Microphones FAQs

A microphone ("mic" for short) is a device for converting audible sound into an electrical signal. To accomplish this task with optimum efficiency and accuracy requires a type of mic that is appropriate to the particular situation. There are many different types of mics some are designed for very specific applications and others have a more general purpose.

Microphones can be categorised in several different ways. The most important of these categories are described below.

All types of microphone incorporate some form of diaphragm. This is a small surface which vibrates in sympathy with the sound pressure waves reaching the microphone. However, dynamic and condenser mics vary in how these vibrations are used to produce an electrical signal. In a dynamic microphone, sound is converted to an electrical signal by the vibrations of the diaphragm causing the vibration of a coil in a magnetic field, effectively an electrical generator on a very small scale. As this produces sufficient signal level for direct connection to a PA system, no amplification of the signal is required within the mic. Dynamic mics are most useful for close-proximity applications (i.e. 0 to 15 cm) such as lead vocals, guitar amplifiers, etc. In a condenser mic (also called a capacitor mic), sound is converted to an electrical signal by the vibrations of the diaphragm causing changes in the capacitance of a charged capacitor. This is achieved by the diaphragm itself being one of the plates of the capacitor. As this produces a very small signal level, some initial amplification of the signal is required within the mic itself. This internal amplifier may be powered either by an internal battery or by power supplied from the mixer. Condenser mics are most useful for larger distances between the sound source and the mic (i.e. 15 cm upwards), such are encountered with lecterns and with overhead miking of drum kits, choirs, theatre stages etc. They can be more prone than dynamic mics to making a "popping" sound when used close-up with a "breathy" sound source such as a voice or a wind instrument, though this problem can be reduced with the windshield fitted. They are capable of a higher quality sound than dynamic mics, and the best versions are therefore extensively used in studio recording work. They are the best type of microphone for use with an Audio Frequency Induction Loop System (AFILS).

Omni-directional mics pick up sound with equal sensitivity from all directions. This is not normally useful for PA work, because in PA work each mic is targeted at a single sound source (so that the amplification given to that sound can be controlled separately from others, and so that pick-up of unwanted sounds can be minimised). Their application is generally limited to recording work (particularly of ambient sounds) and to sound-level measurement. Uni-directional mics pick up sound with greater sensitivity from the front than from other directions. There are several variations on this theme: Cardioid mics have a gradually reducing sensitivity from the front to the back, with very little sensitivity at the back. Super-cardioid mics reduce their sensitivity from the front to the sides at a faster rate than cardioid types, reaching a minimum sensitivity at an angle of around 120-140°, measured from the front. The sensitivity then increases again towards the back, but the sensitivity at the back is still very much less than at the front. Hyper-cardioid mics provide even less sensitivity at the sides than do super-cardioid types, at the expense of a little more sensitivity at the back. Therefore, a monitor speaker should never be placed directly behind this type of mic. Their minimum sensitivity is at an angle of around 100-120°, measured from the front. 'Rifle' or 'shotgun' mics are the most directional type, so-called because of their long rifle-like barrels. They are generally used only for long-distance miking (more than 2 metres from the source), e.g. for theatrical work, and should be located such that the back of the mic is not exposed to unwanted sounds.

Unbalanced mics, where one of the signal-carrying conductors of the connection between the mic and the system is provided by the signal earth or 'screen' of the connection. With this arrangement, the inter-connecting cable is prone to pick-up of interference from stray magnetic fields and radio signals, and so these types of mic are suitable only for use with moderate lengths of cable (up to around 10 metres). Balanced, where the two signal-carrying conductors of the connection are separate conductors from the signal earth or 'screen' of the connection. This arrangement is highly immune to pick-up of interference, and so may be used with very long lengths of cable (up to 200 metres), provided it is of good quality.

A boundary mic is a special type which when placed on a surface utilises the sound energy collected at that surface. Such mics are generally equally sensitive to sounds in all directions above the surface. Typically used for speech, where a convenient surface such as a desk or lectern is available.

Leaded or wired mics connect to the PA system by means of a cable. The cable usually attaches to the mic by means of a 3-pin XLR or 1/4" jack plug. Radio (or "wireless") mics contain a battery-powered radio transmitter. The radio signal from this transmitter is picked up by a receiver which is connected to the PA system. The mic and the receiver are purchased as a pair and are referred to as a "radio mic system". Radio mic systems use a frequency-modulated (FM) radio signal at VHF or UHF frequencies. (Strictly, each system does not operate at a single frequency, but rather uses a narrow range of frequencies called a 'channel'. The frequency that is quoted is the carrier frequency, the frequency at the centre of the channel.) The frequencies used are either "licensed" or "de-regulated". Use of a licensed frequency requires payment of an annual license fee. Use of the de-regulated frequencies is free.

Non diversity means that the radio signal (usually picked up on a single aerial) is processed by a single set of receiving electronics. These types are prone to "drop-outs" - temporary interruptions of the audio signal caused by temporary reductions in the received radio signal strength (due to reflections of the signal and physical obstacles in its path). The receiver aerial is generally best set vertically. Some single-channel receivers are equipped with two aerials, even though they have only one set of receiving electronics - these may perform a little better than single-channel receivers with only one aerial, but fall far short of the performance obtained from true diversity receivers.

True or space diversity means that the radio signal is picked up on two aerials, each connected to a separate set of receiving electronics - the output of the receiver is provided from the set which is giving the best quality signal at any moment in time, or is a combination of the two. True or space diversity receivers are much less prone to drop-outs than single-channel types. The aerials are generally best set at between + and - 30 to 45 degrees from the vertical, i.e. spreading apart at between 60 and 90 degrees to each other.

Some receivers provide an audio output intended for connection to a line input of the PA system, whilst others have outputs intended for connection to a mic input. Some types may provide both kinds of output, or a single output of adjustable level.

When several radio mics need to be operated simultaneously, each system must be set to a different frequency. Furthermore, in order to avoid intermodulation interference between the systems, the frequencies selected must be chosen from a compatible set for the particular make and type of system being used. The maximum number of frequencies in a compatible set will depend upon the quality of the system.

Hand-held mics are generally about 6 to 7 inches (15 to 18 cm) long and 1.25 inches (3 cm) in diameter. They may be held in the hand or placed in a clip on a mic stand. Note that many radio mics have a slightly larger diameter than leaded (wired) types, and therefore will not fit into 'standard' sized mic clips. Mic clips fix to the stand by means of a screw thread, of which there are three common types: 5/8 inch 27 turns per inch (a large diameter fine thread) 1/2 inch (a medium diameter fine thread) 3/8 inch Whitworth (a small diameter coarse thread) In case your mic clip doesn't fit your stand, thread adaptors are available. Lapel (tie clip) mics are generally much smaller and are either body-worn (e.g. clipped to a lapel or tie, or attached to a head-set). A body-worn mic is also known as a lavalier mic.

To get the best results, it is important to choose an appropriate type of mic for the job, and to use it correctly.

Most PA mics are uni-directional types, and all uni-directional mics exhibit what is known as the "proximity effect". The result of this effect is that sounds which are made very close to the mic are picked up with a greater bass response than sounds which are made further away. This is most important for presenters and vocalists to understand, because the difference that a change in working distance makes to the sound of their voice can be quite dramatic. It is especially significant for deep-voiced vocalists (usually male), because a greater proportion of their voice is in the frequency range which is subject to the proximity effect. At a working distance of greater than about 4 to 6 inches (10 to 15 cm), the proximity effect can be ignored. As the distance decreases from this down to zero, the amount of bass emphasis increases.

Mics contain delicate precision-engineered components, and if you want your mics to continue to perform as well as when they were new, you must look after them very carefully. Even ruggedised stage mics will benefit from careful treatment. Following these simple do's and don'ts will help considerably: 5/8 inch 27 turns per inch (a large diameter fine thread) 1/2 inch (a medium diameter fine thread) 3/8 inch Whitworth (a small diameter coarse thread) • DO keep them in padded protective boxes or pouches - preferably individually - when not in use (especially during transport). • DO clean the integral windshield from time to time, when this is accessible. Follow the maker's instructions (especially for expensive mics!), but in the absence of any instructions the wire-meshed end of most types can be unscrewed and then gently washed in warm soapy water - allow to dry thoroughly before re-attaching to the main part of the mic. • DON'T check them by tapping them or by blowing into them - speak (or sing) into them instead, and educate users to do the same. • DON'T drop them or allow them to be subjected to other sudden shocks. • DON'T store them in damp conditions or expose them to extremes of temperature.