Transducers for Music Dr Ian Drumm www'aee'salford'ac'uk Student Area Undergraduate Username: Studen

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Transducers for MusicDr Ian Drummwww.aee.salford
.ac.uk -gt Student Area UndergraduateUsername
Students Password Module

• Aims
• To discuss condenser and electret microphones
• To discuss crystal microphones
• Learning Outcomes
• Capacitors for Transduction
• Piezoelectric effect.

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Condenser microphone
Diaphragm and back plate form Capacitor
Charged with 48 Volts DC phantom power
Capacitance varies with distance
High Z to achieve useful signal which then must
be amplified (pre-amp stage).
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About Condensers
No moving coil in magnetic field, diaphragm can
be a very light disk (12-25mm diameter)
High quality standard for many studio
applications instrument grade microphones
Nueman U87
Need for phantom power Susceptible to humidity
Bruel and Kjaer 4190
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e0 is the permittivity of free space (8.8542 x
10-12 C2/(N m2) S is the surface area of a
plate of the capacitor x is the separation
distance between the plates. C is capacitance
without material between plates Q is charge
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Equilibrium component and Fluctuating component
Displacement/Pressure relationship at low
frequency
Hence substituting for x
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If the polarizing and load impedances are high,
"no" currents flow in the steady state - the
charge Q is constant THIS TECHNIQUE IS CALLED
CONSTANT CHARGE
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Factors influencing open circuit sensitivity

• polarizing field Vpol/x0 can't be too big or
• SPARKS (Puncture the thin metal diaphragm)
• displacement response S/k can't be too big or
• Lower bandwidth
• Disrupt the field (polar effects)
• Distortion

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Design Precautions
Blocking Capacitor removes phantom power from
signal
Pressure release vent protects from low
frequencies
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Electret Microphones
Permanent (electrostatic) charge removing need
for phantom power Cheap, compact and easy to
mass produce. Classic applications include
portable cassettes, tie clips Modern high
quality examples (Back Electrets) apply charge to
back plate
AKG C1000S
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Crystal Microphones

• Utilises piezoelectric effect
• Mechanical stress causes voltage
• Rochelle Salt, Quartz, Some Ceramics
• Diaphragm couples vibration to crystal
• Cheap
• Restricted band width
• Telephone Handsets and Specialised Uses
• Guitar pickups and accelerometers

CADs HM50 For harmonica