Microphones of all types are designed to convert sound pressure variations into electrical signals. There are pressure and pressure-gradient microphones that react slightly differently to pressure in order to do so.
What is a pressure microphone? A pressure microphone is a mic that has one side of its diaphragm open to external sound waves and the other side closed in a fixed pressure system. Pressure mics are omnidirectional (sound pressure is a scalar quantity); exhibit no proximity effect, and are fairly resistant to plosives.
What is a pressure-gradient microphone? A pressure-gradient microphone has both sides of its diaphragm open to sound waves (at least partially). Pressure-gradient capsules are found in all directional mics (including ribbon and multi-directional condensers).
In this article, we’ll take a closer look at the designs of pressure and pressure-gradient microphones and discuss a few examples of each of these mic types.
Pressure And Pressure-Gradient Microphones
The diaphragm and capsule/element of a microphone is essential for the mic to act properly as a transducer (converting sound wave energy to audio signal energy).
The way in which the diaphragm acts when subjected to varying sound pressure is critical in the performance of the microphone.
Because a microphone diaphragm is essentially a two-sided thin membrane, there are essentially two methods a diaphragm may interact with sound waves.
The first way is to have only one side of the microphone diaphragm exposed to the environment and reacting with sound waves. The other side of the microphone would be part of a sealed container at a constant pressure. These microphones are known as pressure microphones.
The second way is the have both sides of the microphone diaphragm exposed to the environment and reacting with sound waves. These microphones are known as pressure-gradient microphones.
To learn more about microphone diaphragms and capsules, check out my articles What Is A Microphone Diaphragm? (An In-Depth Guide) and What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules), respectively.
Let’s now look at each of these mic types in greater detail:
Pressure microphones are designed so that only one side of their diaphragms is open to varying pressure. The other side (the backside) of the diaphragm is closed off in a “container” at a constant pressure.
Note that these containers are not totally closed systems. They generally have tiny holes so that the pressure within the container will match the ambient pressure of the environment. These pressure-adjusting containers are not, however, affected by variation in ambient pressure caused by sound waves.
So the rear of the pressure microphone diaphragm is effectively set to the ambient pressure of the environment. In this case, the sound waves will only react with the front side of the diaphragm.
Sound waves naturally cause localized variation in whatever medium they travel through. Sound waves cycle through moments of rarefaction and compression. Within a typical cycle, a sound wave will cause an increase and a decrease in localized pressure within a medium.
A sound wave at the diaphragm causes the diaphragm to move since it causes a pressure difference between the two sides of the diaphragm.
With one side closed off to ambient pressure, an increase in pressure will cause the diaphragm to move inward while a decrease in pressure will cause the diaphragm to move outward.
In this way, the sound causes the pressure microphone diaphragm to move.
Because the pressure mic diaphragm is only open on one side, there are no phasing issues with the mic. Sound waves from all directions will effectively react with diaphragm similarly.
For this reason, all pressure microphones exhibit an omnidirectional polar pattern!
For everything you need to know about the omnidirectional microphone polar pattern, check out my article What Is An Omnidirectional Microphone? (Polar Pattern + Mic Examples).
Pressure Microphone Examples
It’s always great to have real-world examples of the microphones we study. In this section, we’ll have a look at 3 fairly distinct pressure microphones:
- Neumann KM 183
- Rode Reporter
- Sanken COS-11D
Neumann KM 183
The Neumann KM 183 (link to check the price on Amazon) is a small-diaphragm externally-polarized condenser microphone with an omnidirectional polar pattern.
The KM 183 is a top-address “pencil mic” and so the top of the microphone diaphragm is exposed to sound pressure while the backside (bottom) is not. This makes the 183 a pressure microphone and gives it its consistent omnidirectional polar pattern.
Neumann is featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.
To learn more about pencil microphones, check out my article What Are Pencil Microphones And What Are They Used For?
The Rode Reporter (link to check the price on Amazon) is a moving-coil dynamic microphone with an omnidirectional polar pattern.
This mic, like the KM 183, is a top-address pressure microphone. Only the front side of the Reporter diaphragm is exposed to sound waves and varying sound pressure.
Rode is also featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.
To learn more about top-address microphones, check out my article What Are Top, End & Side-Address Microphones? (+ Examples).
The Sanken COS-11D (link to check the price on Amazon) is a miniature electret lavalier microphone with an omnidirectional polar pattern.
This microphone is very tiny but is still designed so that its diaphragm is only exposed to sound waves on one side.
To learn more about lavalier microphones, check out my article How And Where To Attach A Lavalier/Lapel Microphone along with My New Microphone’s recommended Best Lavalier Microphones For Interviews/News/Presentations and Best Lavalier Microphones For Actors.
Pressure-gradient microphones are much more complex than pressure microphones since they have both sides of their diaphragms open to sound pressure variations (sound waves). This may not seem too complicated but the intricate designs of pressure-gradient capsules would say otherwise.
Opening both sides of the diaphragm up to the environment opens up a whole new world for microphone polar patterns. With pressure-gradient microphones, we have many more options than just omnidirectional.
The Ribbon Mic And The Bidirectional Polar Pattern
The simplest pressure-gradient polar pattern is the bidirectional pattern and the simplest microphone transducer type to explain the term pressure-gradient is the ribbon microphone.
The typical ribbon microphone has a ribbon diaphragm suspended in a magnetic structure. There is just enough space between the perimeter of the ribbon and the magnetic structure for the ribbon diaphragm to oscillate.
This symmetrical design (along with a symmetrical mic body and grille) means that both sides of the ribbon diaphragm are equally exposed to the environment.
This is as “equal” a pressure-gradient microphone as we can get.
These microphones exhibit a symmetrical bidirectional polar pattern.
Let’s look at why this is by looking at the direction of the sound waves.
Remember that with pressure microphones, it doesn’t matter where the sound waves come from since they all only react with the front side of the diaphragm. However, with pressure-gradient mics, sound waves from any direction will react with both sides of the diaphragm. Phase consistencies and inconsistencies cause directional polar patterns.
Sound From The Front
As sound arrives from the front of a ribbon mic, it reaches the front of the diaphragm first and then, after time t, it hits the rear of the diaphragm. This causes phase and amplitude differences, which makes the diaphragm move.
Sound From The Rear
Similarly, sound waves from the rear will hit the rear of the diaphragm before reaching the front. This, too, causes phase and amplitude differences that make the diaphragm move.
Sound From The Side
Sound waves coming directly from the side of the bidirectional microphone will reach both sides of the diaphragm at the same time with the same phase and same amplitude.
This means that the sound wave will exert equal pressure on both sides of the diaphragm and, therefore, the diaphragm will not move. In other words, sound waves from the side will not cause any microphone signal.
This covers the basics of microphone directionality.
Sounds at various other angles will be somewhere between having a maximum effect on the microphone diaphragm (like the direct front and rear) and no effect on the microphone diaphragm (like the sides). The sensitivity of the microphone to directional sound waves is drawn out in the bidirectional polar pattern graph. Here it is once again:
For everything you need to know about the bidirectional microphone polar pattern, check out my article What Is A Bidirectional/Figure-8 Microphone? (With Mic Examples).
The Cardioid Pattern And Other Polar Patterns
The cardioid pattern is produced by a pressure-gradient microphone.
However, unlike the bidirectional polar pattern, the cardioid mic’s diaphragm is not equally exposed at both sides.
There are obstructions (acoustic labyrinths, acoustic foam, etc.) that cause calculated phase and amplitude differences between the front and rear of the diaphragm. These differences cause the cardioid polar pattern, which is unidirectional. The cardioid pattern is most sensitive to its front and actually has a null-point with no sensitivity directly to its rear.
Here is a diagram of the cardioid microphone polar pattern:
To read more about the cardioid microphone polar pattern, check out my article What Is A Cardioid Microphone? (Polar Pattern + Mic Examples).
By altering the differences in amplitude and phase between the front and rear of the diaphragm, many other polar patterns are achievable. Remember that these patterns are only possible with pressure-gradient microphones and not with the omnidirectional pressure microphones.
For an in-depth read on microphone polar patterns, please consider reading My New Microphone’s Complete Guide To Microphone Polar Patterns.
Pressure-Gradient Microphone Examples
Let’s now take a look a 4 pressure-gradient microphone examples to get an idea of the wide variety of these mics:
- Neumann KM 184
- Shure SM57
- Rode NTR
- AKG C 414 XLII
Neumann KM 184
The Neumann KM 184 (link to check the price on Amazon), like the aforementioned KM 183, is a microphone in Neumann’s KM 180 line of microphones.
The 184, like its family member, is a small-diaphragm pencil condenser microphone.
However, this microphone is a pressure-gradient microphone. The front side of its diaphragm is fully exposed to the environment (though it’s protected by the mic grille). The backside of the diaphragm is still exposed to sound pressure variation, though sound waves must enter through carefully designed acoustic labyrinths in order to reach the backside of the diaphragm.
The careful design of the KM 184 capsule allows for its consistent cardioid polar pattern.
The Shure SM57 (link to check the price on Amazon) is one of the world’s most popular microphones. It’s a top-address moving-coil dynamic microphone with a cardioid polar pattern.
In order to achieve this pattern, Shure developed the pressure-gradient Unidyne III capsule for its microphones. The SM57 is built with an updated version of this capsule and is, therefore, a pressure-gradient microphone.
Shure is another brand featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.
To learn more about moving-coil dynamic mics, check out my article Moving-Coil Dynamic Microphones: The In-Depth Guide.
The Rode NTR (link to check the price on Amazon) is a ribbon microphone, which makes it, by default, a pressure-gradient microphone.
Like the vast majority of ribbon mics, the NTR has its diaphragm suspended within its magnetic structure. The diaphragm is equally exposed to the environment on its front and back sides.
The NTR is just one example of a ribbon mic with a true pressure-gradient element and bidirectional polar pattern.
To learn more about ribbon microphones, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.
AKG C 414 XLII
The AKG C 414 XLII (link to check the price on Amazon) is a multi-pattern large-diaphragm condenser microphone. This microphone is built around AKG’s famed dual-diaphragm CK12 capsule which allows for a whopping 9 selectable polar patterns.
Each of the diaphragms of the CK 12 capsule is set up to be pressure-gradient. Even in omnidirectional mode, the C 414 XLII is a pressure-gradient microphone (its two diaphragm signals are combined in such a way as to produce a pseudo-omnidirectional pattern.
AKG is also featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.
A Note On Multi-Pattern Microphones
It bears repeating that multi-pattern microphones set to omnidirectional mode are not exactly pressure microphones. This is a common assumption to make when learning about pressure and pressure-gradient mics but is simply not the case.
Rather, the omnidirectional polar pattern in multi-pattern mics is achieved by summing up the signals of the two back-to-back diaphragms in a dual-diaphragm capsule. These diaphragms typically exhibit cardioid patterns and so combining them with equal amplitude and phase would yield an omnidirectional polar pattern.
What is a line + gradient microphone? A line + gradient microphone is the same as a shotgun microphone and is the term used by the famous mic manufacturer Audio-Technica. With these shotgun mics, the “line” refers to the long interference tube in front of the mic capsule and the “gradient” refers to the directional capsule of the mic.
To learn more about shotgun microphones, check out my article The Lobar/Shotgun Microphone Polar Pattern (With Mic Examples).
Do condenser mics have proximity effect? Directional condenser (pressure-gradient) mics do exhibit proximity effect while omnidirectional condenser (pressure) mics do not. Note that, due to their dual-diaphragm capsules, multipattern condenser microphones often exhibit proximity effect when set to omnidirectional mode.
To learn more about the microphone proximity effect, please read my article titled In-Depth Guide To Microphone Proximity Effect.