Microphones

Basic types 

Dynamic

Dynamic Microphones or Moving Coil Microphones, as they’re known in technological circles, work by electro-magnetic induction. This is to say they work in the same principal as a loudspeaker, only reversed. A diaphragm captures the vibrations of a sound wave. Attached to this diaphragm is an induction coil that vibrates within the magnetic field of a permanent magnet. This provides a varying current within the induction coil. Though not as sensitive or not having as wide range of frequency response as other microphones types, they are extremely durable, inexpensive, and do not require external power for operation. They also have their own unique characteristics when incorporating proximity effect and off axis rejection (see below). Because of these attributes, Dynamic mics are often used in live sound applications as well as in the studio where high sound pressure levels occur such as snare, toms, and the proverbial Marshall guitar cabinet “cranked to 11”. Examples of popular dynamic mics would be: Shure SM57/SM58, Shure SM7B, Sennheiser MD421, AKG D112, Electrovoice RE-20.

Condenser

Condenser, or capacitor microphones, operate by having the diaphragm work as one electrically charged plate of a capacitor. The vibrations change the distance between this and the static plate, creating a variance in capacitance, thereby variance of voltage within the output.  Because these plates must maintain a constant voltage, these microphones need a power source to work. Some such mics, particularly those used in field applications, can be powered by a battery. Studio mics, however, incorporate “phantom power” which is a 48v charge that can be sent through the recording console or through an external power source. Because condenser mics use this method of constant voltage, they are more sensitive and have a wider frequency response than dynamics, making them a staple in studio recording. They are also relatively much more delicate than dynamics. There are two subtypes of condensers: Large diaphragm and small diaphragm.

 Large diaphragm condenser mics provide a very accurate, clear and open sound that is ideal for vocals. Because of their sensitivity, a pop filter should be used in vocal applications to filter out the harsh “Ps”. Also, this high sensitivity can result in a “hot” signal going into the console which could possibly overdrive the console’s preamp. Some of these “hotter” mics incorporate a “pad” switch that reduces the signal a given amount to prevent this. Many consoles also have pads built into them for this purpose. Because of their wide frequency response, they are also popular for orchestral instruments and piano. Examples of large diaphragm condensers are: Neumann U87, Neumann TLM103, AKG414, Shure KSM44, Audio Technica AT4050.

 Small diaphragm condenser mics also provide a clear sound, but tend to be more sensitive at higher frequencies. They’re most commonly used for cymbals on a drum kit (high hat, overheads), but also may be used in conjunction with large diaphragm mics on some instruments to add “brightness”. Examples of small diaphragms: Neumann KM184, Shure SM81, AKG C-1000.

 Another sub-genre of large diaphragm condenser microphones would be tube mics. These mics incorporate their own proprietary preamp that use tubes as opposed to solid state circuitry as most modern condenser mics do. These mics deserve mention because of their legendary status within the industry. Inspite of the increased harmonic distortion inherent with tubes, these mics are considered to have a “warmer” characteristic according to many engineers. As such, classic tube mics tend to command an exorbitant price. Some microphone manufacturers have released modern versions of these classic mics. As to whether these modern reissues effectively emulate their classic counterparts is a point of contention. One word of warning about the use of these mics: Their proprietary preamp cable carries a significant voltage. They can give you quite a jolt if you forget to power the mic down when connecting/disconnecting! Examples of classic tube mics would be: Neumann U-47, AKG C-12. Modern reissue examples: AKG Perception 820, Rode K2.

Ribbon

Ribbon, or pressure gradient microphones, incorporate the same basic principal as dynamic microphones. But rather than using a diaphragm, these mics use an extremely thin aluminum ribbon suspended between two poles of a magnet. Ribbon mics are known to have a good transient response and a smooth proximity effect which give them a nice, mellow sound. They are legendary for recording jazz and blues vocals. Because of how they’re internally constructed, ribbon mics, particularly the classic ones, are very delicate. Modern ribbon mics, internally constructed with modern materials, are somewhat more robust, but are still relatively delicate and extreme care should be used when handling. Care should also be considered in the use of phantom power. While dynamic mics will “ignore” phantom power if it’s accidentally engaged, a ribbon mic given phantom power when it’s not required can be “blown”. That being said, there are some modern ribbons, such as the Royer R-122, that require phantom power. Be careful! When in doubt, don’t engage phantom power! Classic ribbons are expensive to replace! Examples of ribbon mics are: Royer R-121/122, RCA 44/77, Blue Woodpecker.

 Basic Microphone placement 

 On-axis vs. off-axis

 On-axis refers to mic placement in such a way that the sound source enters the microphone directly at its most sensitive point. Obviously, off-axis would then refer to the sound source deviating from the point of highest sensitivity of the microphone. If a sound source enters a microphone off-axis, two things occur. Firstly, there is a reduction in amplitude, or signal strength. This is known as off-axis rejection. Secondly, the microphone is less sensitive to certain frequencies with a sound source entering off-axis. This is known as off-axis coloration. It would be obvious that, with a few rare exceptions, a mic should be placed where the sound source is on-axis to the microphone to get the strongest, most transparent signal.

 One item to be considered in proper mic placement is whether the microphone is front address or side address. This is usually pretty obvious in that the grill of the microphone typically indicates on-axis. But there are exceptions. One case being that of the Sennheiser MD421, which appears from it’s casing to be side address but is, in fact, front address. Another example is that of most large diaphragm condenser mics that have a grill on either side of the casing. Most of the nicer large diaphragm condensers have switchable polar patterns (see below). In the bi-directional setting, on-axis would be both sides of the microphone. In the cardioid setting, on-axis would be on the side where the logo or trademark label is located.

 So what about those rare exceptions? Sometimes a situation calls for creative use of off-axis rejection. For example, let’s say you’ve been tasked to mic a choir. The best situation, of course, is to mic every person or at least each row in the choir. But the problem is the more microphones used, the less headroom you have in the sound system before feedback. You need to use as few microphones as possible. One possible solution is to place the microphones close to the front rows, yet pointed directly at the back rows. Though the back rows are further away, they’re on-axis. Consequently, though the front rows are closer, off-axis rejection is taking place. With experimentation, it’s possible to equalize the amplitude of all rows simply on how the mics are aimed.

 The key to proper mic placement is your ears. When placing a mic, it is essential that you listen and experiment with different mic positions.

Proximity Effect

 Another characteristic of most directional microphones is proximity effect. This is the phenomenon that occurs when, as the distance from a sound source is decreased, there is a significant boost in the bass frequencies. The amount of proximity effect varies with make and model of different microphones.

 In many cases, this proximity effect is seen as a negative, particularly when trying to do close micing on an instrument. To achieve a flat frequency response at various distances, many nicer microphones will incorporate a “roll off filter” which attenuates, or brings down, lower frequencies to compensate for close micing situations.

 In other cases, proximity effect is seen as a positive. As mentioned earlier, dynamic mics have a more pronounced proximity effect. Many male radio personalities prefer dynamic mics that have an extremely pronounced proximity effect, such as an Electrovoice RE-20 or a Shure SM7B, to give their voice a deeper, larger than life quality. Skilled vocalists also learn to use to the pronounced proximity effect of handheld dynamic mics to their advantage. When singing in lower registers, they’ll bring the mics closer to emphasize lower notes. As they move to higher registers, they’ll bring the mic further away to reduce low frequencies, thereby accentuating high notes.

Polar Patterns

The polar pattern is how a microphone responds in sensitivity and frequency coloration relative to its axis. Different microphones incorporate different polar patterns for different applications. Common polar patterns are:

Omnidirectional

A microphone with an omnidirectional polar pattern will pickup sound uniformly from all directions. Because these mics aren’t subject to off-axis rejection or coloration and have a virtually non-existent proximity effect, they tend to sound very natural. The down side to this is that if there are other instruments in the room, they’ll “bleed” or pick these up as well. Also, if there are speakers in the room, such as FOH cabinets and monitors in a live situation, they’ll be highly prone to feedback. Another characteristic to keep in mind when placing these mics is that they do have a tendency to become directional at higher frequencies. They are great for capturing the “ambient” sound of a room.

Cardioid

 This most common pattern of microphones gets its name from the relatively “heart shaped” appearance of it’s pattern. Rejection and coloration increases gradually as the sound source is moved off axis. Rear axis rejection is typically a 15 to 25 dB drop in sensitivity.

Supercardioid

 This pattern is similar to the cardioid, but with much more off axis rejection at the sides and not as much at the rear. These mics are used when it’s desirable to isolate the instrument directly on-axis and capture some ambience from the rear while rejecting instruments on the side.

Hypercardioid

 This pattern is a more exaggerated version of a supercardioid. They’re used when a maximum of side rejection is needed. They have the least amount of rear axis rejection of the three. 

Bidirectional/Figure Eight

 A bidirectional, or figure eight as it’s often called, is a polar pattern indicative of side address mics. On-axis is on either side of the mic with a high amount of off-axis rejection everywhere else. As mentioned earlier, many higher end large diaphragm condensers are switchable between bidirectional and cardioid.

Specialty Microphones 

Boundary layer

 The Boundary Layer, or sometimes referred to as simply Boundary microphone is an omnidirectional mic that has a hemispherical pattern like a ball that’s been cut in half. It has a flat casing that allows it to be mounted to a flat surface. These are often the choice of omnidirectional mics in the studio to capture ambience. They can also be mounted to a piano lid, a conference table, or on a sheet of plexiglass or directly on the stage for orchestral recording. Examples include: Crown PZM185, AKG CBL410. Sennheiser e901

Instrument

 While the basic types of microphones covered earlier are used for recording in the studio, there are mics designed specifically for certain instruments for live performance applications. These mics, like their direct mount piezo cousins, allow an instrumentalist freedom of movement while performing, much like the headset mic would offer a vocalist.

 Direct mount piezo transducers, or pickups, capture the direct vibrations off the instrument. They are mounted, either at the factory or as an after market product, on acoustic guitars or orchestral string instruments (not to be confused with electric guitar or bass pickups which are magnetic transducers and are a totally different animal, capturing the vibrations of steel strings as opposed to the body). The advantage of the piezo is that, since it is direct mounted, there is no bleed from other instruments. Also, they are not susceptible to the feedback that can be problematic in a live situation.

 The downside to these is that many require permanent modification to the instrument for mounting. This would be highly undesirable on a vintage instrument such as a 300 year old Stradivarius or Matthias Alban. Also, true microphones tend to produce a much more natural, open sound that many engineers and artists alike prefer. To quote famed Celtic violinist, Máiréad Nesbitt: “Mic over pickups always! Even though there’s a risk of feedback, the mic gives better quality of sound.” (M. Nesbitt-2014, Twitter correspondence) These mics are available for woodwind and brass instruments as well as string. Examples include: Sennheiser AMT VSW, Sennheiser Evolution e908 B, Audio Technica ATM-350, AKG C516ML

 Shotgun

 Shotgun microphones have a highly directional polar pattern. This pattern is achieved by incorporating sound cancellation vents built into the sides of the microphone. They were designed to be used on film and television sets as well as field news gathering and “man on the street” interviews. They’re most commonly mounted to a long “Fish Pole”, allowing the operator to point the mic directly at the talent. They’re also mounted in a protective plastic “Zeppelin” which, in turn, is covered by a furry “Dead Cat” (film people love colorful nomenclature) to reduce noise from movement or wind in the case of outdoor applications. These are also condenser mics, allowing a high level of sensitivity due to the fact they may have to pick up sound from a relatively long distance in order to keep the mic out of the shot. However, many popular models have a much lower power requirement than their studio brethren to save battery power when used in field applications. Examples include: Sennheiser MKH416/816, Audio Technica AT8035.

Parabolic

 The parabolic mic is not technically a microphone per se, but rather a microphone mounting device designed for a specific purpose. That being said, most parabolic mic “kits” come with their own proprietary mic. The parabolic “dish” captures sound and focuses it onto a “receiver” where the mic is mounted, much as a satellite dish captures radio waves. They are extremely directional and are best known for capturing sound at exceptionally long distances. They are commonly used in sporting events for capturing such elements as a referee’s call from across the playing field. Another commonly used application is capturing nature sounds such as bird calls. They’re also commonly sold at spy shops. But really, have you ever seen James Bond wielding a device with all the subtle appearance of a satellite dish? These are never used in musical applications because of their low fidelity response, particularly in the low frequency range.

A Word on Impedance

Impedance, by definition, is the amount of resistance encountered by the signal. This is measured in ohms denoted by the Omega symbol, or “Ω”. The shorthand denotation is “Z”. This can be a confusing subject because the “technical” implications used by audio technicians and “colloquial” implications used by audio engineers often differ. For example, input of a professional console is commonly considered “low impedance”. However, the “low Z” input of a console’s preamp ranges from 1000Ω to 2000Ω, which is considered “high impedance” in technical circles.

 To simplify this, let’s consider the impedance of microphones:

 A “low impedance” microphone is rated at, or less than 600Ω. These are higher end “professional” microphones.

 A “high impedance” microphone is rated at, or greater than 10,000Ω. These are lower end “consumer grade” microphones.

 Yes, “medium impedance” mics with values between the two exist, but mics are typically classified as either “low impedance” or “high impedance”.

 Here’s the deal in practical terms: The microphone must have a lower impedance rating than the preamp it’s being plugged into. Hence, a “low impedance” mic can be plugged into a “HiZ” input without negative consequences (though a connection adapter would be required). However, if a “high impedance” mic is plugged into a “LoZ” input, signal degradation may occur and it can cause damage to the preamp! Many consoles and outboard preamps will have both “LoZ” and “HiZ” inputs to accommodate both.

 Another consideration is the output of an electric guitar or bass. It’s not uncommon to plug these, particularly bass, directly into a console’s preamp. The magnetic pickups on these instruments are rated “high impedance”. If no “HiZ” input is available, a “direct input box” must be used to lower the impedance of the instrument’s signal.

Connections

The standard connection for a “low impedance” microphone and the “LoZ” input on a preamp is the exchanged line receptacle commonly referred to as the “XLR”. This is a three conductor connector consisting of three “pins”. This allows for a “balanced” cable which has the characteristic of having a high resistance to noise induction. Pin 1 on the cable is the “chassis ground” connected to the cable’s shielding. Pin 2 carries the positive polarity, or “hot” side of the circuit. Pin 3 carries the negative polarity, or “cold” side of the circuit.

 The standard connection for a “high impedance” microphone, an electric instrument cable, and the “HiZ” input on a preamp is the ¼” TS Phone plug (tip, sleeve). This is a two conductor connector resulting in an “unbalanced” connection which is much more susceptible to noise induction.

 High end specialty instrument microphones discussed above commonly use a 3.5mm TRS Phone plug (tip, ring, sleeve). This allows the three connector “balanced” connection, yet much smaller for the miniscule sized mic.