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Microphones - Types, Construction & Performance

© 2006, Rod Elliott (ESP)
Page Created 10 Jan 2006

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1.0 - Introduction

Microphones are often poorly understood, and this article seeks to provide some basic details about the various types, how they work, and the interfacing of the microphone with a suitable preamplifier.  While it would be tempting to explain mic techniques, proper usage, etc., these are not topics that will be covered.  There are several reasons, but the main one is that there are so many possibilities that it is impossible to cover them all.

Instead, the focus will be on the microphone basics - how each type works, along with its advantages and disadvantages.  The following brief summary is a warm-up for the real thing - and even though it looks like a lot to cover, there are (and will remain) several omissions.  For example, carbon microphones will not be covered because they are no longer used in new equipment, and 'esoteric' microphones (such as the so-called shotgun mic) will not be explained in any detail.

With microphones, the terms Directional, Cardioid, Omni-directional (or just omni), Hyper (or Super) Cardoid, etc.  refer to the polar response, but these terms are sometimes loosely applied.  The directivity of all microphones is frequency dependent, and becomes spherical (omni-directional) as the frequency decreases.  There are exceptions, and these will be looked at as we progress.

The microphone, abbreviated to 'mic' or 'mike' is an essential part of the process of getting our music from the performers to our listening rooms.  Mics are also used for sound reinforcement, ensuring we can hear everything at a concert (and also often ensuring that we can hear very little for hours afterwards).  Correct microphone selection and placement during recording minimises the amount of equalisation that is needed, because the sound is already the way the producer intends.  The choice is enormous, as the brief summary below indicates.

This article is mainly focussed on performance mics, rather than those used for test and measurement.  The latter are almost exclusively either 'true' capacitor mics or electret types.  Almost all measurement mics are omnidirectional.  Directional mics are not used because their response is unpredictable (especially for low frequencies) and SPL (sound pressure level) must include sound coming from all directions.  Measurement mics are a complete topic unto themselves, and are only mentioned here in passing.

2.0 - Basic Microphone Characteristics

Although the number of different microphones looks daunting, they are all based on common parameters ... these are directional patterns and transducer types, and almost every microphone made is covered by the two listings below.

2.1 - Directional Patterns

The directional characteristics of microphones are defined in the capsule (or capsules, in the case of dual capsule mics).  Contrary to what some may claim, any type of microphone can be configured to have any of the listed configuration patterns.  The directional characteristics are frequency dependent, and refer to the free field response - placing a microphone very close to any surface changes its directional characteristics, and they become unpredictable because of the almost infinite number of possibilities.  Directional microphones are also called 'pressure gradient' mics, because their directional characteristics are created by means of varying pressure to the front and rear of the diaphragm (the pressure gradient).

In the drawings below, the mic position is shown by a dot.

Omni-directional ...
omni Pick up sound (more or less) equally from any direction.  Omni-directional refers to the frequency response being essentially flat, regardless of the direction of the arriving sound waves.  Omni mics can often give fewer feedback problems compared to most cardioids, but this is highly dependent on correct usage.  Omni mics have minimal proximity effect, and are (generally) better suited to instruments.  These mics are not commonly used for live production - partly because of limited understanding.

Measurement microphones are exclusively omnidirectional, with no significant exceptions that I could find.  Sometimes they may be arranged in an array to obtain the required directional characteristics, but this is only common for infrasound measurements as used for detecting volcanic activity or missile launches.

Cardioid ...
cardioid The most common directional pattern.  These usually have a proximity affect that colours and enhances the bass end of vocals at close range.  Different cardioid mics may suit male and female singers.  Singers should own their microphones and be skilled in the techniques of using them, in the same way that musicians own their instruments.  Cardioid mics are often misused for instruments, typically used in very close proximity to drum skins (among other misuses).  Naturally, if this gives you a specific sound that you want, then it is no longer misuse.

Hyper-Cardioid ...
hyper This is an exaggerated version of the cardioid mic, so it is more directional.  A side-effect is that a small lobe is created at the rear of the microphone, so these mics must never be 'aimed' so that the rear lobe points towards a floor monitor (for example).  Sometimes a distinction is made between 'super' and 'hyper' cardioid microphones, but other descriptions will consider them to be equivalent.

Figure-8 ...
fig8 The figure-8 mic picks up sound equally well from the front and back, but rejects sound coming from the sides (as well as top, bottom, etc.).  The pattern can be looked at as an extreme form of hyper-cardioid, where the front and rear lobes are equal in amplitude and frequency response.  Many dual element microphones combine an omni and figure 8 capsule to allow switchable directivity.

2.2 - Transducer Types

At the heart of every microphone is a transducer - simply a mechanism that converts one form of energy to another.  The source (input) energy is sound, and the output is electricity.  An electrical waveform is produced, which matches the acoustic input with as little modification as possible.  All directional microphones must (by definition) alter the received sound to some extent.  It is not possible to modify the directional characteristics without also altering the nature of the sound that is picked up.  This is not necessarily 'bad', just different.

Likewise, many cartridge / capsule types (the actual transducer) have their own sound, whether real or imagined.  This often influences the choice of microphone type for different tasks - for example, there are mics that are favoured for bass (kick) drums that may be deemed unsuited for anything else.  This is not necessarily true, as experimentation can often demonstrate.

Condenser ...
More correctly called capacitor mics, these are generally considered to be the ultimate.  They have exceptional detail, and can usually tolerate very high sound levels.  Distortion is very low, because the diaphragm movement is so small (comparable to that of the human ear drum).  Capacitor mics most commonly use a high DC voltage to polarise the 'plates' of the capacitor sensor, although some use the change in capacitance to modulate a radio frequency oscillator.  The frequency modulated 'carrier' is then fed to a detector stage to be converted back to audio.  Another form is called MEMS (Micro Electro-Mechanical Systems), which typically use a charge-pump to provide the polarising voltage.

Capacitor mics (of all types) require power - this may be supplied via the P48 (48V phantom feed) from a mixing desk, or may be an external power supply.  Electret and MEMS mics are low voltage (between 1.2 and 5V) and are normally supplied with power by the equipment in which they are installed, or from a single 1.5V cell (common with self-contained electret microphones).

In audio production, probably the most famous of all capacitor mics is the Neumann U47.

Dynamic ...
The dynamic mic uses a mechanism that is very similar to speakers.  The majority are robust and can accept extreme sound levels, ideally suited for live productions.  Most are cardioid, although omni-directional and hyper-cardioid types are also available.  Dynamic mics are the most common of all types used in live work, and they are often used for studio recording as well.  One of the best known is the venerable Shure SM58.

They are usually very rugged, and can handle more abuse than almost any other type of microphone.  The ideal dynamic mic has a low impedance voicecoil, and uses a small transformer to provide the required output level and impedance.  High impedance voicecoils are fragile, and can't handle the rough treatment common with live performances.

Electret ...
Also called 'Electret Condenser' or 'Electret Capacitor' microphones use a permanently 'charged' plastic membrane so that a high voltage polarising signal is not needed (as is the case with 'true' capacitor mics).  Most are omni-directional, although cardioid inserts are also made.  Like capacitor mics, an impedance conversion stage is essential because of the extremely high intrinsic impedance.  While electrets can be used for stage work, they may distort at high sound pressure level (SPL).  Many vocalists are capable of driving electret mics well into distortion.  Temperature and humidity (such as from the breath of vocalists) can adversely affect them.  Professional electret microphones are excellent for recording.

Electret mics (also known as 'pre-polarised') are now very common for sound level meters and other precision measurements.  Many do not use an internal FET, relying on an external preamp to provide the several gigohm input impedance needed to measure low frequencies.  The capacitance is very small, often no more than 10pF for a miniature capsule.  To get down to 20Hz, the preamp input impedance needs to be around 1G ohm (1,000Meg).

Ribbon ...
These are common in recording studios, but less so in live work because they are comparatively fragile.  A very thin (usually aluminium) ribbon is suspended in an intense magnetic field, and generates a small current when it is moved by sound.  Ribbon mics have extremely low impedance, typically (much) less than 1 Ohm.  A transformer is used to raise the impedance (and output voltage) to a usable level.  Although Ribbon mics have an inherent Figure-8 pattern, they are also available with cardioid or hyper-cardioid patterns.  'Planar ribbon' mics are a variation of the theme.  These use a thin membrane with a planar (flat) coil deposited on the membrane.

Carbon ...
These microphones used to be very common - every old style telephone had one.  The carbon mic has one major advantage over every other type - it has gain! Because the microphone element is made up of carbon granules, speech activating the diaphragm will compress and release the granules, changing the resistance significantly.  Since power is needed by these mics, this is provided by the telephone line.  The microphone gain is such that no additional amplification is needed to allow a normal phone call - even over a considerable distance.  In the early days of telephony, this was essential to the operation of the 'phone network - so much so that if it were not for the carbon microphone, the telephone would never have been even useful (let alone gain acceptance) in those early days.  Cheap and reliable amplification has made them redundant now.

As a short side-note, it is worthwhile mentioning that the telephone system uses a nominal 48V supply (see phantom feed, below).  The influence of telephony on electronics as we know it is huge - so much so that the development of the phone system drove many of the inventions that we now take for granted.  Have a look at the vast contribution of Bell Laboratories (which used to be an integral part of AT&T).  Bell labs invented the transistor - the very cornerstone of every electronic product we use, as well as the electret microphone (plus countless other things we now treat as commonplace).

2.3 - Specifications

The output level of microphones should ideally be rated in millivolts per Pascal (mV /Pa), although there are many variations.  Other conventions used include dBm at 0.1 Pa (this will always be a negative number).  All new microphones will generally be be rated in dBV at Pa, where 0dB is 1V.  For example, a mic may state its sensitivity as -44dBV (the 1Pa reference is sometimes assumed), which translates to 6.31mV at 94dB SPL.  Other standards may persist in some countries.

1 Pascal = 10 micro-Bar = 94dB SPL
0.1 Pascal = 1 µBar = 74dB SPL
1 dyne/ cm² = 0.1 Pascal = 1 µBar

There are also noise ratings (these vary widely, both in output noise and the way it is specified), output impedance, recommended load impedance, polar response, frequency response, etc.  Frequency response claims are meaningless without a graph showing the actual response, and for directional mics this should also indicate the distance of the mic from the sound source.  Cheap microphones are particularly bad in this respect, and it is not uncommon to see the frequency response stated as (for example) 50 - 20,000Hz.  Because no limits are quoted (such as ±3dB) this is pointless - any microphone will react to that frequency range, but may be -20dB at the frequency extremes, with wide variations in between.

A proper graph showing the response at all frequencies will quickly show the actual response, although it is uncommon for any manufacturer of general purpose mics to state the distance between mic and sound source, or the method used to take the measurement.

A polar response graph will also show the directivity at a number of different frequencies.  As frequency decreases, the directional pattern commonly approaches omni-directional, although some mics maintain excellent directivity even at very low frequencies (the secret is in the rear chamber).

3.0 - Sub-Types of Microphones

There are some microphones that appear to be completely different from those described above.  Not the case, as the essential characteristics and transducer types don't change, but more/ different hardware or electronics are added to give additional functionality.

RF (Transmitting) Mics ...
These are available in many variations and professional types can be very expensive.  A conventional microphone transducer having one of the directional characteristics listed above is connected to a small radio frequency (RF) transmitter so the mic can be used without the need for cables.  The transmitter for professional radio mics requires excellent frequency stability, and receivers are highly specialised to ensure no 'drop-outs' and maintain a good signal to noise ratio (SNR) at all times.

These mics used to require specialist knowledge and experience to use them correctly, but they are now commonplace and few people have issues with them.  Many have automatic limiting and compression that has to be managed carefully, because compression limits the dynamic expression of good singers, causing them to sound comparatively flat and lifeless.

PZM™ ...
The Pressure Zone Microphone™ (also known as a boundary mic) is a special application of the electret mic.  A miniature electret sensor is mounted a small distance (typically less than 1mm) from (and facing towards) a flat plate.  They are often used on the floor or walls, tables (for conferencing and the like), but can also be attached to large flat discs or plates.  They have exceptional performance, and can effectively reduce reverberation if used carefully.

There are several variations on the basic technique, allowing for a single stereo mic unit, a cheap 'knock-off' made by Radio Shack (Tandy in Australia) called a boundary mic, but lacking the characteristics of a true PZM, and a few others.

Dummy Head ...
The dummy head mic technique yields extraordinary performance, but the recording can only give the full effect when listened to through headphones.  Electret mic capsules are either embedded in a true dummy head (wig-carriers can be used ... meet Yorick below), or miniature capsules are worn in the ears of the sound recordist.  When played back through headphones, the original sound field is essentially restored, and the listener hears the sound as if s/he were there.

The requirement for headphones has limited the appeal of the technique.

Shotgun ...
Shotgun mics are worthy of a complete article to themselves.  Usually fairly long, they have an extreme directional pattern, and typically only pick up sound from the general vicinity directly in front of the mic.  There are several ways to make shotgun microphones - techniques include a long 'barrel' with slots designed to create an interference pattern that rejects sound from the side, and multi-element designs with phase and amplitude balance between elements.  An old method was to use multiple thin tubes of differing lengths, arranged so that the longest tube is in the centre, with smaller tubes surrounding it.

Some shotgun mics use a combination of methods, as well as careful attention to the mic capsule's rear chamber.  These mics are useful for location sound recording (for movies or TV), nature recordings, and anywhere else where very high discrimination is needed.

4.0 - Element Construction

The following is intended to give you an idea of the basic techniques used to make various microphone elements.  These are the basic building blocks, and while some (such as dynamic mics) can be used with no additional circuitry other than a small transformer (not always used), most others require some additional components to be useful.

Because the dynamic mic is one of the most prolific (or so it would appear to the uninitiated), it will be covered first.

fig 1 The general arrangement of a dynamic mic is shown to the left.  A diaphragm is coupled to a voicecoil that is suspended in a strong magnetic field.  As the diaphragm (and thence the coil) moves in sympathy with the arriving sound waves, an electric current is generated.  In a perfect microphone, the electrical current will be an exact replica of the acoustic signal, but in reality this is never the case.

The element (also known as a capsule) shown has a vented pole-piece (indicated with a *), and this is typically done to create the required directional characteristic.  For an omni-directional dynamic microphone, the back would be sealed.  However, as with all omnidirectional microphones there must be a small vent to allow air pressure to equalise on both sides of the diaphragm.  Without the vent, the diaphragm would be displaced by changes in atmospheric pressure.

As you can see, this is very similar to the construction of a small speaker, and indeed, a speaker will work as a microphone (and a dynamic mic can also make noise).  Naturally, the speaker and mic are each optimised for their intended application, and neither works particularly well when its role is reversed.  99% of basic intercom systems use the speaker as a microphone.

Typical dynamic microphones have an impedance of around 150 - 300 ohms, although some are higher or lower than that.  While it may seem tempting to match the impedance of the microphone and preamplifier, this is ill advised, as it will reduce the signal level by 6dB, and thus reduce the signal to noise ratio.

fig 2 A capacitor microphone is much simpler mechanically, but the material quality is critical for good performance.  Because the capacitance is so small, the insulation resistance must be very high, as must the impedance of the following stage.  It is not uncommon to find well in excess of 1 Gigohm input impedance for the impedance conversion stage.

While the capsule shown has damping material, this may not always be the case.  The distance between the diaphragm and the rear of the housing can be made small enough so that no ill effects occur within the audio range.  Like the omnidirectional dynamic mic, a vent is provided to equalise air pressure.

The backplate must be polarised so the microphone will work.  While this may be as low as 48V, this may not be insufficient to allow a worthwhile signal level.  Voltages up to 200V will be found in some examples.  This places great constraints on the insulation, and means that such mics can be adversely affected by moisture.

In some cases, the microphone capsule may have two diaphragms, each spaced as close as possible to the backplate.  This will create a microphone with a figure-8 directional pattern.  The one shown is omni-directional - this may come as a surprise because sound coming from the rear of the mic is shielded from the diaphragm by the mic itself, but this only applies at very high frequencies.  Many Neumann mics use a dual diaphragm capsule, and switch one diaphragm to change the directional characteristic from cardioid to omni-directional.

The diaphragm of capacitor mics must be conductive, and it is common to use metallised plastic film (Mylar is popular).  The metallisation film must be protected from moisture, so may be on the inside of the capsule.  In almost all cases, the insert will have a tiny bleed hole to allow the air pressure inside the housing to match that of the outside atmosphere.  If this were not provided, the diaphragm to backplate spacing would vary with atmospheric pressure.

Electrically, a capacitor mic can be represented by a signal source in series with a capacitance equal to that of the capsule itself.  As noted above, this will be very low.  A typical capacitor microphone (such as the Neumann U47) has a capacitance of around 80pF (See References.)

Historically, these mics have been known as 'condenser' mics for many years.  'Condenser' is the old term for a capacitor.

fig 3 Electret (sometimes referred to as 'ECMs' - electret capacitor microphone) mics work using the same general principles as a traditional capacitor mic.  Instead of using a DC polarising voltage, the backplate is an electret material (this is a so-called 'back electret').  This material is a plastic that is subjected to an intense electrical field during processing.  This causes the plastic material to retain a charge (more or less) permanently.  The electret surface must be metallised to make it conductive.  Some electret mics use the diaphragm as the electret element (and use a conventional backplate), and while this works very well, they do not have an indefinite life.  As before, the vent is required.

The FET shown is almost always included in the capsule itself for consumer electret mics.  This is the impedance converter, and in most cases there is no resistor from the gate to common (ground, mic housing).  This is one reason that electret mics can react badly to a sudden loud sound, and may lose sensitivity for a few seconds.  The FET gate circuit relies on surface leakage alone to bias the FET correctly.

While I stated earlier that dynamic mics seem to be the most common, they are soundly (pun intended) beaten by electret microphones.  All modern telephones use electret mics, including mobiles (aka cell phones), answering machines, computer headsets, and virtually every piece of electronic equipment that needs to hear voice commands, noise, etc.  The electret has been the most successful mic capsule ever developed - over 100 million are produced every year! However, MEMS mics (see below) are now starting to take over, and will capture even more of the market in time.

fig 4 The ribbon mic has a special place in the heart of many a sound engineer.  They have an inherent figure-8 pattern, although this is often modified to produce more 'conventional' patterns.  Because the impedance of the ribbon is so low, all such microphones use a transformer to raise the impedance and output voltage to more usable levels.  The transformer is almost always in the same housing as the microphone element itself.

Ribbon mics are often though to be fragile, and many of them are.  There are others that are very robust indeed.  Because ribbon mics use a relatively large diaphragm (much larger than most other mics), they can be very sensitive to air movement - even at subsonic frequencies.

However, high SPL does not usually bother a ribbon mic in the least.  Provided the ribbon remains in the gap, almost nothing will cause a ribbon mic to distort - apart from the aforementioned air movement which must be avoided.  Even apparently gentle air movement can distort the ribbon, which then must be replaced.  'Planar' ribbons are used by some manufacturers - a planar ribbon is not a ribbon in the true sense of the term, but uses a metallised coil printed on a thin plastic carrier.  These are very rugged according to the literature.

Because of relatively low output level (even after the transformer), you need a very quiet preamp for ribbons.  They have very low self noise, so preamp noise can easily exceed the microphone noise.

fig 5 MEMS (Micro Electro-Mechanical Systems) microphones are now replacing electrets in many applications.  They are made using traditional silicon etching processes, where layers of different materials are deposited onto a silicon wafer and the unwanted material is then etched away.  This creates a moveable membrane and a fixed backplate over a cavity in the base wafer.  The sensor backplate is a stiff perforated structure that allows air to move easily through it, while the membrane is a thin solid structure that flexes in response to the change in air pressure caused by sound waves.

Changes in air pressure created by sound waves cause the thin membrane to flex while the thicker backplate remains stationary as the air moves through its perforations.  The movement of the membrane creates a change in the capacitance between the membrane and the backplate, which is translated into an electrical signal by the ASIC (application specific IC).  MEMS mics always require power, typically 3.3V at a few hundred microamps.

MEMS mics are rugged, and are almost always made as SMD (surface-mount devices) allowing them to be placed on a PCB along with the other SMD circuitry.  While some have good low frequency response, most are tailored for use with speech signals only.  They can have an analogue output, although many provide a digital output in the form of pulse-density modulation (PDM), which is easily converted to a 'traditional' digital data stream by a microprocessor.

MEMS mics are available with the sound port at the top or bottom.  A bottom port as shown in the drawing provides a reasonably large back-chamber, which improves low frequency response and sensitivity.  Top port types mean that the back chamber is very small (just the size of the front chamber in the drawing), generally resulting in reduced sensitivity.  The small cavities (chambers) also act as Helmholtz resonators, and can be used to tailor the frequency response, especially at high frequencies where the chamber size becomes significant compared to wavelength.  Most MEMS mics are tiny, with a typical package size being only 3 x 4 x 1mm, with some being smaller still.  As the package size is reduced, it becomes more difficult to achieve good performance because the back chamber (in particular) is so small.

5.0 - Powering Schemes

For those microphones that require power, the most common option is phantom feed (P48).  This uses a nominal 48V DC applied to both signal leads via a 6.81k resistor.  A good example of a P48 powered microphone is described in Project 93, and Project 96 describes a 48V power supply and P48 distribution scheme.  For the sake of completeness, Figure 5 shows the general arrangement of a 48V phantom feed system.  Although the feed resistors are shown as 6.81k, 6.8k resistors can be used instead.  It is recommended that they be matched to within 0.1% so common mode rejection is not compromised.

fig 6
Figure 6 - 48V Phantom Powering

Although the phantom feed supply voltage has been standardised at 48V, there are many supplies that do not comply, with some operating at 30V or even less.  While mics designed for P48 power might work with these low supplies, they may not.  In general, phantom feed power supply must be able to supply 48V.  The accepted voltage range for P48 is between 38V and 52V.  A 'new' sub-standard has arisen, called P24 (20V - 26V), but this is (IMHO) a seriously retrograde step, creating potentially disastrous incompatibilities between competing standards.

Some time in the late 1960s, Neumann (of microphone fame) converted its valve (tube) capacitor microphones to solid-state.  They decided upon a remote powering system that they called Phantom Power, and this was a trade mark of Neumann.  Although other manufacturers originally avoided the trade mark (using terms such as 'simplex' instead), with time the term Phantom Power has become generic.  DIN standard 45596 describes the powering of any device that uses the P48 phantom powering scheme.

Because phantom power is a common mode signal (it appears equally on both mic leads), plugging a balanced dynamic mic into a 'live' P48 powered mixer channel will not harm the microphone.  The mic may make strange and/or loud and/or rude noises if the internal insulation is degraded (by age, saliva, beer, rum+cola, etc., etc.).  In general, it is better to switch off the P48 supply unless it is needed.

Phantom powering is not the only way that power is supplied to microphones.  Another standard is called T12 - as well as transverse feed, A-B powering, parallel powering, and occasionally by its full name ... 12V Tonader (it originated in Germany).  It is not commonly found outside the film industry, and is totally incompatible with P48 powering.  Adaptors can be fabricated, but require a transformer.

The T12 system uses 180 ohm feed resistors and a 12V supply, but the DC is not sent as a common mode signal like phantom feed.  Referring to an XLR mic connector, the positive DC is applied on pin 2, negative on pin 3, and earth (ground) on pin 1.  However, there is also a reverse version, with positive on pin 3 and negative on pin2.  T12 powering will probably damage dynamic mics that are inadvertently connected while the T12 power is on.

Capacitor microphones using valves (tubes) will almost always require a special outboard power supply, and multi-pin connectors are common.  Because of the current needed by the valve heater, the 2 - 4mA available from P48 is completely unsuitable.  These power supplies will be specific to the microphone - as far as I know, there is/was no standard adopted by manufacturers, so each will be different.

6.0 - General Usage Guidelines

For live applications, the number of 'open' microphones (i.e. connected and picking up sound) should be kept to minimum.  Unnecessary use of a large number of open mics creates excessive comb filter distortion.  This reduces intelligibility and increases feedback problems.  There are many recommendations that you may find - you may be advised to minimise the number of different microphones, for example.  Exceptions are (directional overhead) for percussion and (dynamic high velocity) for bass drums.  Placing any mic too close to an instrument, sound source or surface affects its response.  This effect may be good or bad, depending on what you are trying to achieve.

There are many sites on the Net that give some general idea of what microphone to use where, but these are mainly a matter of opinion.  Everyone who uses mics has different ideas on optimum placement and type.  Some are reasonable, a few good, and a lot that are (IMO) just plain wrong.  One thing that is almost never mentioned is that where you place a microphone may change its characteristics.

If a mic is placed very close to a surface (be it a wall, floor, drum skin or singer's face) it will no longer have the directional characteristics you purchased it with.  Likewise, holding a mic in such a way that your hand cups the back of the mic ball will change directionality radically and unpredictably.

Something that is not well understood is just how much signal you can get from a microphone.  A typical dynamic mic is easily capable of 0.5V RMS (500mV) when held close and singing (or in my case yelling) loudly.  This may seem extreme, but look at the specification for the SM58 as an example.  1.85mV at 1 Pascal (94dB SPL), so 185mV results at 114dB SPL - anyone can yell that loud at close range.  You will get 500mV at just under 123dB SPL.  While this may seem pretty extreme, many vocalists can achieve such levels at close range - good mic technique includes 'pulling back' from the mic when singing loudly, and getting in close for soft passages.  This is a vocalist's natural compressor, but many singers don't have any mic technique at all (there seems to be an increasing number that don't have any singing technique either, but that's a different matter). 

At these levels, you can completely forget using electret mics, as they will just distort badly.  Because sensitivity is much higher than a typical dynamic mic, the mic may attempt to produce perhaps 3-5V RMS at the same SPL (123dB), and this is not possible with standard electret capsules.  This is especially the case if it is powered by a 1.5V battery! Such mics are very common (and very useless for most applications).

7.0 - Conclusion

This article has only scratched the surface, but is a good starting place.  Although there are a great many variations, the details above cover the majority of microphones in general use.

As an experiment, I was recently forced (i.e. it was something I'd been planning to do for well over a year) to build Yorick (as in "Alas poor Yorick, I knew him well" - Shakespeare).  Yorick is a dummy head microphone, and details are available in Project 112 so you can build your own version.  Tests are very encouraging, with an amazing ability to locate the sound source.

Yorick - My Dummy Head Microphone System

Please note that any resemblance between 'Yorick' and a certain well-known (and now deceased) American entertainer (who seemed to be rather over-fond of cosmetic surgery) is entirely coincidental. 

Although you can purchase a Neumann, Gras or Brüel & Kjær dummy head mic already made, I suspect the price will be a fairly strong deterrent.  There are other methods of achieving much the same result, but there is something rather nice about having a 'real' head rather than a plastic or MDF disc with mics on each side (the hair is optional of course).  Each capsule uses a P93 mic amplifier board.  In my case, I already had a suitable preamp that is multi-purpose, but the P93 mic amp is the easiest way to build the unit.

8.0 - References
  1. Phantom Powering for Condenser Microphones -
  2. Shure SM58 Data Sheet -
  3. Microphone History - ...  (No longer available)
  4. Neumann U47 Schematic -


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Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2006.  Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws.  The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference.  Commercial use is prohibited without express written authorisation from Rod Elliott.
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