 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