The Ultimate Microphone Guide
TIME WAS THAT a
major proportion of the recording engineer's art consisted of choosing the
right mic for a given application and placing it in exactly the right position
to capture the full potential of the desired sound. These days exactly the same
care is needed with mic choice and placement but with a lot of music being made
using synths, samples and digitally modelled amp simulators, the use of
microphones is perhaps not the everyday task it once was.
Nevertheless, using a microphone is still the only way to record anything that moves a bit of air, and that includes acoustic instruments, amplified instruments and voices. So a basic knowledge of microphones is knowledge worth having, even if all you use a mic for is to add vocals to your sequenced tracks.This guide (continued in next month's issue) aims to give you the lowdown on exactly how microphones work, the characteristics of the various types, selecting and buying your own microphones, choosing the right mics for the job and where to place them for the optimum sound. This month we are concerned with how and why microphones work and the various types that are available.
When recording an acoustic instrument or a voice, a microphone is the first item in the signal chain and, as we all know, any signal chain is only as good as it's weakest link. That being the case, the choice of microphone is extremely important, as is its position in relation to the sound source.
Disregarding the placement for the time being (I'll be covering that in next month's instalment), the fact is that if you use a crap mic to start with, or simply one that is unsuitable for the job, there's nothing you can do about it afterwards that is going to improve the quality of the signal. So it's not only necessary to use as good a mic as possible but also the type of mic you use will make a difference.
There are loads of different makes and models of mics out there, each with their own characteristics that make them suitable for particular jobs, something that a knowledgeable engineer with a well-stocked mic cupboard can exploit to full advantage. Fortunately, an intimate knowledge of every single available mic model is not necessary to make good recordings because, although no two models will sound identical, there are types of mics designed to do an identical job that exhibit similar characteristics to each other, so a basic knowledge of each type as demonstrated in this guide will allow you to make an informed choice of microphone for any task.
How a Microphone Works
Technically speaking, a microphone is a transducer: something that converts energy from one form to another. In the case of the microphone that is to convert soundwaves (acoustic energy) to electrical energy, which is the exact opposite of what a loudspeaker does.
The basic modus operandi is that soundwaves cause changes in air pressure which cause motion in a microphone's diaphragm. These motions or vibrations are then used to generate an alternating voltage, the exact means of doing this being dependent on the type of microphone. Although each type has variations, there are really only two basic types of microphones that are used in studios. They are the dynamic mic and the condenser mic.
A condenser microphone, also known as capacitor mic, works on the electrostatic rather than the electromagnetic principle that is used by a dynamic mic. The basic principle is that the mic's capsule is made up of two electrically conductive surfaces separated by a minute air gap. One surface is a lightweight membrane (the diaphragm) while the other is solid (the backplate). These two surfaces, close but not touching, act as a capacitor - an electrical device capable of storing an electrical charge. Sound pressure on the diaphragm causes it to move, thereby changing the capacitance in the circuit and creating an electrical output. This electrical output is amplified by an impedance matching head amplifier in the mic's body.
For a condenser mic to work it has to have a power supply. This is known as 48V phantom power and is provided from the preamp on a mixing desk channel or a standalone mic preamp. It is sent from the XLR socket down the mic cable. Condenser mics are valued for their uniform frequency response which results in a natural sound. They are generally more sensitive than dynamic mics, they have an increased response to transient sounds and they exhibit an extended high frequency response.
Generally condensers are not as robust as dynamics so need to be treated more gently. Condenser mics come in different shapes and sizes. Side-entry large diaphragm models are the preferred choice for recording vocals, but smaller diaphragmed tubular shaped models that take in sound from the end are also available.
A dynamic mic uses electromagnetic induction to generate its output signal and works on the principle of the moving coil. The mic's thin metal diaphragm is attached to a coil of wire (voice coil) surrounded by magnets and effectively suspended in a magnetic field. When the diaphragm moves in response to changing sound pressure, the coil moves in the magnetic field, cutting the lines of flux causing current to flow.
Dynamics do not have the top-end frequency response that condensers do, (for example, a Shure SM57 only has a frequency response up to 15kHz), so may be lacking a bit of air or transparency on really high frequency sounds such as cymbals or acoustic guitar, they also tend to be less sensitive.Generally, dynamic mics are more rugged than condensers and need no external power source to work. They are used on loud sound sources like drums and guitar amps and in situations where extended high end is not a priority.
The electret mic: An electret mic is very similar to a standard condenser mic but differs in that it uses permanently charged material (usually fixed to the capsule backplate) to charge the diaphragm rather than phantom power. Electret mics can generally be made smaller and lighter than standard condensers, allowing them to be placed in tight spaces. Power is still needed for an electret as it features an impedance changing amplifier but this can be supplied by a battery (AKG's C1000S uses a PP3), although phantom power will produce a better performance.
The ribbon mic: Ribbon microphones have been having a resurgence of late with several new models appearing on the market including reworkings of the RCA 44 (widely regarded as one of the classics). A ribbon mic is a special form of dynamic mic that uses a thin aluminium ribbon suspended between the poles of a magnetic circuit. The relatively fragile ribbon gives an excellent transient response but is susceptible to high acoustic pressure or wind.
The valve mic: A valve (tube) mic is essentially a condenser mic that uses a valve inside the mic for its head amplifier rather than transistor circuitry. Due, in part, to the even harmonic distortion resulting from valve circuitry, this type of mic is perceived as having a smooth and warm sonic character and is favoured for vocal recording. Valve mics are not run from phantom power but have their own power supply that is connected to the mic via a special cable. The standard mic cable is connected to an XLR output on the power supply box which also usually houses any pad, roll-off or polar pattern switches.
Everything you wanted to know about direction
Directivity: Microphones do not just pick up sound from the direction they are pointed at, in fact mics can be classified by their directional properties ie the manner in which they pick up sound from various directions. Some mics are omni-directional and can pick up sound from every direction equally, while some are directional and are designed to respond to sound from a specific direction while rejecting sound that arrives from other directions. The directivity of a mic can be represented by a 360° graphical plot known as a polar pattern. The polar pattern for an omni-directional mic would show a whole circle around the mic indicating that it receives sound equally from all directions (see above).
directional mics, the most common of is the cardioid type, so named because the
plot of its polar pattern would be heart shaped (below). This type of mic is
designed to pick up sound from in front of its capsule with minimum sound
acceptance from the rear. Hypercardioid and supercardioid are both more
pronounced versions of the basic cardioid pattern and accept sound from the
front in a tighter angle.
A bidirectional mic can pick up sound equally from both the front and the rear while rejecting it from the sides, something that is known as a figure-of-eight pattern
Some condenser mics are fitted with a dual diaphragm and this allows them to work in a range of directional modes allowing greater versatility. The mic's polar response pattern can be changed using a switch set into its body and will usually provide omni-directional and bidirectional operation plus variations on the cardioid shape.
Sensitivity: The sensitivity of a mic is defined as the electrical output for a given sound pressure level, it is a way of judging the relative output levels between two mics. If two mics are driven by the same sound pressure level (SPL), the mic with the higher sensitivity will produce a higher output.The maximum sound pressure level a mic can take before a specific amount of distortion is reached is another figure quoted by manufacturers. Whereas dynamic mics can take high sound pressure levels without distortion, it is possible that the output of a condenser, when used on loud sound sources, might be high enough to overload its preamp. To overcome this, some condensers have built in overload protection switches, otherwise known as pad switches, that can be used to reduce the output signal level by (usually) 10 or 20 dB enabling the mic to handle higher SPLs.
Buying a microphone
Unless you produce completely sequenced instrumental music, chances are that at least one microphone will be a vital part of your kit list. How many mics you need to own and of what type will be dependent on the type of music you make and how you go about making it. Having a cupboard full of upmarket mics might be a nice little fantasy but realistically the average home studio owner could probably get by with one all-purpose mic, that would be mainly for vocals but could be occasionally put to use on other instruments. If you were to plan on buying just one mic, probably the most versatile option would be a high-quality, dual-diaphragm condenser with switchable polar patterns, although the number of times you would need omni-directional and figure-of-eight patterns would most likely be small.
For most applications a condenser with just a cardioid response would do the job just fine. Fortunately for us, the competition between mic manufacturers for our custom is so great that you can now buy an excellent quality condenser mic for much less than a week's wages.
If you intend to buy two mics, the two most likely versatile scenarios would be to have a matched pair of condensers or one good condenser plus one all-purpose dynamic like a Shure SM57. Owning more than two mics opens up a wealth of possibilities that will depend on exactly what you are intending to record, but a stereo pair of condensers plus an all-purpose dynamic and large diaphragm dynamic for bass instruments would cover a multitude of tasks including recording a drumkit.Come back next month when we'll be setting up the mics. FM
Favourite studio mics
One defining factor of the top rank of recording studios regardless of what desk, recorder, outboard, effects and monitoring they have is their mic collection. Any studio worth its salt will have mics suitable for many jobs and plenty of them. While there are many mics that can be used to record vocals, the two classic vocal mics, both in use for several decades are the Neumann U47 and the AKG C12, valve mics both. Most photos of the Beatles recording at Abbey Road have a favoured U47 in the frame.
Studios will also have a selection of quality large diaphragm condenser mics primarily the AKG C414 and the Neumann U87, both of these have switchable polar patterns and are suitable for a wide variety of jobs. Other mics to be found in the pro studio cupboard will be a selection of condensers with a narrower diaphragm like the AKG C451 or the B&K 4000 series. Everyday dynamic mics will include a selection of Shure SM57s (snares, guitar amps) and Sennheiser MD421s which are a favourite for use on toms. Bottom end sounds like kick drums are usually tackled by specialist mics, predominantly dynamics with a large diaphragms. The AKG D12 has been a staple for a few decades but has been joined by it's distinctively egg shaped sibling the D112. The Electrovoice RE20 is another mic favoured for use on kick drums.
Choosing a live vocal mic
While the preferred mic for vocals in the studio is usually a condenser, a live vocalist is more likely to use a hand held dynamic. Stage mics can be used for recording but their features are specifically tailored for live work.
Firstly, a stage mic should be built to withstand knocks or being being dropped, and to withstand regular transport from venue to venue. Most of these mics will have built-in shock protection which also helps to cut down handling noise transmitted into the mic when hand held.
Secondly, stage vocal mics have to withstand wind and spit the extraneous noises they generate. In a studio setting, practically all vocals are recorded with a pop shield between the vocalist and the mic to stop any blasts of air which could cause popping sounds. Stage mics usually compensate by having a solid metal grille, lined with a thin layer of foam.Thirdly, stage vocal mics have a cardioid frequency response. If the singer sings directly into the mic it picks up the voice clearly but rejects any sound bleeding from the backline or monitors allowing a little more volume before the onset of feedback.
Finally, stage mics also have one other built-in factor that can help a vocal cut through the mix: a tailored frequency response known as a presence peak or lift. Part of the frequency range, usually the upper mid range where the main energy of a vocal resides, is given a slight boost so that the vocal cuts through loud and clear.
Six of the best on a budget
Fortunately you don't have to pay an arm and a leg to get quality mics these days and here are half a dozen with street prices that won't stretch your wallet too far...
Shure SM58 For years the SM58 has been the industry standard mic for live vocals. A dynamic cardioid mic, with an upper-mid presence peak to allow the voice to cut through a mix, it might lack the top end detail of a good condenser but is a good choice for recording a punchy rock vocal or for when a singer wants to record using a hand-held mic.
Shure SM57 The SM57 is usually the Number 1 choice for miking up guitar cabs and snares both live and in the studio, but can be used for many other jobs, it may just be the best all round dynamic mic available in this price category.
AKG C1000s are back electret mics that can be used with a battery if you don't have a phantom power source. It's a good choice for all those condenser mic jobs if you are on a tight budget and will produce good results on vocals, acoustic guitars or as a drum overhead.
Rode NT1: A large diaphragm, no-frills, cardioid, large diaphragm condenser that will do the business on vocals.
The AudioTechnica 4033 is a highly respected large diaphragm cardioid condenser with pad and bass roll-off switches and a suspension cradle mount.
Rode NTK There are quite a few new valve mics on the market now, but the NTK was awarded an FM Platinum award. A rich full sound great for vocals but useful on loads of other stuff. Not exactly a budget mic.