Transcript Microphones - Music Technology 2

Microphones and Cables
S
What is a microphone?
S Transducer = changes one form of energy into another
S Initial energy = Sound waves
S Transduced energy = electrical impulses (voltage)
Microphone Quality Variables
S Placement
S Distance
S Acoustic Environment
S Microphone operate type
S Microphone design
S Microphone quality
Microphone General Guidelines
S #1 - There are no rules! Only guidelines…
S #2 - The overall quality of an audio signal is no better than
the weakest link in the signal
S Microphone, cable, mixer, poor placement, poor performance
S #3 – The “good” rule; good musicians, room acoustics,
microphone and placement = good sound
Microphone Design
S 3 Main transducer types
S DYNAMIC
S RIBBON
S CONDENSER
Dynamic Microphone
S Electromagnetic induction
S Mylar diaphragm and voice
coil suspended in magnetic
field
S Acoustic pressure hits
diaphragm, displacing
voice coil; movement along
magnetic field create
electrical signal
Dynamic microphone
S Lower dynamic range
S Can push louder signals
and are ideal for louder or
outdoor performances
S Not ideal for high-quality
audio production or studio
recording.
Ribbon Microphone
S Electromagnetic
induction
S Corrugated aluminum
ribbon diaphragm
suspended in magnetic
flux
S Diaphragm cuts across
flux lines to create current
Ribbon Microphone
S Wider dynamic range than
dynamic microphones
S Often used in radio
broadcasts or to amplify
speaking voice
Condenser Microphone
S Electrostatic principle
S 2 thin plates - one moveable
and one fixed store an
electrical charge (capacitor)
S Direct Current (DC)
power supply provides
voltage to capacitor
S Capacitance changes
with sound pressure
Condenser (contd.)
S Signal has high impedance (more on that later)
S Amplifier on mic’s body prevents hum, noise, and signal-
level losses
S Some use vacuum tubes
S ELECTRET-CONDENSER
S Same principles but doesn’t require external power, referred to
as phantom power
Condenser Microphone
S Widest dynamic range
S Most sensitive frequency
and transient response
S Good for live performance
and recording (studio and
live)
S Type of microphone found
in many handheld
recorders
Phantom Power
S Positive DC supply of
voltage
S +48 Volts
S Supplied through
microphone cable;
activated through audio
interface and/or mixer.
S Powers modern condenser
microphones
Frequency Response
S Measurement of OUTPUT over audible frequency range
when driven by a constant signal
S Gives clues about how a microphone will react at different
frequencies
Flat Frequency Response
S Responds equally to all frequencies
Shaped Frequency Response
S Enhances or reduces certain frequencies
Low-Frequency Response
Characteristics
S Rumble (3-25Hz) can occur in a studio along floor space
from
S Trucks or other outside automobiles/heavy machinery
S Air Conditioners
S Avoid this by
S Using a shock mount for the microphones
S Choose mic with restricted low frequency response
S Use filter to restrict frequency range
Low-Frequency Response
Characteristics
S Proximity Effect
S Bass response when directional mic is brought within 1 foot of
sound source
S Bass boost increases as distance decreases
S Avoid this by
S Low-frequency roll-off filter switch on some mics
S Use EQ to remove low end
S Use omni-directional mic rather than cardioid
Transient Response
S How quickly the diaphragm reacts when hit by an acoustic
wavefront
S Varies widely!
S Major reason for differences in sound quality among
microphones
Transient Response (contd.)
S Dynamic mic - large diaphragm; slow response; rugged,
gutsy, less accurate
S Ribbon mic - much lighter diaphragm; reacts more quickly;
cleaner sound
S Condenser - very light diaphragm; accurately tracks waves
over entire frequency range
Microphone Characteristics
S Directionality
S Output level (sensitivity) at various angles of incidence
S Polar response - polar pattern
S Graphically plots mic’s sensitivity in 360 degrees
S 2 Directionality types
S Omnidirectional
S Directional (uni- and bi-)
Microphone Polar Patterns:
Omnidirectional
Microphone Polar Patterns:
Cardioid (unidirectional)
Microphone Polar Patterns: BiDirectional
Microphone Polar Patterns:
other cardioid flavors
Hyper cardioid
Super cardioid
Polar Patterns Compared
Microphone Output
Characteristics
S Sensitivity Rating
S Output level in volts, given specific standardized input
S Equivalent Noise Rating
S Device’s electrical self-noise
S Overload Characteristics
S Distortion capabilities (eg. Dynamic range of dynamic mic =
140dB)
Microphone Impedance
S Rating used to match signal-providing capability of one
device to signal-drawing requirements of another device
S Measured in OHMS (Ω)
S Low impedance = 50, 150, 250 OHMs
S High impedance = 25 OHMs
High Impedance Mics
S Lower cost
S Maximum cable length =
10’
S Uses unbalanced cable
S Not useable in high
quality audio
applications
Low Impedance Mics
S Can drive long cable
lengths
S Balanced output
S Shielded - provides
protection from noise
and interference
S Best option for high
quality sound
What about condenser mics?
S Condenser microphones have high impedance signal, but
are ideal of high quality audio. How can this be?
S Wide dynamic range and light frequency/transient response
S Built-in impedance converters. Operated using external
phantom power.
Balanced vs. unbalanced
Audio Cables: Balanced
S 2 wires carry signal; 3rd wire is neutral ground (no voltage)
S Neither signal wire is connected to the ground
Balanced connectors
S XLR - pin 2=hot;
pin3=negative; pin1=ground
S 1/4” TRS
Audio cables: Unbalanced
S Line-level and high-impedence mics
S 1 wire carry the signal; 2nd is ground (no voltage)
S Can be noisy at low levels
Unbalanced connectors
S 1/4”
S RCA
Audio snake
Stereo Recording
S Most recordings use a stereo set up
S Three basic types
S Coincident
S Near-coincident (or quasi-coincident)
S Spaced
Spaced Pair (or more)
S Two (or three) mics spaced apart
S Between 8” and 60”
S Usually Omnidirectional
S Cardioid if a noisy crowd!
S Uses time-of-arrival cues for stereo image
S Good for large ensembles in large rooms
Decca Tree
S Classical, time-tested technique
S Although not used as much
S Time and Amplitude cues
S 3 omni-mics
S L and R 3 ft.(or 2m) apart, 3rd 1.5 ft. (or 1.5m) front
Coincident
S Two closely spaced mics at the same location oriented
differently
S Stereo imaging due to amplitude
S Tend to produce more precise spatial imaging
S Trade-off is decreased sense of room spaciousness
Near-Coincident
S Pairs of directional mics placed close together
S Separated by a distance of up to 30”
S Uses time and amplitude cues
S Precise imaging of coincident
S Sense of spaciousness from from spaced
Stereo microphone techniques