More Microphone Design/Application

1
More Microphone Design/Application

• Transient Response to
• Phantom Power

2
Transient Response

• Measure of how quickly a microphones diaphragm
  will react to acoustic waveform.
• No accepted measure.
• Dynamic mics usually have larger diaphragm,
  coil, and core. All add up to slower transient
  response.
• Ribbon mics have thinner, lighter diaphragms.
  Better transient response.
• Condenser mics have extremely lightweight
  diaphragms, offering little mechanical
  resistance. Generally have most accurate
  transient response.

3
Microphone Output Characteristics

• Sensitivity Rating
• Equivalent Noise Rating
• Overload characteristics
• Microphone Impedance

4
Sensitivity Rating

• The output level (in volts) produced by a given
  (standardized) acoustic input signal (in dB SPL).
• Also specifies the amount of amplification
  required to bring signal up line level (-10 dBV
  in consumer systems 4 dBm in pro systems)
• Allows comparison of mics.
• Sensitivity proportional to output-signal voltage.

5
Equivalent Noise Rating

• Self-noise of device, measured in dB SPL or dBA
  (weighted).
• Generally not as pronounced as self-noise of
  other stages in recording chain, but more
  noticeable with technological advancements.
• Dynamic and Ribbon mics s-n from movement of
  electrons in ribbon or coil.
• Condenser s-n from built-in preamp.

6
Overload Characteristics

• Upper limit of SPL that wont distort.
• Dynamic typically around 140 dB
• Condenser typically doesnt distort except at
  very high levels. However, mic preamp can be
  overloaded.
• Pad or attenuation switch.
• Pad reduces overall signal-to-noise ratio by
  amount of pad.

7
Microphone Impedance

• Output impedance (sometimes called Z)
• Impedance measures resistance to signal flow.
• Important to match output (signal providing)
  capability to input (signal receiving) capability
  (flow).
• Typical low 50 ?, 150 to 250 ?
• Typical high 20 to 50 k? (high)
• High impedance likely to pick up noise through
  cable.
• Low impedance can have long cable runs, but can
  pick up electromagnetic noise. (shielded, twisted
  pair, balanced line, 200 ? used to prevent this)

8
Balanced Lines

• Balanced line 3 wires to carry audio signal.
• Audio signal carried on two pins (wires), with
  different polarities
• Third pin is a ground.
• Transformer/summing amp eliminates added noise
  (equal polarity on both pins), responds to
  difference in voltage between two pins.
• XLR 2 (), 3 (-), 1 (ground)
• TRS tip-ring-sleeve 1/4, ring (), tip (-),
  sleeve (ground)
• Less susceptible to external noise.

9
Unbalanced Lines

• Two wires to carry audio signal.
• XLR 2 is still , but (-) pin connected to
  ground (along with 1).
• TS tip-sleeve, tip (), sleeve (ground)
• More susceptible to external noise.

10
Microphone Preamps

• Mic output levels too low to drive line level
  input of recording systems.
• Boost needed (30 - 60 dB)
• Most mixing consoles have stock pres.
• Recording professionals usually use outboard
  (separate from mixer) preamps.
• Higher quality (lower noise/distortion)
• Special sound characteristics
• Special circuitry, or tube designs
• More control over input gain, filtering, perhaps
  compression.
• With the general demise of large, analog
  consoles, classic pres from these boards are
  being marketed separately.

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Phantom Power

• Power needed for condenser (and other active)
  microphones.
• 48V, DC, sent to mic condenser and impedance
  preamp over pins 2 and 3.
• Equally distributed through the use of identical
  value resistors (often hand-chosen)
• Input preamp transformer looks for voltage
  differences.
• Matched 48V over both leads is eliminated at
  preamp transformer. Doesnt reach preamp gain
  stage.