Microphone Guide - The Basics
Microphone Guide - The Basics from sE Electronics
Like the human ear, the microphone is designed to capture sound. But whilst there is a single optimal design for the human ear, the same isn’t true of the microphone; not least because it doesn’t have the human brain’s ability to process and filter the sound being captured. For example, the human hearing system can pick out a single voice in a group of voices and, to a large extent, can ignore room acoustics. On the other hand, a microphone simply picks up what it hears and passes it on. Furthermore, a microphone may be called upon to collect sound in environments that would be hazardous to the human ear - such as inside a kick drum, looking down the bell of a trumpet or touching the speaker grille of a 200 Watt guitar stack. The solution is to build different types of microphone which are optimised for different tasks, though in reality, most well- designed microphones can cover a reasonably wide range of applications. There are three types of mechanism commonly used in microphone design: the Dynamic or “moving coil” Transducer, the Condenser Transducer and the Ribbon Transducer, all of which have specific characteristics and strengths.
Dynamic microphones (mics) comprise a voice coil attached to a lightweight diaphragm which is suspended in a magnetic field. When sound causes the diaphragm to vibrate, the coil moves within the magnetic field, and consequently a small alternating electrical voltage is generated which is proportional to the sound being picked up. Dynamic mics require no external power (phantom power, for example), they are robust, and as well as being used extensively in live sound for vocal and instrument use, they suit the sound of certain instruments such as electric guitar and bass, close-mic’ed drums and some brass instruments. They produce a punchy sound that cuts through busy mixes, but they are less effective at capturing high frequency (transient) detail. Most have a response that rolls-off at around 16kHz and they are not particularly sensitive, which means that they need a lot of preamplifier gain when used with quieter or more distant sound sources. The vast majority of dynamic microphones have a fixed Cardioid or Hypercardioid pickup pattern, which means that they pick up sound predominantly from in front of them.
Condenser microphones use a much lighter diaphragm, usually made from extremely thin Mylar with a coating of pure gold, and require no voice coil. Their much lower diaphragm mass means that they are much more responsive to high frequency sounds. A polarising voltage is applied between the diaphragm and a fixed back- plate, such that the voltage is modulated when air vibrations force the diaphragm to move. The electrical output from the capsule requires a preamplifier with an extremely high input impedance, so FET or tube amplifiers are usually built into the microphone body. Many models cover the full 20Hz to 20kHz range which is deemed adequate to satisfy the human hearing system, and they are more sensitive than dynamic models – meaning that they need less amplification. All amplifiers introduce some noise, so the less amplification is needed, the quieter the end result is likely to be. Condenser microphones have on board impedance- matching and amplification circuitry that requires power, and they also require a polarising voltage for the capsule, so most (other than tube mics) run from 48 Volt “phantom power” which is supplied by the preamp along a standard balanced XLR cable. With very few exceptions, tube microphones come with their own power supplies.
Some companies use capsules based on the Back-Electret principle, where a material carrying a permanent electrical charge is fixed to the back-plate in order to avoid the need for a separate polarising voltage. An internal preamplifier is again necessary, so either phantom power or battery power is still required. These microphones can work extremely well though the ‘permanent’ charge may eventually leak away, resulting in reduced sensitivity over a period of a decade or two. This may sound like a long time, but some of the traditional condenser microphones still used in studios can be up to 50 years old or more. Large-diaphragm cardioid capsules (usually around one inch and above) tend to be favoured for vocal recording as they can be tuned to add character to the sound, while small-diaphragm models (usually around half an inch) are often preferred when greater accuracy is required.
The psychological perception that bigger diaphragms must somehow produce a “bigger” sound is simply not true and excellent, full-sounding vocal recordings can be made using small diaphragm models. Technically speaking, small-diaphragm models tend to have a better off-axis response than large-diaphragm models; the larger the diaphragm, the greater the time arrival difference between sounds arriving at opposite edges of the diaphragm when originating off-axis. This time difference leads to high-frequency cancellation, which is why the off-axis sound of a large diaphragm microphone can sound dull when compared with the on-axis sound.
It is possible to build condenser mics with any of the three main pickup patterns (Cardioid/Unidirectional, Figure-of-Eight or Omnidirectional). Dynamic mics are mainly Cardioid or Hypercardioid, while Ribbon mics have a natural Figure-of-Eight response. Perhaps the simplest type of microphone is the Figure-of-Eight, in which the diaphragm is open to the air on both sides. It picks up sound equally well at the front and rear, but is insensitive to sounds approaching from the sides. By contrast, an Omnidirectional microphone comprises a diaphragm placed over the mouth of an acoustic chamber and picks up sounds from all directions. An Omnidirectional mic can sometimes be referred to as a Pressure-Gradient mic, as it essentially senses air pressure changes in its vicinity (though the acoustic chamber does have a small vent to equalise the air pressure inside, otherwise the diaphragm would bulge as the outside air pressure changed). While Cardioid and Figure-of-Eight mics also operate on the pressure gradient principle, they sense the difference in air pressure between the front and rear of the microphone.
Note that both Cardioid and Figure-of-Eight-pattern microphones exhibit what is known as the “proximity effect”, where low frequencies are boosted if the mic is used very close to the sound source; typically 100mm or closer. The Cardioid pattern is really a modification of the Figure-of-Eight principle, where the addition of acoustic porting reduces the sensitivity of the capsule at the rear and sides. This porting can affect the natural tonality of the microphone to some extent, which is why Omni-pattern mics generally have a more natural and open sound than cardioid models. However, a well- designed Cardioid microphone can still sound extremely good and has the obvious spill-rejection benefits associated with uni-directionality. By building a condenser mic using two back-to-back cardioid capsules (usually large-diameter), the outputs from the two capsules can be mixed electronically to generate any desired pickup pattern from cardioid, via figure-of-eight, right through to omni; and all points in between. However, there is an argument for choosing a single-diaphragm, dedicated Omni microphone for critical Omni-mic’ing applications as their simpler construction usually gives a more even, natural sound. With dual large- diaphragm capsules, the high frequency response of the microphone often fluctuates very slightly with angle, being most accurate when the sound source is directly in front of them. In many cases, this small compromise is offset by the microphone’s versatility.
Ribbon microphones are based on one of the first microphone principles ever devised, where a thin ribbon of conductive metal (usually aluminium) is suspended in a magnetic field. This construction produces a natural figure-of-eight pickup pattern. When the ribbon moves in response to sound, a very small electric current is generated which is fed out via an impedance-matching transformer to produce an electrical signal. Traditional ribbon microphones are fairly insensitive (little signal is produced for a given sound level) and their high frequency response rolls off significantly above 10kHz or thereabouts. The reason they remain popular is that they have a musically pleasing sound, especially for use on strings, electric guitar amplifiers and drum overheads. Modern advances in ribbon design have made these microphones more robust than before, and the sE RNR1 has a much extended high frequency response - but they still need to be handled with reasonable care as a fall could break the ribbon. It is important to check what manufacturer backup is available for ribbon microphones, though you’ll find sE is very generous in this respect. Active ribbon models can be made so that they’re more sensitive and can have their high frequency response extended somewhat, but they still retain the smoothness of tone and musicality that made them popular in the first place. The introduction of the sE RNR1 means that ribbon microphones may now be used in some applications where previously a capacitor model would have been the only logical choice.
The Importance of Patterns
Unidirectional or cardioid mics are useful in limiting the level of unwanted off-axis sound, such as room reverberation or spill from other instruments being picked up by the microphone. In the studio this helps create better separation when several musicians are playing together, each with their own microphone. However, it should be borne in mind that off- axis sounds will be reduced in level but not eliminated entirely, and a characteristic of a typical cardioid pattern mic is that high frequencies fall off more rapidly than low ones as you move away from the frontal axis. In practice this means that off-axis spill can sound dull and coloured.
The figure-of-eight pattern microphone is used for some specialist applications such as stereophonic mic’ing but the fact that they are almost totally ‘deaf’ to sounds arriving 90 degrees off-axis can be exploited in maintaining good separation simply by ensuring the ‘deaf’ axis aims directly at the unwanted sound. Often you won’t need to pick up sound from the rear of the mic (which is just as sensitive as the front) so this can be screened to some extent using acoustic gobos or an sE Reflexion Filter TM.
Omnidirectional microphones have an inherently more natural sound than cardioids, as their simple construction eliminates the need for complex rear porting to shape their directional response. Off-axis sounds are reproduced reasonably faithfully and with small diaphragm models the sound quality is virtually consistent regardless of the direction from which the sound approaches. Other advantages include a higher immunity to handling noise than cardioid mics, extended low end response, lack of proximity effect, and often a greater ability for handling high sound levels. In situations where separation is a prime requirement, cardioid microphones are usually the instinctive choice but as omnis don’t colour the off-axis sound in the way cardioids do, any off-axis spill will be captured faithfully. In practice, a recording made using omni mics can be easier to mix; spill levels may be a few dBs higher than when using cardioid microphones but the quality of the spill isn’t compromised so it integrates with the wanted sound better. Note that if you place an omni mic at between half and two thirds the distance from the source than you would for a cardioid mic, the amount of spill will be similar so the use of omnis doesn’t necessarily signify poor separation.
MIcrophone Glossary Of Terms
- Sensitivity: This is a means of stating how much signal output a mic produces for a given level of sound input, usually specified at 1 Pa or Pascal of sound pressure at 1kHz.
- Max SPL: The maximum sound pressure a mic can accommodate before the output becomes significantly distorted. A typical figure would be around 130dB, but for high sound level applications some models can handle up to 145dB or more and this may be further extended if the mic has a pad switch.
- Pad Switch: Used to reduce the output from a microphone, usually by 10 or 20dB when used in high sound level environments. This prevents the internal electronics and any subsequently connected preamplifier from being overloaded.
- Noise: All electronic circuitry produces random noise and in the case of microphones this is often specified as an equivalent input noise or EIN. Typical EIN noise figures for capacitor microphones are between 12 and 20dB where the higher the number, the noisier the mic. Extremely low noise models may produce as little as 5dB of self noise.
- Signal-to-noise ratio: This is another way of expressing noise performance by expressing it as the ratio between the noise and the nominal operating level of the microphone. This figure is expressed in dBs and anything higher than 75dB is reasonably good with larger values indicating lower noise.
- Low Cut Filter: If a mic doesn’t need to pick up very low frequency sounds from bass instruments, it can be advantageous to switch in a filter to reduce the effect of vibration, rumble, vocal popping and so on. Many mics have a low cut filter, often at around 80Hz with a 12 or 18dB/octave slope.
- Pop Shield: Close mic’ed vocals can suffer popping when the singer pronounces B and P sounds as these tend to expel a blast of air that can slam into the diaphragm causing a high level, low frequency thump at the output. These can sometimes be called “plosives”. While a low cut filter will help reduce popping, a more effective solution is to use a perforated nylon or metal mesh in front of the mic to disperse the air blasts before they reach the microphone. These are normally placed 50 to 75mm from the microphone, directly in front of the singer’s mouth.
- Shock Mounts: Microphones are sensitive to vibrations picked up by their casework and even a well-designed flexible capsule suspension system can’t eliminate it entirely. Where stand-borne vibrations might be a problem, a shock mount cradle that holds the mic via a series of elastic belts makes a significant improvement.
- Tube Mics: Typical condenser microphones use Field Effect Transistors or FETs as amplifiers as these have the very high input impedance necessary to match the capsule. Before FETs, tubes or valves were used, and though these require additional power supplies and are more costly to build, they are still popular because of the warm, solid tonality imparted by the valve circuitry. This is partly due to the progressive distortion characteristics of valves, which add pleasing harmonics to the sound and add some natural compression. Tube microphones have an external power supply and so don’t require phantom power. While vintage tube microphones often use exotic tubes that are difficult to find and very expensive to replace, modern versions use more commonly available tubes so getting spares presents no problem.
- Phantom Power: Condenser microphones and other active microphones are often designed to run from a standard 48 Volt phantom power source. This is supplied by the mixer or mic preamp and is passed along the conductors of a standard balanced XLR mic cable. The term ‘phantom power’ came about because no additional cabling is needed. When considering the purchase of a condenser microphone, check that your mixer or mic preamp can supply phantom power.
Care of Microphones
Studio microphones need to be protected from dust, mechanical shock and excess humidity. The best strategy is to put them back in their cases when not in use and to store these in a domestic environment or other low humidity area such as a typical home. If the microphone must be left out on its stand, a plastic bag placed over it when not in use will protect it from dust. Ribbon mics are particularly vulnerable to mechanical damage so avoid deliberately blowing into them or dropping them. The outer case of most microphones can be cleaned quite effectively using a damp (not wet) cloth with a drop of household detergent (washing up liquid) added to the water if necessary. Avoid the use of alcohol or spirit cleaners and don’t spray household polish near the capsule basket. With reasonable care, a good microphone can give trouble-free operation for many years or even several decades.