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Microphone Basics

Published: Thu March 29, 2007  News Feed

Microphones are used to record sound or to connect a sound source to an amplification device. A singer could, for example, make his or her voice louder by singing into a microphone that is connected to an amplifier and speaker. Recording sound can be achieved by connecting microphones to a recording device such as a Digital Audio Workstation [DAW].

Microphones work by picking up the vibrations made by sounds which they then convert to an electrical signal. This signal can be sent to an amplifier or recording device. Microphones are often connected to Mixing Desks that offer a number of different channels with sound level, amplification, pan, EQ and effects controls. These controls can be applied to the sound signal and are useful both in recording and for ‘live’ sound engineering.

Microphones contain a diaphragm [a thin, flexible surface] that moves backwards and forwards when struck by the vibrations made by sound [sound waves]. The movements of the diaphragm are converted to an electrical signal that is analogous to the original sound source [an analogue sound signal].

There two main types of microphones: dynamic microphones and condenser microphones.

Dynamic microphones use a diaphragm attached to a coil of wire and suspended in a magnetic field. When the diaphragm moves in response to sound vibrations the magnetic field is affected. The affect on the magnetic field generates the analogous sound signal. Dynamic mics. do not need a power source to operate.

Condenser microphones use a power source [batteries or phantom power supplied by a mixing desk or mic. input device] to charge a capacitor diaphragm. This is a design that uses two electrified surfaces spaced slightly apart. As the flexible diaphragm is struck by sound vibrations it moves closer to and further away from the second fixed surface. The varying distance between the two surfaces [the capacitance] generates a signal which is also analogous to the original sound source.

Another kind of microphone technology is the PZM or boundary microphone. PZM stands for pressure zone microphone which refers to the position of the condenser mic. ‘capsule’ just above a boundary plate [see the design illustration below]. The idea behind the PZM mic. is to eliminate the sound reflections picked up by normal microphones: when a ‘normal’ microphone is mounted on a stand it picks up the sound waves from the sound source but also picks up any sound ‘bounced’ off surfaces in the recording room. The PZM mic. is placed close to the floor [or on a flat surface] so that the sound waves from the sound source reach the mic. at closer to the same time as the reflections. This results in improved recording results [because the vibrations from the sound source are in phase with the reflected sound vibrations].

Pick Up Pattern

As well as being of different types, microphones can have different characteristics. The main characteristics are the pick-up pattern and frequency response. Pick-up pattern refers to the direction [or directions] from which the mic will be most sensitive to sound vibrations. The different mic pick-up patterns are shown below:

 

An Omni-directional microphone is sensitive to sound vibrations from all around it.

 

The Cardioid mic picks up sound vibrations from the front.

 

The Hypercardioid mic picks up sound vibrations from the front and close rear.

 

The Bi-directional [or figure of eight] mic picks up sound vibrations from the front and rear but not sides.

 

The ‘Shotgun’ mic picks up from mostly from the front but also from the rear and has some sensitivity to sound

 

Frequency Response and Sensitivity

The frequency response of a microphone describes the sensitivity of the microphone to certain frequencies of sound. Ideally, a microphone would be equally sensitive to the whole range of sound frequencies [this would be a ‘flat’ frequency response]. However, microphones have a varied sensitivity to sound frequencies and as such offer a varied frequency response. In better quality microphones this is reduced to improve the quality of the recording.

As well as being more or less sensitive to certain frequencies, microphones vary in their overall sensitivity to sound vibrations. A condenser microphone, for example, would normally be much more sensitive to sound vibrations than a dynamic microphone. This would mean that the signal from a condenser mic. would be louder and need less amplification from the mixer of mic. input device. 

Connecting the Microphone

Microphones usually connect to a mixer, input device [such as a microphone pre-amp or ‘channel strip’] or amplification device. The cable used is usually an XLR type microphone cable which is balanced to reduce cable ‘noise’. This means that the cable has two cable ‘cores’ with the advantage of being able to carry a ground signal both to and from the input device which reduces the potential for noise interference. Unbalanced cables, like those used on home hi-fi systems for example, can produce unwanted noise because the ground signal has be connected at both ends of the cable. The ’hum’ heard from poorly earthed record turntables is a good example of this ‘ground loop’ phenomenon.

Mixing Desk

A mixing desk is commonly used as an input device for microphones in both recording and ‘live’ sound control or amplification situations. The mixing desk will have channels with signal amplifiers, effects applicators, EQ, pan [locating left or right stereo placement] into which the microphone cable can be connected. The signals from numerous microphone inputs can then be manipulated, ‘mixed’ and routed to an output. The output could be further effects processors, a recording device [such as a computer DAW] or amplification.

 

 

 

 

 

 

 

 

Routing Boxes/Channel Strips

Microphones can also be connected to other input devices that offer signal amplifiers, effects, pan and/or EQ. An example of this is the channel strip. This device is similar to the channel of a mixer and can be used as an alternative way of signal processing. Channel strips often offer specialised signal processing to give specific effects on the sound source. A channel strip designed for recording and processing vocal microphone signals is an example of this kind of device. Other signal processing and routing devices include the following examples: Amplifier emulators and Pre-amps, which are used to amplify signals and ‘copy’ types of amplifiers [such as a vintage valve amplifier]; and signal splitters or mergers that can route signals to multiple outputs or merge multiple inputs into a single output. Also effects processors and digital audio converters [converting the analogue signal to digital audio] are examples or signal processors.

 

 

Phantom Power

To use a condenser microphone the device to which the microphone is connected will send a charge through the mic cable. This charge is a very small static energy charge that powers the condenser microphone. It is called the phantom power charge. You will find a phantom power switch on mixing desks, channel strips, routing devices, pre-amps and audio capture devices because the charge is needed to operate condenser microphones.

 

 

 

 

 

Sound Quality

Sound quality from a microphone signal, whether recorded or amplified, can be affected in many ways:

Background sounds and ‘spillage’

Microphones can pick up unwanted sounds such as ‘background’ sound from an audience if recording a live performance. Sometimes even studio recordings can include unwanted sound. This is often  known as ‘spillage’, which is sound from sources other than the one you want to record. For example, backing music.

Noise

Noise is any unwanted sound interference. This could be, for example, a ‘hum’ generated by poor grounding or electrical faults. Using balanced cables and not over amplifying signals are ways of overcoming excessive noise interference.

Distortion

Distortion might occur if the signal is set too high or over amplified. Distortion literally means that the signal begins to distort and sound unclear. Keeping signal levels and amplification down are ways to avoid distortion. Pad switches are features often included on microphones or microphone input devices to reduce microphone sensitivity or sound levels when used with sound sources that are very loud or amplified. The pad switch reduces the signal level and can help to avoid distortion.

Low end rumble

Microphones are often very sensitive to low frequencies. This can sometimes result in unwanted low end noise on the recorded or amplified signal. Some microphones and microphone input devices offer roll-off filters to eliminate these unwanted low frequency sounds.

Popping

Popping can occur when using a microphone to record or amplify vocals. The performer can cause sudden blasts of air when pronouncing certain sounds [particularly the ‘P’ sound]. This is usually overcome by using a pop shield which is a thin fabric stretched across a frame that obstructs the blast of air.

Proximity effect

The proximity effect occurs when a cardioid microphone is placed close to a sound source. The low frequency sound from that sound source is boosted more and more the closer the microphone gets. Sometimes this can be used to the advantage of the recording or amplification because it gives more bass to the signal. At other times it is unwanted and must be controlled by either moving the microphone away from the sound source or by using a filter, EQ or effects processor.

Damping

Damping is used to reduce the vibrations of the sound being recorded or amplified. It can be easily imagined by thinking of a guitar string which when plucked vibrates to create a sound. If you were then to touch that string gently the vibrations would be restricted and the sound would reduce. This is how damping works: by reducing sound vibrations. Damping can be used by sound engineers in many ways from acoustic treatment for the recording environment [such as textured wall tiles and sound ‘traps’], microphone placement or the use of damping devices.

 

 

 

The following are examples of how to set up microphones to record different sound sources. A selection of four  microphones will be used in the examples. The characteristics of each mic. will also be given.

 The Shure SM58

This is a dynamic microphone that offers a frequency response suitable for vocals with bass roll off and high sensitivity to the midrange. It has a solid design that helps reduce and unwanted noise from being held and includes a built in pop shield. Ideally used for performance recording and amplification settings but also good for general use. A cardioid pickup pattern isolates the main sound source while minimizing unwanted background noise.

Shure SM58
Shure SM58

The Shure SM58 is probably the best-known dynamic microphone in the entire world, the "ball" 58 is popular as a vocal mic because (a) it's virtually indestructible (b) if you get real close to it, it accentuates bass frequencies and thus is great for vocalists with weak delivery.

 

The Shure SM57

This is also a dynamic cardioid microphone used both for instruments and for vocals. It is suitable for guitar, bass, drums, woodwind, brass and strings. The unidirectional pick-up pattern and solid design means reduced background noise.

Shure SM57
Shure SM57

Instrument Mic - Guitar/Bass Amps, Brass, Saxophone, Harmonica, Snare/Tom, Congas.

The AKG1000s

This is a condenser microphone and is as such much more sensitive than the dynamic mics listed above. It can be used with a cardioid or hypercardioid pick up pattern. Suitable for recording both vocals and instruments. It can be used further away from the sound source because it is highly sensitive. This means that the 1000s would be ideal for ambient microphone techniques [when the performance space is recorded rather than specific sound sources within that space]. Care should be taken with this microphone because it is much more sensitive to background noise.

 

AKG C 1000 S
AKG C 1000 S

The AKG C1000S is a large diaphragm condenser microphone - vocal / instrument microphone. Now includes a SA 63 Stand adapter, W 1000 Windscreen and a Hard shell road case.

The Rode NT1-A

A large diaphragm condenser microphone with cardioid pick up. Very sensitive and gives a clear recording. Ideally suited to recording vocals but can be used for other sound sources as well. The frequency response of the Rode NT1-A is extremely good because it is mostly ‘flat’ [this means that the microphone is sensitive to the sound frequency range quite evenly]. As with all condenser microphones the NT1-A is very sensitive and must be used with care to avoid unwanted background noise.

Rode NT1-A Condenser Including Shockmount
Rode NT1-A Condenser Including Shockmount

This redesigned version of RODE’s classic NT1 offers the lowest self-noise on the market (5dBA), while providing the warmth, extended dynamic range and incredible clarity of the most expensive large-diaphragm condensers.

 

Microphone Set Up Examples

‘Close microphone’ techniques

These techniques use microphones to record specific instruments close up

Vocals

The best microphone for recording vocals is a large diaphragm condenser microphone with a cardioid pick up pattern. A pop shield should be used to reduce the popping sounds that can be made by the blasts of air caused by the singers breath. The mic would be best used on a stand positioned close to the singer whose face should be about a hand’s span distance from the microphone. Damping materials [such as blankets or a duvet] could be hung to ‘deaden’ the environment and improve the quality of the recording.

Electric Guitar

A dynamic microphone should be used [such as the Shure sm57] pointed at a 45 degree angle at the centre of the amplifier being used. Moving the mic to the edge of the speaker would give a softer sound and changing the angle of the mic will alter the tone of the recording. If micing the electric guitar from further away a more sensitive condenser microphone would be better.

Acoustic Guitar

A condenser mic should be used for an acoustic guitar because it will capture the higher frequencies of the instruments better. Angle the mic towards the strings at a distance of about 10cm and positioned somewhere about mid way between the soundhole and 12th fret. Moving the mic away from the soundhole will reduce the lowed frequencies recorded and moving it towards will add lower frequencies.

Drums

A range of microphones should be used for recording a drum kit: Various microphones for close up recording and large diaphragm condensers for overhead recording cymbals plus an overall sound from the kit as a whole.

Kick drums are usually recorded with a condenser mic placed in the drum facing the beater.

Dynamic microphones are used for the snare and toms. These would be positioned a few centimetres above the drums, angled downwards just in from the rim.

Hi Hats would be best recorded using a condenser mic above [angled down] the edge of the top cymbal. Angling the mic away from the snare drum would help reduce spillage.

Ambient Mic Techniques

The techniques are used to record a sound and the space in which that sound is made, such as recording a concert using just a pair of microphones placed a distance away from the performers.

An example of an ambient mic technique is the spaced pair. This uses two identical condenser microphones positioned a distance apart away from the sound source. The distance between the microphones will determine the stereo width of the recording and the distance from the sound source should be sufficient to record the sound sources without too much bias in either microphone. A good rule to follow is positioning the microphones  where a listener might choose to sit to best hear the performance. The spaced pair gives a good stereo recording.

 

As a general rule you would not use condenser microphones for live amplification situations because they would likely give too much background noise. Likewise you would not normally use low sensitivity dynamic microphones for ambient microphone recording because they would lack the sensitivity required to capture sound vibrations clearly from a distance.
 
 
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