Microphones and Cables

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

Microphone Design

3 Main transducer types

DYNAMIC

RIBBON

CONDENSER

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

Flat Frequency Response

Responds equally to all frequencies

Shaped Frequency Response

Enhances or reduces certain 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 Polar Patterns:Omnidirectional

Microphone Polar Patterns:Cardioid (unidirectional)

Microphone Polar Patterns: Bi-Directional

Microphone Polar Patterns: other cardioid flavors

Hyper cardioid

Super cardioid

Polar Patterns Compared

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.

Balanced vs. unbalanced

Audio Cables: Balanced

2 wires carry signal; 3rd wire is neutral ground (no voltage)

Neither signal wire is connected to the ground

Balanced connectors

XLR - pin 2=hot; pin3=negative; pin1=ground

1/4” TRS

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

Unbalanced connectors

1/4”

RCA

Audio snake

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

Stereo microphone techniques