Microphones in Live Sound

The Microphone is an integral part of live sound production. Those of us who have been exposed to Top Of The Pops at an early age will know that every singer uses a microphone. The camera never focuses on the console, the amp rack, the monitors or the speaker system. People have been known to ask about hiring a microphone when what they actually want is a complete system.

What it is

Technically, a microphone is a device that converts variations in sound (either airborne sound or sound from a resonating body, such as the front of an acoustic guitar) into variations in voltage. Although most people would recognise the shape of a Shure SM58, microphones also come in other shapes and sizes (lapel mics, headphone mics, boundary mics, contact mics).

What it does

Microphones are used to convert sound into an electrical form so that it can be amplified, processed, and mixed with other sounds before it is converted back into sound by the loudspeakers.

How it works

In common use, there are three basic ways in which microphones convert sound into electricity:

1.  By using changes in sound pressure to move a coil in a magnetic field. This is how dynamic microphones (the most common form of microphone) work: variations in air pressure cause a diaphragm to move. The diaphragm is attached to a coil, which either surrounds or is surrounded by a magnet. Movement of the coil within the magnet's field creates a very small potential difference (voltage) between the two ends of the coil. Ribbon microphones work in the same way, except that a thin conductive ribbon acts as both diaphragm and coil (movement of the ribbon within a magnetic field creates a small potential difference across the ribbon).

2.  By using changes in sound pressure to influence the reactance of a capacitor in an electrical circuit. This is how condenser (and electret) microphones work: variations in air pressure cause a diaphragm to move. The diaphragm forms one plate of a capacitor. The distance between the plates of the capacitor (and hence its reactance) is altered by movement in the diaphragm. If a D.C. voltage is applied to the circuit, the voltage across the capacitor varies as the sound pressure varies. Some budget condenser microphones (known as electret microphones) have one plate of the capacitor permanently charged.

3.  By using changes in sound pressure to distort a piezoelectric element.  This is how most contact microphones work: the piezoelectric element produces a small voltage when distorted, and this varies as the sound pressure varies. While these are commonly known as transducers, all microphones are in fact transducers: a transducer is something that converts one form of energy (in this case sound) into another (in this case electrical energy).

How do you use it?

If all else fails, read the manual! There are also useful notes on most manufacturers' websites (see e.g. Shure's knowledge base).

Almost without exception, microphones should be connected directly to a mixer's microphone (XLR) inputs - usually via the stage box & multicore - using balanced XLR to XLR cable. If you are using a condenser microphone you will also need to switch on phantom power (either on that microphone's channel, or globally).

Generally, you point the business end at the thing you want to amplify, and get it as close to the thing you want to amplify as practically possible. This will improve separation: the mic will pick up more of the thing it is pointing at and less of everything else. It will also allow more gain before feedback occurs. However, some instruments (e.g. pianos, acoustic guitars) create sound over quite a large area, and moving the mic closer will inevitably focus more on one element of its sound than on another. Achieving a sound that is relatively natural may mean moving the mic further away, compromising separation and available gain. Adding another mic may help, but this will itself reduce separation and available gain, and may also have unwanted side effects (primarily comb-filtering, if the microphones are relatively close together). Generally it is best to use one microphone or pickup - and only one microphone or pickup - for each sound source. The main exceptions are:

It is also fairly common practice to use more than one microphone with any instrument (e.g. marimba) that generates sound over an area that is too large to cover with a single mic.

Inbuilt pickups on acoustic instruments (such as under-saddle pickups on acoustic guitars, or bridge pickups on fiddles) often have limited high-frequency output, and it is sometimes necessary to use a mic as well as the inbuilt pickup. Condensers are the rule-of-thumb for high-frequency detail on acoustic instruments. However, it is still generally preferable to use one method (pickup OR mic), not both. Good-quality pickups - see below - are advisable for live applications.

When using microphones close together (e.g. with a group of backing singers), follow the three-to-one rule: have each microphone at least three times as far from its neighbour as it is from the sound source (voice or instrument). This allows a margin of about 10dB between neighbouring sound sources: enough to minimise the effects of comb-filtering and afford a usable degree of separation.

If you can't get the sound you want, it is worth trying different microphone positions and different microphones before reaching for the EQ.

Do you need one?

Simply, yes. For every instrument or voice you want to make louder, you need a microphone. If there are four singers and two guitarists that need to be louder, you probably need a microphone for each of them. "Probably" means you could in some circumstances get away with having two singers sharing one microphone, but ideally you would still use one microphone for each. The trombone might be loud enough on its own, but you would still need a microphone for it if:

•  you needed the sound to get to a wider area than the instrument covered on its own;

•  you were recording the show.

What sort do you need?

For general guidelines on the type of microphone suitable for any particular instrument, most manufacturers provide useful information (e.g. the Product Advisor on the AKG homepage, or Shure's Product Selection Guides; there is also a useful pdf booklet on the Shure website). For other general information, read on.

For acoustic instruments with low acoustic output (quieter instruments, e.g. acoustic guitar), a contact microphone (a.k.a. transducer or "bug") is often useful. Because the mic is in contact with the instrument, the instrument's own sound will be very much more prominent than surrounding sounds (even very loud ones, like drums & electric guitars). This helps to increase separation and reduce feedback. However, contact microphones often sound less natural than other types of microphone, and may need additional processing - e.g. dedicated preamplifiers or parametric equalisers - to compensate for this.

For everything else, unless it generates an electric signal already, a dynamic or condenser microphone is the thing. Ribbon microphones are seldom appropriate for live use.

The main differences between dynamic and condenser microphones are:

•  Power supply. Condenser microphones (including electrets) always require a D.C. voltage in order to work. Usually this comes in the form of phantom power from the main desk. All our desks (and most modern desks) can supply phantom power, but some older desks and budget mixer-amplifiers do not include this facility. A few condenser microphones (e.g. the AKG C1000S) can use a battery instead.

•  Frequency response. Because the physical mass of a coil is greater, dynamic microphones are generally less effective than condenser microphones at very high frequencies. Because of the limitations of PA systems, human hearing, and the actual output of most instruments and voices, output above 16kHz is not generally important, so this difference is less crucial than it might first appear. Even so, condenser microphones often sound crisper and more transparent than dynamic microphones. Conversely, dynamic microphones can sound "warmer" than condensers.

•  Sensitivity. Again, this is partly related to the physical mass of the diaphragm. Condenser microphones are usually more sensitive than dynamics, which makes them more suitable for quieter sound sources. However, all condensers (including electrets) include a preamplifier, and this may distort where very high sound pressure levels are involved (e.g. with drums). Many condensers therefore incorporate a gain reduction switch, which reduces the signal before it arrives at the microphone's preamplifier. This allows them to be used with very loud sound sources, as well as quieter ones.

•  Robustness. It has been claimed that a Shure SM58 (the dynamic mic everyone has heard of) will survive a 6' drop onto concrete. While we wouldn't advise you to try it, the chances are you would only dent the basket if you did. Condenser mics are more fragile.

•  The effect of condensation. Dynamic microphones will let you breathe water vapour into them all night without apparent damage (we disinfect our baskets and pop filters routinely, in case you wondered). Condensers sometimes object to large amounts of moisture. Given Time and an Encouraging Climate (an airing cupboard is fairly good) a condenser will dry out and return to normal, but this is not much use if the lead singer has started to sound Strange without medication and there are four more numbers in the set.

For these reasons, dynamics are generally favoured for:

•  Vocals;

•  Instruments that don't need much very high-frequency response (e.g. electric guitar amps, drums);

•  Instruments that have a high acoustic output - so sensitivity isn't an issue - (e.g. electric guitar amps, kick drums, brass);

•  Situations where the mic could be dropped or otherwise damaged (e.g. hand-held vocals, Fierce drummers).

Condensers, on the other hand, are good for:

•  Quieter instruments (e.g. acoustic guitars, harpsichords);

•  Instruments needing more high-frequency detail (e.g. acoustic guitars, harpsichords, fiddles, flutes, cymbals);

Microphones also vary in their directional pattern.

Ribbon microphones always have a figure-of-eight pattern: they are equally sensitive to sounds arriving from directly in front of or directly behind the diaphragm, and completely insensitive to sounds arriving from the sides. Because they are also easily damaged, they are not generally appropriate for live use, and are seldom seen on live stages.

A dynamic or condenser microphone generally has one of three patterns (although some condensers - e.g. the AKG C414 - have switches that enable more than one pattern, and some of these also have a figure-of-eight option):

The most common form in live use is the cardioid pattern, although hypercardioid mics are also common. Omnidirectional mics are less often seen. Hypercardioid mics have the best feedback rejection.

Omnidirectional microphones are also known as pressure microphones: the rear of the capsule is enclosed, so the microphone responds directly to changes in pressure at the front of the diaphragm, irrespective of direction.

Directional microphones are also known as pressure gradient (or simply gradient) or velocity microphones: there are vents in the rear of the capsule, so the microphone responds to the difference (i.e. the gradient) in pressure between the front and back of the diaphragm. This is affected by the angle between the microphone and the sound source, and by the source frequency (there is usually some cancellation of lower frequencies unless the microphone is very close to the sound source). This has two consequences:

1.  Cardioid mics are usually more sensitive to low frequencies when used closer to the sound source (this is known as the Proximity Effect);

2.  Covering the vents reduces or cancels the microphone's directional properties.

For these reasons, omnidirectional mics are sometimes favoured for close-up use on loud sound sources (there is no increase in bass response) and CUPPING A CARDIOID MICROPHONE BASKET IN YOUR HANDS CAUSES FEEDBACK.

Radio Microphones

Radio Microphones are simply microphones that use radio waves instead of wire to transmit a signal to the desk. They have a transmitter (either built-in or plugged into the microphone's XLR socket), and a receiver (to convert the radio wave back into a normal signal that the desk can use). They come in pretty much all the shapes and sizes available in standard microphones, but the most common are:

•  Hand-held: like an ordinary hand-held microphone, but allowing greater freedom of movement;

•  Headset: mounted on a headset, often used in dance routines;

•  Lapel (also known as Lavaliere): clip-on microphones, often used for interview and broadcast applications.

Transmitter/receiver combinations can also be used with other signal-generating equipment (electric guitars often use wireless systems).

Radio microphones and other transmitter/receiver combinations also involve a choice between:

•  UHF (Ultra High Frequency). These are generally expensive, but usually have more possible transmission bands, a greater usable range, and are less prone to interference and dropout than...

•  VHF (Very High Frequency) microphones. These are cheaper to manufacture than UHF (typically retailing for about half the price). In many situations they work well enough, however.

•  Diversity. Diversity systems have more than one receiver channel, and will automatically use the strongest signal available. This makes them much less prone to transmission problems (most notably dropout) in situations where the microphone is moved about, which is generally the reason you want a radio mic in the first place.

All radio microphones use a form of noise reduction known as a compander (compressor/expander). This is similar to Dolby noise reduction systems used in tape recording, and is used because the dynamic range of the radio signal is less than the dynamic range of the microphone. Ordinarily the effect of this will be inaudible.

Controls that come with a radio mic & receiver may include:

•  Frequency. Many systems allow variation of the radio frequency. The transmitter frequency needs to be the same as the receiver frequency, so if you change the frequency on the microphone, you need to change the receiver frequency to match it.

•  Sensitivity. Some transmitters have a sensitivity control (the equivalent of a gain control). Use the lowest setting that gives you an adequate signal. Increasing the sensitivity of the microphone may cause compander effects to become more noticeable, as well as increasing the risk of feedback. There may also be a sensitivity control on the receiver.

•  Output level. The receiver may well have a level control ("Output", or "AF Level") which sets the level at the receiver output. Set this so that the level at the mixer input is within the usable range of the channel gain control.

•  Squelch. This is the equivalent of a noise gate acting on the radio signal, and mutes the receiver when there is no signal present. Setting the squelch threshold too high can cause signal dropout. With the transmitter switched off and the squelch control at its minimum setting, gradually increase the squelch setting until the hiss is silenced. Then switch the transmitter on, & check that the signal is OK. It is better to have no squelch than no signal.

Radio microphones can be only used without a licence on licence-exempt channels (which in practice means only four radio microphones can be used simultaneously without the risk of interference). Information about channels and licences can be obtained from the Joint Frequency Management Group (JFMG), who can also supply a licence to operate radio systems on a greater range of frequencies. However, even with extra licensed channels there is a practical limit of about 8 frequencies that can be used simultaneously without interference. Some radio microphones can be programmed to operate on unlicensed as well as licensed channels, so it is important to check that any radio microphone you intend to use will be operating on exempt or licensed channels.