Crystal earplug

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The simplest speaker to use in a crystal receiver is the crystal earplug.

-Sensitivity
The sensitivity of a crystal earplug is fearly good
Klick here to see the sensitivity compared with other speakers.
At frequencies below 1 kHz however the sensitivity will drop rapidly.
Also at high frequencies the sensitivity will decrease.

-Sound quality
The sound quality of a crystal earplug is poor, it has almost no low frequency responce.

-Impedance
The DC resistance of a crystal earplug is very high (more than 20 Mega Ohm).
For audio frequencies the impedance depends on the capacitance of the earplug.
Normaly the capacitance is about 500 pF.
With the formula: Impedance (Z)= 1/(2.pi.f.C) we can calculate the impedance at a certain audio frequency.

Z = impedance in Ohm.
pi = 3.14
f = frequency of audio signal in Hertz
C= capacitance of the crystal earplug in Farad.

With 500 pF we find Z= 318 k.Ohm at 1000 Hz, and Z= 106 k.Ohm at 3000 Hz.
The higher the audio frequency, the lower the impedance.

Parallel to the crystal earplug we must connect a resistor, this resistor provides a return path to ground for the current rectified by the diode.
When we don't use the resistor, the capacitance of the earplug will be charged, and once it is fully charged the earplug wont't give sound until it is discharged.

The parallel resistor and the capacitance of the earplug together form a lowpassfilter with frequency: f=1/(2.pi.R.C).

If we take R=100 k.Ohm and the capacitance is 500 pF than we have a lowpassfilter with a 3200 Hz passband, this is a useable value.
A higher value of resistor wil give a smaller audio bandwidth and reduced sound quality.

A part of the audio energy wil get lost in the resistor, and don't reach the crystal earplug.
The lower the resistor value, the more energy wil be lost.

 Connection of a crystal earplug

-Reliability
A common problem with crystal earplugs is that the sound suddenly fals out. This has to do with warming up of the speaker (in your ear), sometimes the sound comes back after ticking against the earplug.
For the rest there is not much we can do to this problem.

 Crystal earplug.Inside is a piece of piëzo crystal, this bends when you apply voltage to it. It gives sound when we put a audiosignal on it.

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