What Are Centre And Edge-Terminated Microphone Capsules?

There are many types of microphones and nearly countless methods to differentiate one mic from another. One such differentiating factor with condenser mic capsules is the way in which they are terminated. They are either centre-terminated or edge-terminated.

What is a centre-terminated condenser capsule? A centre-terminated condenser mic capsule has its electrical leads terminating from the centre of the diaphragm and/or backplate. Though not a major factor in performance, centre-terminated capsules, in theory, are less prone to plosives and exhibit less resonance.

What is an edge-terminated condenser capsule? An edge-terminated condenser mic capsule has its electrical leads terminated at the edge of the diaphragm at the ring or body of the capsule housing. Though not a major factor, edge-terminated capsules, in theory, are more prone to plosives and resonance but have extended frequency response.

In this short article, we'll discuss centre-terminated and edge-terminated capsules in more detail and take a look at some example capsules (and microphones) with edge and centre termination.

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Centre-Terminated And Edge-Terminated Capsules

Before we start, it's important to note that the distinction between centre and edge-terminated capsules only really applies to large-diaphragm condenser microphones.

Though it's easy to tell the difference between a centre-terminated capsule and an edge-terminated capsule, it doesn't hurt to go over how each capsule type is designed.

Let's preface the simple explanations with a basic description of how condenser capsules are designed.

Condenser capsules, like all microphone capsules, have diaphragms*. These diaphragms are nearly always circular** and are attached (around their circumference) to a circular tension ring.

*Laser microphones, which are fairly rare and relatively new technology, do not have conventional diaphragms.
*Ehrlund microphones from Sweden are known for their non-circular diaphragm shapes.

A condenser diaphragm is stretched to a calculated tension around its tension ring. This ring is part of the greater capsule design and holds the diaphragm in its appropriate position relative to its backplate and capsule in general.

Note that the diaphragm is still able to oscillate inward and outward. This is necessary for the microphone to function properly. This preface was to explain that the diaphragm is tensioned around an outer tension ring.

Now for the big difference between the centre and edge-terminated condenser capsules.

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The Differences Between Centre And Edge-Terminated Capsules

So the big difference between centre and edge-terminated capsules, as their names suggest, is where the electrical leads are terminated in each capsule type.

Centre-terminated condenser capsules like the K67 (which we'll discuss later) have their electrical leads terminated in the centre of their diaphragms.

Edge-terminated condenser capsules like the CK 12 (which we'll also discuss later) have their electrical leads terminated at the edge of their capsules. These electrical leads are attached to the mic's tensioning ring and/or the great capsule body.

So it's quite obvious to tell the two termination types apart.

The termination type is also not a majorly significant factor in microphone performance. However, there are some differences between capsule termination types that could, in theory, alter the specifications of a microphone.

These differences can be summed up in the following table:

Centre-Terminated	Edge-Terminated
Less resonant	More resonant
Less prone to plosives	More prone to plosives
Variation in high-end frequency response	Extended frequency responses

Let's look at each of these differences in more detail:

Capsule Resonance

Resonance in solids (like microphone capsules) is a physical phenomenon that causes an increase in vibration at certain frequencies. These frequencies are known as resonant frequencies.

Resonant frequencies are a product of many factors, including the material and shape of the physical object in question.

The resonant frequency of a microphone diaphragm, for example, is a product of the diaphragm material; the mass and thickness of the diaphragm; the diameter of the diaphragm; the tension of the diaphragm; and the points at which the microphone diaphragm connects to the microphone.

Let's imagine all the diaphragm factors are the same except for the fact that the centre-terminated capsule has a physical connection at the centre of its diaphragm while the edge-terminated capsule does not.

If we think of an edge-terminated microphone, we have a circular diaphragm with a fixed radius tensioned around a perfectly circular ring. It is quite easy for standing waves to cause constructive interference and boost a frequency with a wavelength that matches half the diameter of the diaphragm. This is one way to cause resonance in the mic's diaphragm.

With a center-terminated microphone, we have a physical connection in the center of the diaphragm that effectively breaks up the standing waves that would arise at wavelengths equal to half the diameter of the diaphragm.

Therefore, in theory, centre-terminated capsules have less resonance than edge-terminated capsules. That being said, capsules of both types are designed and tuned to rid of as much resonance as possible.

Plosive Sensitivity

Plosives are the blasts of air that are caused by human speech. We hear these plosives as “pops” when they overload the microphone. In English, these pops are most often found on the hard consonants of P, B, T, G, D, and K.

As plosive energy hits the diaphragm of a microphone, it will likely cause the diaphragm to overload. Plosives will make the diaphragm move in a non-linear fashion and cause distortion or “popping” in the mic signal.

An edge-terminated capsule's diaphragm is more sensitive to plosives because the diaphragm is completely free to move from its circumference inward. The plosive energy can really push and pull the diaphragm and cause it to overload.

However, with centre-terminated capsules, the diaphragm is a bit more difficult to move since it is fixed in the centre. Plosives, too, have a more difficult time moving the diaphragm and, therefore, a harder time overloading the diaphragm.

To learn more about reducing plosives in your microphone signals, check out my article Top 10 Tips For Eliminating Microphone Pops And Plosives.

High-End Frequency Response

Though centre-terminated diaphragms have seemed better in the abovementioned factors, there is one performance factor where the edge goes to edge-terminated capsules (pun intended). This is that edge-terminated capsules tend to have more accurate and extended frequency responses.

High-frequency responses are tough to get right in microphone design.

Center-terminated capsules are more difficult to move, which often hurts their natural responsiveness to short-wavelength high-frequency sound waves.

Edge-terminated capsules are more easily moved by high-frequency sound waves and naturally have more extended frequency responses.

Something similar happens with low frequencies.

Centre terminated capsules are, in effect, oddly shaped in terms of their 2-dimensional movement capabilities. They are essentially donut-shaped, in this case with connections in their centre and around their circumference. This makes it difficult for long-wavelength low-frequency sound waves to move the diaphragm.

Again, the simple shape of the edge-terminated capsule diaphragm allows for better responsiveness to low-end sounds.

Of course, whether the mic is edge or centre-terminated, it will typically be designed in a way that adjusts for its shortcomings.

To learn everything you need to know about microphone frequency response, check out my article Complete Guide To Microphone Frequency Response (With Mic Examples).

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How Do Condenser Microphone Capsules Function?

The electrical termination of a condenser capsule marks the final stage and output of the mic capsule. These electrical leads then send the mic signal through an impedance converter (FET or vacuum tube) and through the rest of the microphone circuitry before the mic's output.

That being said, it's useful to know how a condenser microphone capsule works to know why we would take electrical leads from it in the first place.

A condenser capsule is essentially a parallel-plate capacitor. It has a moveable front plate (the diaphragm) and a backplate. These plates are held together in the capsule housing, and the diaphragm is tensioned with a tension ring.

Because the condenser capsule is a capacitor (“condenser” is a British term for capacitor), the plates need to be charged in order for the capsule to function.

This charge is supplied by the mic's power supply unit (in the case of a tube mic), by phantom power (in the case of “true” externally-polarized condenser mics), or by electret material within the capsule (in the case of electret mics).

The microphone capsule is designed to hold a fixed charge. With a fixed charge, any change in capacitance will cause an inversely proportionate change in voltage.

The distance between the diaphragm and the backplate is a factor in the capsule capacitance. Therefore, as sound causes oscillation in the diaphragm, the distance between the plates oscillates as well, and a coinciding AC voltage is produced.

This AC voltage is essentially the audio signal! For the audio signal to be processed and outputted from the microphone, it needs to be effectively outputted from the capsule and sent through the rest of the microphone's circuitry. This is where electrical leads (otherwise known as terminals) come into play.

As you can likely guess by now, the electrical leads are either terminated at the centre of the edge of the capsule!

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Edge-Terminated Capsule Examples

Let's take a look at two examples of edge-terminated microphone capsules:

The AKG CK 12

The AKG CK 12 is perhaps the most famous large-diaphragm condenser microphone capsule in the world. It has been around since its first introduction to the market in AKG's legendary C 12 tube microphone.

Since then, the CK 12 has been cloned by too many manufacturers to count. If the capsule in question is not labelled as a CK 12, it was probably still based on the design of this capsule.

Note that the original CK 12 is a difficult and expensive microphone capsule to produce. Though many manufacturers claim to produce CK 12s, very few can match the quality of the original.

Countless microphones utilize the CK 12 or a clone of the CK 12 as their capsule.

Rode HF6

The HF6 is the edge-terminated capsule of the famous Rode NT1 large-diaphragm condenser mic. It is not a clone of the CK 12.

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Centre-Terminated Capsule Examples

Now let's take a look at two examples of centre-terminated microphone capsules:

Neumann K67

The Neumann K67 is, like the AKG CK 12, is a benchmark for condenser capsule quality.

The K67 was designed as a multi-pattern alternative to Neumann's K47 capsule (which we'll get to in a minute).

The centre-terminated design of the K67 is often imitated, but the high quality of this capsule is rarely replicated.

Neumann K47

The Neumann K47 was first introduced to the world as a replacement for the lesser-quality M7 capsule in Neumann's U47 tube condenser microphone.

The aforementioned AKG C 12 and Neumann U 47 and U 49 microphones are 3 of the most famous and beloved microphones ever. All 3 of these microphones are 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).

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Related Questions

What are condenser microphones? Condenser mics are active microphone transducers that convert sound into audio via electrostatic principles. Condenser capsules are essentially parallel-plate capacitors. When these capsules hold a fixed charge, any diaphragm movement will cause a proportionate audio signal.

What are dynamic microphones? Dynamic mic transducers convert sound into audio via electromagnetic induction. An electrically conductive diaphragm (whether it's a ribbon or it holds a moving coil) moves within a magnetic field due to sound pressure variation and outputs a proportionate mic signal.

To learn about the differences between condenser and dynamic microphones, check out my article Differences Between Dynamic & Condenser Microphones.