Physics
Microphone is a device which transforms the

Light signal into sound signal
Sound signal into light signal
Current signal into sound signal
Sound signal into current signal

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Physics
When a piece of pure silicon is doped with aluminium, then

The doped silicon piece will become n-type
The doped silicon piece will become p-type
The resistivity of the doped silicon piece will increase
The conductivity of the doped silicon piece will remain the same

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