What Is A Microphone Diaphragm? (An In-Depth Guide)


Neumann K47 Capsule
With Centre-Terminated Diaphragm

Diaphragms have a lot to do with sound. We “sing from the diaphragm” of our physical bodies, and, if we’re singing into a microphone, we’re interacting with a diaphragm as well! Every practical microphone has a diaphragm and an understanding of diaphragms is crucial for microphone mastery.

So what is a microphone diaphragm? A microphone diaphragm is a thin membrane that moves in reaction to external sound pressure variation. A microphone diaphragm is a key transducer component in converting acoustic energy into electrical energy. The three main diaphragm types are the moving-coil, ribbon, and condenser.

There’s a lot to discuss when talking about microphone diaphragms. This article will go into detail about the popular diaphragm types and the considerations we take when dealing with microphone diaphragms!


What Is A Microphone Diaphragm?

As mentioned, a microphone diaphragm is a thin membrane that moves in reaction to sound pressure variation (sound waves). The diaphragm is a critical ingredient in the microphone recipe. In fact, without a movable diaphragm, a microphone would not be able to do its job as a transducer. The coinciding motion of the diaphragm with sound pressure is the first step in changing acoustic energy into electrical energy.

Since a microphone diaphragm is so thin, we observe it as have only two sides. The movement of the diaphragm is predicated on the difference in sound pressure between its two sides.

The microphone diaphragm is part of a bigger unit within microphones called the capsule. Capsule design is of utmost importance in microphone performance. The capsule is, ultimately, the transducer element in any microphone.

For an in-depth read on microphone capsules, check out my article What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules).

The arrangement of the capsule and the diaphragm makes up much of a microphone’s characteristic sound. Frequency response, sensitivity, and polar pattern are a few of the characteristics heavily determined by the capsule and diaphragm design.

There are 3 main types of microphone diaphragms:

  1. Moving-coil diaphragm (dynamic)
  2. Ribbon diaphragm (dynamic)
  3. Front plate diaphragm (condenser)

How Does A Diaphragm Move?

Microphone diaphragms are extremely thin (some less than 5 microns). This thinness makes them very sensitive to vibrating air molecules in their immediate surroundings. The “bombardment” of vibrating air molecules on a microphone diaphragm causes it to move. In turn, this mechanical movement in and out from the resting position is converted into an electrical AC voltage (audio signal).

A microphone diaphragm moves in accordance with the sound pressure difference between its two sides. If one side is “bombarded” by air molecules more than the other, that side will be pushed in. If both sides are subject to an equal amount of pressure, the diaphragm will stay put.

Another way to explain the diaphragm movement is by looking at a simple sine wave. In a sine wave, we have null points, peaks, and troughs.

As the sine wave travels through the air, it affects the air molecules it passes through. The same happens as the sound wave reaches the diaphragm.

  • At its peaks, the sine wave causes maximal compression on the diaphragm, pushing the diaphragm in.
  • At its troughs, the sine wave causes maximal rarefaction on the diaphragm, pulling the diaphragm out.
  • And at the null points, the sine wave doesn’t cause the diaphragm to move.

Sound waves travel at 343 m/s (1125 ft/s) and are infinitely more complex than a simple sine wave. As you can imagine, they cause the diaphragm to vibrate quickly in response to external sound pressure variation!

Microphone diaphragms are designed to move in accordance with sound pressure variation so that they may produce an audio signal that is an accurate representation of the sound happening around the microphone.


Relating Microphone, Loudspeaker, And Thoracic Diaphragms

Comparisons are sometimes useful in explanations of things. We’re probably all familiar with a loudspeaker diaphragm, and we most certainly have thoracic diaphragms within our bodies. The microphone diaphragm is similar to these two diaphragms! Let me explain.

The Loudspeaker Diaphragm

Loudspeakers, like microphones, are transducers. Microphones convert mechanical wave energy (sound) into electrical energy (audio signal). Loudspeakers, conversely, convert electrical energy (audio signal) into mechanical wave energy (sound).

Loudspeakers work on the principle of electromagnetic induction, the same principle governing dynamic microphones (which we’ll discuss later in this article). An audio signal in the form of an AC voltage is sent to the loudspeaker. This signal travels through a stationary conductive coil of wire that surrounds a magnet. The electricity flowing through the conductive wire causes the magnet to move through the principle of electromagnetic induction. Since the audio is AC, the magnet moves forward and backward. This magnet is attached to a diaphragm.

The diaphragm of a loudspeaker moves along with the magnet it is attached to. As the diaphragm vibrates, it pushes and pulls air around it, projecting sound waves through space.

A dynamic microphone works in the opposite manner of a loudspeaker. If we were to wire a loudspeaker in reverse, the diaphragm would essentially be a microphone diaphragm! Although, since loudspeaker diaphragms are typically thicker, wider, and heavier than professional mic diaphragms, they wouldn’t be nearly as sensitive. This would result in a muffled sound.

To learn how to wire a speaker so that it becomes a microphone, check out my article How To Turn A Loudspeaker Into A Microphone In 2 Easy Steps.

The Thoracic Diaphragm

The thoracic diaphragm is a thin sheet of skeletal muscle in humans and other mammals.

In the case of this biological diaphragm, it’s the diaphragm muscle itself that contracts and expands. The thoracic diaphragm plays a paramount role in respiration. As the diaphragm muscle contracts, it helps draw air into the lungs. As the diaphragm relaxes, it pushes air out of the lungs.

Breathing happens at a much slower rate than air vibrations. However, the idea of the diaphragm moving air is the same.

Let quickly recap the three mentioned diaphragms:

  • The thoracic diaphragm contracts and expands, moving air in and out of the lungs.
  • The loudspeaker diaphragm is attached to a magnet and moves according to an applied AC voltage by means of electromagnetic induction.
  • The microphone diaphragm moves in accordance with sound pressure variation around it.

Acoustic Principle: Pressure Versus Pressure-Gradient

Though not a characteristic of the diaphragm itself, it’s worth mentioning the capsule design and how it alters the way sound interacts with the diaphragm.

There are two basic types of polar patterns:

  1. Omnidirectional – which works on the pressure principle.
  2. Bidirectional – which works on the pressure-gradient principle.

A capsule design may expose its diaphragm based on either of these principles or on a combination thereof. Combinations give rise to the cardioid-type polar patterns.

Pressure Principle

The pressure principle has one side of the diaphragm open to external sound pressure. The other side is closed off to a fixed pressure.

We know the movement of the diaphragm is due to the pressure difference between its front and back sides. Because only one side of the diaphragm is exposed to sound vibrations, the diaphragm will react pretty well equally to sound from all directions. Hence the omnidirectional polar pattern!

For more information on the omnidirectional polar pattern, check out my article What Is An Omnidirectional Microphone? (Polar Pattern + Mic Examples).

Pressure-Gradient Principle

The pressure-gradient principle has both sides of the diaphragm open to external sound pressure.

Sound waves coming directly from the front of the diaphragm hit the front first and the back sometime later. This phase difference causes a small pressure difference, causing the diaphragm to move. Sound waves coming directly from the back of the diaphragm work in an opposite fashion.

Sound waves coming directly from the side of the diaphragm hit both the front and back simultaneously, causing no difference in pressure and therefore no diaphragmic movement!

Thus, the pressure-gradient principle yields a bidirectional or “figure-8” polar pattern. The microphone is sensitive to sound coming from the front and back while it rejects sound from the sides.

For more information on the bidirectional polar pattern, check out my article What Is A Bidirectional/Figure-8 Microphone? (With Mic Examples).

Combining Pressure & Pressure-Gradient

Often times capsules are designed in a way that combines both these principles.

The most popular microphone polar pattern is the cardioid pattern. This is basically a 1:1 ratio of pressure and pressure-gradient principles.

By restricting the pathway of sound reaching the back of the diaphragm, manufacturers cleverly access combinations of both principles. Manipulating the amount of air vibration at each side of the diaphragm results in a variety of polar patterns!

To learn more about the cardioid polar pattern and all other microphone polar patterns, check out my articles What Is A Cardioid Microphone? (Polar Pattern + Mic Examples) and The Complete Guide To Microphone Polar Patterns, respectively.


Face Of The Diaphragm: Top-Address Versus Side-Address

Another point to make about diaphragms and their capsules is how to address them. In other words, in what directions do the microphone diaphragms point?

The two most common address types are top-address and side-address.

Top-Address

Top-address microphones have diaphragms “facing” the top of the microphone. Typically the microphone will look as if it is pointing in the direction where it is the most sensitive.

Typically top address microphones are limited to omnidirectional and cardioid-type polar patterns since it’s practically impossible to have both sides of the diaphragm evenly exposed to sound pressure.

Common examples of top address microphones are the Shure SM57 and SM58, as well as the Neumann KM 184 (with links to check their prices on Amazon).

Side-Address

Side-address microphones are designed with their diaphragms facing “to the side.” The “front” and “back” of the diaphragm point to the sides of microphones, making the mic most sensitive to sound from the side directions.

With side-address microphones, all polar patterns are relatively easy to achieve. This setup also allows for two diaphragms to be designed back-to-back in order to create a variable multi-pattern microphone.

Common examples of side-address microphones are the Neumann U87, Rode NT1-A, and the AKG C414 (with links to check their prices on Amazon). The U87 and the C414 are multi-pattern microphones.

Shure, AKG and Neumann are featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.


The Moving-Coil Diaphragm

The moving-coil diaphragm is actually made of two separate parts: the diaphragm itself and the moving coil. However, since they’re attached to one another, it’s useful to think of them as a single moving piece. Moving-coil diaphragms find themselves in moving-coil dynamic microphones.

The diaphragm/conductive coil combination vibrates in reaction to external sound waves. The diaphragm is responsible for being sensitive enough to pick up on the air pressure variance between its two sides. The conductive coil is responsible for the conversion of this vibration into an audio signal. The moving-coil diaphragm and capsule act as a transducer on the principle of electromagnetic induction.

Moving-coil diaphragms are nearly all circular in shape and are stretched tightly around a stationary ring in the microphone capsule. Tension is a crucial factor in the diaphragm’s sensitivity to incoming sound waves.

In a typical design, the coil is roughly half the diameter of the diaphragm. The connection of these two elements creates a tiny dip or corrugation in the diaphragm. Therefore, the diaphragm is not perfectly flat. The diaphragm may also have tiny leaf slots cut out of it and extra corrugation to improve its performance by fixing inherent issues with the diaphragm and capsule structure.

Material

The “moving-coil” (often referred to as the voice coil) is typically made from thin copper wire coiled into a hollow cylindrical shape. There are magnets on either side of the moving-coil to allow maximal electromagnetic induction.

The diaphragm itself does not need to be electrically conductive at all. The typical material used to make the diaphragm is a polyester film (Mylar is a common brand name). This polyester film (plastic sheet) is thin and strong enough to act as an effective diaphragm material!

So the coil is attached to the diaphragm and therefore moves with it. This added weight does a few things to the overall characteristics of the diaphragm. The weight and the shape or moving-coil dynamic microphones generally give rise to the following traits:

  • Decreased sensitivity in the high-frequency range.
  • A resonant frequency in the audible range of human hearing.
  • A slower transient response than condenser and ribbon diaphragms.

For more information on moving-coil dynamic mics, check out my articles What Is A Microphone Voice Coil? and Moving-Coil Dynamic Microphones: The In-Depth Guide.


The Ribbon Diaphragm

The ribbon diaphragm is perhaps the most interesting diaphragm type. Ribbon diaphragms are long, thin, rectangular diaphragms that are only attached to their capsule/baffle at either side of their length. They are most often corrugated instead of perfectly flat and are under relatively low tension compared to moving-coil and condenser diaphragms.

Ribbon mics are also considered dynamic. Just like moving-coil microphones, ribbon mics work on the principle of electromagnetism. However, instead of having a separate diaphragm and conductive piece fused together, the ribbon acts are both these elements simultaneously. The ribbon moves in reaction to the sound pressure difference between its back and front sides. Magnets are placed around the perimeter of the ribbon so as the diaphragm moves, electromagnetic induction generates an audio signal!

Material

Ribbon diaphragms need to be conductive and extremely thin (typically less than 5 microns). Aluminum is great at achieving both these needs. Corrugated aluminum foil makes up many of the ribbon microphone diaphragms on the market. Some manufacturers utilize stronger plastic polymers as the base of the ribbon and coat them with conductive aluminum. Other times, you’ll find aluminum foil covered in a thin layer of gold. Gold layering helps to prevent oxidation of the ribbon while gold itself is a better conductor of aluminum (only not as strong).

Fragility

A Ribbon diaphragm is fragile. Gusts of wind and the air movement associated with kick drums and even vocal plosives have the potential to stretch the diaphragm, causing permanent damage. Phantom power, if sent through bad cables or connections, also has the potential to blow out or stretch the diaphragm. To add to the list, physical trauma (mic drops) also have a high likelihood of damaging the ribbon diaphragm. It goes without saying that caution should be exercised when handling and recording with ribbon microphones. The good news is often times the repair only requires a “re-ribboning” of the microphone. The bad news is the price of a repair can run upwards of $350.

By nature, ribbon microphones are set up as side-address and have bidirectional polar patterns. Because of the bidirectional (figure-8) pattern, they also exhibit the greatest amounts of proximity effect.

For an in-depth read on microphone proximity effect, check out my article In-Depth Guide To Microphone Proximity Effect.

The characteristics of a ribbon diaphragm give the ribbon microphones the following qualities (generally speaking):

  • The low tension of the diaphragm yields a resonant frequency well below the audible range of human hearing.
  • The thinness of the diaphragm gives an accurate transient response.
  • General shape and transducer principle gives a gentle, natural roll-off of high frequencies.

For more information on dynamic ribbon mics, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.


The Condenser (Capacitor) Diaphragm

It’s easiest to explain the diaphragm of a condenser microphone along with its full capsule design.

Condenser capsules are basically capacitors (condenser used to be the term for capacitor). There are two parallel plates spaced apart from one another in the form of a capacitor. In the case of a condenser microphone, these two parallel plates are:

  1. A stationary solid backplate.
  2. A movable front plate, known as the diaphragm!

Capacitors are designed to hold a charge (Q) when supplied with a voltage. The DC voltage is most often supplied through phantom power (in the case of true condenser microphones) or is permanently held in electret material in the plates (in the case of electret condenser microphones). The charge (Q), in an ideal design, stays constant.

The audio signal (AC voltage) output of the capacitor is measured with the formula V = Q / C

Since (Q) is constant, the audio signal (V) is inversely proportional to the capacitance (C). So let’s discuss capacitance.

Capacitance is the ability of the capacitor to store an electric charge. The capacitance of the condenser capsules depends on the area of the plates, the insulator between the plates (air), and the distance between the plates. Of these three influencing factors, the distance between the plates is the only variable!

As the condenser diaphragm vibrates, the distance between the two plates changes, resulting in a varying AC voltage (audio signal)!

Material

The backplate of a true condenser is typically made of solid metal alloys such as brass. The diaphragm plate is often made from either gold-sputtered mylar or exceptionally thin aluminum foil.

Electret condensers are typically made of the same material, only with an electret coating over one of their plates. “Back electrets” are the most efficient and have a thin coat of electret material on their backplates. Electret materials can be any dielectric material, including plastic or wax.

A common differentiation of condenser microphones is by diaphragm size. There are basically two camps: small-diaphragm condensers and large-diaphragm condensers. Let’s discuss both in more detail.

Small-Diaphragm Condensers

Small condenser diaphragms are typically less than 1 inch in diameter, though this only a generalization.

Small-diaphragm condensers (SDCs) are usually built in a “pencil mic” design, meaning they are top-address microphones. For this reason, you typically won’t find small-diaphragm bidirectional or multi-directional microphones.

A smaller diaphragm usually means lower mass. This translates to increased transient response accuracy and extended high-frequency response. Because the diaphragm is smaller, the capsule can also be designed smaller, allowing for a more consistent polar response.

The cons of small diaphragms are lower sensitivity ratings and, therefore, poorer signal-to-noise ratios. The condenser capsule output signal is proportional to the distance between the diaphragm and the backplate. Smaller diaphragms don’t move across as much distance as large diaphragms (to visualize this, I like to think of small and big trampolines). Since the sensitivity is less, the self-noise of the microphone electronics is more pronounced in SDC than it is in LDC with signals of the same level.

Large-Diaphragm Condensers

Larger condenser diaphragms are typically 1 inch or more in diameter, though this is only a generalization.

Large-diaphragm condensers (LDCs) are usually built as side-address microphones. This allows for the design of any polar pattern in the microphone capsule. It is even possible to create multi-pattern microphones by designing a capsule with multiple diaphragms.

The larger size of the diaphragm means greater mass. LDCs have low resonant frequencies, typically creating a bass boost in the bass frequency range. The larger size of the diaphragm also means its displacement when subjected to sound waves is more than the SDC counterparts. A bigger range of diaphragm displacement means a stronger audio signal so LDCs are more sensitive than SDCs. A louder output when subject to the same sound pressure level gives large-diaphragm condensers a better signal-to-noise ratio.

A drawback of LDC and side-address mics is that their large grilles allow for short wavelengths to bounce around inside the grille casing. If not properly damped, these frequencies will create an erratic high-end frequency response.

There are some more downsides to large diaphragms. The size and mass of LDCs make them less responsive to high-end frequencies. The greater displacement that increases sensitivity actually hinders the accuracy of the large diaphragm’s transient response. Lastly, the capsules need to be bigger to host larger diaphragms. LDCs tend to have less consistency in their polar patterns across their frequency responses compared to SDCs.

Another interesting note about LDC is that some diaphragms are edge-terminated while others are center-terminated. Edge-terminated means the audio signal is taken from the capsule edge and so the diaphragm is one full piece. Center-terminated diaphragms have their electrodes in the center of the diaphragm. In theory, center-terminated diaphragms have fewer resonant frequencies, meaning their frequency responses are less erratic. Though center-terminated designs are a bit more complex.

Let’s recap the generalities between small diaphragm condensers (SDC) versus large-diaphragm condensers (LDC):

  • SDCs are less sensitive than LDCs
  • SDCs have worse signal-to-noise ratios than LDCs
  • SDCs have stronger high-frequency responses than LDCs
  • SDCs have weaker low-frequency responses than LDCs
  • SDCs have more accurate transient responses than LDCs
  • SDCs have more consistent polar patterns than LDCs

For a detailed account of the differences between SDCs and LDCs, check out my article Large-Diaphragm Vs. Small-Diaphragm Condenser Microphones.

The general characteristics of a condenser diaphragm give the condenser microphones the following qualities:

  • The light weight of the diaphragm yields a bright upper-frequency response.
  • The tension and thinness of the diaphragm give an accurate transient response.
  • General shape and transducer principle gives a gentle, natural roll-off of high frequencies.

Diaphragm Performance Factors

Let’s discuss the main factors that affect a diaphragm’s performance:

  • Mass of the diaphragm
  • Shape and size of the diaphragm
  • Tension of the diaphragm
  • Material of the diaphragm
  • Conductivity of the diaphragm

The Mass of the Diaphragm

The mass of the diaphragm plays a big role in determining the frequency and transient responses. Both of which are critical characteristics of a microphone.

All else being equal, the heavier a diaphragm is, the lower its resonant frequency. Resonant frequencies offer a boost in a diaphragm’s frequency response. Heavier diaphragms also suffer from a lack of clarity in high frequencies. This is due to inertia and the difficulty that high-frequency sound waves experience in overcoming inertia.

The increased inertia that comes with larger masses also affects the transient response of the diaphragm. The heavier the diaphragm, the more it will resist motion. This resistance against external sound waves worsens the accuracy of the microphone’s transient response.

The Shape and Size of the Diaphragm

The shape and size of the diaphragm influence the frequency response and sensitivity of the microphone.

Most microphone diaphragms are circular shaped. This is true of practically all moving-coil and condenser diaphragms. Ribbon diaphragms are shaped like long strips of ribbon.

In circular diaphragms, the diameter relates to specific resonant frequencies. These resonant frequencies have a wavelength equal to multiples and fractions of the diameter length. It’s easiest to visualize these resonant frequencies along the diaphragm diameter like we would standing waves in a room. The wavelengths that fit within the diameter bounds of the diaphragm perimeter will either interfere constructively or destructively with themselves. Constructive and destructive interference effects frequency-specific sensitivity positively and negatively, respectively.

The size of the circular diaphragm also helps determine the microphone sensitivity. All else being equal, the bigger the diaphragm, the more distance it can be displaced from resting position (think of a small vs large trampoline). The greater the movement of the diaphragm, the more audio signal output from the capsule!

The shape of the ribbon diaphragm is that of a long, thin rectangle rather than a circle. This strip of the diaphragm is also corrugated and under much less tension than its circular counterparts.

The non-circular shape combined with the corrugation makes it so ribbon microphones have very few resonant frequencies. And those frequencies that do resonate, do so weakly. This makes for a smoother frequency response!

The Tension of the Diaphragm

The tension of the diaphragm affects a microphone’s frequency response and sensitivity.

The best analogy to help explain microphone tension is a snare drum. When tuning a snare drum, increasing the tension of the drumhead (membrane) increases the fundamental and resonant frequencies of the drum. The same is true with a microphone diaphragm membrane (though we don’t hit these with drumsticks)!

All else remaining the same, increase in tension increases a diaphragm’s resonant frequencies. The tension in circular diaphragms typically yields a resonant frequency in the bass or sub-bass frequency range. Ribbon diaphragms are typically under such a low amount of tension that their fundamental resonant frequency is below the audible range of human hearing!

Diaphragm tension also affects the sensitivity of a microphone. The tighter a diaphragm is pulled, the less displacement it will experience at a given sound pressure level.

The Diaphragm Material

The diaphragm material plays a critical role in determining a diaphragm’s overall response to sound.

Diaphragms need to be thin, moveable, and, most of the time, conductive. The material used in manufacturing diaphragms has to have high tensile strength and be able to react accurately to sound pressure variation. This reduces the number of viable options for diaphragm material.

Polyester film (Mylar is a common brand name) is an effective material. Though Mylar isn’t conductive, it is strong and flexible enough to excel as a diaphragm material. Moving-coil diaphragms are often made of polyester film exclusively since they are not required to be conductive. Condenser microphone diaphragms are often made of gold-covered polyester film in order to add a conductive element to this material.

Aluminum foil is another frequently used material in diaphragm construction. Aluminum is both strong and conductive and shows up most often in ribbon diaphragm.

The Conductivity of the Diaphragm

As an extra note on diaphragm material, the conductivity is of critical importance to a microphone’s functionality. The conductivity of the diaphragm/capsule is directly proportional to the microphone’s efficacy as a transducer.

Aluminum, gold, and copper are the three most common conductive materials used in microphone diaphragms.

Moving-coil diaphragms do not need to be conductive. However, the attached coil must be. Copper is the material used in this case.

Ribbon diaphragms are typically made of aluminum foil. They will at the very least have a coating of either aluminum or gold if not made from aluminum foil.

Condenser diaphragms are usually made of polyester film with gold or electret material laid over top for conductivity reasons.


Related Questions

Do USB microphones have the same kind of diaphragms as XLR microphones? Yes. There is no special “USB mic diaphragm.” USB mic capsules and diaphragms are built just like those in XLR mics. Common USB diaphragm/capsule designs are the moving-coil, ribbon, and condenser style. The diaphragm has nothing to do with the conversion of the audio signal to digital data.

Does a microphone need a diaphragm? All practical microphones need diaphragms to act effectively as transducers. However, there are experimental mics designed without diaphragms. The laser microphone projects a laser through an exposed stream of smoke. A laser sensor detects the variations in the smoke and outputs an audio signal.

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