The Hemispherical Boundary Microphone/PZM Polar Pattern

The boundary microphone (aka pressure zone microphone) is an often overlooked microphone with a misunderstood polar pattern. Setting these mics up properly will help tremendously to capture the character of an acoustic space, so it's worth knowing about the polar patterns of boundary/PZM mics.

What is a hemispherical boundary/PZM microphone polar pattern? A boundary or pressure zone microphone (PZM) has a hemispherical polar/pickup pattern. A flat boundary surface is made flush with the mic capsule to eliminate reflected sounds and phase cancellation. Boundary mics are placed against room boundaries and are excellent as room mics.

In this in-depth article, we'll discuss the hemispherical boundary/PZM microphone polar pattern in great detail to answer any questions you may have about boundary/PZM microphone polar patterns!

The Hemispherical Polar Pattern

A picture is worth a thousand words. Let's start with a diagram of the cardioid microphone polar pattern:

The hemispherical polar pattern is a theoretical pattern achievable in boundary mics (also known as pressure zone mics or PZMs). Basically, these mics place a capsule near-flush with a boundary (solid surface) to achieve a phase coherent capture of the sound around it.

The hemispherical polar pattern is really like a specialty pattern extension of a standard polar response.

Boundary mics can use a capsule with any kind of polar response. Usually, these capsules are omnidirectional, which yields the closest approximation of a true hemispherical pickup.

Placing PZMs on wide/long surfaces is great for capturing the acoustic environment. This is true in recording studios, live sound stages, sports arenas for broadcast, and many more practical applications.

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Hemispherical Microphone Generalities And Characteristics

1. Only achievable by placing the capsule flush to a flat surface
2. An extension or modification to other polar patterns rather than a polar pattern in and of itself
3. Works on either the pressure or pressure-gradient principle (Will work with any capsule polar pattern)
4. Typically designed with omnidirectional or cardioid, and sometimes designed with supercardioid or hypercardioid capsules
5. Very common in studio and stage environments as room mics
6. Complete phase coherence when positioned at a boundary in an acoustic space

Only Achievable By Placing The Capsule Flush To A Flat Surface

The hemispherical polar pattern is really only obtainable in boundary mics.

These specialty mics are designed with a flat surface and capsules that lay as close as possible to the surface. Furthermore, boundary mics work optimally when they themselves are positioned flush against a large surface.

What this capsule placement does is eliminates the possibility for sound reflections to enter from the rear. Therefore there is phase coherence and no comb filtering or delay effects (which often happen when a mic is positioned close to but not flush against a surface).

An Extension Or Modification To Other Polar Patterns Rather Than A Polar Pattern In And Of Itself

As previously discussed, the “hemispherical” boundary mic polar pattern is really a modification of another polar pattern.

There are no hemispherical microphone capsules out there. Rather, these hemisphere patterns come about when we design capsules to be flush against surfaces.

Works On Either The Pressure Or Pressure-Gradient Principle

Boundary mics are often designed with omnidirectional (pressure principle) capsules. However, there are also many boundary mics that have cardioid-type (pressure-gradient) capsules.

Again, the boundary polar pattern is a modification rather than a standard polar pattern you'd find in a microphone capsule alone.

Related article: Pressure Microphones Vs. Pressure-Gradient Microphones

Typically Designed With Omnidirectional Or Cardioid Capsules

As mentioned above, omnidirectional and cardioid are common base patterns with boundary mics.

Very Common In Studio And Stage Environments As Room Mics

Boundary mics find their niche as room mics. They are often used to record the space in studio rooms, stages, and even broadcasting venues.

Complete Phase Coherence When Positioned At A Boundary In An Acoustic Space

The best way to position a boundary mic or PZM is against a large flat surface. This effectively eliminates any initial reflections from entering the mic capsule, ensuring phase coherence and a good, clean mic signal.

Related article: Microphone Polarity & Phase: How They Affect Mic Signals

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How Is The Hemispherical Polar Pattern Achieved?

The hemispherical polar pattern of boundary and pressure zone microphones is only truly achievable via the acoustic construction of the microphone bodies.

All PZMs are constructed with a flat surface. The mic capsule is positioned nearly flush with this surface to achieve the hemispherical polar pattern.

Placing the capsule flush to the surface or “boundary” effectively rids of rear sound reflections. The lack of any reflections affecting the rear side of the mic diaphragm causes phase coherence and a strong mic signal.

Note that the true hemispherical pattern of boundary mics will work best when the mic itself is positioned on a greater surface or boundary. A large boundary means that absolutely no rear reflections will enter the mic. Therefore the PZM will act as it is designed to.

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When Should You Use A Hemispherical Microphone?

The hemispherical polar pattern is really only possible in pressure zone or boundary microphones. These are specialty microphones that are really only applied in specific situations. Let's talk about when to use a “hemispherical microphone” and when not to.

Best Applications For A Boundary Microphone

• To record the acoustic environment in studio, broadcast, and stage applications.
• On long and wide surfaces like walls, floors, and ceilings.

When Shouldn't You Use A Boundary Microphone?

• For close-miking sound sources that do not have significant surface areas.

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Hemispherical Microphone Examples

• AKG C 547 BL
• Audio-Technica U851A
• Shure Beta 91A

AKG C 547 BL

The AKG C 547 BL is a boundary layer condenser microphone with a hypercardioid capsule. The combination of AKG's patented Phase Coherent Cardioid technology and the flush hypercardioid capsule of the C 547 BL provides this top-of-the-line boundary mic's high directionality and natural sound.

The AKG C 547 BL Polar Response Graph

As we see above, the hypercardioid capsule of the C 547 BL does not cause a true hemispherical polar pattern. Rather it yields directionality in a helf-spheroid type shape. The rear rejection of the C 547 BL can be used to great advantage in studio and live environments to attenuate unwanted sound from entering this boundary mic.

Audio-Technica U841R

The Audio-Technica U841R is an electret condenser boundary microphone with an omnidirectional capsule. This mic's polar pattern is described as “omnidirectional in the hemisphere above the mounting surface.” The U841R is capable of capturing clean, accurate audio from all around its boundary.

The Audio-Technica U841R Polar Response Graph

The polar response pattern of the Audio-Technica U841R is simply the polar pattern of the mic's omnidirectional capsule.

By imagining the U841R as a boundary mic or pressure zone mic, we can see that it will yield a fairly hemispherical polar pattern if positioned correctly against a surface.

Shure Beta 91A

The Shure Beta 91A is a low-profile half-cardioid boundary-style electret condenser microphone. Like most boundary mics, its real polar pattern takes place in a hemisphere above its mounting surface. The Shure Beta 91A has an incredible low-frequency response and is actually marketed as a kick drum and bass instrument microphone.

The Shure Beta 91A Polar Response Graph

The Shure Beta 91A polar response graphs give us the response of the mic's capsule. In practice, this microphone will yield a more hemisphere-like pattern when positioned correctly against a boundary or surface.

The lack of rear sensitivity in the Beta 91A means there will be less potential noise in the signal due to “non-boundary” positioning or extraneous vibrations. This makes it excellent for miking non-ideal surfaces like drums and other instruments.

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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.

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Related Questions

What is the cardioid microphone polar pattern? The cardioid polar pattern is the most popular and basic unidirectional mic polar pattern. It is most sensitive to on-axis sounds, roughly 6 dB less sensitive at its sides, and has a null point to its rear. It is essentially a superposition of the omnidirectional and bidirectional polar patterns.

For a more in-depth explanation of the cardioid microphone polar pattern, check out my article What Is A Cardioid Microphone? (Polar Pattern + Mic Examples).

What are parabolic microphones used for? Parabolic microphones are used to capture specific sounds at a distance. The parabolic dish focuses sound waves from a distance, creating a very narrow pickup pattern with extended reach. Parabolic mics are popular in sports broadcasting and ambience recording.

Related article: Why Are Parabolic Microphone Dishes Used In Football?

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