Microphones Flashcards
Which microphone design works on the electrostatic principle?
Condenser
Microphone
A transducer which changes
acoustical energy into electrical
impulses
Dynamic Microphones
An electrical signal is produced by moving a conductor within a magnetic field
(Electromagnetic Theory of Induction)
2 Basic Designs of Dynamic Microphones
Moving Coil
Ribbon
Which microphone needs a special cable between the mic and power supply?
Tube type condensers
(Cable contains extra conductors which carry the voltage separate from the
audio)
Phantom Power
The audio and DC supply voltage travel
in the same conductors (48 Vdc)
Electret-Condenser
The polarizing charge is stored permanently within the diaphragm or
back plate
✤ No form of powering is required to charge the diaphragm
✤ The pre-amp will still require power
Polar Charts
A graph showing a mics relative sensitivity to sounds
originating at various locations
Omni-Directional
(Pressure) Microphone
Reacts equally to all sound sources, regardless of direction
Contains a diaphragm and a sealed enclosure
Bi-Directional Microphone
Pressure Gradient
✤ Equally sensitive to sounds originating 0° and 180°
✤ Least sensitive to sounds originating from 90° and 270°
✤ Both sides of the diaphragm are exposed
(Figure-8)
Uni-Directional Microphone
Phase Shift
Known as a Cardioid, due to its “heart” shape
✤ Most sensitive at 0°
✤ As the mic is rotated through a 360° arc the level continually decreases until 180° then increases again from 180° to 360°
Specialty Microphones
Parabolic Reflector
Boundary
Lavalier
Shotgun
Direct Box
Direct Injection (DI) box
✤ Reduces line level source to mic
level
✤ Change a high-Z source to low-Z
✤ Isolation of audio signals
Frequency Response
On axis frequency response curve is the measurement of the output over the audible range at a constant input
✤ Output vs. frequency
Proximity Effect
✤ A build up in the bass response of a directional mic used at close proximity
(Many directional microphones have a hi-pass filter to compensate for the proximity effect)
Sensitivity
Tells about the efficiency of converting acoustic energy to electrical energy at a specific standardized input level
Tells the degree of amplification required to raise the signal to operating line level (-10 dBV or +4 dBm)
Typical ratings for Sensitivity
✤ Condenser = -45 dBm per Pa
✤ Dynamic = -55 dBm per Pa
or
✤ Condenser = -65 dBV per µbar
✤ Dynamic = -75 dBV per µbarMicrophone Impedance
The opposition to current flow- measured in ohms
Output or source impedance: How easily the audio power will flow from the source.
Input or load impedance: How much power the input will tend to draw
Low-Z Mic Impedance
50 ohms, 150, or 250
Low-Z mics are fairly insensitive to electrostatic noise
High-Z Impedance
20k ohms to 50k
Decca Tree
Left and right mic pair placed 3 feet apart, and a third mic is placed 1.5ft in front and panned to the center
Ambient Miking
Places the pickup at such a distance that the reverberant or room sound is equally more prominent than the direct signal. The ambient pickup is often a cardioid stereo pair or crossed figure-8 (Bumlein) pair that can be mixed into a stereo or surround-sound production to provide a natural reverb and/or ambience
Stereo Miking Technique
The use of two microphones in order to obtain a coherent stereo image.
4 Fundamental Stereo Miking Techniques
Spaced Pair
X/Y
M/S
Decca Tree
Spaced Pair
Can be placed in front of an instrument or ensemble (in a left/right fashion) to obtain an overall stereo image.
X/Y
An intensity-dependent system that uses only the cue of amplitude to discriminate direction. Facing at angles from 90 to 135 degrees
M/S
Similar to X/Y technique, however, it requires the use of an external transformer, active matrix, or software plug-in in order to work
Two mics, cardioid that faces sound source, S (side) oriented sideways at 90 degrees and 270 degrees
Ribbon Mic
Works on principle of electromagnetic induction
Dynamic Mic
Uses electromagnetic induction
Theory of Electromagnetic Induction
States that 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 that metal
Specialty Microphones
Shotgun
Parabolic Reflector
Boundary
Lavalier
Unbalanced Lines
A single conductor caries the positive (in phase) signal
✤ Shield carries the negative (return) signal to ground
✤ Susceptible to picking up noise
Balanced Lines
Audio signal is carried by two conductors (positive and negative)
✤ Neither is tied to ground
✤ Ground is carried by the outer shield
Sensitivity Refrences
10 dynes/cm2 = 10 ubars = 1 Pa = 94 DBSPL
1 dynes/cm2 = 1 ubar = 0.1 Pa = 74 DBSPL
In a Moving Coil Microphone, attached to the diaphragm is the ______ which is suspended in a high flux magnetic field
voice coil
resistive damping felt
achieve flat frequency response
capacitance determined by
composition and surface areas of plates (fixed value)
dielectric or substance between the plates, air (fixed)
distance between the plates (variable with changing air pressure)
In Condensor microphone, capsule consists of 2 thin plates
one movable (diaphram) one fixed (back plate)
In condensor microphones, when distance between plate decreases
capacitance increases
vise versa
FET
field effect transitor
electret-condensor will stay charged for
about 10 years
predicted 30-100 years for capsule sensitivity to drop 3 dB
directional characteristics
humans have selective hearing, even with distractions
mics do not and pick up and transmit anything in their path
Anechoic Chamber
reflection free environment
used for testing mics
potentiometer
ability to create many variable patterns
cardioid + cardioid=
omni-
cardioid - cardioid=
Bi-
cardioid - cardioid (lesser value)=
super- to hyper-
cardioid + 0 =
Cardioid
In High Z, as cable length increases, so does its capacitance. what is the formula
f=1/2(pie)RC
common mode rejection
eliminating any noise at the input transformer
pin 1 in balanced lines is
always the shield
never use more than how many microphones
2
3-1 rule
every unit of distance measured between a mic should be at least 3 times unit of measure of the first mic
2-1 Rule
every unit of distance measured between a mic should be at least 2 times the unit of measure of the first mic
make sure mic lines are properly
shielded
balanced
terminated
