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