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Download Microphones - Music Technology 2
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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