Microphone Guide

Last updated: 23 January 2009
Published in: Creating new digital media |
Tags: audio | business & community engagement | digitisation | hardware | microphones | sound recordings | video |

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Summary

A guide to microphone characteristics, selection and care

Introduction

Microphones represent the primary point of interface between the world of real acoustic sound, and digital recording devices. There can be many stages in the chain between the two, but the process of plucking soundwaves from the air is - by definition - the sole domain of the microphone.

Microphones are the first stage in the recording chain, and as such their performance will define much of the quality "ceiling" of the remaining production process. Care should be taken in choosing the correct type of microphone(s) for the application, in setting them up, and in using them correctly and to their full potential.

This guide will give information on the types of microphone commonly available to the sound recordist, and their respective strengths, weaknesses and general characteristics. It will also advise on their suitability for different recording tasks and environments, and in caring for these delicate instruments, to maximise their performance and longevity.

A related paper, outlining some of the most important aspects of Microphone Technique is also available from our advice documents pages, which covers issues relating to connecting, powering and adjusting the various types of microphone, configuring and placing them for different tasks.

Update: A screencast about microphone selection and technique

#6 Online surgery - Microphone technique and placement from JISC Digital Media on Vimeo.

Of course there is more to capturing a good recording than simply choosing the right microphone, but it is an important component in the recording chain. Microphones need a lot of amplification, which can again vary greatly in quality, and their signal can then be heavily processed - both in the analogue and digital domains - even if the intention is to make it sound as though it has not been touched; for example, every voice you hear on BBC Radio is subject to considerable processing, to make it sound clear and natural, so you should not expect just to plug in a good condenser and have your voice magically transformed into the dulcet tones of David Attenborough or Joanna Lumley (unfortunately!). There is more to it than that, but it is a good start...

As noted, microphones are delicate and sensitive instruments. Furthermore, the more expensive and esoteric, generally the more delicate they become, rather than more robust. If you are going to be handling and using microphones regularly, then it is vital to be aware of good practice - if for no other reason than to protect your investment! Irrespective of the relevance to you of the rest of this paper, you should make sure to read the short section on microphone care.

Types of microphone

1: Dynamic microphone

Dynamic microphones are one of the simplest and most rugged of microphone designs. The pickup diaphragm is connected to a coil/magnet assembly - similarly to a loudspeaker cone, but in reverse. Vibration of the diaphragm, caused by sound pressure waves, in turn moves the coil relative to the magnet and produces an electrical signal. The dynamic microphone produces current mechanically, and therefore does not require a power source.

Dynamic microphones generally favour one or more frequency bands, and hence are less suited to sensitive studio work. They are, however, ideal for some applications where a limited response is preferable, or high volume handling is required - for example, a bass drum mic, recording an electric guitar amplifier or a stage vocal mic for use in loud environments.

Common uses: Microphones for live performance, or requiring the ability to handle high volumes and/or particular frequency bands

Advantages: Small, inexpensive, rugged and relatively moisture-resistant

Disadvantages: Uneven frequency response

Shure SM58 dynamic microphone

2: Condenser microphone

Condensers are generally acknowledged to be the best microphones for recording speech, acoustic instruments, and indeed all sources where subjective sound quality and musicality are paramount. Their cost, delicacy and sensitivity, however, make them unsuitable for some acoustic environments.

They use a small sealed capsule - usually up to one inch in diameter - one side of which is a very thin membrane pickup. Using either an external current (phantom power) or a electrostatic magnet to provide a difference in electrical charge between the two sides of the capsule, the variation in distance between the two, caused by sound waves, is output as a continuous signal.

n.b. Moisture or condensation trapped in the capsule of a condenser microphone can permanently damage it, so use caution in positioning and storage [see Microphone care]

2.1 Large diaphragm condenser

The workhorse of recording studios, the large diaphragm condenser relies on a large (usually 1" or thereabouts) and lightweight diaphragm, often coated with a very thin layer of gold, to pick up sound waves. Diaphragm vibration is in turn converted into an electrical signal, using the capacitance condenser principle. Some large diaphragm condenser capsules combine two diaphragms facing in opposite directions, and allow the two signals - which will be out of phase with each other - to be mixed together, both in and out of phase, either continuously or in a set of predefined ratios, to create a variety of composite polar patterns [see pickup patterns].

Common uses: Wide range of uses, from domestic mono and stereo mics to professional studio microphones - a versatile design capable of very high quality results

Advantages: Breadth and consistency of frequency response, sensitivity

Disadvantages: Delicate mechanism can be expensive to produce, and is sensitive to moisture

Neumann U87 multi-pattern large diaphragm condenser (with shockmount)

2.2 Small diaphragm condenser

Though similar in many ways to its large diaphragm counterpart, reducing the size of diaphragm in a condenser design has two main effects. Firstly, the frequency response is further improved, giving a flatter and more neutral response across the whole audio spectrum - this characteristic means that the design is well suited to use in measurement microphones. Unfortunately, reducing the diaphragm size also adversely affects the self-noise of the microphone, so signal-to-noise ratio will be lower, unless designed and manufactured to stringently high standards.

In general terms, large diaphragm mics are generally (though not exclusively) favoured for vocal and many types of instrumental recording, and small diaphragms for instrumental recording and acoustic measurement

Common uses: Speech, choral, acoustic and classical recording. Measurement microphones

Advantages: Sensitivity, dynamic and frequency range. Subjective "musicality" and "warmth" (especially large diaphragm)

Disadvantages: Cost, power requirements, delicacy, sensitivity to moisture

Earthworks QTC1 omnidirectional small diaphragm condenser

2.3 Shotgun microphone

A condenser capsule will be positioned in a long slotted tube, which serves - by means of a complex system of diffraction and phase cancellation - to tighten the pickup pattern into a highly directional "beam". This particular arrangement of microphone elements is referred to as a shotgun microphone.

Most professional camera-mounted microphones are of the shotgun type, as they exhibit an extreme form of hypercardioid pattern, and so allow pickup in a tight forward-facing field, with minimal spill of ambient noise [see pickup patterns]. Shotguns are often mounted on a boom arm, in combination with a windshield for on-location recording. The length of the slotted tube is an indication of the degree of directionality - generally, the longer the tube, the tighter the pattern.

Common uses: On-location interviews, field recording, film & television

Advantages: Very tight pattern, highly directional, good off-axis rejection

Disadvantages: Tight pattern unsuitable for many applications. Comb filtering artefacts imparted by slotted tube mounting

Shotgun microphone

3: USB microphone

Over the last few years USB microphones have become available, which in many cases make an excellent choice for podcast production etc. They connect directly to a computer USB port, and draw any necessary power from this USB connection. They amplify and digitise the signal internally, and incorporate a built-in computer audio interface, thus enabling direct connection to a computer based digital audio workstation, without the need for a pre-amplifier or soundcard. This makes them supremely easy to use, though slightly lacking in flexibility in a professional environment.

USB mics can be of either condenser or dynamic type, and are often available bundled with recording software, desktop stands, cables etc, making them a good package choice for the beginner wanting to start making podcast recordings, screeencasts, or simple voiceovers.

Our USB Microphone Guide looks specifically at USB microphones in more depth, with some examples and audio comparisons.

Common uses: Podcasting, screencasting, mobile laptop recording, home recording

Advantages: No soundcard necessary, powered via USB, ease of setup, some have built-in headphone output

Disadvantages: Lack of flexibility in choosing pre-amp/soundcard etc

Røde Podcaster USB microphone

Samson Go Mic USB microphone

4: Lavalier microphone

The term "lavalier" refers to a miniature and usually body-worn microphone. Lavaliers can be of condenser or dynamic types, and often come paired with a beltpack to transmit the signal wirelessly [see wireless systems]. Their small size allows them to be clipped to clothing or worn on a lanyard, and in combination with a radio system give the speaker complete freedom of movement. Though many provide excellent sound quality, their size and design mean that they do not equal larger microphones in quality of output and frequency response, and their signal-to-noise ratio is generally somewhat lower. However, in an environment where a mobile speaker requires a discrete mic with good clarity, where absolute sound quality is not a priority, they are an ideal choice.

Common uses: Television interviews, lectures, seminars etc

Advantages: Small size, clip-on. Combined with radio beltpack give complete freedom of movement

Disadvantages: Limited frequency response, self-noise

Lavalier microphone

5: Radio microphone

Wireless radio microphones transmit the microphone signal from a battery powered mobile transmitter to a matching receiver, attached to the camera/recorder/laptop etc. This allows a degree of freedom to the subject which is not available with a conventional wired mic. Wireless microphones fall into two broad categories :

5.1 Handheld transmitter

The familiar handheld radio mic, incorporating a radio transmitter and batteries into the body of the microphone. Can also be used in combination with a microphone boom [see ‘boom microphones']

5.2 Beltpack transmitter

Used in combination with a miniature clip-on lavalier (see above) or a headband worn mic, the beltpack transmitter is worn by the subject, and attached by a short cable to the mic itself, and again contains batteries and radio transmitter

Radio microphones will use one of a set of allowed frequencies - either switchable on mic and receiver, or fixed by the manufacturers. If multiple mics are to be used in the same venue, then they must all be set to different frequencies, to avoid interference. Fixed frequency mics can therefore usually be ordered in a range of different frequencies.

It is advisable to use fresh batteries in the radio mic transmitter for each event.

Common uses: Lectures, broadcast, live performance

Advantages: No wires! Freedom of movement, unobtrusive

Disadvantages: More complex setup, reliance on batteries, lower signal-to-noise ratio

Radio system components (clockwise from top left): clip-on Lavalier microphone, beltpack transmitter, headworn mic, handheld transmitters, diversity receiver

6: Ribbon microphone

A corrugated metal ribbon is suspended in a magnetic field, and its vibration within this field, when subjected to sound waves, generates the electrical output signal, in a similar way to the magnetic induction produced by the coil of a dynamic microphone. Because of this design, and its inherent inability to distinguish which side of the ribbon the signal is coming from, ribbon microphones invariably exhibit a bipolar or "figure of eight" pickup pattern [see pickup patterns]

Common uses: Part of a Blumlein pair or Mid-Side M-S stereo pair

Advantages: Often favoured for their "natural" sound. Their figure of 8 bidirectional pattern is also very useful in some common stereo recording techniques [see Microphone Techniques]

Disadvantages: Delicacy - can be damaged by plosives or high sound pressure levels (especially vintage ribbon microphones), and phantom power. Reduced high frequency response

Royer ribbon microphone

7: Piezoelectric microphone

Certain chemical compounds will, when pressure is applied to them, generate an electrical impulse. In a piezo-electric microphone, sound waves apply varying pressure to the pickup, which contains this type of substance (often a crystalline material) and produce the output signal.

Common uses: Contact microphones/pickups for guitars, drums and acoustic instruments;hydrophones

Advantages: No exposed moving parts, so can be used in extreme conditions where a conventional microphone is unsuitable (underwater, for example)

Disadvantages: Limited frequency response

8: Boundary microphone (aka PZM: Pressure Zone Microphone)

Often, in theatre or live event recording and/or sound reinforcement, the acoustic properties of the space make it desirable to place a microphone as if it were flush with a floor or wall surface (or boundary). This effectively eliminates audible reflections from at least that surface, and also controls phase cancellation problems and associated comb filtering in the mid and bass ranges, caused by these short delays.

In a boundary microphone an omnidirectional capsule (generally a condenser) is placed very close to an acoustically reflective plate, such that the reflected sound waves reach the capsule almost coincidentally with the direct waves. In this way the small delays which can cause unwanted phasing of the signal are greatly reduced or eliminated, and those that do exist are so short that they do not interfere significantly with audible frequencies. This property gives boundary microphones a more even frequency response in acoustically challenging environments. As an additional bonus, the reflected sound adds to the direct signal to double the signal strength. Phase cancellation does still occur, but only at very high frequencies, giving boundary microphones a slightly dull sound. The pickup pattern of most boundary mics is effectively hemispherical.

As one of their advantages, boundary microphones are among the least physically imposing of microphones, as their design allows them to be placed discretely on the floor or other surface, without requiring a stand, and the damping and low mass of the diaphragm make them less prone to handling or surface-borne vibration.

Common uses: Ambient pickup and reinforcement in theatres, conferences etc

Advantages: Flat low/mid range frequency response. Increased sensitivity. Physically discreet.

Disadvantages: Reduced accuracy at higher frequencies

Boundary microphone

9: 3D surround sound microphone

Some very specialised microphones are available (most notably made by Soundfield) which use a tetrahedral array of four diaphragms to pick up sound waves arriving at the capsule from all directions. A proprietary hardware or software decoder is then used to combine these four signals, and derive a three dimensional sound picture from the correlations between their relative levels and phase.

As one of its many unique benefits, it is possible to virtually reposition the microphone within the recording space after recording has taken place, by altering the balance between these various levels. Multi-channel output makes these microphones ideal for recording for surround sound applications, though they may also (and simultaneously) be configured for mono and M/S stereo outputs, which can be positioned in any plane without repositioning the mic itself. As you might expect, these mics are also reassuringly expensive!

Common uses: Surround sound recording, location broadcast

Advantages: Hugely versatile, very high quality of build and sound

Disadvantages: Cost, delicacy, cost, size, cost

The Soundfield tetrahedral capsule. Image © Soundfield - used with permission

Polar response patterns

The axis perpendicular to the centre of the pickup defines the direction of the microphone (indicated at 0° on fig 1-5), and also usually its greatest sensitivity to sound waves. All other directions, throughout a 360° plane, are measured in an anechoic chamber with a series of reference tones, to ascertain the microphone's directional sensitivity relative to the index value (0dB), and to plot its polar response. Microphone polar response differs with signals at different frequencies, so sometimes you will see several polar patterns superimposed on top of each other to represent the response of the mic at different frequencies (100Hz, 500Hz, 1kHz, 5kHz, 10kHz etc). All professional and semi-professional microphones should be supplied with a polar plot (often generic for that model, but in some cases measured for that specific microphone).

The following diagrams represent idealised versions of the common response patterns found in the vast majority of microphones:

1. Cardioid (heart-shaped)

The pattern of most single-diaphragm directional microphones

Cardioid polar response pattern

2. Omnidirectional

A spherical pickup pattern, with no directional characteristics. Responds equally to sounds from any direction

Omnidirectional polar response pattern

3. Bi-directional (figure-of-eight)

Derived from either a ribbon mic or a dual-diaphragm condenser. Sounds from the front or rear of the mic are sensed equally, with total rejection at the sides. Although equal in level, the two ‘lobes' are in opposite phase to each other, making the figure 8 mic very useful in many stereo applications [see Microphone Techniques]

Bidirectional polar response pattern

4. Supercardioid / Hypercardioid

Derived by mixing different amounts of signal from opposite facing diaphragms, these patterns offer enhanced directionality, compared to cardioid response, accompanied by a small rise in rear-facing sensitivity. The hypercardioid response has the more directional characteristics of the two.

Supercardioid polar response pattern

Hypercardioid polar response pattern

5. Shotgun

Highly directional pattern with small lobes to the sides

Shotgun microphone polar response pattern

Polar diagram anatomy

If you imagine the microphone seen from above, the polar diagram represents the sensitivity of the mic to sound from each direction in the 360° plane - the greater the distance of the polar plot from the centre of the circle, where the mic is positioned, the greater the sensitivity in that direction. For example, the cardioid pattern has its greatest sensitivity (a reference 0dB) on axis, diminishing to nothing (-∞dB) at 180° off-axis (i.e. from the rear). Similarly the bidirectional pattern has equal and maximum 0dB sensitivity to front and rear, and none from the sides

Some common terminology

Diaphragm

The key component of the most common types of microphone pickup, used by dynamic and condenser microphones. Like a small drum skin - or eardrum - the diaphragm vibrates when struck by air pressure waves caused by sound. This vibration is then translated into an electrical signal by various different methods, for amplification and recording.

Signal-to noise ratio

All electronic audio equipment will have an element of background noise, no matter how small. In the case of microphones, signal-to-noise is particularly important, due to the need to amplify the very weak initial signal of the microphone by a large factor. In the process of amplification, all background noise will be made louder along with the signal. A high signal-to-noises ratio (measured in decibels) is therefore desirable.

Phantom power

Many microphones require a small power source, usually needed either to polarize the capsule or to amplify the signal internally. This is often achieved by using an internal battery. If your microphone needs a battery always carry a spare.

Alternatively, a method was developed for delivering power through the microphone's audio cable which does not interfere in any way with the audio signal itself, hence it's name "phantom power". This 48V supply will meet the power needs of most professional and semi-professional microphones, and is provided by the microphone pre-amplifier. There will usually be a "48V" button on each channel, or a single global switch for a set of channels or the entire mixing console.

You can refer to your mic's instructions if you are unsure whether it needs phantom power, but almost all microphones are ambivalent to phantom power - if they need it, they use it, if not it is ignored. However, care must be taken not to enable phantom power to ribbon microphones, as it can damage some vintage designs.

Microphone care

The chief concerns when handling and storing sensitive microphones - particularly all types of condenser and ribbon mics - are to avoid physical shock and moisture. Avoid leaving mics exposed in environments where large variations on temperature are likely, as this can cause condensation to form on sensitive capsule parts, causing permanent damage. Similarly, if a microphone is to be used for close-up vocal work, use a pop-shield where possible, as this not only controls plosives, but also helps minimise any breath-borne moisture reaching the microphone.

Microphones should be put out on stands only as necessary. If setting up a day in advance of the recording event, then - once you are satisfied with your configuration - switch off phantom power, and power to the equipment into which the microphones are plugged, unplug the cable from the socket on the microphone, and store mics overnight in their cases. Do not leave them out on their stands. When plugging back in the next day, connect their cables before powering up other equipment and switching on phantom power, then perform a line check to confirm that they are operational.

Always handle microphones with care.

Professional studios will have a temperature and humidity controlled cupboard for their microphones, and when storing your mics for longer periods, you should try to find a place unlikely to experience any damp or condensation, or large variation in temperature. Pack a small moisture absorbing silica gel bag in each case.

Accessories

Cases:

Padded cases or bags are essential for transporting sensitive audio equipment - especially microphones. Moisture-absorbing silica gel bags in each mic case also help keep capsules damp free and will maintain the performance and prolong the life of your equipment

Shockmounts:

An elastic suspension mount (aka shockmount or spider-mount) helps isolate the microphone from physical vibration and floor-borne noise (footsteps etc) - especially useful if the stand is placed on a suspended floor. Some generic shockmounts are available, but most professional microphones will be matched to a specific model, or have one bundled with the microphone.

Stands:

All stands are not created equal. A heavy-duty stand (e.g. Konig & Meyer) will hold your microphone more securely, and will help minimise movement and reduce the risk of it being knocked over. The base of exposed mic stands can be weighed down with sandbags to improve stability

Pop shield:

A thin mesh shield, which can be placed a couple of inches in front of the microphone head to eliminate ‘popping' noise caused by plosives (‘b' and ‘p') in close-up vocal recording. Negligible effect on audio quality.

Windshield:

A hairy cover (sometimes called a ‘dead cat') will reduce the effects of wind noise in an outdoor location

Boom:

Used in combination with digital cameras, a directional microphone is attached to the end of a long pole (or ‘boom') and can then be held just out of shot by the boom operator and directed towards the subject(s). If used in combination with a radio transmitter - as is increasingly common - the radio receiver is then connected to the camera/recorder microphone input.

Stereo bar:

Some stereo techniques - most notably X-Y pair or Blumlein pair - use a pair of microphones side by side. Special bars are available to allow two mics to be mounted in this way on a single heavy-duty stand or ceiling mount.

Connection

Almost all microphones will terminate in one of two connection types:

XLR

The most common connector for professional microphones is the XLR connector, which is available in both ‘plug' and ‘socket' (usually called ‘male' and ‘female') forms, and full-size and miniature sizes. Mini XLRs are usually only found on miniature radio mics or similar, and the full sized version (pictured) is the far more common of the two.

Three pins (and therefore three cable cores) are used to transmit two identical but phase-inverted signals with a common ground, which are then summed at the other end, after again flipping the phase of one side. This process serves to double signal strength and to phase-cancel any noise picked up in the cable run, and is called ‘signal balancing'. It enables much longer cable runs than a conventional two-core cable will allow, and lower noise.

XLR connectors are also used for delivering phantom power to condenser microphones, and are the professional standard for much high-end audio hardware. XLR cables (male to female) can also usefully and securely be chained together to cover larger distances.

In almost all cases, male connectors are used for signal output, and female connectors for signal input.

Female (left) and Male (right) XLR connectors

Jack plug

Some microphones use the familiar ‘headphone' jack as their connector. Again this is available in full (1/4") and miniature (3.5mm) sizes - for reference, the headphone connector found on an iPod, for example, is a 3.5mm stereo mini-jack - and two and three core versions, often referred to as mono or stereo. The three-band version is also referred to as TRS (Tip - Ring - Sleeve). Close inspection of the bands around the tip of the plug will indicate which type it is [see image].

Many consumer microphones use jack plugs for connection, as this is the ‘mic' socket found on many domestic PC soundcards and handheld recorders.

1/4" Stereo/TRS (left) and Mono (right) jack plugs

Last updated: 23 January 2009
Published in: Creating new digital media |
Tags: audio | business & community engagement | digitisation | hardware | microphones | sound recordings | video |

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