Microphones and recording
As elsewhere I’m going to concentrate on mic’ing guitar – if anyone wants information about vocals and/or other instruments shout. I’m also going to try to keep the physics and maths down to a minimum.
Also – although I mention a couple of specific mics don’t see that necessarily as a recommendation – they’re ones that either I use a lot and/or are common. Mics are like guitars – try lots and see which work for you. IF I had the money I’d buy a Neumann KMS – sadly I have no money though.
There are many different types of microphones that are commonly available but despite this they may be classified in several distinct groups: dynamic and capacitor. In addition to classifying by type, microphones also have different filtering patterns – omni-directional or unidirectional (cardioid). So what are the main differences between these types and patterns and how might it relate to recording guitars?
Dynamic microphones, for instance the Shure SM58, are usually good, robust all round general microphones. They are used widely in live venues and also for recording instruments, like drums and guitar amps, that can produce high sound pressure levels. A dynamic mic contains a small plastic film diaphragm attached to a small coil of wire suspended in a magnetic field. A sound makes the diaphragm vibrate and the coil then moves in the magnetic field producing an electric current that can then be amplified.The big advantages of a dynamic mic are low cost, robustness and the ability to cope with sudden high sound levels.
One different form of dynamic microphone that has started to become fashionable again is the ribbon. Here the diaphragm and coil are replaced by a thin metal ribbon suspended in a magnetic field. Ribbon mics are renowned for their smooth and detailed sound. However they generally are quite fragile and cost considerably more then a conventional dynamic mic.
Capacitor mics, for instance Rode NT1A and AKG C3000 condensers, are often preferred by recording studios because they offer a wider frequency range and more accuracy for recording. In a capacitor mic is a pair of conducting plates, one fixed and the other acts as a moving diaphragm. The distance between the plates changes when the diaphragm vibrates and this causes the capacitance to vary. If a fixed electrical charge is applied to the capacitor an electrical signal will be produced that accurately represents the vibration of the diaphragm, and thus the recorded sound.
The diaphragm is often a thin gold-coated plastic film that is much lighter (and more delicate) then the diaphragm and coil of a dynamic mic. This lightness result in a system that is able to record higher frequencies, including high order harmonics, above and beyond the human hearing range. Small diaphragm capacitor mics (1/2 inch or smaller) produce a very accurate recording whilst large diameter ones (1 inch and over) produce a warm, often flattering sound. The lightness of the film however also results in it being quite delicate and because of this capacitor mics are not as well suited as dynamic mics for recording high sound pressures.
Capacitor mics produce a small electrical signal that has to be amplified via a special, usually in-built, pre-amp. They also need a polarizing voltage to work – sometimes referred to as ‘phantom power’. ‘Phantom power’ – usually found on mixing consoles and some audio cards - can only be supplied by a balanced source and cable – unbalanced inputs and standard guitar jacks are not suitable. The pre-amp and ‘phantom power’ requirements generally mean that capacitor mics are more expensive then their dynamic counterparts. However they are also more sensitive and more able to capture high-frequency detail.
One further type of capacitor mike is the tube mic (this also differs from the others as they are solid state FET but that’s a different story.) Tube mics require an output transformer and usually come with their own power supply – no phantom power needed. The exception here are tube mics sometimes called ‘starved circuit’ where a high voltage tube is deliberately run at a reduced voltage. Tube mics are supposed to deliver a more musical sound and can produce a nice saturated/musical distortion if overloaded ( FETs in contrast will sound harsh)
An omni-directional microphone works in principle as a pressure transducer and will pick up sound from any direction. Technically omni-directional mics are the most accurate but can often pick up unwanted sounds. They work best in sound proofed venues where a single source is recorded – for instance a solo singer or guitar.
For recordings where there is more then one source a cardioid microphone pattern may be best because these are more directional. Cardioid mics work principly in terms of pressure gradient and as such are usually least sensitive from behind and most sensitive on-axis. (A Rode NT1A actually has a small gold dot on the body to indicate which way to point it at the sound source.) Cardioid mics often suffer form a phenomena called the ‘proximity effect’. This is a low frequency boost that occurs if a cardioid mic is placed very close to the sound source. The effect is minimized on some mics by applying a low frequency cut – some cardioid mics offer a range of filters to achieve this.
One other pattern is the multi-pattern mic that allows you to switch between specific patterns, for instance figure of eight (where on-axis is the same directly in front and directly behind the mic but the sides are ‘tuned’ out). Figure of eight multi-pattern mics can be particularly useful when recording a solo vocalist with acoustic guitar.
Recording an electric guitar
From the discussion above we know that a dynamic mic is robust and able to handle high sound pressure levels but may lack some high frequency response. A capacitor mic meanwhile should give a detailed sound with lots of high end but may lack some low end and cannot be placed too close to a guitar amp. One way to record an electric guitar then is to use both mics in such a way as to use their strengths to produce a rounded guitar recording.
A dynamic mic (ie a Shure SM58) can be placed directly in front of a guitar amp cabinet, right up to the speaker cone. This is called close mic’ing. A capacitor mic (ie a Rode NT1A) can then be placed further away from the amp and slightly off axis in relation to the amp’s speaker cone.
To start mic'ing
Turn off phantom power on your mixing console/audio source (nb you can damage a capcitor mic if you plug in an XLR lead that already has ‘phantom power’). Fit the capacitor mic in its shock mount (this will help isolate it from any unintended bumps or bangs in the guitar stand) and attach the XLR cable to the mic and the mic input and the shock mount to a mic stand. Once the mic is plugged in you can then turn on phantom power and allow the mic to warm up for at least 15 seconds.
Place the mic approximately 12 inches from the amp’s speaker cabinet with the mic on-axis with the cabinet. If there is one speaker the mic should be positioned slightly off center in relation to the speaker cone. If there are two or more speakers then position the mic so that it is orientated more to one speaker then the others. If you place the mic centrally between speakers you will get a weaker sound.
Set the input level as described in the lesson on Setting up a project in Reaper. Put on a good pair of stereo headphones and connect them to your desk and you should be able to hear your guitar amp through them. Turn up the headphone volume to a comfortable level -not too loud though as you need to protect your hearing! Now move the capacitor mic until you find its sweet-spot for the guitar amp. (If your recording room is big enough you can move the mike as far as 6 or more feet from the cabinet. Often 3 foot/1 metre works well.) At the sweet-spot it should sound more ‘airy’ and open’ then in other positions.
Now try it with your monitors just to make sure that you’ve found the best position. Take some time as time spent here getting a good input sound will save you having to re-record a poor sounding take. Once you are happy you can then set up the dynamic mic to add more depth and some grit to the sound.
Attach the dynamic mic to its clip or mount and fix to a stand. Plug the mic in to the desk – remember that it doesn’t need phantom power. Place the mic close to the speaker cabinet – say about 1-2 inched away from the grill cloth – and slightly of axis, that is at an angle, but pointing towards the center of a speaker. If you point an on-axis mic at the center you may overdrive the mic. Pointing the mic at an angle lets it pick up more bass but without the treble becoming harsh.
Note – you can experiment with the position of the capacitor mic. If you move it further away from the speaker you may get some natural reverb and ambience. If you place the near field dynamic mic further back from the cabinet then a rule of 3:1 might be best – place the capacitor mic 3 times further away from the cabinet then the dynamic mic. If you have an open backed speaker cab also try placing the capacitor behind rather then in front of the cab.
For single mics – UK heavy rock is often recorded by a dynamic mic for a darker and heavier sound whilst a more US rock sound is brighter and so favours a capacitor. In either instance reduce the amp’s distortion a little from your preferred setting – you will get a much better sound with much more detail. If you’re picking up too much bass no matter where you place the mics then either get the speaker cabinet off the floor or tilt in backwards at an angle.
Mic'ing an Acoustic
Acoustic guitar is often best recorded using a small diaphragm capacitor mic to reduce room colouration. Try placing it about 2-3 feet away (approximately 1 meter) and experiment with it pointing on-axis at the sound hole, at the body and at the 12th fret. If you move the mic in to close to the guitar you will get the ‘proximity effect’ and the guitar will sound boomy. Bringing it out makes the sound brighter. (There is a mathematical principle involved here – basically the mic should be placed at a distance that is at least equivalent to the vibrating section of the instrument. In this case it’s the length of the body of the acoustic guitar – about 2 feet.)
If the acoustic is a solo instrument – ie not as background for a singer – use a pair of cardioids set in an x-y pattern (capsules nearly touching and with an angle of about 110 degrees between the mics) and placed 2 feet or so back. Note – the mics for x-y must be identical or your stereo image will wander about!
If the acoustic is backing a singer whose close to the guitar place the cardioid again about 2 feet back but lower then the guitar and pointing up at it. Put the vocal mic high and as close to the singer as you can (proximity effect on vocals can actually help by adding bass and therefore depth to the vocal.).
Editorial note: published 2007-10-29 by TonyMiro, published with his permission.