This document covers Crystal Microphone

1. Crystal Microphone
By Prof. Hitesh Dholakiya
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2. Outlines
❖ Basics of Crystal Microphone
❖ Structure of Crystal Microphone
❖ Elements of Crystal Microphone
❖ Working of Crystal Microphone
❖ Directivity of Crystal Microphone
❖ Characteristics of Crystal Microphone
❖ Features of Crystal Microphone
❖ Applications of Crystal Microphone
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3. Basics
❖ based on the principle of ‘Piezo Electric Effect’
❖ ‘Difference of potential between the opposite faces of some crystals is
produced when these are subjected to mechanical pressure.’
❖ crystals : Quartz, Tourmaline, Rochelle salt and ceramic.
❖ Rochelle Salt : high piezo electric effect but is susceptible to moisture –
cannot withstand with high temperature of 50o C
❖ Quartz and Tourmaline : low piezo electric effect
❖ Ceramic : most suitable for crystal microphones as it is not susceptible to
moisture and can withstand high temp up to 100o C
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4. Structure of Crystal Microphone
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5. Elements
❖ Crystal is cut along certain planes to form a slice.
❖ Metallic foil electrodes are attached to two surfaces to carry the potential
difference to the output terminals.
❖ Two thin crystal slices suitably cut are placed in an insulating holder with
an air space between them. Large number of such elements are combined
to increase emf.
❖ Diaphragm : made of Aluminum, is attached to the crystal surface through
a push road.
❖ The whole unit is enclosed in a protective case.
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6. Working
❖ sound wave compression – compresses crystal
❖ rarefaction – converse takes place – crystal is extended and is
under tension
❖ due to this compression and extension – varying potential
difference is generated which is proportional to the mechanical
pressure applied to the crystal by the sound waves – Pressure
Microphone
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7. Characteristics
❖ Sensitivity : good – about 50 mV for 0.1 Pa
❖ SNR : high – about 40 dB
❖ Freq Resp : 100-8000 Hz
❖ Distortion : low – about 1%
❖ Directivity : Omni-directional
❖ Out Imp : high – about 1 MΩ
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8. Features
❖ can be spoken into at close range
❖ high imp – requires relatively short leads to the input circuit of the
amplifier to prevent loss of higher audio frequencies, or pick of hum.
❖ mixer circuit will load it and cause severe loss of bass – hence cannot
be used in multi microphone system
❖ does not need a bias supply
❖ should not be exposed to direct sun light for long time
❖ cost is low
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9. Applications
❖ Home recording systems
❖ Amateur Communication
❖ Mobile Communication
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