Stand 2 inches from a ribbon microphone and the bass can jump 16 dB or more below 100 Hz. Move to 12 inches and it largely disappears. Most engineers call this "proximity effect" and treat it as a simple distance rule, get close, get bass. But the actual mechanism is stranger than that, and it determines exactly why certain microphone designs are completely immune while others cannot escape it.
Two Ways to Detect Sound
All microphones convert air pressure variations into electrical signals, but the capsule design determines whether proximity effect occurs at all.
An omnidirectional microphone is a pressure transducer. Its diaphragm faces incoming sound from one side; the rear is sealed in a closed acoustic chamber. The diaphragm responds to the instantaneous air pressure at a single point in space. Because it measures one point, it doesn't care whether a sound wave is curved or flat. The same physics apply at 2 inches as at 20 feet. No proximity effect.
A directional microphone, cardioid, hypercardioid, figure-8, works differently. It exposes both sides of the diaphragm to incoming sound through acoustic ports at front and rear. The diaphragm responds not to absolute pressure, but to the pressure difference (the gradient) between its two faces. This differential design is what creates the directional polar pattern. It is also what makes proximity effect unavoidable.
The Near-Field Problem
Far from any sound source, sound behaves like a plane wave: wavefronts are essentially flat, pressure and particle velocity are in phase, and amplitude drops at 6 dB per doubling of distance. A pressure-gradient microphone in this environment sees consistent behavior, the frequency response is flat because the gradient across the capsule is proportional to frequency throughout the audible range.
Close to a sound source, the physics change. The wave curvature becomes significant: these are spherical waves, not plane waves. In a spherical near-field, particle velocity and pressure are no longer in phase. At low frequencies, particle velocity leads pressure by up to 90 degrees. And the amplitude drop with distance follows a steeper curve for low frequencies, because their long wavelengths produce high wave-front curvature even at moderate distances.
For a pressure-gradient capsule, this means the pressure difference between its front and rear ports is exaggerated at low frequencies when the source is close. The capsule responds to a gradient that is larger than it "expects" for those frequencies. The result: bass frequencies are amplified relative to higher frequencies. That is proximity effect, physically described.
Why the Polar Pattern Determines the Degree
A true figure-8 (bidirectional) microphone is a pure pressure-gradient transducer, both sides of the diaphragm are fully open to the acoustic field. It feels the full proximity effect. This is why ribbon mics (which are almost always figure-8) can produce 16–20 dB bass boosts below 100 Hz at close range.
A cardioid microphone is not a pure pressure-gradient device. Acoustically, it is approximately a 50/50 combination of a pressure transducer and a pressure-gradient transducer. The pressure component is completely immune to proximity effect; the pressure-gradient component is fully susceptible. The result is that a cardioid at close range experiences approximately half the proximity effect of a figure-8 mic.
Hypercardioid and supercardioid patterns sit between figure-8 and cardioid, with more pressure-gradient weighting than a standard cardioid. The hierarchy is direct: Figure-8 > Hypercardioid > Supercardioid > Cardioid. Omni mics, pure pressure transducers, have zero proximity effect by design.
On multi-pattern condenser microphones, this hierarchy is selectable. Switching from omnidirectional to cardioid to figure-8 at the same distance progressively increases the proximity effect. You are literally adjusting the ratio of pressure to pressure-gradient sensitivity.
The 6 dB/Octave Mechanism
The frequency-dependent behavior of pressure-gradient response follows a 6 dB per octave slope in the near field. Moving toward the source, the bass-frequency gradient across the capsule increases at 6 dB per octave relative to far-field behavior. Capsule designers partially compensate for this through acoustic damping built into the rear port, but at very close distances, the compensation is overwhelmed and the bass tip-up becomes audible.
This is why proximity effect is not a cliff but a slope. At 12 inches (30 cm) from a cardioid, it becomes measurable. At 6 inches, it is clearly audible. At 2 inches, it dominates the sound. Distance and polar pattern together set the degree.
The Neumann U87's low-cut switch is designed to compensate for proximity effect at close range in cardioid mode, a dedicated shelving filter built into the mic body for exactly this use case.
Using It Deliberately
Once you understand proximity effect mechanically, you can control it.
Frank Sinatra was known for precise distance management at the mic. He worked close enough to add weight to his voice, far enough to avoid muddiness, and moved his position during performance to shape dynamic character. The effect was intentional, not accidental.
Radio broadcasters exploit the full effect. A large-diaphragm cardioid 3–4 inches from the mouth adds exactly the low-end weight that reads as authority through a speaker. It is not a processing trick; it is an acoustic reality built into how the capsule responds.
Ribbon mics in particular reward close-mic technique for this reason. A vocal through an AEA R84 or Coles 4038 at 4–6 inches produces low-end weight that is difficult to replicate with EQ because it affects the velocity component of the sound, not just the steady-state pressure levels. The texture is different, not just louder in the bass.
The Counter-Move
Once proximity effect is recorded, you can reduce it but not fully undo it. A high-pass filter or low-shelf cut below 150–200 Hz addresses the energy buildup, but the tonal character of a close-mic'd recording persists in how transients interact with the room and in the velocity component embedded in the waveform. That said, a low shelf at 120–150 Hz and a high-pass around 60–80 Hz is a standard starting point for corrective work.
If proximity effect is a consistent problem in your setup, the structural fix is either more distance or switching to an omnidirectional capsule. DPA builds many of its lavalier and boundary mics as omnidirectional specifically to eliminate proximity effect in situations where distance control is impossible.
The physics are not complicated, but they are easy to misread as a simple distance rule. Get the mental model right, wave curvature, gradient transducers, polar pattern weighting, and proximity effect becomes a precise tool rather than an unwanted artifact.