Piezoelectric crystals Optical axis Z Mechanical axis Y

Piezoelectric crystals Optical axis Z Mechanical axis Y Electrical axis X Plate with Xcutting Quartz KNT (Seignette salt)

Vibration types of piezoelectric plates Compressional – longitudinal Compressional – thickness Shear – longitudinal Shear - thickness

Piezoelectric materials • • • acid amonium phosphate (ADP) Seignette-Rochelle’s salt (KNT) barian titanete (BAT) lead titanete zirconato (PZT) polymers vinylidene polyfluoride

Piezoelectric bimorphs Bending bimorph Torsional (twisting) bimorph

Piezoresistive transducer Housing Membrane Beam Piezoresistor

Wheatstone’s bridge with piezoresistors

Carbon (coal) transducer Membrane Semispherical electrode Coal pulver chamber Fixed electrode

ELECTROACOUSTICS PART IV MICROPHONES

Microphones – the general classification • Pressure mics – the electrical response is caused by variations in pressure of actuating sound wave • Velocity mics – the electrical response corresponds to particle velocity resulting from the propagation of sound wave through acoustic medium the difference in pressure between two points in space pressure gradient • Pressure-velocity mics – a combination of the above

Microphones • - Pressure microphones carbon mics condenser mics piezoelectric mics moving conductor mics (dynamic – moving coil)

Microphones • Velocity mics - ribbon dynamic mics (with both sides of membrane actuated by the sound wave) - condenser mics (with both sides of membrane actuated by the sound wave)

Microphones – kind of transducer • Dynamic microphones (moving coil, ribbon) • Condenser microphones („real” condenser, electret)

Sensitivity of microphone • Sensitivity in free field • Sensitivity in diffused field • Pressure sensitivity

Other parameters • • Output impedance Frequency response Directivity (polar pattern) Noise level

Pressure microphones

Carbon microphone

Microphones – the voltage (at open circuit)

Dynamic pressure microphone

Frequency response

Diffraction effect and its correction

bass-reflex system

bass-reflex system (cont. )

Helmholtz resonator

Helmholtz resonator (cont. )

Condenser microphone

Condenser microphone

Electret microphone

Equivalent circuit Ce

Preamplifier for condenser mics

Frequency responses Diffractional effects diaphragm The aperture for pressures equalization

Gradient microphones – basic idea d

Polar pattern of gradient mic

Condenser gradient mic

Equivalent circuit

Pressure –gradient microphones

Polar patterns (directivity)

Polar patterns

Variable polar patterns

Cardioid condenser microphone with 2 membranes

The schematic action of condenser cardioid microphone w. 2 diaphragms

Equivalent circuit