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Microphone
Notes
Microphone – a transducer that
converts one type of energy (sound waves) into another corresponding form
of energy (electric signal).
The overall sound is only as good
as the weakest link in the signal path.
Good musician + good acoustics + good mike + good
placement = good sound
DYNAMIC MICS
Dynamic microphones use electromagnetic induction to
generate an output signal.
In theory: whenever an electrically conductive metal
cuts across the flux lines of a magnetic field, a current of a specific magnitude
and direction will be generated within the metal.
RIBBON MICS
Similar principle to dynamic mic except that the diaphragm
is made of extremely thin aluminum.
Because the diaphragm is light (compared
to the coil of the dynamic mic), a step-up transformer must be used to bring
the output impedance up from 2 ohms to the acceptable range of 150 – 600 W.
CONDENSER MICS
Operate on electrostatic instead of electromagnetic
principle. The capsule consists of two thin plates, one fixed and one movable.
The plates form a capacitor or condenser. A condenser is an electric device
capable of storing an electric charge.
Because the diaphragmÕs output is of extremely
high impedance, it is fed through
an impedance conversion amplifier which is placed into the circuit at close
proximity to the diaphragm in order to prevent hum and noise. This is why
condenser microphones require a power supply voltage in order to operate.
Electret-Condenser
microphones feature a permanently stored polarizing charge on the microphoneÕs
back-plate or diaphragm. Since the capsule still has a very high output impedance,
phantom power or n external power supply is still required.
MICROPHONE CHARACTERISTICS
Directional response refers to the micÕs output level at
various angles of incidence with the respect to the front side of the mic.
Patterns:
Omnidirectional
polar response – pressure
sensitive device (mic) responsive to sound waves from all directions.
Directional microphones are pressure-gradient. They discern pressure differences coming from different directions.
Bi-directional microphone is one that is purely pressure gradient. Many ribbon mics are bi-directional.
Some very commonly known and used patterns are:
| PATTERN | ICON | DESCRIPTION | MICROPHONE |
| Cardioid |  |
The cardioid pattern, sometimes called unidirectional, looks like an upside-down heart. These highly directional microphones pick up sound from the front and reject sound from the rear. Thusly, this pattern is a favorite for live situations. |  Shure SM-57 |
| Hypercardioid |  |
A highly directional microphone whose minimum sensitivity
begins at 110 degrees. Microphones called "line" or "shotgun" microphones
which are used often on sound stages have a hypercardioid pattern. |
 AKG D-409 |
| Omnidirectional |  |
The omnidirectional pattern picks up sound equally from all directions. This pattern is an excellent choice for a ÒliveÓ recording feel, acoustic bass and surround sound applications. |  AKG C4000B Omni, Cardioid or Hypercardioid |
| Bidirectional |  |
The bidirectional microphone pattern picks up form the front and rear equally and eliminates sound from the left or right side. This mic is great when two people are facing each other while performing or in the case of a panel discussion when a performer or lecturer is interacting with the audience. A figure-8 polar pattern is an excellent esample of a bidirectional microphone. |
 Shure KSM-44 Omni, Cardioid or Bidirectional |
| Hemispheric |  |
Also known as a "half-omni" pattern. A PZM zone pressure microphone is the most common example of a hemispheric pattern microphone. The hemispheric pattern picks up sound in the shape of a half-sphere. For this reason, PZM mics are often set on the studio glass at about 5 to 6 feet in height. This enable the hemispheric pattern to capture sound from floor to ceiling in a three-dimensional way. |  Crown PZM |
| Supercardioid |  |
The supercardioid is similar to the cardioid except that this pattern adds sensitivity to the rear. This microphone is useful for picking up the direct sound and some of the reflections from the rear of the microphone. The supercardioid picks up a little more than 180¡ from the front. The supercardioid pattern pick up a very broad picture from the front. From the rear, the microphone picks up a narrow beam of sound waves. This microphone is very useful when recording a large horn section or vocal ensemble. |  Shure Beta 58 |
| Subcardioid | |
A highly directional microphone whose minimum sensitivity begins at 100 degrees. |  AT 808G |
| Spherical |  |
The KU 100 dummy head is a binaural stereo
microphone. It resembles the human head and has two microphone capsules
built into the ears. When listening through high-quality headphones it
gives the illusion of being right at the scene of the acoustic events.
When using the KU 100 dummy head, the binaural stereo experience moves
the listener into the scene of the original performance, in contrast
to other space-related recording techniques, where the acoustic event
is moved to the listener. The dummy head is also used in many industrial
applications as a measuring device, for example in acoustic research.
The KU 100 can be operated with typical 48 V phantom powering, or from
an external power supply unit, or from the built-in battery. At the bottom
ofthe unit is a switch for the different power supply modes, as well
as connectors for balanced and unbalanced output signals. Inside the
head are additional switches for 10 dB attenuation and the high-pass
filter.
Ê |
|
| Wide cardioid |  |
For amzing discussions on
microphones, microphone images, sound clips, connectors and patterns, please
visit:
http://www.indiana.edu/~emusic/micropho.htm
Microphone data
Audio
Technica
VTA
Vintage microphones history
Answers.com
Coutant.org
(This is an amazing site with pictures and information)
FREQUENCY RESPONSE
Response is measured against the standard 20
- 20,000 Hz Frequency range.
Flat response means equal output at all levels.
Low freq. (3 – 25
Hz) can cause rumble. Use a shock mount to eliminate or reduce effect.
Proximity
effect is an increased bass response when sound source is closer than 1Õ to
the mic.
Transient response – how quickly
does mic react to the sound wave.
RESPONSE CHARACTERISTICS
Dynamic mics, because they have large diaphragms, have slow responses producing rugged, gutsy and less accurate sound than smaller diaphragm mics.
Ribbon mics have very small diaphragms producing a very fast and accurate response.
OUTPUT CHARACTERISTICS
Sensitivity rating – is the output level in volts that a microphone will produce given a specific and standardized input level (rated in dB). The measurement specifies the amount of amplification needed to raise the micÕs signal to line level (-10 or +4).
The higher the micÕs sensitivity level, the higher itÕs output level.
EQUIVALENT NOISE RATING – the deviceÕs electrical self-noise.
Overload Characteristics – at what point does the input level cause distortion. Condenser mics are very sensitive and therefore usually have a built in pad.
MIC IMPEDANCE
Dynamic mics are low impedance devices that use a built-in step-up transformer. These mics cables often pickup electrostatic noise. They use unbalanced lines.
High-impedance mics
MICROPHONE PREAMPS
Used because most mics canÕt put enough signal for line level. A mic preamp is used to boost the signal (30 – 60 dB) to an acceptable level.
Most preamps today are very inexpensive and sound great. Most offer features such as:
Input gain, high-pass filtering and phantom power.
Phantom Power
Phantom power supplies a positive DC supply voltage of 48+ V
to both conductors
(pins 2 and 3) of a balanced mic line. Most condenser mics
need phantom power in order to work correctly.
MICROPHONE TECHNIQUES
All mics have a distinctive sound based on their design.
MIC PLACEMENT
Distant placement – more than
3Õ from source.
- Picks up a large portion of the sound from an individual instrument or ensemble. This technique helps to preserve overall tonal balance. A natural balance can often be achieved by placing the mic at a distance thatÕs roughly equal to the size of the instrument or sound source. Used for vocal ensembles, string ensembles and large groups.
- This technique allows the Òroom soundÓ to be picked up and incorporated into the overall sound.
Close placement – 1Õ – 3Õ
from source.
- Creates, tight, present sound quality.
- Effectively excludes the acoustic environment (room sound).
Leakage is defined as when sound from one instrument is picked up by a nearby microphone. Happens often with horn sessions and live rhythm section recordings.
Techniques used in avoiding leakage:
- Bring mics closer to respective instruments.
- Place an acoustic barrier between instruments. (Baffle, gobo, flat, divider, etc.)
- Use directional mics.
- Spread the instruments farther apart.
Whenever individual instruments are being recorded close or semi-close, itÕs generally wise to follow the 3:1 distance rule. The rule states that in order to maintain phase integrity É for every unit of distance between a mic and its source, a nearby mic (or mics) should be separated by at least three times that distance.
Accent Placement
Accent mic – a mic placed by an instrument or section, inside of a larger ensemble, in a close but not so close position as to record an unnatural sound. Used to add presence to those instruments without changing the balance or spatial relationship of the overall recording.
Ex. Placing a mic close to the double-reed section in an orchestra.
Ambient Placement
Ambient mic is placed at a distance so that the room sound is more prominent than the source sound.
Reasons for use of ambient mics:
- In a live concert, to restore natural reverberation and room sound lost by close mic techniques.
- In live recordings, ambient mics pick up audience sound.
- In studios, used to add a sense of natural acoustics back into the sound. This is particularly useful when recording a ÒsectionÓ such as horns, strings or vocals.
Stereo Techniques
The use of two microphones in one of three ways:
- Spaced
pair – Uses time and amplitude cues to create a stereo image. Two mics of the same manufacturer, type and
model can be placed from 3Õ to 30Õ in front of an ensemble in left to
right fashion in order to achieve an overall stereo image. The primary
drawback of this technique is the possibility of phase discrepancies due
to the difference in arrival time of the sound between the two mics.
- XY
coincident pair technique – Uses only amplitude cues to create a
stereo image. Two directional mics of
the exact same type, manufacturer and model are placed with their grills
as close together as possible without touching. The should face between 90¡
and 135¡.
The midpoint between the the mics is faced toward the source and the mic
outputs are equally panned left and right. The stereo imaging in this case
is excellent and often better than the spaced pair. The generally accepted
polar pattern is cardiod. The Blumlein technique uses two crossed
bi-directional mics. (See page 119 for drawing.)
- M-S
method – Mid-side technique. M/S classic technique uses one mic to
be the mid-positioned pickup, generally oriented towards the sound source
and using a cardioid pick-up pattern. The S (side) capsule is generally
chosen to be a figure-8 pattern thatÕs oriented sideways (with the 90¡
null side facing the cardioidÕs main axis). ÒMÓ picks up direct sound and
ÒSÓ picks up the ambient sound.
DIRECT RECORDING
D.I. (direct injection) box serves to interface in the following ways:
- Reduces an instrumentÕs line-level output to mic level for direct insertion into console.
- Changes an instrumentÕs unbalanced, high-source impedance line to a balanced, low-impedance signal.
- Electrically isolates audio signal paths reducing ground loop hum.
- It is
common to use both the direct sound and a mic sound in combination in
recording. Especially used in recordings of bass.