Microphones: Basic Understanding Of The Different Types
Brief Introduction to How Microphones Work
Microphones convert acoustical energy or sound waves into electrical energy, thus reproducing the audio signal that we hear. They are therefore transducers, in that they convert energy from one form to another. Microphones have different ways of converting acoustical energy, but one thing that is common to all types, is that they have a diaphragm. This diaphram is a thin membrane that mimics the human ear, and is a piece of material that vibrates when struck by sound waves. The transducer elements of the microphone are housed in the mic capsule. In a typical handheld micophone the capsule is found in the microphone’s head.
Microphones are usually primarily categorised according to their transducer principle. For example, they may be classed as condenser, dynamic, ribbon, etc.
They can also be labelled according to their directional characteristics, diaphragm size, how they are intended to be orientated (end- or side-address), or used ( boundary, boom, lapel etc).
Dynamic, or moving coil microphones work like a speaker in reverse. In these microphones the diaphragm is attached to an induction coil, and as the diaphragm vibrates responding to incoming sound waves, that coil moves backwards and forwards past the magnet. The variable current that is created in the coil is then channeled along the wires in the microphone.
Dynamic microphones are robust and can take a bit more of a beating than other types. They do not require phantom power. Most hand-held microphones are of the dynamic type. However, they do have a more restricted frequency response than other types of microphones, and can’t reproduce very high frequencies with accuracy. Often this doesn’t matter too much, but if you want to capture an instrument or voice that has lots of tonal detail found in it’s upper harmonics, using dynamic microphones may not be the best choice. They also require more gain to obtain a useful level of signal than condensor mics. The downside of this is, that the more gain you need to use to get a good signal level, the more the noise potentially also add to the signal rises. For close micing this is not usually an issue, but for distant recording, or the recording of quiet sources it may become a problem.
Condensor microphones are also sometimes known as capacitor microphones.
A condenser or capacitor is an electronic component that stores energy in an electrostatic field. Condensor microphones use capacitors to convert acoustical energy into electrical energy. One of the two plates that is found in the capacitor is made of a very light material and acts as a diaphram.
The frequency response and transient response of these mics is considerably better than most dynamic mics. They are therefore able to reproduce the character and subtleties of an instrument or voice with more accuracy and detail. The output of these microphones is also usually higher than dynamic mics, but they are correspondingly more sensitive to loud sounds, where they may distort the signal, and are also more easily damaged. Condensor mics require the input of phantom power in order to operate. This power is supplied by batteries or by an external source of phantom power.
Large Diaphragm Condensor Microphones
Large diaphragm condensor microphones (LDCs) generally tend to “flatter” the sound source to some extent, and are often chosen for studio vocals, or instrument recording where a warm sound is desired. They are perhaps not as clinically accurate, or have as flat a response as small diaphram condensor mics, but in many cases the flattery may be more appealing and musical than simply flat. They are very sensitive to plosives such as “P’s” and “Bs,” and when used for speech or vocals it is advisable to have a “Pop” filter infront of the microphone. Many LDCs have switches to change the polar patterns of the mic’s pickup, included on them.
Small Diaphram Condensor Microphones
Small diaphragm condensor microphones (SDCs) are often the best choice when you want a natural sound to be captured on sources that have a wide frequency range, or that require a rapid response to transients. They also tend to have the flattest response to all frequencies, and that includes bass, as well as high ones. Microphones of this type are often chosen for recording acoustic stringed instruments, choirs, cymbals, and the recording of concerts. Good SDCs can reproduce a wide range of sounds with great accuracy and detail. Some SDCs are available with screw-on capsules of different polar patterns.
Tube or Valve Microphones
Tube microphones are usually a type of condensor mic that combines capacitor capsules with valve preamplifiers. Valves (or tubes as they are known in some places) are often used to provide a subjective warmth to the tonality of sound. They often sound very smoothe and warm, but generally cost more than most other condensor microphones. They also tend to be more susceptible to damage They require their own external power supply, with a special cable that runs from the power supply to the microphone, and cannot be powered by a conventional phantom power source. They also tend to be noisier than solid state microphones.
The electret microphone is another type of capacitor microphone. Most lavalier microphones, consumer cam-corder microphones and many other low cost microphones are electret microphones. The diaphragm is usually quite thick in these mics and has a permanent electrical charge attached to it.
Another type is known as the back-electret microphone, and these have the permanent electrical charge attached to a back plate rather than to the diaphram This means that a thinner and more sensitive diaphram can be used. These are often quite good sounding mics that are relatively cheap to buy .Electret and back electret microphones require a very low voltage input from a preamplifier, and often can be run on batteries. One downside is that the electrical charge does weaken over time and eventually the capsule needs to be replaced.
Ribbon microphones employ a thin strip of metallic foil that is suspended in front of a magnetic plate. Upon entering the capsule, the sound waves cause vibrations in the foil or “Ribbon”, and they produce fluctuations in the electrical current, which creates the audio signal. The ribbon is a delicate mechanism, with great sensitivity. These microphones generally reproduce a wonderful rich sound over a wide range of frequencies. Ribbon mics tend to capture high frequencies in great detail, but without the brittleness of some condensor microphones. They are also generally the most sensitive to damage of all the microphone types. They do not require phantom power, and some may even be damaged if it is introduced to them inadvertently. Older ribbon microphones particularly, were very delicate, and also costly to buy. However, nowadays less expensive and hardier ribbon mics have found their way into production. Most ribbon microphones are made with a bi-directional polar pattern.
Microphone polar patterns
A Microphone’s polar pattern or directionality, relative to it’s axis, refers to it’s sensitivity to sound as it relates to the direction or angle from which that the sound arrives. Put simply, it is the way that the micophone hears the sound that hits it.
Omni directional microphones generally produce the most natural sound, but since they pick up sound from all around them, and this is not always desirable, they should be used in rooms that are well treated or contolled acoustically. They can also be used outdoors for gathering ambiences.
Unidirectional microphones come in several polar patterns, the most common of which is the cardioid microphone. The sensitivity pattern of a cardiod mic is heart-shaped. Hyper-cardioid microphones are similar but have a tighter area of sensitivity towards the front, and the lobe towards the rear has less sensitivity. A super-cardioid microphone is similar to a hyper-cardioid one, but has more frontal pickup and less rear pickup.
Because unidirectional microphones are relatively efficient at rejecting sounds form directions other than straight in front of them, they are often used for recording vocals and speech.
Bi-directional microphones which are commonly known as figure-of-eight microphones pick up sound with a high degree of sensitivity, from the front and back, but with low sensitivity, from the sides. They are therefore used in applications where sound you want sound to be recorded from the front and back of the micrphone, but not from the sides. Sound from right angles to the mic “nulls” well.
Special Purpose Microphones
Shotgun or Boom Microphones
The Shotgun microphone is a highly directional one. Often also known as boom mics on account of the fact that they are generally suspended by a boom, which is a special stand that enables the microphone to be hung above the subject, or fitted to a boom pole held by a boom operator. The object of these stands is to keep the microphone out of the camera shot on a film or TV set. Actually, it’s not technically correct to call shotgun mics “boom mics,” because a mic of any polar pattern can be mounted on a boom, and infact other types of mics are often used this way. But most times when people refer to the term “boom mic” they are talking about a shotgun microphone suspended on a boom. The stands have adjustments for height and angle, and can also be swivelled. Portable boom poles are also often known as fish poles. Because they are the most highly directional microphones, shotguns pick up very little background noise. However, conversely because they are so highly directional, they must be positioned very carefully to keep sounds on axis.
The capsule element is placed at the back end of a tube that has slots cut along it’s sides; Much of the off axis sound is nullified by wave cancellation. Because they have such a narrow area of sensitivity and great off axis rejection, shotgun microphones are ideal for use on television and film sets.They are also very handy for the field recording of wildlife, or at sports events. The longer the shotgun tube the more directional the microphone is. Contrary to some misguided thinking, they are not like a telephoto lense. In other words, distant sounds will still sound distant, and not magnified as in the case of a telephoto camera lense. Ideally when recording a spoken voice they should be positioned just out of camera shot, and preferably no more than 18 inches (45 cm) from the subject’s mouth.
A lavalier microphone, lav or lapel mic is a small electret condensor mic or dynamic microphone. The are used in television or theatre, and for public speaking applications. Usually they employ small clips for attaching them to collars and other clothing. The mic cable is thin, and usually gets hidden by the subject’s clothes. Lav mics may be wireless microphones with a transmitter attached to a person’s belt, or cabled and running straight to a stage box or a mixer’s inputs. When lavalier microphones are concealed under clothing in order to be inconspicuous, scratching of clothing against the microphone is a potential problem. Taping the cable is to the person is sometimes done, and often times it is advisable to wrap the head of the microphone in moleskin, or find some other way of preventing clothes or hair from touching the capsule and therefore causing annoying scratching sounds every time the subject moves.
Microphone preamps that have a phantom power supplying +48V on the balanced signal pins are designed for use with condensor microphones. However, if you apply this voltage directly to an electret lavalier microphone capsule, it can be damaged to the point of being ruined in a nano-second. Electret Lavalier microphones usually have their own power supply that provides them with a low DC voltage (usually between 1.5V and 5V). A lavalier power supply usually looks like a barrel or an enlongated XLR connector, and usually contains a battery. If the mic is hard wired to the power supply, then on one end of this power supply is a wire that runs directly to the microphone. If it is not hard wired, but has a detachable power supply, then it will have a connector that mates to the microphone’s cable. With wireless lavalier microphones that are connected to a transmitter, it is usually the transmitter that supplies the low vottage current that the microphone requires to operate.
Boundary microphones are somewhat unconventional, because they are designed to lay on a table, be placed on the floor or attached to a wall. They use the entire surface to facilitate the pickup of sound. The larger the surface on which they are placed, the better the sound is likely to be. The microphone capsule is incorporated into the body, so that it sits just a fraction of an inch above the surface. This eliminates phase cancellations that can happen to other types of microphones. Boundary mics are designed to avoid the coloration of the peaks and troughs and comb filtering that other microphones types placed close to hard surfaces introduce. Where they can really shine is in the picking up of ambiences, crowds of people, audience applause, stage sounds, and room tones. They are also often attached to the underside of the lids of pianos. But because they are so good at picking up ambiences, they are also prone to pickup air conditioning sounds, and other unwanted noise too. Boundary microphones are often referred to as PZM microphones. This is an abreviation for the term “Pressure Zone Microphone.” The PZM trademark belongs to Crown International, but the term has become widely used to describe boundary mics made by other manufacturers as well.
They are miniature condensor microphones, and are available in omni-directional or bi-directional configurations.
Power Supplies:Phantom Power and T-Power
Condensor microphones require an external power source in order to operate. Phantom power is a DC voltage, that is supplied down pins two and three of an XLR cable to power condenser microphones, active DI boxes and effects pedals. The standard phantom power voltage supplied by
most mixers, preamps and consoles is +48V, however some manufacturers power supplies for their microphones may be less than that. Mixers and preamps usually have a switch for turning this power supply on or off. The switches may be on individual channels, or on groups of channels.
T-Power ( also sometimes known as A-B power) was developed for use in portable applications, mainly for film and television sound equipment. It is sometimes still used in field equipment.
T-power is usually 12 volts, and only T-power mics should be connected to T-power inputs. Dynamic microphones and ribbon microphones may be damaged if T-Power is introduced to them, and normal condensor mics will not operate properly on it either.
All the different types of microphones, and their varying polar charcteristics have their place and uses. Learning which mics best suit which purpose takes a bit of experience. I hope this tutorial has covered many of the basics to help with understanding the concept of microphone choice, and the different applications for which they are suitable.
Written by Tony Koretz © copyright March 2012