and a strong output. Sound arriving exactly from the side reaches both faces at the same time and with the same pressure, so the forces cancel and the output falls to almost nothing.
Mathematically, the sensitivity follows the cosine of the arrival angle. At 0 degrees it is maximal, at 60 degrees it is half, or about 6 dB down, and at 90 degrees it is theoretically zero. Sound from the rear at 180 degrees is picked up at full level but with inverted polarity, because it pushes the diaphragm in the opposite direction.
A ribbon of thin corrugated aluminum suspended in a magnetic field is the most natural figure-8 transducer. Condenser figure-8 designs usually combine two cardioid capsules back to back and subtract their outputs, or use a single double-sided capsule with a center backplate.
Figure-8 compared with other polar patterns
First-order polar patterns are all mixtures of an omnidirectional and a figure-8 component. Comparing their key properties shows where the bidirectional pattern fits and why it is as directional as a cardioid in terms of ambient rejection.
| Pattern | Null angle | Ambient pickup versus omni | Rear pickup |
|---|---|---|---|
| Omnidirectional | None | 0 dB, reference | Full |
| Cardioid | 180 degrees | About -4.8 dB | Rejected |
| Supercardioid | About 126 degrees | About -5.7 dB | Small rear lobe |
| Hypercardioid | About 110 degrees | About -6 dB | Larger rear lobe |
| Figure-8 | 90 degrees | About -4.8 dB | Full, opposite polarity |
The figure-8 rejects diffuse room sound as well as a cardioid does, and its side nulls are usually deeper and more consistent across frequency than the rear null of a typical cardioid. That makes the side null a precise tool for keeping a nearby instrument out of the signal.
Proximity effect and frequency behavior
Because it is a pure pressure-gradient device, a figure-8 microphone shows the strongest proximity effect of all first-order patterns. Close to a source, the gradient at low frequencies rises, so bass increases markedly when the source is within roughly 30 centimeters. Engineers use this for warmth on voices and instruments, or reduce it with distance and high-pass filtering.
| Implementation | Pattern consistency | Typical character |
|---|---|---|
| Passive ribbon | Very symmetrical front and rear | Smooth highs, low output, needs a quiet high-gain preamp |
| Active ribbon | Very symmetrical | Higher output, easier to drive |
| Dual-diaphragm condenser | Good, can vary at high frequencies | Bright, sensitive, multi-pattern |
| Small-diaphragm condenser | Consistent across frequency | Accurate, used for stereo arrays |
Passive ribbons can be damaged by strong blasts of air and, with some older designs, by phantom power applied through faulty cables, so handling rules differ from those for condensers.
Stereo and studio techniques
The pattern’s geometry underpins several important recording methods. Each one relies on the cosine response, the side nulls or the polarity of the rear lobe.
| Technique | Setup | What it achieves |
|---|---|---|
| Blumlein pair | Two figure-8s crossed at 90 degrees, coincident | Natural stereo image with room ambience |
| Mid-side stereo | Forward mid microphone plus a sideways figure-8 | Stereo width adjustable after recording |
| Face-to-face duet | One microphone between two performers | Balanced pickup of both voices |
| Side-null isolation | Null aimed at a loud neighboring source | Reduced bleed between instruments |
| Room microphone | Rear lobe facing the room | Controlled mix of direct and ambient sound |
In mid-side recording, the figure-8 forms the side channel. Its signal is added to the mid for the left channel and subtracted for the right, so the stereo width is set in the mix simply by changing the side level. The method also gives excellent mono compatibility, because the side signal cancels completely when the channels are summed.
Pitfalls when using a figure-8 microphone
Most problems with bidirectional microphones come from treating them like cardioids. The rear lobe is fully live, the nulls are narrow, and the output of passive designs is low, so placement and gain staging need more care than with a typical studio condenser.
- Ignoring the rear lobe: in a reflective room, a hard wall behind the microphone returns strong reflections at full sensitivity.
- Polarity surprises: a source captured by the rear lobe and another microphone at the front can cancel partially in the mix.
- Placing the source in the null: slight side positioning makes the sound thin and inconsistent.
- Too little preamp gain: passive ribbons often need about 50-60 dB of clean gain for quiet sources.
- Excessive bass at close range: the strong proximity effect can make voices boomy without distance or filtering.
A plain-language summary
Most microphones listen mainly in one direction. A figure-8 microphone listens equally well in two opposite directions, to the front and to the back, but is almost deaf to the sides.
Think of two people sitting across a table with the microphone between them: both are heard clearly, while a noisy fan off to the side is largely ignored. That simple shape makes it useful for interviews, duets and stereo recording.
An example from practice
A small studio records a singer-guitarist who wants to perform live rather than in separate takes. The engineer places a ribbon figure-8 microphone in front of the singer’s mouth, angled so that its side null points at the guitar, and a small-diaphragm cardioid on the guitar with its rear null toward the voice.
The rear lobe of the ribbon faces a thick absorber panel so that the room does not color the vocal. Compared with using two cardioids, the guitar bleed in the vocal track drops by roughly 10 to 15 dB, enough to adjust vocal level and tone independently in the mix. The engineer then adds a short high-pass filter at about 80 Hz to tame the proximity effect, and the final recording keeps the energy of a live performance with clean, separate tracks.